Brought to you by that maniac that brings you the ANTLR parser generator!
Terence Parr
University of San Francisco
parrt at cs.usfca.edu
Copyright 2003-2005
http://www.stringtemplate.org
(StringTemplate released under BSD License)
Version 2.2, August 5, 2005
Note that the TestStringTemplates.java file has many tests that are an excellent set of examples. Also, I highly recommend you read the (academically-oriented) paper, Enforcing Model-View Separation in Template Engines. There are some more examples including nested menu generation that will be of interest.
Most programs that emit source code or other text output are unstructured blobs of generation logic interspersed with print statements. The primary reason is the lack of suitable tools and formalisms. The proper formalism is that of an output grammar because you are not generating random characters--you are generating sentences in an output language. This is analogous to using a grammar to describe the structure of input sentences. Rather than building a parser by hand, most programmers will use a parser generator. Similarly, we need some form of unparser generator to generate text. The most convenient manifestation of the output grammar is a template engine such as StringTemplate.
A template engine is a simply a code generator that emits text using
templates, which are really just "documents with holes" in them where
you can stick values. StringTemplate breaks up your template into
chunks of text and attribute expressions, which are by default
enclosed in dollar signs $attribute-expression$ (to make them
easy to see in HTML files). StringTemplate ignores everything
outside of attribute expressions, treating it as just text to spit out
when you call StringTemplate.toString(). For example, the following
template has two chunks, a literal and a reference to attribute
name:
Hello, $name$
Using templates in Java code is very easy. Here is the requisite example that prints "Hello, World":
StringTemplate hello = new StringTemplate("Hello, $name$");
hello.setAttribute("name", "World");
System.out.println(hello.toString());
StringTemplate is not a "system" or "engine" or "server"; it's a library with two primary classes of interest: StringTemplate and StringTemplateGroup. You can directly create a StringTemplate in Java code, you can load a template from a file, and you can load a single file with many templates (a template group file).
StringTemplate was born and evolved during the development of http://www.jGuru.com. The need for such dynamically-generated web pages has led to the development of numerous other template engines in an attempt to make web application development easier, improve flexibility, reduce maintenance costs, and allow parallel code and HTML development. These enticing benefits, which have driven the proliferation of template engines, derive entirely from a single principle: separating the specification of a page's business logic and data computations from the specification of how a page displays such information. These template engines are in a sense are a reaction to the completely entangled specifications encouraged by JSP (Java Server Pages). With separate encapsulated specifications, template engines promote component reuse, pluggable site "looks", single-points-of-change for common components, and high overall system clarity. In the code generation realm, model-view separation guarantees retargetability.
When developing StringTemplate, I recalled Frederick Brook's book, "Mythical Man Month", where he identified conceptual integrity as a crucial product ingredient. For example, in UNIX everything is a stream. My concept, if you will, is strict model-view separation. My mission statement is therefore:
"StringTemplate shall be as simple, consistent, and powerful as possible without sacrificing strict model-view separation."
I ruthlessly evaluate all potential features and functionality against this standard. Over the years, however, I have made certain concessions to practicality that one could consider as infringing ever-so-slightly into potential model-view entanglement. That said, StringTemplate still seems to enforce separation while providing excellent functionality.
I let my needs dictate the language and tool feature set. The tool evolved as my needs evolved. I have done almost no feature "backtracking". Further, I have worked really hard to make this little language self-consistent and consistent with existing syntax/metaphors from other languages. There are very few special cases and attribute/template scoping rules make a lot of sense even if they are unfamiliar or strange at first glance. Everything in the language exists to solve a very real need.
After examining hundreds of template files that I created over years of jGuru.com (and now in ANTLR v3) development, I found that I needed only the following four basic canonical operations (with some variations):
where template references can be recursive.
Language theory supports my premise that even a minimal StringTemplate engine with only these features is very powerful--such an engine can generate the context-free languages (see Enforcing Strict Model-View Separation in Template Engines); e.g., most programming languages are context-free as are any XML pages whose form can be expressed with a DTD.
The normal imperative programming language features like setting variables, loops, arithmetic expressions, arbitrary method calls into the model, etc... are not only unnecessary, but they are very specifically what is wrong with JSP. Recall that JSP allows arbitrary Java expressions and statements, allowing programmers to incorporate computations and logic in their templates. A quick scan of template engines reveals an unfortunate truth--all but a few are Turing-complete languages just like JSP. One can argue that they are worse than JSP because they use languages peculiar to that template engine. Many tool builders have clearly lost sight of the original problem we were all trying to solve. We programmers often get caught up in cool implementations, but we should focus on what should be built not what can be built.
The fact that StringTemplate does not allow such things as assignments (no side-effects) should make you suspicious of engines that do allow it. I guarantee that the templates in ANTLR v3's code generator are vastly more complicated than any web page, for example, created for use with another template engine and I have yet to find a situation where I needed assignments. If your template looks like a program, it probably is--you have totally entangled your model and view.
While providing all sorts of dangerous features like assignment that promote the use of computations and logic in templates, many engines miss the key elements. Certain language semantics are absolutely required for generative programming and language translation. One is recursion. A template engine without recursion seems unlikely to be capable of generating recursive output structures such as nested tables or nested code blocks.
Another distinctive StringTemplate language feature lacking in other engines is lazy-evaluation. StringTemplate's attributes are lazily evaluated in the sense that referencing attribute "a" does not actually invoke the data lookup mechanism until the template is asked to render itself to text. Lazy evaluation is surprising useful in both the web and code generation worlds because such order decoupling allows code to set attributes when it is convenient or efficient not necessarily before a template that references those attributes is created. For example, a complicated web page may consist of many nested templates many of which reference $userName$, but the value of userName does not need to be set by the model until right before the entire page is rendered to text via toString(). You can build up the complicated page, setting attribute values in any convenient order.
StringTemplate implements a "poor man's" form of lazy evaluation by
simply requiring that all attributes be computed a priori. That is,
all attributes must be computed and pushed into a template before it
is written to text; this is the so-called "push method" whereas most
template engines use the "pull method". The pull method appears
more conventional because programmers mistakenly regard templates as
programs, but pulling attributes introduces order-of-computation
dependencies. Imagine a simple web page that displays a list of
names (using some mythical template engine notation):
<html>
<body>
<ol>
$foreach n in names$
<li>$n$</li>
$end$
</ol>
There are $numberNames$ names.
</body>
</html>
Using the pull method, the reference to names invokes model.getNames(), which presumably loads a list of names from the database. The reference to numberNames invokes model.getNumberNames() which necessarily uses the internal data structure computed by getNames() to compute names.size() or whatever. Now, suppose a designer moves the numberNames reference to the <title> tag, which is before the reference to names in the foreach statement. The names will not yet have been loaded, yielding a null pointer exception at worst or a blank title at best. You have to anticipate these dependencies and have getNumberNames() invoke getNames() because of a change in the template. I'm stunned that other template engine authors with whom I've spoken think this is ok. Any time I can get the computer to do something automatically for me that removes an entire class of programming errors, I'll take it! Automatic garbage collection is the obvious analogy here.
