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<h1><img src="images/Idea.jpg" align=CENTER width="120" height="86" /></h1>
<h1 align="center">JET Tutorial Part 2 (Write Code that Writes Code)</h1>
<p>In Part 2 of this JET (Java Emitter Templates) tutorial, we will take a look
at the JET engine API. You will learn how to write plug-ins that use the classes
in the JET package to generate Java source code.</p>
<p>As a real-world example, we will create a plug-in that takes user input and
generates a Typesafe Enumeration class. The generated source code is based
on a JET template that can be distributed with the plug-in, allowing users
of the plug-in to customize the generated code by editing the template.</p>
<p>This article also provides a short reference to the JET API.</p>
<p>Contributed by Remko Popma, Azzurri Ltd., remko.popma at azzurri dot jp,
August 26, 2003. Last update: May 31, 2004 for EMF 2.0 (Eclipse 3.0).</p>
<hr width="100%"/>
<h2>Source Code </h2>
<p>To run the example or view the source for code for this article you can unzip
<a href=""></a>
into your <i>plugins/</i> subdirectory. To use the example plug-in, you will
need <a href="" target="_blank">EMF</a> plug-in version
2.0 installed.</p>
<h2>Introduction </h2>
<table border="1" cellspacing="0" width="80%" cellpadding="3">
<p><b>Translation vs. Generation</b></p>
<p>An aspect of JET templates that is at first confusing is that generating
text takes two steps: translation and generation. The first step is
translating the template to a template implementation class. The second
step is using this template implementation class to generate the text.
<p>If your goal with JET is to generate Java source code, it can be confusing
that the template translation step also results in Java source code. Remember
that this source code is <i>not</i> the generated text. The source code
that is the result of the translation step is simply another form of the
template. </p>
<p>If you have used JSP and servlets before, you can think of a JET template
as being equivalent to a JSP page. A JET template is translated to a
template implementation class, just like a JSP page is translated to
a servlet. The second step, where the template implementation class
generates text, is equivalent to the servlet creating and returning
<p><a href="../Article-JET/jet_tutorial1.html">Part 1</a> of this tutorial introduced
JET templates and explained how you can convert a project to a JET project to
have the JET Builder automatically translate templates in your project to template
implementation classes. </p>
<p>In part 2 of this tutorial, we will focus on writing a plug-in that uses the
classes in the JET package to generate Java source code. A plug-in that generates
text from a JET template can no longer rely on the JET Nature and JET Builder
to automatically translate templates. This is because JET Nature and JET Builder
operate only on workspace projects, not on plug-ins. Plug-ins need to use the
classes in the JET package to translate their templates. </p>
<p>The next section will discuss some of the classes in the <tt class="code">org.eclipse.emf.codegen</tt>
package. We will see what the steps are to generate source code with JET, and
how the JET engine classes fit in. If you are anxious to see some code that
shows how to use these classes in practice, you can go straight to <a href="#example_plugin">A
Plug-in that Generates Source Code</a>. </p>
<h2>Some JET Classes</h2>
<p>In this section, we will take a closer look at some of the classes in the JET
package. They can roughly be divided into two groups: </p>
<li>Lower-level classes dealing with the nuts and bolts of translating a template
to a template implementation class. The <tt class="code">JETCompiler</tt> class brings
all these lower-level classes together to provide a single API for template
translation. </li>
<li>Higher-level classes that build on top of <tt class="code">JETCompiler</tt> to accomplish
user tasks. In Part 1 of this tutorial, we have already seen <tt class="code">JETNature</tt>
and <tt class="code">JETBuilder</tt> at work. Other high-level classes are <tt class="code">CodeGen</tt>
and <tt class="code">JETEmitter</tt>. </li>
<p>The lower-level classes are not discussed in-depth in this article. For a description
of all classes in the <tt class="code">org.eclipse.emf.codegen</tt> plug-in, see the <a href="#jet_api_overview">JET
API Overview</a> section below. In the rest of this section, we will focus on
a few of the higher-level classes.</p>
<h4><tt class="code">org.eclipse.emf.codegen.jet.JETCompiler</tt></h4>
<p><tt class="code">JETCompiler</tt> is the core class for template translation. This class
is responsible for translating templates to the Java source code of a template
implementation class. The actual translation is delegated to other classes in
the same package. Clients create a JETCompiler object for a particular template
and then call the <tt class="code">parse</tt> method followed by the <tt class="code">generate</tt>
method to write the Java source code for the resulting template implementation
class to a specified stream.</p>
<h4><tt class="code">org.eclipse.emf.codegen.jet.JETEmitter</tt></h4>
<p><tt class="code">JETEmitter</tt> provides a convenient high-level API for users of the
JET package. The <tt class="code">generate</tt> method of this class combines template
translation and text generation into a single step. By taking care of the gory
details of translating templates and compiling the Java source code of the translated
template implementation class, JETEmitter lets you focus on the final generator
<p>Another way of looking at JETEmitter is that it abstracts away the translation
step and lets you pretend that you can directly generate text with a template.
Following the <a href="" target="_blank">Law
of Leaky Abstractions</a>, we cannot always get away with this, and the <a href="#jetemitter_gotchas">JETEmitter
Gotchas</a> section below points to a few places where you have to be careful.</p>
<p>JETEmitter is the class we will be using in our plug-in, so we will go into
a little more detail here.</p>
<p>A JETEmitter object is constructed with the uri of the template used to generate
text. Any type of uri is acceptable as long as a protocol handler is available.
