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Scripting, Compiling, and Annotation Processing in Java

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This chapter introduces three techniques for processing Java code: The Scripting API, the Compiler API, and Annotation processors.
This chapter is from the book

This chapter introduces three techniques for processing code. The scripting API lets you invoke code in a scripting language such as JavaScript or Groovy. You can use the compiler API when you want to compile Java code inside your application. Annotation processors operate on Java source or class files that contain annotations. As you will see, there are many applications for annotation processing, ranging from simple diagnostics to “bytecode engineering”—the insertion of bytecodes into class files or even running programs.

10.1. Scripting for the Java Platform

A scripting language is a language that avoids the usual edit/compile/link/run cycle by interpreting the program text at runtime. Scripting languages have a number of advantages:

  • Rapid turnaround, encouraging experimentation
  • Changing the behavior of a running program
  • Enabling customization by program users

On the other hand, most scripting languages lack features that are beneficial for programming complex applications, such as strong typing, encapsulation, and modularity.

It is therefore tempting to combine the advantages of scripting and traditional languages. The scripting API lets you do just that for the Java platform. It enables you to invoke scripts written in JavaScript, Groovy, Ruby, and even exotic languages such as Scheme and Haskell, from a Java program. (The other direction—accessing Java from the scripting language—is the responsibility of the scripting language provider. Most scripting languages that run on the Java virtual machine have this capability.)

In the following sections, we’ll show you how to select an engine for a particular language, how to execute scripts, and how to take advantage of advanced features that some scripting engines offer.

10.1.1. Getting a Scripting Engine

A scripting engine is a library that can execute scripts in a particular language. When the virtual machine starts, it discovers the available scripting engines. To enumerate them, construct a ScriptEngineManager and invoke the getEngineFactories method. You can ask each engine factory for the supported engine names, MIME types, and file extensions. Table 10.1 shows typical values.

Table 10.1. Properties of Scripting Engine Factories



MIME types


Rhino (included with Java SE)

js, rhino, JavaScript, javascript, ECMAScript, ecmascript

application/javascript, application/ecmascript, text/javascript, text/ecmascript






SISC Scheme

scheme, sisc


scc, sce, scm, shp

Usually, you know which engine you need, and you can simply request it by name, MIME type, or extension. For example:

ScriptEngine engine = manager.getEngineByName("JavaScript");

Java SE 7 includes a version of Rhino, a JavaScript interpreter developed by the Mozilla foundation. You can add more languages by providing the necessary JAR files on the class path. You will generally need two sets of JAR files. The scripting language itself is implemented by a single JAR file or a set of JARs. The engine that adapts the language to the scripting API usually requires an additional JAR. The site http://java.net/projects/scripting provides engines for a wide range of scripting languages. For example, to add support for Groovy, the class path should contain groovy/lib/* (from http://groovy.codehaus.org) and groovy-engine.jar (from http://java.net/projects/scripting).

10.1.2. Script Evaluation and Bindings

Once you have an engine, you can call a script simply by invoking

Object result = engine.eval(scriptString);

If the script is stored in a file, open a Reader and call

Object result = engine.eval(reader);

You can invoke multiple scripts on the same engine. If one script defines variables, functions, or classes, most scripting engines retain the definitions for later use. For example,

engine.eval("n = 1728");
Object result = engine.eval("n + 1");

will return 1729.

You will often want to add variable bindings to the engine. A binding consists of a name and an associated Java object. For example, consider these statements:

engine.put(k, 1728);
Object result = engine.eval("k + 1");

The script code reads the definition of k from the bindings in the “engine scope.” This is particularly important because most scripting languages can access Java objects, often with a syntax that is simpler than the Java syntax. For example,

engine.put(b, new JButton());
engine.eval("b.text = 'Ok'");

Conversely, you can retrieve variables that were bound by scripting statements:

engine.eval("n = 1728");
Object result = engine.get("n");

In addition to the engine scope, there is also a global scope. Any bindings that you add to the ScriptEngineManager are visible to all engines.

Instead of adding bindings to the engine or global scope, you can collect them in an object of type Bindings and pass it to the eval method:

Bindings scope = engine.createBindings();
scope.put(b, new JButton());
engine.eval(scriptString, scope);

This is useful if a set of bindings should not persist for future calls to the eval method.

10.1.3. Redirecting Input and Output

You can redirect the standard input and output of a script by calling the setReader and setWriter methods of the script context. For example,

StringWriter writer = new StringWriter();
engine.getContext().setWriter(new PrintWriter(writer, true));

Any output written with the JavaScript print or println functions is sent to writer.

The setReader and setWriter methods only affect the scripting engine’s standard input and output sources. For example, if you execute the JavaScript code


only the first output is redirected.

The Rhino engine does not have the notion of a standard input source. Calling setReader has no effect.

10.1.4. Calling Scripting Functions and Methods

With many script engines, you can invoke a function in the scripting language without having to evaluate the actual script code. This is useful if you allow users to implement a service in a scripting language of their choice.

The script engines that offer this functionality implement the Invocable interface. In particular, the Rhino engine implements Invocable.

