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8.3 Using Annotations

Annotations are tags that you insert into your source code so that some tool can process them. The tools can operate on the source level, or they can process class files into which the compiler has placed annotations.

Annotations do not change the way in which your programs are compiled. The Java compiler generates the same virtual machine instructions with or without the annotations.

To benefit from annotations, you need to select a processing tool. Use annotations that your processing tool understands, then apply the processing tool to your code.

There is a wide range of uses for annotations, and that generality can be confusing at first. Here are some uses for annotations:

  • Automatic generation of auxiliary files, such as deployment descriptors or bean information classes

  • Automatic generation of code for testing, logging, transaction semantics, and so on

8.3.1 An Introduction into Annotations

We’ll start our discussion of annotations with the basic concepts and put them to use in a concrete example: We will mark methods as event listeners for AWT components, and show you an annotation processor that analyzes the annotations and hooks up the listeners. We’ll then discuss the syntax rules in detail and finish the chapter with two advanced examples of annotation processing. One of them processes source-level annotations, the other uses the Apache Bytecode Engineering Library to process class files, injecting additional bytecodes into annotated methods.

Here is an example of a simple annotation:

public class MyClass
{
   . . .
   @Test public void checkRandomInsertions()
}

The annotation @Test annotates the checkRandomInsertions method.

In Java, an annotation is used like a modifier and is placed before the annotated item without a semicolon. (A modifier is a keyword such as public or static.) The name of each annotation is preceded by an @ symbol, similar to Javadoc comments.

However, Javadoc comments occur inside /** . . . */ delimiters, whereas annotations are part of the code.

By itself, the @Test annotation does not do anything. It needs a tool to be useful. For example, the JUnit 4 testing tool (available at http://junit.org) calls all methods that are labeled @Test when testing a class. Another tool might remove all test methods from a class file so they are not shipped with the program after it has been tested.

Annotations can be defined to have elements, such as

@Test(timeout="10000")

These elements can be processed by the tools that read the annotations. Other forms of elements are possible; we’ll discuss them later in this chapter.

Besides methods, you can annotate classes, fields, and local variables—an annotation can be anywhere you could put a modifier such as public or static. In addition, as you will see in Section 8.4, “Annotation Syntax,” on p. 462, you can annotate packages, parameter variables, type parameters, and type uses.

Each annotation must be defined by an annotation interface. The methods of the interface correspond to the elements of the annotation. For example, the JUnit Test annotation is defined by the following interface:

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Test
{
   long timeout() default 0L;
   . . .
}

The @interface declaration creates an actual Java interface. Tools that process annotations receive objects that implement the annotation interface. A tool would call the timeout method to retrieve the timeout element of a particular Test annotation.

The Target and Retention annotations are meta-annotations. They annotate the Test annotation, marking it as an annotation that can be applied to methods only and is retained when the class file is loaded into the virtual machine. We’ll discuss these in detail in Section 8.5.3, “Meta-Annotations,” on p. 472.

You have now seen the basic concepts of program metadata and annotations. In the next section, we’ll walk through a concrete example of annotation processing.

8.3.2 An Example: Annotating Event Handlers

One of the more boring tasks in user interface programming is the wiring of listeners to event sources. Many listeners are of the form

myButton.addActionListener(() -> doSomething());

In this section, we’ll design an annotation to reverse the wiring. The annotation, defined in Listing 8.11, is used as follows:

@ActionListenerFor(source="myButton") void doSomething() { . . . }

The programmer no longer has to make calls to addActionListener. Instead, each method is tagged with an annotation. Listing 8.10 shows the ButtonFrame class from Volume I, Chapter 11, reimplemented with these annotations.

We also need to define an annotation interface. The code is in Listing 8.11.

Of course, the annotations don’t do anything by themselves. They sit in the source file. The compiler places them in the class file, and the virtual machine loads them. We now need a mechanism to analyze them and install action listeners. That is the job of the ActionListenerInstaller class. The ButtonFrame constructor calls

ActionListenerInstaller.processAnnotations(this);

The static processAnnotations method enumerates all methods of the object it received. For each method, it gets the ActionListenerFor annotation object and processes it.