This means that <tt class="code">file:/</tt> uris, <tt class="code">ftp:/</tt> uris and <tt class="code">http:/</tt><i>
</i>uris can all be used. Eclipse adds special protocol handlers for <tt class="code">platform:/base</tt>/,
<tt class="code">platform:/plugin/</tt>, <tt class="code">platform:/fragment/</tt> and <tt class="code">platform:/resource/</tt>
uris, so plug-ins can use a uri like <tt class="code">platform:/resource/myproject/myfolder/mytemplate.jet</tt>
to specify a template file. Note: Eclipse 3.0 has introduced <tt class="code">bundleentry</tt> to its list
of special protocols. It should be used in references to Eclipse elements such as plugins and features.</p>
<p>In our example plug-in, we will distribute our template file together with our
plug-in, so the template file will be located in a <i>myplugin/templates/</i>
folder under the Eclipse <i>plugins/</i> folder. The following code can then
be used to locate and generate a template from this folder:</p>
<pre class="code"> String pluginId = "";
String base = Platform.getBundle(pluginId).getEntry("/").toString();
String uri = base + "templates/myTemplate.javajet";
JETEmitter emitter = new JETEmitter(uri);
String generatedText = emitter.generate(new Object[] {parameter});</pre>
<p>After constructing a JETEmitter object, clients then call <tt class="code">generate</tt>
on it to generate text. The <tt class="code">generate</tt> method will perform the following
<li>Create a project called <i>.JETEmitters</i> in the workspace </li>
<li>Prepare this project by giving it the Java Nature and adding classpath variables
to its classpath </li>
<li>Translate the template to a template implementation Java source file in
the <i>.JETEmitters</i> project </li>
<li>Build the project to compile the template implementation source code to
a Java <i>.class</i> file </li>
<li>Call the <tt class="code">generate</tt> method on the translated Java template implementation
class and return the generated text as a String</li>
<p>* <i>.JETEmitters</i> is the default name for the project that is created during the template
translation. This value can be changed by the <tt class="code">setProjectName</tt> method.</p>
<p>Our example plug-in will use JETEmitter and save the generated text to a Java
source file in the workspace. The figure below shows the steps for generating
source code using JETEmitter. </p>
<p><img alt="Using JETEmitter to generate text from a plugin" src="images/jetemitter.gif"/> </p>
<h3><a name="jetemitter_gotchas"></a>JETEmitter Gotchas</h3>
<p>The JETEmitter class combines template translation and text generation into
a single step, which makes it a very convenient tool. However, it is important
that you know what takes place under the hood, otherwise you might be in for
some nasty surprises. This section highlights some &quot;gotchas&quot; that
I ran into, so that you don't make the same mistakes.</p>
<h4>1. Plug-in Initialization Required </h4>
<p>It is not easy to use JET outside of Eclipse. JET is designed to run only as
a workspace application. Any application using JET must minimally run as an
Eclipse &quot;headless&quot; application so that plug-in initialization takes
place. (The term headless refers to running Eclipse without the user interface.)</p>
<p>This means that using JETEmitter from a simple standalone application (a standard
Java class with a <tt class="code">main</tt> method) <i>will not work</i>:</p>
<pre class="code"> // This fails: cannot use JETEmitter from a standalone application
public static void main(String[] args) {
JETEmitter emitter = new JETEmitter("/myproject/templates/HelloWorld.txtjet");
// this will throw a NullPointerException
String result = emitter.generate(new NullProgressMonitor(), {"hi" });
<p>Note that this is not a restriction of just the JETEmitter class, many of the
classes in the <tt class="code">org.eclipse.emf.codegen</tt> plug-in have dependencies
on other plug-ins. The <a href="#appendix">Appendix</a> section below has more
details on using JET from standalone applications.</p>
<p>In the rest of this article we will assume that our code is running from inside
a plug-in.</p>
<h4>2. Classloader Issues </h4>
<p>You may get a NoClassDefFoundError when you pass a custom object as the argument
to the <tt class="code">JETEmitter.generate</tt> method. This can happen if the object
you pass as the argument is not one of the java "bootstrap" classes (the bootstrap
classes are the runtime classes in rt.jar and internationalization classes in
<p>To prevent this error you must specify the classloader of your plug-in when
using JETEmitter. If no classloader is specified, JETEmitter uses the classloader
of its own class, which is usually the classloader for the <tt class="code">org.eclipse.emf.codegen</tt>
plug-in, and this classloader can't see much. In recent versions of EMF (since
version 1.1.0 build 20030527_0913VL), JETEmitter has a constructor that takes
a classloader argument.</p>
<p>Note that another way to specify a classloader is to subclass JETEmitter in
your own project; if no classloader is specified, JETEmitter will use the classloader
of this subclass. (If you are using an older version of EMF, there are no constructors
that take a classloader argument and you will have no choice but to subclass
JETEmitter in your own project.)</p>
<p>The example below shows an action class that translates and invokes a selected
template using JETEmitter. The example shows how a JETEmitter can be constructed
<img src="images/tag_1.gif" width="24" height="13"/>with a classloader parameter or by <img src="images/tag_2.gif" width="24" height="13"/>constructing