The pull method requires that programmers do a topological sort in their minds anticipating any order that a programmer or designer could induce. To ensure attribute computation safety (i.e., avoid hidden dependency landmines), I have shown trivially in my academic paper that pull reduces to push in the worst case. With a complicated mesh of templates, you will miss a dependency, thus, creating a really nasty, difficult-to-find bug.
Just so you know, I've never been a big fan of functional languages and I laughed really hard when I realized (while writing the academic paper) that I had implemented a functional language. The nature of the problem simply dictated a particular solution. We are generating sentences in an output language so we should use something akin to a grammar. Output grammars are inconvenient so tool builders created template engines. Restricted template engines that enforce the universally-agreed-upon goal of strict model-view separation also look remarkably like output grammars as I have shown. So, the very nature of the language generation problem dictates the solution: a template engine that is restricted to support a mutually-recursive set of templates with side-effect-free and order-independent attribute references.
Here is a simple piece of Java that creates and uses a template on the fly:
StringTemplate query =
new StringTemplate("SELECT $column$ FROM $table$;");
query.setAttribute("column", "name");
query.setAttribute("table", "User");
where StringTemplate considers anything in $...$ to be something
it needs to pay attention to. By setting attributes, you are
"pushing" values into the template for use when the template is
printed out. The attribute values are set by referencing their names.
Invoking query.toString() would yield
SELECT name FROM User;
You can set an attribute multiple times, which simply means that the
attribute is multi-valued. For example, adding another
setAttribute() call makes attribute column multi-valued:
StringTemplate query =
new StringTemplate("SELECT $column$ FROM $table$;");
query.setAttribute("column", "name");
query.setAttribute("column", "email");
query.setAttribute("table", "User");
Invoking query.toString() would now yield
SELECT nameemail FROM User;
Ooops...there is no separator between the multiple values. If you want a comma, say, between the column names, then change the template to record that formatting information:
StringTemplate query =
new StringTemplate("SELECT $column; separator=\",\"$ FROM $table$;");
query.setAttribute("column", "name");
query.setAttribute("column", "email");
query.setAttribute("table", "User");
Note that the right-hand-side of the separator specification in this case is a string literal; therefore, I have escaped the double-quotes as the template is specified in a Java string. In general, the right-hand-side can be any attribute expression. Invoking query.toString() would now yield
SELECT name,email FROM User;
Attributes can be any object at all. StringTemplate calls toString() on each object as it writes the template out. The separator is not used unless the attribute is multi-valued.
To load a template from the disk you must use a StringTemplateGroup
that will manage all the templates you load, caching them so you do
not waste time talking to the disk for each template fetch request
(you can change it to not cache; see below). You may have multiple
template groups. Here is a simple example that loads the previous SQL
template from file /tmp/theQuery.st:
SELECT $column; separator=","$ FROM $table$;
The Java code creates a StringTemplateGroup called myGroup rooted
at /tmp so that requests for template theQuery forces a load of
file /tmp/theQuery.st.
StringTemplateGroup group =
new StringTemplateGroup("myGroup", "/tmp");
StringTemplate query = group.getInstanceOf("theQuery");
query.setAttribute("column", "name");
query.setAttribute("column", "email");
query.setAttribute("table", "User");
If you have a directory hierarchy of templates such as file /tmp/jguru/bullet.st, you would reference them relative to the root; in this case, you would ask for template jguru/bullet().
Note: StringTemplate strips whitespace from the front and back of all loaded template files. You can add, for example, <\n> at the end of the file to get an extra carriage return.
When deploying applications or providing a library for use by other programmers, you will not know where your templates files live specifically on the disk. You will, however, know relative to the classpath where your templates reside. For example, if your code is in package com.mycompany.server you might put your templates in a templates subdirectory of server. If you do not specify an absolute directory with the StringTemplateGroup constructor, future loads via that group will happen relative to the CLASSPATH. For example, to load template file page.st you would do the following:
// Look for templates in CLASSPATH as resources
StringTemplateGroup group = new StringTemplateGroup("mygroup");
StringTemplate st = group.getInstanceOf("com/mycompany/server/templates/page");
If page.st references, say, searchbox template, it must be fully qualified as:
<font size=2>SEARCH</font>: $com/mycompany/server/templates/page/searchbox()$
This is inconvenient and I may add the invoking template's path prefix automatically in the future.
StringTemplate 2.0 introduced the notion of a group file that has
two main attractions. First, it allows you to define lots of small
templates more conveniently because they may all be defined within a
single file. Second, you may specify formal template arguments that
help StringTemplate detect errors (such as setting unknown
attributes) and make the templates easier to read. Here is a sample
group file (I'm using <...> delimiters) with two templates, vardef
and method, that could be used to generate C files:
group simple;
vardef(type,name) ::= "<type> <name>;"
method(type,name,args) ::= <<
<type> <name>(<args; separator=",">) {
<statements; separator="\n">
}
>>
Single line templates are enclosed in double quotes while multi-line templates are enclosed in double angle-brackets. Every template must define arguments even if the formal argument list if blank.
Using templates in a group file is straightforwards. A StringTemplateGroup constructor accepts a Reader so you can pass in a string or file or whatever:
String templates =
"group simple; vardef(type,name) ..."; // templates from above
StringTemplateGroup group = new StringTemplateGroup(new StringReader(templates));
StringTemplate t = group.getInstanceOf("vardef");
t.setAttribute("type", "int");
t.setAttribute("name", "foo");
System.out.println(t);
The output would be: "int foo;".
Sometimes it is convenient to have default values for formal arguments
that are used when no value is set by the model. For example, when
generating a parser in Java from ANTLR, I want the super class of the
generated object to be Parser unless the ANTLR user uses an option
to set the super class to some custom class. For example, here is a
partial parser template definition:
parser(name, rules, superClass="Parser") ::= ...
Any argument may be given a default value by following the name with an equals sign and a string or an anonymous template.
When using a group file format to specify templates, you must specify the formal arguments for that template. If you try to set an attribute via setAttribute that is not specifically formally defined in that template, you will get a NoSuchElementException.
If you reference an attribute that is not formally defined in that template or any enclosing template, you also get a NoSuchElementException exception.
There are situations where you need to translate a string in one
language to a string in another language. For example, you might want
to translate integer to int when translating Pascal to C. You
could pass a Map (hashtable) from the model into the templates, but
then you have output literals in your model! The only solution is to
have StringTemplate support mappings. For example, here is how
ANTLR v3 knows how to initialize local variables to their default
values:
typeInitMap ::= [
"int":"0",
"long":"0",
"float":"0.0",
"double":"0.0",
"boolean":"false",
"byte":"0",
"short":"0",
"char":"0",
default:"null" // anything other than an atomic type
]
To use the map in a template, refer to it as you would an attribute. For example, <typeInitMap.int> which returns "0". If your type name is an attribute not a constant like int, then use an indirect field access: <typeInitMap.(typeName)>.
Maps are defined in the group's scope and are visible if no attribute hides them. For example, if you define a formal argument called typeInitMap in template foo then foo cannot see the map defined in the group (though you could pass it in as another parameter). If a name is not an attribute and it's not in the group's maps table, then the super group is consulted etc... You may not redefine a map and it may not have the same name as a template in that group. The default value is used if you use a key as a property that doesn't exist. For example <typeInitMap.foo> returns "null".
You'll note that the square brackets will denote data structure in other areas too such as [a,b,c,...] which makes a singe multi-valued attribute out of other attributes so you can iterate across them.