To call a function, call the invokeFunction method with the function name, followed by the function parameters:

if (engine implements Invocable)
   ((Invocable) engine).invokeFunction("aFunction", param1, param2);

If the scripting language is object-oriented, you call can a method like this:

((Invocable) engine).invokeMethod(implicitParam, "aMethod", explicitParam1, explicitParam2);

Here, the implicitParam object is a proxy to an object in the scripting language. It must be the result of a prior call to the scripting engine.

You can go a step further and ask the scripting engine to implement a Java interface. Then you can call scripting functions and methods with the Java method call syntax.

The details depend on the scripting engine, but typically you need to supply a function for each method of the interface. For example, consider a Java interface

public interface Greeter
   String greet(String whom);

In Rhino, you provide a function

function greet(x) { return "Hello, " + x + "!"; }

This code must be evaluated first. Then you can call

Greeter g = ((Invocable) engine).getInterface(Greeter.class);

Now you can make a plain Java method call

String result = g.greet("World");

Behind the scenes, the JavaScript greet method is invoked. This approach is similar to making a remote method call, as discussed in Chapter 11.

In an object-oriented scripting language, you can access a script class through a matching Java interface. For example, consider this JavaScript code, which defines a SimpleGreeter class.

function SimpleGreeter(salutation) { this.salutation = salutation; }
SimpleGreeter.prototype.greet = function(whom) { return this.salutation + ", " + whom + "!"; }

You can use this class to construct greeters with different salutations (such as “Hello”, “Goodbye”, and so on).

After evaluating the JavaScript class definition, call

Object goodbyeGreeter = engine.eval("new SimpleGreeter('Goodbye')");
Greeter g = ((Invocable) engine).getInterface(goodbyeGreeter, Greeter.class);

When you call g.greet("World"), the greet method is invoked on the JavaScript object goodbyeGreeter. The result is a string "Goodbye, World!".

In summary, the Invocable interface is useful if you want to call scripting code from Java without worrying about the scripting language syntax.

10.1.5. Compiling a Script

Some scripting engines can compile scripting code into an intermediate form for efficient execution. Those engines implement the Compilable interface. The following example shows how to compile and evaluate code contained in a script file:

Reader reader = new FileReader("myscript.js");
CompiledScript script = null;
if (engine implements Compilable)
   CompiledScript script = ((Compilable) engine).compile(reader);

Once the script is compiled, you can execute it. The following code executes the compiled script if compilation was successful, or the original script if the engine didn’t support compilation.

if (script != null)

Of course, it only makes sense to compile a script if you need to execute it repeatedly.

10.1.6. An Example: Scripting GUI Events

To illustrate the scripting API, we will write a sample program that allows users to specify event handlers in a scripting language of their choice.

Have a look at the program in Listing 10.1 that adds scripting to an arbitrary frame class. By default it reads the ButtonFrame class in Listing 10.2, which is similar to the event handling demo in Volume I, with two differences:

  • Each component has its name property set.
  • There are no event handlers.

The event handlers are defined in a property file. Each property definition has the form

componentName.eventName = scriptCode

For example, if you choose to use JavaScript, supply the event handlers in a file js.properties, like this:

yellowButton.action=panel.background = java.awt.Color.YELLOW
blueButton.action=panel.background = java.awt.Color.BLUE
redButton.action=panel.background = java.awt.Color.RED

The companion code also has files for Groovy and SISC Scheme.

The program starts by loading an engine for the language specified on the command line. If no language is specified, we use JavaScript.

We then process a script init.language if it is present. This seems like a good idea in general; moreover, the Scheme interpreter needs some cumbersome initializations that we did not want to include in every event handler script.

Next, we recursively traverse all child components and add the bindings (name, object) into the engine scope.

Then we read the file language.properties. For each property, we synthesize an event handler proxy that causes the script code to be executed. The details are a bit technical. You might want to read the section on proxies in Volume I, Chapter 6, together with the section on JavaBeans events in Chapter 8 of this volume, if you want to follow the implementation in detail. The essential part, however, is that each event handler calls


Let us look at the yellowButton in more detail. When the line

yellowButton.action=panel.background = java.awt.Color.YELLOW

is processed, we find the JButton component with the name "yellowButton". We then attach an ActionListener with an actionPerformed method that executes the script

panel.background = java.awt.Color.YELLOW

The engine contains a binding that binds the name "panel" to the JPanel object. When the event occurs, the setBackground method of the panel is executed, and the color changes.

You can run this program with the JavaScript event handlers, simply by executing

java ScriptTest

For the Groovy handlers, use

java -classpath .:groovy/lib/*:jsr223-engines/groovy/build/groovy-engine.jar ScriptTest groovy

Here, groovy is the directory into which you installed Groovy, and jsr223-engines is the directory that contains the engine adapters from http://java.net/projects/scripting.

To try out Scheme, download SISC Scheme from http://sisc-scheme.org and run

java -classpath .:sisc/*:jsr223-engines/scheme/build/scheme-engine.jar ScriptTest scheme

This application demonstrates how to use scripting for Java GUI programming. One could go a step further and describe the GUI with an XML file, as you have seen in Chapter 2. Then our program would become an interpreter for GUIs that have visual presentation defined by XML and behavior defined by a scripting language. Note the similarity to a dynamic HTML page or a dynamic server-side scripting environment.