an anonymous subclass.</p>
<pre class="code">package org.eclipse.emf.examples.jet.article2.actionexample;
// imports omitted
public class EmitAction implements IActionDelegate {
protected ISelection selection;
public void selectionChanged(IAction action, ISelection selection) {
this.selection = selection;
public void run(IAction action) {
List files = (selection instanceof IStructuredSelection)
? ((IStructuredSelection) selection).toList()
: Collections.EMPTY_LIST;
for (Iterator i = files.iterator(); i.hasNext();) {
IFile file = (IFile);
IPath fullPath = file.getFullPath();
String templateURI = "platform:/resource" + fullPath;
<b>Class</b><b>Loader classloader = getClass().getClassLoader();</b>
<img src="images/tag_1.gif" width="24" height="13"/> JETEmitter emitter = new JETEmitter(templateURI, <b>classloader</b>);
// <b>or: use an anonymous subclass</b>
<img src="images/tag_2.gif" width="24" height="13"/> // emitter = <b>new JETEmitter(templateURI) {}</b>; // notice the brackets
try {
IProgressMonitor monitor = new NullProgressMonitor();
String[] arguments = new String[] { "hi" };
String result = emitter.generate(monitor, arguments);
saveGenerated(result, file);
} catch (Exception e) {
throw new RuntimeException(e);
// saveGenerated method omitted
<h4>3. Classpath Issues </h4>
<p>JETEmitter translates your templates to Java source files in the <i>.JETEmitters</i>
project, and invokes the JavaBuilder to compile these source files. If your
templates use classes that are not standard Java classes, or not in the EMF
plug-in, you will need to add these classes to the classpath of the <i>.JETEmitters</i>
project, or the JavaBuilder cannot compile the template implementation source
files. Fortunately, JETEmitter provides a simple way to do this through the method
<tt class="code">addVariable</tt> which adds a <i>Classpath Variable</i> to the <i>.JETEmiter</i> project.</p>
<p>A <i>Classpath Variable</i> is a workspace-wide name that is used in Eclipse to refer to a jar file or
directory. The list of all such variables can be seen using the Window &gt; Preferences &gt; Java &gt;
Classpath Variables menua action.. Your program will need to add a classpath variable for each
jar file or directory that is needed on the classpath of the <i>.JETEmiter</i> project.</p>
<h2><a name="example_plugin"></a>A Plug-in that Generates Source Code </h2>
<p>In this part of the JET Tutorial, we will write an Eclipse plug-in that uses
a JET template to generate Java source code for typesafe enumerations.</p>
<p>Our plug-in has to perform the following tasks:</p>
<li>Collect user input values for the variables in our template: the class name,
the type and name of the attributes of the typesafe enumeration class, and
the values of these attributes for each instance. We will write a simple GUI
to collect these values.</li>
<li>Translate the JET template file to a Java template implementation class</li>
<li>Invoke the template implementation class with an object that contains the
user input values collected by the GUI</li>
<li>Save the resulting generated source code to a location obtained from the
<p>In the following sections we will go through the steps above one by one.</p>
<h3><a name="typesafe_enum_example"></a>Typesafe Enumerations</h3>
<p>Let's have a look at a typesafe enumeration class to see what kind of source
code we want to generate. The Digit class below is an example typesafe enum.</p>
<pre class="code"> // an example typesafe enum
package x.y.z;
public class Digit {
<img src="images/tag_1.gif" width="24" height="13"/> public static final Digit ZERO = new Digit(0, "zero");
public static final Digit ONE = new Digit(1, "one");
public static final Digit TWO = new Digit(2, "two");
public static final Digit THREE = new Digit(3, "three");
// ...
public static final Digit NINE = new Digit(9, "nine");
private static final Digit[] ALL =
<img src="images/tag_2.gif" width="24" height="13"/> private final int value;
private final String name;
private Digit(int value, String name) {
this.value = value; = name;
<img src="images/tag_3.gif" width="24" height="13"/> public static Digit lookup(int key) {
for (int i = 0; i &lt; ALL.length; i++) {
if (key == ALL[i].getValue()) { return ALL[i]; }
// lookup failed:
// we have no default Digit, so we throw an exception
<img src="images/tag_4.gif" width="24" height="13"/> throw new IllegalArgumentException("No digit exists for " + key);
public int getValue() { return value; }
public int getName() { return name; }
public String toString() { return getName(); }
<p>Let's take a closer look at this class. First of all, the Digit class has several
<b><img src="images/tag_1.gif" width="24" height="13"/>instances</b> - the constants
ZERO, ONE, TWO, etc. Each instance is defined by its Java variable name, "ZERO",
"ONE", "TWO"..., and the values for each <b><img src="images/tag_2.gif" width="24" height="13"/>attribute</b>
of the enumeration class. Most typesafe enums have one or more attributes. The
Digit class has two attributes: a <tt class="code">value</tt> integer and a <tt class="code">name</tt>
<p>Our example Digit class also has a <b><img src="images/tag_3.gif" width="24" height="13"/><tt class="code">lookup</tt></b>
method, which returns the instance whose <tt class="code">value</tt> attribute equals
the specified int parameter. A lookup method introduces the concept of <b>key
attributes</b>. Many typesafe enums have one or more attributes that uniquely
distinguish one instance from another.</p>
<p><i>Note that key attributes are not required: the Java VM guarantees that every