The first newline following the << in a template definition is
ignored as it is usually used just to get the first line of text for
the template at the start of a line. In other words, if you want to
have a blank line at the start of your template, use:
foo() ::= <<
2nd line is not blank, but first is
>>
or
foo() ::= <<<\n>
same as before; newline then this line
>>
The last newline before the >> is also ignored and is included in
the output. To add a final newline, add an extra line or <\n>
before the >>:
foo() ::= <<
rodent
>>
or
foo() ::= <<
rodent<\n>
>>
Template:
foo() ::= <<
rodent
>>
on the other hand, is identical to
foo() ::= "rodent"
group
: "group" ID ';' (template|mapdef)*
;
template
: ID '(" (args)? ')' "::=" TEMPLATE
| ID "::=" ID // alias one template to be another
;
args: arg (',' arg)*
;
arg : ID '=' STRING // "..."
| ID '=' ANONYMOUS_TEMPLATE // {...}
;
mapdef
: ID "::=" map
;
map : '[' keyValuePair (',' keyValuePair)* ']'
;
keyValuePair
: STRING ':' STRING
| "default" ':' STRING
;
| An aside: All along, during my website construction days, I kept in mind that any text output follows a format and, thus, output sentences conform to a language. Consequently, a grammar should describe the output rather than a bunch of ad hoc print statements in code. This helped me formalize the study of templates because I could compare templates (output grammars) to well established ideas from formal language theory and context-free grammars. This allowed me to show, among other things, that StringTemplate can easily generate any document describable with an XML DTD even though it is deliberately limited. The group file format should look very much like a grammar to you. |
See the scoping rules section for information on how formal arguments affect attribute look up.
I use .stg file extension, which you might also want to use.
The most common thing in a template besides plain text is a simple named attribute reference such as:
Your email: $email$
The template will look up the value of email and insert it into the output stream when you ask the template to print itself out. If email has no value, then it evaluates to the empty string and nothing is printed out for that attribute expression.
If a named attribute is an aggregate with a property (ala JavaBeans)
or a simple data field, you may reference that property using
attribute.property. For example:
Your name: $person.name$
Your email: $person.email$
StringTemplate ignores the actual object type stored in attribute person and simply invokes getName() and getEmail() via reflection. As a special case, StringTemplate tries x.isFoo() for x.foo if x is Boolean.
If the property is not accessible ala JavaBeans, StringTemplate attempts to find a field with the property name. In the above example, StringTemplate would look for fields name and email without the capitalization used with JavaBeans property access methods.
Because the type is ignored, you can pass in whatever existing Java
aggregate (class) you have such as User or Person:
User u = database.lookupPerson("parrt@jguru.com");
st.setAttribute("person", u);
Or, if a suitable aggregate doesn't exist, you can make a connector or "glue" object and pass that in instead:
st.setAttribute("person", new Connector());
where Connector is defined as:
public class Connector {
public String getName() { return "Terence"; }
public String getEmail() { return "parrt@jguru.com"; }
}
The ability to reference aggregrate properties saves you the trouble
of having to pull out the properties with Java code like this:
User u = database.lookupPerson("parrt@jguru.com");
st.setAttribute("name", u.getName());
st.setAttribute("email", u.getEmail());
and having template:
Your name: $name$
Your email: $email$
Warning: The latter is more widely applicable and totally decoupled from code and logic; i.e., it's "better" but much less convenient. Be very careful that the property methods do not have any side-effects like updating a counter or whatever. This breaks the rule of order of evaluation independence.
Sometimes the property name is itself in which case you need to use
indirect property access notation:
$person.(propertyName)$
where propertyName is an attribute whose value is the name of a property to fetch from person. Using the examples from above, propertyName could hold the value of either name or email.
propertyName may actually be an expression instead of a simple attribute name.
You may pass in instances of type HashMap and Hashtable but cannot
pass in objects implementing the Map because that would allow all
sorts of wacky stuff like database access. Rather than creating an
aggregate object (though I make it easier for you with the automatic
aggregate creation in the next section) you can pass in a HashMap
that has keys referencable within templates. For example,
StringTemplate a = new StringTemplate("$user.name$, $user.phone$");
HashMap user = new HashMap();
user.put("name", "Terence");
user.put("phone", "none-of-your-business");
a.setAttribute("user", user);
String results = a.toString();
yields a result of "Terence, none-of-your-business".
Creating one-off data aggregates in Java is a pain, you have to define
a new class just to associate two pieces of data. StringTemplate
makes it easy to group data during setAttribute() calls. You may
pass in an aggregrate attribute name to setAttribute() with the data
to aggregate:
StringTemplate st =
new StringTemplate("$items:{$it.last$, $it.first$\n}$");
st.setAttribute("items.{first,last}", "John", "Smith");
st.setAttribute("items.{first,last}", "Baron", "Von Munchhausen");
String expecting =
"Smith, John\n" +
"Von Munchhausen, Baron\n";
Note that the template, st, expects the items to be aggregates with properties first and last. By using attribute name
items.{first,last}
You are telling StringTemplate to take the following two arguments as properties first and last.
I have created variants of the setAttribute() method to handle from 2 to 5 properties. This is really handy, at least for Java. :)
As of 2.2, you may combine multiple attributes into a single
multi-valued attribute in a syntax similar to the group map feature.
Catenate attributes by placing them in square brackets in a
comma-separated list. For example,
$[mine,yours]$
creates a new multi-valued attribute (a list) with both elements--all of mine first then all of yours. This feature is handy when the model happens to group attributes differently than you need to access them in the view. This ability to rearrange attributes is consistent with model-view separation because the template cannot alter the data structure nor test its values--the template is merely looking at the data from a new perspective.
Naturally you may combine the list construction with template
application:
$[mine,yours]:{ v | ...}$
Note that this is very different than
$mine,yours:{ x,y | ...}$
which iterates max(n,m) times where n and m are the lengths of mine and yours, respectively. The [mine,yours] version iterates n+m times.
Sometimes you need to treat the first or last element of multi-valued
attribute differently than the others. For example, if you have a
list of integers in an attribute and you need to generate code to sum
those numbers, you could start like this:
<numbers:{ n | sum += <n>;}>
You need to define sum, however:
int sum = 0;
<numbers:{ n | sum += <n>;}>
What if numbers is empty though? No need to create the sum
definition so you could do this:
<if(numbers)>int sum = 0;<endif>
<numbers:{ n | sum += <n>;}>
A more specific strategy (and one that generates slightly better code as
it avoids an unnecessary initialization to 0) is the following:
<first(numbers):{ n | int sum += <n>;}>
<rest(numbers):{ n | sum += <n>;}>
where first(numbers) results in the first value of attribute numbers if any and rest(numbers) results all values in numbers but the first value.
The other operator available to you is last, which naturally results in the last value of a multi-valued attribute.
Special cases:
You may reference other templates to have them included just like the C language preprocessor #include construct behaves. For example, if you are building a web page (page.st) that has a search box, you might want the search box stored in a separate template file, say, searchbox.st. This has two advantages:
Using method call syntax, just reference the foreign template:
<html>
<body>
...
$searchbox()$
...