Listing 10.1. script/ScriptTest.java

 1  package script;
 3  import java.awt.*;
 4  import java.beans.*;
 5  import java.io.*;
 6  import java.lang.reflect.*;
 7  import java.util.*;
 8  import javax.script.*;
 9  import javax.swing.*;
11  /**
12   * @version 1.01 2012-01-28
13   * @author Cay Horstmann
14   */
15  public class ScriptTest
16  {
17     public static void main(final String[] args)
18     {
19        EventQueue.invokeLater(new Runnable()
20           {
21              public void run()
22              {
23                 try
24                 {
25                    ScriptEngineManager manager = new ScriptEngineManager();
26                    String language;
27                    if (args.length == 0)
28                    {
29                       System.out.println("Available factories: ");
30                       for (ScriptEngineFactory factory : manager.getEngineFactories())
31                          System.out.println(factory.getEngineName());
33                       language = "js";
34                    }
35                    else language = args[0];
37                    final ScriptEngine engine = manager.getEngineByName(language);
38                    if (engine == null)
39                    {
40                       System.err.println("No engine for " + language);
41                       System.exit(1);
42                    }
44                    final String frameClassName = args.length < 2 ? "buttons1.ButtonFrame" : args[1];
46                    JFrame frame = (JFrame) Class.forName(frameClassName).newInstance();
47                    InputStream in = frame.getClass().getResourceAsStream("init." + language);
48                    if (in != null) engine.eval(new InputStreamReader(in));
49                    getComponentBindings(frame, engine);
51                    final Properties events = new Properties();
52                    in = frame.getClass().getResourceAsStream(language + ".properties");
53                    events.load(in);
55                    for (final Object e : events.keySet())
56                    {
57                       String[] s = ((String) e).split("\\.");
58                       addListener(s[0], s[1], (String) events.get(e), engine);
59                    }
60                    frame.setTitle("ScriptTest");
61                    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
62                    frame.setVisible(true);
63                 }
64                 catch (ReflectiveOperationException | IOException
65                   | ScriptException | IntrospectionException ex)
66                 {
67                   ex.printStackTrace();
68                 }
69              }
70           });
71     }
72     /**
73      * Gathers all named components in a container.
74      * @param c the component
75      * @param namedComponents
76      */
77     private static void getComponentBindings(Component c, ScriptEngine engine)
78     {
79        String name = c.getName();
80        if (name != null) engine.put(name, c);
81        if (c instanceof Container)
82        {
83           for (Component child : ((Container) c).getComponents())
84              getComponentBindings(child, engine);
85        }
86     }
88     /**
89      * Adds a listener to an object whose listener method executes a script.
90      * @param beanName the name of the bean to which the listener should be added
91      * @param eventName the name of the listener type, such as "action" or "change"
92      * @param scriptCode the script code to be executed
93      * @param engine the engine that executes the code
94      * @param bindings the bindings for the execution
95      * @throws IntrospectionException
96      */
97     private static void addListener(String beanName, String eventName, final String scriptCode,
98        final ScriptEngine engine) throws ReflectiveOperationException, IntrospectionException
99     {
100       Object bean = engine.get(beanName);
101       EventSetDescriptor descriptor = getEventSetDescriptor(bean, eventName);
102       if (descriptor == null) return;
103       descriptor.getAddListenerMethod().invoke(bean,
104          Proxy.newProxyInstance(null, new Class[] { descriptor.getListenerType() },
105             new InvocationHandler()
106                {
107                   public Object invoke(Object proxy, Method method, Object[] args)
108                         throws Throwable
109                   {
110                      engine.eval(scriptCode);
111                      return null;
112                   }
113                }));
114    }
116    private static EventSetDescriptor getEventSetDescriptor(Object bean, String eventName)
117       throws IntrospectionException
118    {
119       for (EventSetDescriptor descriptor : Introspector.getBeanInfo(bean.getClass())
120             .getEventSetDescriptors())
121          if (descriptor.getName().equals(eventName)) return descriptor;
122       return null;
123    }
124 }

Listing 10.2. buttons1/ButtonFrame.java

 1  package buttons1;
 3  import javax.swing.*;
 5  public class ButtonFrame extends JFrame
 6  {
 7     private static final int DEFAULT_WIDTH = 300;
 8     private static final int DEFAULT_HEIGHT = 200;
10     private JPanel panel;
11     private JButton yellowButton;
12     private JButton blueButton;
13     private JButton redButton;
15     public ButtonFrame()
16     {
19        panel = new JPanel();
20        panel.setName("panel");
21        add(panel);
23        yellowButton = new JButton("Yellow");
24        yellowButton.setName("yellowButton");
25        blueButton = new JButton("Blue");
26        blueButton.setName("blueButton");
27        redButton = new JButton("Red");
28        redButton.setName("redButton");
30        panel.add(yellowButton);
31        panel.add(blueButton);
32        panel.add(redButton);
33     }
34  }
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