newly constructed object is unique, so it is possible to have typesafe enumerations
that have no attributes at all, and simply distinguish their instances with
the <b>==</b> instance identity operator. This works fine, but often it is convenient
to have a key attribute that uniquely identifies an instance, and a <tt class="code">lookup</tt>
method that finds an instance for a specified key value.</i></p>
<p>Our template does have a <tt class="code">lookup</tt> method, so we need to decide what
to do if <img src="images/tag_4.gif" width="24" height="13"/>no instance is found
for the specified key value. Basically there are three options: throwing an
Exception, returning a designated &quot;default&quot; instance, or returning
<tt class="code">null</tt>. Which option is best depends on the application in which
the class is used, so we should probably let the user decide.</p>
<p>Now that we've studied typesafe enums in more detail, let's summarize what
is customizable in a typesafe enumeration:</p>
<li>package name </li>
<li>class name </li>
<li>attributes, where each attribute </li>
<li>has a type </li>
<li>has a name </li>
<li>may be a key attribute </li>
<li>instances, where each instance </li>
<li>has a name </li>
<li>has a value for every attribute </li>
<li>may be the default instance to return for failed lookups</li>
<h3><a name="typesafe_enum_model"></a>A Simple Typesafe Enumeration Model </h3>
<p>A simple model for the customizable parts of a typesafe enum could look something
like this:</p>
<table border="1" cellspacing="0">
<td><tt class="code">TypesafeEnum</tt></td>
<td><tt class="code">getInstances() : Instance[]<br/>
getAttributes() : Attribute[]<br/>
getKeyAttributes() : Attribute[]<br/>
getDefaultInstance() : Instance<br/>
getPackageName() : String<br/>
getClassName() : String</tt></td>
<table border="1" cellspacing="0">
<td><tt class="code">Instance</tt></td>
<td><tt class="code"> getName() : String<br/>
getAttributeValues() : Properties<br/>
getAttributeValue(Attribute) : String<br/>
isDefault() : boolean</tt></td>
<table border="1" cellspacing="0">
<td><tt class="code">Attribute</tt></td>
<td><tt class="code"> getName() : String<br/>
getType() : String<br/>
isKey() : boolean</tt></td>
<p>In the next section we will use these classes to convert our Digit class to
a JET template for typesafe enumerations.</p>
<h3><a name="typesafe_enum_template"></a>A Typesafe Enumeration Template </h3>
<p>Now that we have a model, we can take our Digit class and replace all Digit-specific
code with JET scriptlets and expressions that call our model classes. The resulting
template could look something like this:</p>
<pre class="code"> <span class="highlight"><span class="highlight">&lt;%@ jet package="translated" imports="java.util.* article2.model.*" class="TypeSafeEnumeration" %&gt;</span></span>
<span class="highlight"><span class="highlight">&lt;% TypesafeEnum enum = (TypesafeEnum) argument; %&gt;</span></span>
package <span class="highlight">&lt;%=enum.getPackageName()%&gt;</span>;
* This final class implements a type-safe enumeration
* over the valid instances of a <span class="highlight">&lt;%=enum.getClassName()%&gt;</span>.
* Instances of this class are immutable.
public final class <span class="highlight">&lt;%=enum.getClassName()%&gt;</span> {
<span class="highlight">&lt;% for (Iterator i = enum.instances(); i.hasNext(); ) { %&gt;</span>
<span class="highlight">&lt;% Instance instance = (Instance); %&gt;</span>
// <b>instance definition</b>
public static final <span class="highlight">&lt;%=enum.getClassName()%&gt;</span> <span class="highlight">&lt;%=instance.getName()%&gt;</span> =
<img src="images/tag_1.gif" width="24" height="13"/> new <span class="highlight">&lt;%=enum.getClassName()%&gt;</span>(<span class="highlight">&lt;%=instance.constructorValues()%&gt;</span>);
<span class="highlight">&lt;% } %&gt;</span>
<span class="highlight">&lt;% for (Iterator i = enum.attributes(); i.hasNext(); ) { %&gt;</span>
<span class="highlight">&lt;% Attribute attribute = (Attribute); %&gt;</span>
// <b>attribute declaration</b>
<img src="images/tag_2.gif" width="24" height="13"/> private final <span class="highlight">&lt;%=attribute.getType()%&gt;</span> m<span class="highlight">&lt;%=attribute.getCappedName()%&gt;</span>;
<span class="highlight">&lt;% } %&gt;</span>
* Private <b>constructor</b>.
<img src="images/tag_3.gif" width="24" height="13"/> private <span class="highlight">&lt;%=enum.getClassName()%&gt;</span>(<span class="highlight">&lt;%=enum.constructorParameterDescription()%&gt;</span>) {
<span class="highlight">&lt;% for (Iterator i = enum.attributes(); i.hasNext(); ) { %&gt;</span>
<span class="highlight">&lt;% Attribute attribute = (Attribute); %&gt;</span>
<img src="images/tag_4.gif" width="24" height="13"/> m<span class="highlight">&lt;%=attribute.getCappedName()%&gt;</span> = <span class="highlight">&lt;%=attribute.getUncappedName()%&gt;</span>;
<span class="highlight">&lt;% } %&gt;</span>
// <b>getter accessor methods</b>
<span class="highlight">&lt;% for (Iterator i = enum.attributes(); i.hasNext(); ) { %&gt;</span>
<span class="highlight">&lt;% Attribute attribute = (Attribute); %&gt;</span>
* Returns the <span class="highlight">&lt;%=attribute.getName()%&gt;</span>.
* @return the <span class="highlight">&lt;%=attribute.getName()%&gt;</span>.
public <span class="highlight">&lt;%=attribute.getType()%&gt;</span> get<span class="highlight">&lt;%=attribute.getCappedName()%&gt;</span>() {
return m<span class="highlight">&lt;%=attribute.getCappedName()%&gt;</span>;
<span class="highlight">&lt;% } %&gt;</span>
// lookup method omitted...