</body>
</html>
The invoking Java code would still just create the overall page and the enclosing page template would automatically create an instance of the referenced template and insert it:
StringTemplateGroup group =
new StringTemplateGroup("webpages", "/usr/local/site/templates");
StringTemplate page = group.getInstanceOf("page");
If the template you want to reference, say searchbox, is in a subdirectory of the StringTemplateGroup root directory called misc, then you must reference the template as: misc/searchbox().
The included template may access attributes. How can you set the attribute of an included template? There are two ways: inheriting attributes and passing parameters.
Any included template can reference the attributes of the enclosing template instance. So if searchbox references an attribute called resource:
<form ...>
...
<input type=hidden name=resource value=$resource$>
...
</form>
you could set attribute resource in the enclosing template page object:
StringTemplate page = group.getInstanceOf("page");
page.setAttribute("resource", "faqs");
This "inheritance" (dynamic scoping really) of attributes feature is particularly handy for setting generally useful attributes like siteFontTag in the outermost body template and being able to reference it in any nested template in the body.
Another, more obvious, way to set the attributes of an included template is to pass in values as parameters, making them look like C macro invocations rather than includes. The syntax looks like a set of attribute assignments:
<html>
<body>
...
$searchbox(resource="faqs")$
...
</body>
</html>
where I am setting the attribute of the included searchbox to be the string literal "faqs".
The right-hand-side of the assignment may be any expression such as an attribute reference or even a reference to another template like this:
$boldMe(item=copyrightNotice())$
You may also use an anonymous template such as:
$bold(it={$firstName$ $lastName$})$
which first computes the template argument and then assigns it to it.
If you are using StringTemplate groups, then you have formal parameters and for those templates with a sole formal argument, you can pass just an expression instead of doing an assignment to the argument name. For example, if you do $bold(name)$ and bold has one formal argument call item, then item gets the value of name just as if you had said $bold(item=name)$.
When template x calls template y, the formal arguments of y hide any x arguments of the same because the formal parameters force you to define values. This prevents surprises and makes it easy to ensure any parameter value is empty unless you specifically set it for that template. The problem is that you need to factor templates sometimes and want to refine behavior with a subclass or just invoke another shared template but invoking y as <y()> hides all of x's parameters with the same name. Use <y(...)> syntax to indicate y should inherit all values even those with the same name. <y(name="foo", ...)> would set one arg, but the others are inherited whereas <y(name="foo")> only has name set; all other arguments of template y are empty. You can set manually with StringTemplate.setPassThroughAttributes().
The right-hand-side of the argument assignments are evaluated within the scope of the enclosing template whereas the left-hand-side attribute name is the name of an attribute in the target template. Template invocations like $bold(item=item)$ actually make sense because the item on the right is evaluated in a different scope.
When setting parameters or testing IF conditionals, you may find it handy to use the plus "string concatenate" operator. For example, when building web pages, you will find it useful to create a template called link and then use it to generate HTML link tags; you may want to change the way every link looks on your site and it's convenient to have one place to change things. The template might look like:
<a href="$url$"><b>$title$</b></a>
Then in a page template you might reference:
...$link(url="http://www.jguru.com", title="jGuru")$...
or you could use attributes to set the link parameters:
...$link(url=person.homePage, title=person.name)$...
Sometimes you may want to compute the URL; usually it is enough to concatenate strings:
...$link(url="/faq/view?ID="+faqid, title=faqtitle)$...
where faqid and faqtitle are attributes you would have to set for the template that referenced link.
Terence says: I'm a little uncomfortable with this catenation
operation. Please use a template instead:
...$link(url={/faq/view?ID=$faqid$}, title=faqtitle)$...
Imagine a simple template called bold:
<b>$item$</b>
Just as with template link described above, you can reference it from a template by invoking it like a method call:
$bold(item=name)$
What if you want something bold and italicized? You could simply nest the template reference:
$bold(item=italics(item=name))$
where template italics is defined as:
<i>$item$</i>
using a different attribute with the same name, item; the attributes have different values just like you would expect if these template references where Java method calls and item were a local variable. Parameters and attribute references are scoped like a programming language.
Think about what you are really trying to say here. You want to say "make name italics and then make it bold", or "apply italics to the name and then apply bold." There is an "apply template" syntax that is a literal translation:
$name:italics():bold()$
where the templates are applied in the order specified from left to right. This is much more clear, particularly if you had three templates to apply:
$name:courierFont():italics():bold()$
For this syntax to work, however, the applied templates have to reference a standard attribute because you are not setting the attribute in a parameter assignment. In general for syntax expr:template(), an attribute called it is set to the value of expr. So, the definition of bold (and analogously italics), would have to be:
<b>$it$</b>
to pick up the value of name in our examples above.
As of 2.2 StringTemplate, you can avoid using it as a default
parameter by using formal arguments. For expression $x:y()$,
StringTemplate will assign the value of x to it and any sole
formal argument of y. For example, if y is:
y(item) ::= "_$item$_"
then item would also have the value of x.
Where template application really shines though is when an attribute is multi-valued. One of the most common web page generation issues is making lists of items either as bullet lists or table rows etc... Applying a template to a multi-valued attribute means that you want the template applied to each of the values.
Consider a list of names (i.e., you set attribute names multiple times) that you want in a bullet list. If you have a template called listItem:
<li>$it$</li>
then you can do this:
<ul>
$names:listItem()$
</ul>
and each name will appear as a bullet item. For example, if you set names to "Terence", "Tom", and "Jim", then you would see:
<ul>
<li>Terence</li>
<li>Tom</li>
<li>Jim</li>
</ul>
in the output.
Whenever you apply a template to an attribute or multi-valued attribute, the default attribute it is set. Another attribute i (of type Integer) is also set to the value's index number starting from 1. For example, if you wanted to make your own style of numbered list, you could reference i to get the index:
$names:numberedListItem()$
where template numberedListItem is defined as:
$i$. $it$<br>
In this case, the output would be:
1. Terence<br>
2. Tom<br>
3. Jim<br>
If there is only one attribute value, then i will be 1.
As when invoking templates ala "includes", a single formal argument is
also set to the iterated value. For example, you could define
numberedListItem as follows in a StringTemplateGroup file:
numberedListItem(item) ::= "$i$. $item$<br>"
The result of applying a template to a multi-valued attribute is another multi-valued attribute containing the results of the application. You may apply another template to the results of the first template application, which comes in handy when you need to format the elements of a list before they go into the list. For example, to bold the elements of a list do the following (given the appropriate template definitions from above):
$names:bold():listItem()$
If you actually want to apply a template to the combined (string) result of a previous template application, enclose the previous application in parenthesis. The parenthesis will force immediate evaluation of the template application, resulting in a string. For example,
$(names:bold()):listItem()$
results in a single list item full of a bunch of bolded names. Without the parenthesis, you get a list of items that are bolded.
When generating lists of things, you often need to change the color or other formatting instructions depending on the list position. For example, you might want to alternate the color of the background for the elements of a list. The easiest and most natural way to specify this is with an alternating list of templates to apply to an expression of the form: $expr:t1(),t2(),...,tN()$. To make an alternating list of blue and green names, you might say:
$names:blueListItem(),greenListItem()$
where presumably blueListItem template is an HTML <table> or something that lets you change background color. names[0] would get blueListItem() applied to it, names[1] would get greenListItem(), and names[2] would get blueListItem(), etc...