<p>As you can see, the template calls some methods that were not in the simple
model we introduced earlier. We have added a few convenience methods, like the
<tt class="code"><img src="images/tag_2.gif" width="24" height="13"/>Attribute.getCappedName()</tt>
and <tt class="code"><img src="images/tag_4.gif" width="24" height="13"/>getUncappedName()</tt>
methods. Such methods help to keep the template simple.</p>
<p>Another example of methods we added to the model are the <tt class="code"><img src="images/tag_3.gif" width="24" height="13"/>TypesafeEnum.constructorParameterDescription()</tt>
method and the <tt class="code"><img src="images/tag_1.gif" width="24" height="13"/>Instance.constructorValues()</tt>
method. The implementation of the <tt class="code">constructorValues</tt> method is shown
below. </p>
<pre class="code">// class Instance
* Convenience method that returns the attribute values of this instance,
* in the order expected by the constructor of this instance.
* @return a comma-separated list of all attribute values of this instance,
* formatted like <tt class="code">attrib1-value, attrib2-value (, ...)</tt>
public String constructorValues() {
StringBuffer result = new StringBuffer();
for (Iterator i = getType().attributes(); i.hasNext(); ) {
Attribute attribute = (Attribute);
if (i.hasNext()) {
result.append(", ");
return result.toString();
<p>The <tt class="code">constructorValues</tt> method loops through the attributes of the
typesafe enum, looks up the value for each attribute in the instance, and concatenates
these values into a string, separated by commas. For example, in our <tt>Digit</tt>
typesafe enum class above, this method would return <tt>&quot;0, \&quot;zero\&quot;&quot;</tt>
for the &quot;ZERO&quot; instance. </p>
<p>We could have looped through the attribute values in the template, but that
would have made the template much more difficult to read. Pushing this logic
into the model made the template more readable and easier to maintain. On the
other hand, we lost some flexibility because users cannot customize that logic
anymore by editing the template. This is a trade-off you have to make. Which
is better depends on your template and your application.</p>
<h3><a name="gui"></a>A GUI to Collect User Input </h3>
<p>Now that we have a model and a template, we still need two more pieces to finish
our plug-in: we need a GUI to collect values from the user to populate our model
with, and we need to invoke our template with the populated model to generate
source code and save this source code to a location in the workspace.</p>
<p>Let's start with the GUI. The workbench provides a few wizards that do something
similar to what we have in mind, for example the New Class, New Interface and
New JUnit TestCase wizards. It probably makes sense to have our GUI look similar
to these wizards and make it accessible from the standard menu and toolbar locations.</p>
<p>Our wizard has three pages. The first page, shown below, looks like a simplified
version of the New Class wizard. In fact, we are using the same framework that
the New Class wizard uses, the <tt class="code">org.eclipse.jdt.ui.wizards</tt> package.
In the first page, we collect the package name and the class name of the typesafe
enum, and the location where the result should be saved.</p>
<p><img alt="GUI wizard page one: class, package and location of the typesafe enum" src="images/enum_gui_page1.gif"/>
<p>Our second page collects information on the attributes of the typesafe enum
class. Every attribute has a name and a type, and may be one of the key attributes.
Our second wizard page is shown below:</p>
<p><img alt="GUI wizard page two: attributes of the typesafe enum" src="images/enum_gui_page2.gif"/></p>
<p>Our third and last wizard page, shown below, collects information on the instances
of the typesafe enum. The user inputs the instance name, and for each instance
provides values for all attributes.</p>
<p>Finally, one of the instances may be the &quot;default&quot; instance, which
is the instance returned by the <tt class="code">lookup</tt> method if no instance was
found for the specified key attribute values.</p>
<p><img alt="GUI wizard page three: instances of the typesafe enum" src="images/enum_gui_page3.gif"/></p>
<h3><a name="invoke_plugin_template"></a>Invoking the Template </h3>
<p>Now that we have a GUI to populate our model, we can finally use what we learned
in the first part of this article, and generate source code with our template.</p>
<p>When a user presses Finish on the wizard, the <tt class="code">performFinish</tt> method
in our wizard is called. The code below shows how we <img src="images/tag_1.gif" width="24" height="13"/>use
a custom subclass of JETEmitter to <img src="images/tag_2.gif" width="24" height="13"/>add
the jar file of our plug-in to the classpath of the <i>.JETEmitters</i> project
before we <img src="images/tag_3.gif" width="24" height="13"/>call generate on
the JETEmitter. The generated typesafe enum source code is <img src="images/tag_4.gif" width="24" height="13"/>saved
to the location in the workspace that the user specified.</p>
<pre class="code"> // class NewTypesafeEnumCreationWizard
protected void finishPage(IProgressMonitor monitor)
throws InterruptedException, CoreException {
String pluginId = "org.eclipse.emf.examples.jet.article2";
String base = Platform.getBundle(pluginId).getEntry("/").toString();
String relativeUri = "templates/TypeSafeEnumeration.javajet";
<img src="images/tag_1.gif" width="24" height="13"/> JETEmitter emitter = new JETEmitter(base + relativeUri, getClass().getClassLoader());
<img src="images/tag_2.gif" width="24" height="13"/> emitter.addClasspathVariable("JET_TUTORIAL", pluginId);
TypesafeEnum model = mPage1.getTypesafeEnumModel();
IProgressMonitor sub = new SubProgressMonitor(monitor, 1);
<img src="images/tag_3.gif" width="24" height="13"/> String result = emitter.generate(sub, new Object[] { model });
<img src="images/tag_4.gif" width="24" height="13"/> IFile file = save(monitor, result.getBytes());
<h3><a name="pluginxml"></a>Registering our Wizard </h3>
<p>Our final code snippet below shows the part of our <tt class="code">plugin.xml</tt> configuration
file where we register our wizard as a contribution to the workbench.</p>
<pre class="code"> &lt;extension point="org.eclipse.ui.newWizards"&gt;
name="Typesafe Enum"
Create a Typesafe Enumeration
&lt;class class="org.eclipse.emf.examples.jet.article2.ui.NewTypesafeEnumCreationWizard"&gt;
<img src="images/tag_1.gif" width="24" height="13"/> &lt;parameter name="javatype" value="true"/&gt;
<p>Now our wizard is activated when users select File &gt; New &gt; Other &gt;
Java &gt; Typesafe Enum from the workbench, as shown in the image below. </p>
<p><img alt="Typesafe Enum wizard shows up in the New creation wizard" src="images/new_creation_wizard.gif"/>
<p>Note that we set the <tt class="code"><img src="images/tag_1.gif" width="24" height="13"/>javatype</tt>
attribute to true in the wizard extension element in the <tt class="code">plugin.xml</tt>
file. This will cause our wizard to show up as an action on the toolbar in the
Java Perspective, as shown in the image below.</p>
<p><img alt="Typesafe Enum wizard shows up as an action on the toolbar in the Java Perspective" src="images/new_toolbar.gif"/>
<h2><a name="conclusion"></a>Conclusion </h2>
JET can be a great help for applications that need to generate text. Templates
are as much of an improvement to code generation as JSP pages were to old style
<p>When using JET, you need to decide whether you want to distribute your templates
with your application, or distribute only the template implementation classes.</p>
<p>If your goal is to simplify the text generation capabilities of your application,
then using JET Nature and JET Builder to automatically translate your templates
is a good choice. See <a href="../Article-JET/jet_tutorial1.html">JET Tutorial
Part 1</a> for details. In that case you only need to distribute the translated
template implementation classes with your application, not the templates themselves.</p>
<p>On the other hand, if it is important for your application that users have
ultimate control over the generated text, you may want to distribute the template
files themselves with your application. In that case, you will need to translate
these templates every time you generate text. The plug-in we wrote in this article
is an example of this type of application.</p>
<p>This article explained what classes are available in the JET package to achieve
this and showed how to use these classes with an Eclipse plug-in. The appendix
below provides an overview of the JET API and shows how it can be used in headless
or standalone applications.</p>
<h2><a name="appendix"></a>Appendix</h2>
<h3><a name="jet_api_overview"></a>JET API Overview </h3>
<b>Package org.eclipse.emf.codegen</b>
<table border="1" cellspacing="0">
<td valign="top"><tt class="code">CodeGen</tt></td>
<p>The <tt class="code">CodeGen</tt> class can translate a JET template to Java source
code and optionally merge the template implementation Java source code
with an existing Java class. <tt class="code">CodeGen</tt> can be used as an Eclipse
headless application. The <tt class="code">run</tt> method expects a String array parameter
of two or three elements: </p>
<li> the uri of the template to translate </li>
<li>the target path where the translation result should be saved </li>
<li>an optional <tt class="code">JMerge</tt> control model file specifying how to merge
the new translation result with the source code of an existing Java
<td valign="top"><tt class="code">CodeGenPlugin</tt></td>
<td>The plug-in class for the JET package.</td>
<b>Package org.eclipse.emf.codegen.jet</b>
<table border="1" cellspacing="0">
<td valign="top"><tt class="code">IJETNature</tt></td>
<td>Interface extending <tt class="code">org.eclipse.core.resources.IProjectNature</tt>.
Defines some of the properties that a JET nature has. Implemented by <tt class="code">JETNature</tt>.
Used as a filter for project property pages by the <tt class="code">org.eclipse.emf.codegen.ui</tt>
<td valign="top"><tt class="code">JETAddNatureOperation</tt></td>
<td>A <tt class="code">org.eclipse.core.resources.IWorkspaceRunnable</tt> for adding the
JET nature to a project in the workspace. Used by the <tt class="code">AddJETNatureAction</tt>
in the <tt class="code">org.eclipse.emf.codegen.ui</tt> plug-in.</td>
<td valign="top"><tt class="code">JETBuilder</tt></td>
<td>This class extends <tt class="code">org.eclipse.core.resources.IncrementalProjectBuilder</tt>.
When its <tt class="code">build</tt> method is invoked, it delegates to <tt class="code">JETCompileTemplateOperation</tt>
to translate all templates in the workspace project that have been changed
since the previous build. Templates must be located in one of the folders
specified as Template Containers in the JET Nature of the project.</td>
<td valign="top"><tt class="code">JETCharDataGenerator</tt></td>
<td>Responsible for a part of the template translation process. Generates
strings for the character data present in the template file. Used by <tt class="code">JETCompiler</tt>.</td>
<td valign="top"><tt class="code">JETCompiler</tt></td>
<td>This is the core class for template translation. This class is responsible
for translating templates to the Java source code of a template implementation
class. The actual translation is delegated to other classes in this package.
A <tt class="code">JETParser</tt> is used to parse the template into template elements.