If names is single-valued, then blueListItem() is applied and that's it.
Some templates are so simple or so unlikely to be reused that it seems a waste of time making a separate template file and then referencing it. StringTemplate provides anonymous subtemplates to handle this case. The templates are anonymous in the sense that they are not named; they are directly applied in a single instance.
For example, to show a name list do the following:
<ul>
$names:{<li>$it$</li>}$
</ul>
where anything enclosed in curlies is an anonymous subtemplate if, of course, it's within an attribute expression. Note that in the subtemplate, I must enclose the it reference in the template expression delimiters. You have started a new template exactly like the surrounding template and you must distinguish between text and attribute expressions.
You can apply multiple templates very conveniently. Here is the bold list of names again with anonymous templates:
<ul>
$names:{<b>$it$</b>}:{<li>$it$</li>}$
</ul>
The output would look like:
<ul>
<li><b>Terence</b></li>
<li><b>Tom</b></li>
<li><b>Jim</b></li>
</ul>
Anonymous templates work on single-valued attributes as well.
As of 2.2, you may define formal arguments on anonymous templates even
if you are not using StringTemplate groups. I have borrowed the
syntax from SmallTalk though it is identical in function to lamba of
Python. Use a comma-separated list of argument names followed by the
'|' "pipe" symbol. Any single whitespace character immediately
following the pipe is ignored. The following example bolds the names
in a list using an argument to avoid the monotonous use of it:
<ul>
$names:{ n | <b>$n$</b>}$
</ul>
Clearly only one argument may be defined in this situation: the iterated value of a single list.
In some cases, the model may present data to the view as separate
columns of data rather than as a single list of objects, such as
multi-valued attributes names and phones rather than a single
users multi-valued attribute. As of 2.2, you may iterate over
multiple attributes:
$names,phones:{ n,p | $n$: $p$}$
An error is generated if you have too many arguments for the number of attributes. Iteration proceeds while at least one of the attributes (names or phones, in this case) has values.
Sometimes the name of the template you would like to include is itself a variable. So, rather than using "<item:format()>" you want the name of the template, format, to be a variable rather than a literal. Just enclose the template name in parenthesis to indicate you want the immediate value of that attribute and then add () like a normal template invocation and you get "<item:(someFormat)()>", which means "look up attribute someFormat and use its value as a template name; appy to item." This deliberately looks similar to the C function call indirection through a function pointer (e.g., "(*fp)()" where fp is a pointer to a function). A better way to look at it though is that the (someFormat) implies immediately evaluate someFormat and use as the template name.
Usually this "variable template" situation occurs when you have a list of items to format and each element may require a different template. Rather than have the controller code create a bunch of instances, one could consider it better to have StringTemplate do the creation--the controller just names what format to use.
If StringTemplate did not have a map definition, you could simulate
its functionality. Consider generating a list of Java declarations
that are initialized to 0, false, null, etc... You could define a
template for int, Object, Array, etc... declarations and then
pass in an aggregate object that has the variable declaration object
and the format. In a template group file you might have:
group Java;
file(variables,methods) ::= <<
<variables:{ v | <v.decl:(v.format)()>}; separator="\n">
<methods>
>>
intdecl(decl) ::= "int <decl.name> = 0;"
intarray(decl) ::= "int[] <decl.name> = null;"
Your code might look like:
StringTemplateGroup group =
new StringTemplateGroup(new StringReader(templates),
AngleBracketTemplateLexer.class);
StringTemplate f = group.getInstanceOf("file");
f.setAttribute("variables.{decl,format}", new Decl("i","int"), "intdecl");
f.setAttribute("variables.{decl,format}", new Decl("a","int-array"), "intarray");
System.out.println("f="+f);
String expecting = ""+newline+newline;
For this simple unit test, I used the following dummy decl class:
public static class Decl {
String name;
String type;
public Decl(String name, String type) {this.name=name; this.type=type;}
public String getName() {return name;}
public String getType() {return type;}
}
The value of f.toString() is:
int i = 0;
int[] a = null;
Missing attributes (i.e., null valued attributes) used as indirect template attribute generate nothing just like referencing a missing attribute.
There are many situations when you want to conditionally include some text or another template. StringTemplate provides simple IF-statements to let you specify conditional includes. For example, in a dynamic web page you usually want a slightly different look depending on whether or not the viewer is "logged in" or not. Without a conditional include, you would need two templates: page_logged_in and page_logged_out. You can use a single page definition with if(expr) attribute actions instead:
<html>
...
<body>
$if(member)$
$gutter/top_gutter_logged_in()$
$else$
$gutter/top_gutter_logged_out()$
$endif$
...
</body>
</html>
where template top_gutter_logged_in is located in the gutter subdirectory of my StringTemplateGroup.
IF actions test the presence or absence of an attribute unless the object is a Boolean, in which case it tests the attribute for true/false. The only operator allowed is "not" and means either "not present" or "not true". For example, "$if(!member)$...$endif$".
There is a simple, but not perfect rule: kill a single newline after
<if>, <<, <else>, and <endif> (but for <endif> only if it's
on a line by itself) . Kill newlines before <else> and <endif>
and >>. For example,
a <if(foo)>big<else>small<endif> dog
is identical to:
a <if(foo)>
big
<else>
small
<endif>
dog
It is very difficult to get the newline rule to work "properly" because sometimes you want newlines and sometimes you don't. I decided to chew up as many as is reasonable and then let you explicitly say <\n> when you need to.
Sometimes you use or define templates improperly. Either you set an
attribute that is not used or forget to set one or reference the wrong
template etc... I have made a toy visualization tool via that shows
both the attributes and the way StringTemplate breaks up your
template into chunks. It properly handles StringTemplate objects as
attributes and other nested structures. Here is the way to launch a
Swing frame to view your template:
StringTemplate st = ...;
StringTemplateTreeView viz =
new StringTemplateTreeView("sample",st);
viz.setVisible(true);
Here is a snapshot (note that due some weird font thing on my os x box, the first <html> text chunk is missing in the display.) :( The display is associated with the fill-a-table example below.
If you turn on "lint mode" via StringTemplate.setLintMode(true) then you may access attribute attributes, which is a text string that recursively dumps out types, properties, etc... It does not print out their values.
| Syntax | Description |
| <attribute> | Evaluates to the value of attribute.toString() if it exists else empty string. |
| <attribute.property> | Looks for property of attribute via JavaBeans accessor methods like getProperty() or isProperty(). If that fails, StringTemplate looks for a raw field of the attribute called property. Evaluates to the empty string if no such property is found. |
| <attribute.(expr)> | Indirect property lookup. Same as attribute.property except use the value of expr as the property name. Evaluates to the empty string if no such property is found. |
| <multi-valued-attribute> | Concatenation of toString() invoked on each element. If multi-valued-attribute is missing this evaluates to the empty string. |
| <multi-valued-attribute; separator=expr> | Concatenation of toString() invoked on each element separated by expr. |
| <template(argument-list)> | Include template. The argument-list is a list of attribute assignments where each assignment is of the form arg-of-template=expr where expr is evaluated in the context of the surrounding template not of the invoked template. |
| <(expr)(argument-list)> | Include template whose name is computed via expr. The argument-list is a list of attribute assignments where each assignment is of the form attribute=expr. Example $(whichFormat)()$ looks up whichFormat's value and uses that as template name. Can also apply an indirect template to an attribute. |
| <attribute:template(argument-list)> | Apply template to attribute. The optional argument-list is evaluated before application so that you can set attributes referenced within template. The default attribute it is set to the value of attribute. If attribute is multi-valued, then it is set to each element in turn and template is invoked n times where n is the number of values in attribute. Example: $name:bold() applies bold() to name's value. |
| <attribute:(expr)(argument-list)> | Apply a template, whose name is computed from expr, to each value of attribute. Example $data:(name)()$ looks up name's value and uses that as template name to apply to data. |
| <attribute:t1(argument-list): ... :tN(argument-list)> | Apply multiple templates in order from left to right. The result of a template application upon a multi-valued attribute is another multi-valued attribute. The overall expression evaluates to the concatenation of all elements of the final multi-valued attribute resulting from templateN's application. |
| <attribute:{anonymous-template}> | Apply an anonymous template to each element of attribute. The iterated it attribute is set automatically. |
| <attribute:{argument-name | anonymous-template}> | Apply an anonymous template to each element of attribute. Set the argument-name to the iterated value and also set it. |
| <a1,a2,...,aN:{argument-list | anonymous-template}> | Parallel list iteration. March through the values of the attributes a1..aN, setting the values to the arguments in argument-list in the same order. Apply the anonymous template. There is no defined it value unless inherited from an enclosing scope. |
| <attribute:t1(),t2(),...,tN()> | Apply an alternating list of templates to the elements of attribute. The template names may include argument lists. |
| <if(attribute)>subtemplate <else>subtemplate2 <endif> | If attribute has a value or is a true Boolean object, include subtemplate else include subtemplate2. These conditionals may be nested. |
| <if(!attribute)>subtemplate<endif> | If attribute has no value or is a false Boolean object, include subtemplate. These conditionals may be nested. |
| \$ or \< | escaped delimiter prevents $ or < from starting an attribute expression and results in that single character. |
| <\ >, <\n>, <\t>, <\r> | special characters: space, newline, tab, carriage return. |
| <! comment !>, $! comment !$ | Comments, ignored by StringTemplate. |
The atomic element of a template is a simple object that is rendered to text by its toString() method (in Java). For example, an Integer object is converted to text as a sequence of characters representing the numeric value written out. What if you wanted commas to separate the 1000's places like 1,000,000? What if you wanted commas and sometimes periods depending on the locale? Prior to 2.2, there was no means of altering the rendering of objects to text. The controller had to pull data from the model and wrap it on an object whose toString() method rendered it appropriately.
As of StringTemplate 2.2, you may register various attribute renderers associated with object class types. Normally a single renderer will be used for a group of templates so that Date objects are always displayed using the appropriate Locale, for example. There are, however, situations where you might want a template to override the group renderers. You may register renderers with either templates or groups and groups inherit the renderers from super groups (if any).
There is a new interface, AttributeRenderer, that defines how an
object is rendered to string. Here is a renderer that renders
Calendar date objects tersely.
public class DateRenderer implements AttributeRenderer {
public String toString(Object o) {
SimpleDateFormat f = new SimpleDateFormat("yyyy.MM.dd");
return f.format(((Calendar)o).getTime());
}
}
...
StringTemplate st =new StringTemplate(
"date: <created>",
AngleBracketTemplateLexer.class);
st.setAttribute("created", new GregorianCalendar(2005, 07-1, 05));
st.registerRenderer(GregorianCalendar.class, new DateRenderer());
String expecting = "date: 2005.07.05";
String result = st.toString();
All attributes of type GregorianCalendar in template st are rendered using the DateRenderer object.
You will notice that there is no way for the template to say which renderer to use. Allowing such a mechanism would effectively imply an ability to call random code from the template. In StringTemplate's scheme, only the model or controller can set the renderer. The template must still reference a simple attribute such as <created>. If you need the same kind of attribute displayed differently within the same template or group, you must pass in two different attribute types. This would be rare, but if you need it, you can easily still wrap an object in a renderer before sending it to the template as an attribute. For example, if you have a web site that allows editing of some descriptions, you will probably need both an escaped and unescaped version of the description. Send in the unescaped description as one attribute and send it in again wrapped in an HTML escape renderer as a different attribute.
As far as I can tell, this functionality is mostly useful in the web page generation realm rather than code generation; perhaps an opportunity will present it self though.
Recall that a StringTemplateGroup is a collection of related templates such as all templates associated with the look of a web site. If you want to design a similar look for that site (such as for premium users), you don't really want to cut-n-paste the original template files for use in the new look. Changes to the original will not be propogated to the new look.
Just like you would do with a class definition, a template group may inherit templates from another group, the supergroup. If template t is not found in a group, it is looked up in the supergroup, if present. This works regardless of whether you use a group file format or load templates from the disk via a StringTemplateGroup object. Currently you cannot use the group file syntax to specify a supergroup. I am investigating how this should work. In the meantime, you must explicitly set the super.
From my unit tests, here is a simple inheritance of a template, bold:
StringTemplateGroup supergroup = new StringTemplateGroup("super");
StringTemplateGroup subgroup = new StringTemplateGroup("sub");
supergroup.defineTemplate("bold", "<b>$it$</b>");
subgroup.setSuperGroup(supergroup);
StringTemplate st = new StringTemplate(subgroup, "$name:bold()$");
st.setAttribute("name", "Terence");
String expecting = "<b>Terence</b>";
The supergroup has a bold definition but the subgroup does not. Referencing $name:bold()$ works because subgroup looks into its supergroup if it is not found.
You may override templates:
supergroup.defineTemplate("bold", "<b>$it$</b>");
subgroup.defineTemplate("bold", "<strong>$it$</strong>");
And you may refer to a template in a supergroup via super.template():
StringTemplateGroup group = new StringTemplateGroup(...);
StringTemplateGroup subGroup = new StringTemplateGroup(...);
subGroup.setSuperGroup(group);
group.defineTemplate("page", "$font()$:text");
group.defineTemplate("font", "Helvetica");
subGroup.defineTemplate("font", "$super.font()$ and Times");
StringTemplate st = subGroup.getInstanceOf("page");
The string st.toString() results in "Helvetica and Times:text".
Just like object-oriented programming languages, StringTemplate has polymorphism. That is, template names are looked up dynamically relative to the invoking templates group. The classic demonstration of dynamic message sends in Java, for example, would be the following example that catches my students all the time: ;)
class A {
public void page() {bold();}
public void bold() {System.out.println("A.bold");}
}
class B extends A {
public void bold() {System.out.println("B.bold");}
}
...
A a = new B();
a.page();
This prints "B.bold" not "A.bold" because the receiver determines how to answer a message not the type of the variable. So, I have created a B object meaning that any message, such as bold(), invoked will first look in class B for bold().
Similarly, a template's group determines where it starts looking for a template. In this case, both super and sub groups define a bold template mirroring the Java above. Because I create template st as a member of the subGroup and reference to bold starts looking in subGroup even though page is the template referring to bold.