<tt class="code">JETCompiler</tt> implements the <tt class="code">JETParseEventListener</tt> interface
and registers itself with the parser to be notified when the parser recognizes
a template element. For every recognized template element, <tt class="code">JETCompiler</tt>
uses a <tt class="code">JETGenerator</tt> to translate the template element to Java source
code. When the template parsing is complete, <tt class="code">JETCompiler</tt> uses a
<tt class="code">JETSkeleton</tt> to assemble the Java source code elements to a single
compilation unit (a Java class).</td>
<td valign="top"><tt class="code">JETCompileTemplateOperation</tt></td>
<td>This class implements <tt class="code">org.eclipse.core.resources.IWorkspaceRunnable</tt>
so it can execute as a batch operation within the workspace. This operation
takes a workspace project, one or more Template Containers and optionally
a list of specific template files as constructor parameters. When its <tt class="code">run</tt>
method is invoked, it uses a <tt class="code">JETCompiler</tt> to translate the template
files in the specified workspace project folders to Java source files for
template implementation classes. This operation can optionally be configured
to trigger a complete build of the project when it is finished to compile
the Java source files to <i>.class</i> files.</td>
<td valign="top"><tt class="code">JETConstantDataGenerator</tt></td>
<td>Responsible for a part of the template translation process. Extends <tt class="code">JETCharDataGenerator</tt>
to generate constant declarations for the strings with character data present
in the template file.</td>
<td valign="top"><tt class="code">JETCoreElement</tt></td>
<td>Interface for core JET syntax elements (directive, expression, scriptlet
and quote-escape). Used by <tt class="code">JETParser</tt>.</td>
<td valign="top"><tt class="code">JETEmitter</tt></td>
<td>This class provides a convenient high-level API for users of this package.
The <tt class="code">generate</tt> method of this class translates a template to Java
source code, compiles this source code to a template implementation class,
asks the template class to generate text and finally returns the generated
result. This class creates a Java project called <i>.JETEmitters</i> in
the workspace, translates the template into this project, and simply calls
<tt class="code">build</tt> on the <i>.JETEmitters</i> project to compile the source
code. If translation or compilation fails, a <tt class="code">JETException</tt> is thrown.
A template implementation Java class is "executed" by calling its <tt class="code">generate</tt>
<td valign="top"><tt class="code">JETException</tt></td>
<td>Extends <tt class="code">org.eclipse.core.runtime.CoreException</tt>, but provides
more convenient constructors.</td>
<td valign="top"><tt class="code">JETExpressionGenerator</tt></td>
<td>Responsible for a part of the template translation process. Extends <tt class="code">JETScriptletGenerator</tt>
to translate JET expressions (<tt class="code">&lt;%= ... %&gt;</tt> stuff) to Java source
<td valign="top"><tt class="code">JETGenerator</tt></td>
<td>Interface for generators: classes that know how to translate part of a
JET template to a Java source code element.</td>
<td valign="top"><tt class="code">JETMark</tt></td>
<td>A state object used by the <tt class="code">JETParser</tt> to mark points in the JET
character input stream, and delegate the processing of parts of the stream
to other objects.</td>
<td valign="top"><tt class="code">JETNature</tt></td>
<p>This class implements <tt class="code">IJETNature</tt> so that it can configure
a workspace project with the JET Nature. When this nature is added to
a project, it adds a JET Builder to the front of the build spec of the
project. This nature defines two properties: </p>
<li> Template Containers - a list of folders in the project that contain
the JET templates to translate. </li>
<li>Source Container - the target folder in which to save translated template
implementation Java classes. </li>
<p>These properties are used by the JET Builder when performing a build.</p>
<td valign="top"><tt class="code">JETParseEventListener</tt></td>
<td>Interface for objects that know how to process parts of a JET character
input stream.</td>
<td valign="top"><tt class="code">JETParser</tt></td>
<td>The main parser class. Has several inner classes for recognizing core
JET syntax elements (directive, expression, scriptlet and quote-escape).
When a core JET syntax element is recognized, the actual processing of the
element is delegated to a <tt class="code">JETParseEventListener</tt>.</td>
<td valign="top"><tt class="code">JETReader</tt></td>
<td>An input buffer for the JET parser. Provides a <tt class="code">stackStream</tt> method
that others can call with the character stream to an include file. Also
provides many other convenience methods for the parser.</td>
<td valign="top"><tt class="code">JETScriptletGenerator</tt></td>
<td>Responsible for a part of the template translation process. Translates
JET scriptlets (<tt class="code">&lt;% ... %&gt;</tt> stuff) to Java source code.</td>
<td valign="top"><tt class="code">JETSkeleton</tt></td>
<td>This class provides an interface for assembling Java source code elements
into a single Java compilation unit (a Java class). Java source code elements
are assembled according to a class skeleton definition. A skeleton can be
used to add boilerplate code to a translated template implementation class.
This class provides a default custom template implementation class skeleton
definition, but can also assemble Java elements using a custom skeleton.