StringTemplateGroup group = new StringTemplateGroup("super");
StringTemplateGroup subGroup = new StringTemplateGroup("sub");
subGroup.setSuperGroup(group);
group.defineTemplate("bold", "<b>$it$</b>");
group.defineTemplate("page", "$name:bold()$");
subGroup.defineTemplate("bold", "<strong>$it$</strong>");
StringTemplate st = subGroup.getInstanceOf("page");
st.setAttribute("name", "Ter");
String expecting = "<strong>Ter</strong>";
StringTemplate group maps also inherit. If an attribute reference is not found, StringTemplate looks for a map in its group with that name. If not found, the super group is checked.
When you request a named template via StringTemplateGroup.getInstanceOf() or within a template, there is a specific sequence used to locate the template.
If a template, t, references another template and t is not specifically associated with any group, t is implicitly associated with a default group whose root directory is ".", the current directory. The referenced template will be looked up in the current directory.
If a template t is associated with a group, but was not defined via a group file format, lookup a referenced template in the group's template table. If not there, look for it on the disk under the group's root dir or via the CLASSPATH (it might have appeared dynamically on disk). If not found, recursively look at any supergroup of the group. If not found at all, record this fact and don't look again on the disk until refresh interval.
If the template's associated group was defined via a group file, then that group is searched first. If not found, the template is looked up in any supergroup. The refresh interval is not used for group files because the group file is considered complete and enduring.
A StringTemplate is a list of chunks, text literals and attribute expressions, and an attributes table. To render a template to string, the chunks are written out in order; the expressions are evaluated only when asked to during rendering. Attributes referenced in expressions are looked up using a very specific sequence similar to an inheritance mechanism.
When you nest a template within another, such as when a page template references a searchbox template, the nested template may see any attributes of the enclosing instance or its enclosing instances. This mechanism is called dynamic scoping. Contrast this with lexical scoping used in most programming languages like Java where a method may not see the variables defined in invoking methods. Dynamic scoping is very natural for templates. For example, if page has an attribute/value pair font/Times then searchbox could reference $font$ when nested within a page instance.
Reference to attribute a in template t is resolved as follows:
This process is recursively executed until a is found or there are no more enclosing template instances or super groups.
When using a group file format to specify templates, you must specify the formal arguments for that template. If you try to access an attribute that is not formally defined in that template or an enclosing template, you will get a NoSuchElementException.
When building code generators with StringTemplate, large heavily
nested template tree structures are commonplace and, due to dynamic
attribute scoping, a nested template could inadvertently use an
attribute from an enclosing scope. This could lead to infinite
recursion during rendering and other surprises. To prevent this,
formal arguments on template t hide any attribute value with that
name in any enclosing scope. Here is a test case that illustrates the
point.
String templates =
"group test;" +newline+
"block(stats) ::= \"{$stats$}\""
;
StringTemplateGroup group =
new StringTemplateGroup(new StringReader(templates));
StringTemplate b = group.getInstanceOf("block");
b.setAttribute("stats", group.getInstanceOf("block"));
String expecting ="{{}}";
Even though block has a stats value that refers to itself, there is no recursion because each instance of block hides the stats value from above since stats is a formal argument.
Sometimes self-recursive (hence infinitely recursive) structures occur
through programming error and they are nasty to track down. If you
turn on "lint mode", StringTemplate will attempt to find cases where
a template instance is being evaluated during the evaluation of
itself. For example, here is a test case that causes and traps
infinite recursion.
String templates =
"group test;" +newline+
"block(stats) ::= \"$stats$\"" +
"ifstat(stats) ::= \"IF true then $stats$\"\n"
;
StringTemplate.setLintMode(true);
StringTemplateGroup group =
new StringTemplateGroup(new StringReader(templates));
StringTemplate b = group.getInstanceOf("block");
StringTemplate ifstat = group.getInstanceOf("ifstat");
b.setAttribute("stats", ifstat); // block has if stat
ifstat.setAttribute("stats", b); // but make the "if" contain block
try {
String result = b.toString();
}
catch (IllegalStateException ise) {
...
}
The nested template stack trace from ise.getMessage() will be similar to:
infinite recursion to <ifstat([stats])@4> referenced in <block([stats])@3>; stack trace:
<ifstat([stats])@4>, attributes=[stats=<block()@3>]>
<block([stats])@3>, attributes=[stats=<ifstat()@4>], references=[stats]>
<ifstat([stats])@4> (start of recursive cycle)
...
By default, expressions in a template are delimited by dollar signs: $...$. This works great for the most common case of HTML generation because the attribute expressions are clearly highlighted in the text. Sometimes, with other formats like SQL statement generation, you may want to change the template expression delimiters to avoid a conflict and to make the expressions stand out.
As of 2.0, the start and stop strings are limited to either $...$ or <...> (unless you build your own lexical analyzer to break apart templates into chunks).
To use the angle bracket delimiters you must create a StringTemplateGroup:
StringTemplateGroup group =
new StringTemplateGroup("sqlstuff", "/tmp", AngleBracketTemplateLexer.class);
StringTemplate query =
new StringTemplate(group, "SELECT <column> FROM <table>;");
query.setAttribute("column", "name");
query.setAttribute("table", "User");
All templates created through the group or in anyway associated with the group will assume your the angle bracket delimiters. It's smart to be consistent across all files of similar type such as "all HTML templates use $...$" and "all SQL templates use <...>".
By default templates are loaded from disk just once. During development, however, it is convenient to turn caching off. Also, you may want to turn off caching so that you can quickly update a running site. You can set a simple refresh interval using StringTemplateGroup.setRefreshInterval(...). When the interval is reached, all templates are thrown out. Set interval to 0 to refresh constantly (no caching). Set the interval to a huge number like Integer.MAX_INT to have no refreshing at all.
2.0 introduced the notion of a StringTemplateWriter. All text
rendered from a template goes through one of these writers before
being placed in the output buffer. I added this primarily for
auto-indentation for code generation, but it also could be used to
remove whitespace (as a compression) from HTML output. If you don't
care about indentation, you can simply implement write():
public interface StringTemplateWriter {
void pushIndentation(String indent);
String popIndentation();
void write(String str) throws IOException;
}
Here is a "pass through" writer that is already defined:
/** Just pass through the text */
public class NoIndentWriter extends AutoIndentWriter {
public NoIndentWriter(Writer out) {
super(out);
}
public void write(String str) throws IOException {
out.write(str);
}
}
Use it like this:
StringWriter out = new StringWriter();
StringTemplateGroup group =
new StringTemplateGroup("test");
group.defineTemplate("bold", "<b>$x$</b>");
StringTemplate nameST = new StringTemplate(group, "$name:bold(x=name)$");
nameST.setAttribute("name", "Terence");
// write to 'out' with no indentation
nameST.write(new NoIndentWriter(out));
System.out.println("output: "+out.toString());
Instead of using nameST.toString(), which calls write with a string write and returns its value, manually invoke write with your writer.
If you want to always use a particular output filter, then use
StringTemplateGroup.setStringTemplateWriter(Class userSpecifiedWriterClass);
The StringTemplate.toString() method is sensitive to the group's writer class.
StringTemplate has auto-indentation on by default. To turn it off, use NoIndentWriter rather than (the default) AutoIndentWriter.