The actual parsing and generation of Java source code is delegated to classes
in the <tt class="code">org.eclipse.jdt.core.jdom</tt> package.</td>
<b>Package org.eclipse.emf.codegen.jmerge</b>
<table border="1" cellspacing="0">
<td valign="top"><tt class="code">JControlModel</tt></td>
<td>A control model that provides dictionaries and rules to drive a merge
<td valign="top"><tt class="code">JMerger</tt></td>
<td>A class for merging Java source files. Uses classes in the <tt class="code">org.eclipse.jdt.core.jdom</tt>
package to parse the source code. This class can be used by application
code, but can also be run as an Eclipse headless application.</td>
<td valign="top"><tt class="code">JPatternDictionary</tt></td>
<td>A dictionary of signatures and JDOM nodes.</td>
<td valign="top"><tt class="code">PropertyMerger</tt></td>
<td>A class for merging property files. This class can be used by application
code, but can also be run as an Eclipse headless application.</td>
<h3><a name="codegen_headless_application"></a>Running CodeGen as an Eclipse Headless
Application </h3>
<p>The <tt class="code">org.eclipse.emf.codegen.CodeGen</tt> class can translate a JET template to Java source code
and optionally merge the template implementation Java source code with an existing
Java class. CodeGen can be used as an Eclipse headless application ("headless"
means that the Eclipse GUI does not start). The <tt class="code">plugins/org.eclipse.emf.codegen/test</tt>
folder in your Eclipse installation contains some scripts for launching the
CodeGen class as an Eclipse headless application. These scripts are in Unix
<p><img src="images/win_only.gif" width="49" height="13"/> Below is an example
script for Windows. Note that we pass two arguments to the CodeGen class:</p>
<li>the <img src="images/tag_1.gif" width="24" height="13"/>uri of the template
to translate </li>
<li>the <img src="images/tag_2.gif" width="24" height="13"/>target path where
the translation result should be saved </li>
<p>If the target path already contains a previous translation result, and you
want to merge the new translation result with the existing one, you can specify
a <tt class="code">JMerge</tt> control model file as the third argument. The <tt class="code">plugins/org.eclipse.emf.codegen/test</tt>
folder in your Eclipse installation contains an example <tt class="code">merge.xml</tt> file.</p>
<pre class="code"> @echo off
set ECLIPSE_HOME=C:\eclipse-2.1\eclipse
set OPTS=-Xmx900M -Djava.compiler=NONE -verify -cp %ECLIPSE_HOME%\startup.jar
set MAIN=org.eclipse.core.launcher.Main -noupdate -data %WORKSPACE%
<img src="images/tag_1.gif" width="24" height="13"/>set TEMPLATE_URI=test.javajet
<img src="images/tag_2.gif" width="24" height="13"/>set TARGET_FOLDER=C:\temp\jetstandalone\MyProject
echo Shut down Eclipse before running this script.
java %OPTS% %MAIN% -application org.eclipse.emf.codegen.CodeGen %ARGUMENTS%
<h3><a name="jetc"></a>jetc: An ANT Task for Translating JET Templates Outside
of Eclipse </h3>
<p>Author: Knut Wannheden (knut.wannheden at </p>
<p>Binary: <a href="jetc-task.jar">jetc-task.jar</a>. </p>
<p>The source: <a href="" target=_blank></a>.</p>
<p>Some notes: </p>
<li>In the <tt class="code">&lt;taskdef/&gt;</tt> you have to specify the classpath to include
both this task as well as the jars of a number of Eclipse plug-ins (see the
example buildfile). </li>
<li>There are two ways to tell the jetc task which template(s) it should translate:
<li>Use a nested fileset to specify the directory containing the template
files. </li>
<li>Specify the template uri in the "template" attribute of the task. When
used this way, the task supports the "class" and "package" attributes which
overload the JET directives "class" and "package" in the template. It can
also be used if the JET directive is missing altogether. Use this attribute
when you want to specify the class to generate outside the template. </li>
<p>Here's a simple Ant buildfile (the taskdef classpath assumes you have Eclipse
2.1 and EMF 1.1.0): </p>
<pre class="code"> &lt;project default="jetc_multiple_templates"&gt;
&lt;property name="eclipse.plugins.dir" location="C:/eclipse-2.1/eclipse/plugins"/&gt;
&lt;taskdef name="jetc" classname="ch.paranor.epla.structure.JETCTask"&gt;
&lt;pathelement location="jetc-task.jar"/&gt;
&lt;fileset dir="${eclipse.plugins.dir}"&gt;
&lt;include name="org.eclipse.core.boot_2.1.0/boot.jar"/&gt;
&lt;include name="org.eclipse.core.resources_2.1.0/resources.jar"/&gt;
&lt;include name="org.eclipse.core.runtime_2.1.0/runtime.jar"/&gt;
&lt;include name="org.eclipse.emf.codegen_1.1.0/runtime/codegen.jar"/&gt;
&lt;include name="org.eclipse.jdt.core_2.1.0/jdtcore.jar"/&gt;
&lt;!-- Usage example 1: --&gt;
&lt;!-- Specify the template file in the "template" attribute. --&gt;
&lt;!-- You can use the "class" and "package" attributes to override the --&gt;
&lt;!-- "class" and "package" attributes in the template file. --&gt;
&lt;target name="jetc_single_template"&gt;
&lt;mkdir dir="jet-output"/&gt;
&lt;jetc template="test.xmljet" package="" class="Test" destdir="jet-output"/&gt;
&lt;javac srcdir="jet-output" destdir="classes"/&gt;
&lt;!-- Usage example 2: --&gt;
&lt;!-- Translate a bunch of template files at once. --&gt;
&lt;!-- You cannot use the "class" and "package" attributes when using a fileset. --&gt;
&lt;target name="jetc_multiple_templates"&gt;
&lt;mkdir dir="jet-output"/&gt;
&lt;jetc destdir="jet-output"&gt;
&lt;fileset dir="jet-templates" includes="*.*jet"/&gt;
&lt;javac srcdir="jet-output" destdir="classes"/&gt;
<h2>Resources </h2>
<p><a href="" target="_blank">Substitutes
for Missing C Constructs</a> (By Joshua Bloch)</p>
<a href="" target="_blank">
Java Tip 122: Beware of Java typesafe enumerations</a> (By Vladimir Roubtsov)<br/>
<p><a href="" target="_blank">Java
Tip 133: More on typesafe enums</a> (By Philip Bishop)</p>
<p><a href=""> </a></p>
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