At the simplest level, the indentation looks like a simple column count:
My dogs' names
$names; separator="\n"$
The last, unindented line
will yield output like:
My dog's names
Fido
Rex
Stinky
The last, unindented line
where the last line gets "unindented" after displaying the list. StringTemplate tracks the characters to the left of the $ or < rather than the column number so that if you indent with tabs versus spaces, you'll get the same indentation in the output.
When there are nested templates, StringTemplate tracks the combined
indentation:
// <user> is indented two spaces
main(user) ::= <<
Hi
\t$user:quote(); separator="\n"$
>>
quote ::= " '$it$'"
In this case, you would get output like:
Hi
\t 'Bob'
\t 'Ephram'
\t 'Mary'
where the combined indentation is tab plus space for the attribute references in template quote. Expression $user$ is indented by 1 tab and hence any attribute generated from it (in this case the $attr$ of quote()) must have at least the tab.
Consider generating nested statement lists as in C. Any statements
inside must be nested 4 spaces. Here are two templates that could
take care of this:
function(name,body) ::= <<
void $name$() $body$
>>
slist(statements) ::= <<
{
$statements; separator="\n"$
}>>
Your code would create a function template instance and an slist
instance, which gets passed to the function template as attribute
body. The following Java code:
StringTemplate f = group.getInstanceOf("function");
f.setAttribute("name", "foo");
StringTemplate body = group.getInstanceOf("slist");
body.setAttribute("statements", "i=1;");
StringTemplate nestedSList = group.getInstanceOf("slist");
nestedSList.setAttribute("statements", "i=2;");
body.setAttribute("statements", nestedSList);
body.setAttribute("statements", "i=3;");
f.setAttribute("body", body);
should generate something like:
void foo() {
i=1;
{
i=2;
}
i=3;
}
Indentation can only occur at the start of a line so indentation is only tracked in front of attribute expressions following a newline.
The one exception to indentation is that naturally, $if$ actions do
not cause indentation as they do not result in any output. However,
the subtemplates (THEN and ELSE clauses) will see indentations. For
example, in the following template, the two subtemplates are indented
by exactly 1 space each:
$if(foo)$
$x$
\t\t$else
$y$
$endif$
You should look at org.antlr.stringtemplate.test.TestStringTemplate.java, which contains a many tests.
The manner in which a template engine handles filling an HTML table with data often provides good insight into its programming and design strategy. It illustrates the interaction of the model and view via the controller. Using StringTemplate, the view may not access the model directly; rather the view is the passive recipient of data from the model.
First, imagine we have objects of type User that we will pull from a
simulated database:
public static class User {
String name;
int age;
public User(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() { return name; }
public int getAge() { return age; }
}
Our database is just a static list:
static User[] users = new User[] {
new User("Boris", 39),
new User("Natasha", 31),
new User("Jorge", 25),
new User("Vladimir", 28)
};
Here is my simple overall page design template, page.st:
<html>
<head>
<title>$title$</title>
</head>
<body>
<h1>$title$</h1>
$body$
</body>
</html>
The body attribute of page.st will be set to the following template
users.inline.st by my web server infrastructure (part of the controller):
<table border=1>
$users:{
<tr>
<td>$it.name$</td><td>$it.age$</td>
</tr>
}$
</table>
Again, it is the default attribute passed to a template when you apply that template to an attribute or attributes. it.name gets the name property, if it exists, from the it object ala JavaBeans. That is, StringTemplate uses reflection to call the getName() method on the incoming object. By using reflection, I avoid a type dependence between model and view.
Now, imagine the server and templates are set up to format data. My
page definition is part of the controller that pulls data from the
model (the database) and pushes into the view (the template). That is
all the page definition should do--interpret the data and set some
attributes in the view. The view only formats data and does no
interpretation.
public static class UserListPage extends SamplePage {
/** This "controller" pulls from "model" and pushes to "view" */
public void generateBody(StringTemplate bodyST) throws Exception {
User[] list = users; // normally pull from database
// filter list if you want here (not in template)
bodyST.setAttribute("users", list);
}
public String getTitle() { return "User List"; }
}
Notice that the controller and model have no HTML in them at all and that the template has no code with side-effects or logic that can break the model-view separation. If you wanted to only see users with age < 30, you would filter the list in generateBody() rather than alter your template. The template only displays information once the controller pulls the right data from the model.
Pushing factorization further, you could make a row.st component in
order to reuse the table row HTML:
<tr>
<td>$it.name$</td><td>$it.age$</td>
</tr>
Then the user list template reduces to the more readable:
<table border=1>
$users:row()$
</table>
Naturally, you could go one step further and make another component
for the entire table (putting it in file table.st):
<table border=1>
$elements:row()$
</table>
then the body template would simply be:
$table(elements=users)$
StringTemplate has multiple grammars that describe templates at varying degrees of detail. At the grossest level of granularity, the group.g grammar accepts a list of templates with formal template arguments. Each of these templates is broken up into chunks of literal text and attribute expressions via template.g. The default lexer uses $...$ delimiters, but the angle.bracket.template.g lexer provides <...> delimiters. Each of the attribute expression chunks is processed by action.g. It builds trees (ASTs) representing the operation indicated in the expression. These ASTs represent the "precompiled" templates, which are evaluated by the tree grammar eval.g each time a StringTemplate is rendered to string with toString().
The grammar files are:
Anything outside of the StringTemplate start/stop delimiters is ignored.
A word about Strings. Strings are double-quoted with optional
embedded escaped characters that are translated (escapes are not
translated outside of strings; for example, text outside of attribute
expressions do not get escape chars translated except \\$, \\< and
\\>).
STRING
: '"' (ESC_CHAR | ~'"')* '"'
;
The translated escapes are:
ESC_CHAR
: '\\'
( 'n'
| 'r'
| 't'
| 'b'
| 'f'
| '"'
| '\\'
)
;
but other escapes are allowed and ignored.
Please see the actual grammar files for the formal language specification of StringTemplate's various components.
Why have a template like:
Check this faq entry: $link(url="/faq/view?ID="+id, title="A FAQ")$.
instead of
Check this faq entry: $link(url="/faq/view?ID="+$id$, title="A FAQ")$.
using $id$ instead of id for that attribute reference? The simplest answer is that you are already within the scope of an attribute expression between the $...$ and hence StringTemplate knows that id must be a reference to an attribute by the grammar alone. The template delimiters mark the beginning and end of what StringTemplate cares about.
Another way to look at it is the following. Surely syntax (i) makes the most sense:
(i) SELECT $col+blort$ FROM $table$
(ii) SELECT $$col$+$blort$$ FROM $table$
(iii) SELECT $col$+$blort$ FROM $table$
Syntax (ii) is rather verbose and redundant wouldn't you say? Syntax (iii) doesn't even work because the + is outside the realm of StringTemplate delimiters.
Tom Burns (CEO jGuru.com) co-designed StringTemplate and listened to me think out loud incessantly. I would also like to thank Monty Zukowski for planting the template "meme" in my head back in the mid 1990's. Loring Craymer and Monty both helped me hone these ideas for use in source-to-source language translation. Matthew Ford has done a huge amount of thinking about StringTemplate and has submitted numerous suggestions and patches. Anthony Casalena at http://www.squarespace.com has been a big help beta-testing StringTemplate. Thanks to the users who have submitted suggestions, bugs, sample code, etc...