Class<?> cl = obj.getClass();
for (Method m : cl.getDeclaredMethods())
{
   ActionListenerFor a = m.getAnnotation(ActionListenerFor.class);
   if (a != null) . . .
}

Here, we use the getAnnotation method defined in the AnnotatedElement interface. The classes Method, Constructor, Field, Class, and Package implement this interface.

The name of the source field is stored in the annotation object. We retrieve it by calling the source method, and then look up the matching field.

String fieldName = a.source();
Field f = cl.getDeclaredField(fieldName);

This shows a limitation of our annotation. The source element must be the name of a field. It cannot be a local variable.

The remainder of the code is rather technical. For each annotated method, we construct a proxy object, implementing the ActionListener interface, with an actionPerformed method that calls the annotated method. (For more information about proxies, see Volume I, Chapter 6.) The details are not important. The key observation is that the functionality of the annotations was established by the processAnnotations method.

Figure 8.1 shows how annotations are handled in this example.

Figure 8.1

Figure 8.1 Processing annotations at runtime

In this example, the annotations were processed at runtime. It is also possible to process them at the source level: A source code generator would then produce the code for adding the listeners. Alternatively, the annotations can be processed at the bytecode level: A bytecode editor could inject the calls to addActionListener into the frame constructor. This sounds complex, but libraries are available to make this task relatively straightforward. You can see an example in Section 8.7, “Bytecode Engineering,” on p. 481.

Our example was not intended as a serious tool for user interface programmers. A utility method for adding a listener could be just as convenient for the programmer as the annotation. (In fact, the java.beans.EventHandler class tries to do just that. You could make the class truly useful by supplying a method that adds the event handler instead of just constructing it.)

However, this example shows the mechanics of annotating a program and of analyzing the annotations. Having seen a concrete example, you are now more prepared (we hope) for the following sections that describe the annotation syntax in complete detail.

Listing 8.9 runtimeAnnotations/ActionListenerInstaller.java

 1   package runtimeAnnotations;
 2
 3   import java.awt.event.*;
 4   import java.lang.reflect.*;
 5
 6   /**
 7    * @version 1.00 2004-08-17
 8    * @author Cay Horstmann
 9    */
10   public class ActionListenerInstaller
11   {
12      /**
13       * Processes all ActionListenerFor annotations in the given object.
14       * @param obj an object whose methods may have ActionListenerFor annotations
15       */
16      public static void processAnnotations(Object obj)
17      {
18         try
19         {
20            Class<?> cl = obj.getClass();
21            for (Method m : cl.getDeclaredMethods())
22            {
23               ActionListenerFor a = m.getAnnotation(ActionListenerFor.class);
24               if (a != null)
25              {
26                  Field f = cl.getDeclaredField(a.source());
27                  f.setAccessible(true);
28                  addListener(f.get(obj), obj, m);
29              }
30            }
31         }
32         catch (ReflectiveOperationException e)
33         {
34            e.printStackTrace();
35         }
36      }
37
38      /**
39       * Adds an action listener that calls a given method.
40       * @param source the event source to which an action listener is added
41       * @param param the implicit parameter of the method that the listener calls
42       * @param m the method that the listener calls
43       */
44      public static void addListener(Object source, final Object param, final Method m)
45         throws ReflectiveOperationException
46      {
47         InvocationHandler handler = new InvocationHandler()
48            {
49               public Object invoke(Object proxy, Method mm, Object[] args) throws Throwable
50               {
51                  return m.invoke(param);
52               }
53            };
54
55         Object listener = Proxy.newProxyInstance(null,
56               new Class[] { java.awt.event.ActionListener.class }, handler);
57         Method adder = source.getClass().getMethod("addActionListener", ActionListener.class);
58         adder.invoke(source, listener);
59      }
60   }

Listing 8.10 buttons3/ButtonFrame.java

 1   package buttons3;
 2
 3   import java.awt.*;
 4   import javax.swing.*;
 5   import runtimeAnnotations.*;
 6
 7   /**
 8    * A frame with a button panel.
 9    * @version 1.00 2004-08-17
10    * @author Cay Horstmann
11    */
12   public class ButtonFrame extends JFrame
13   {
14      private static final int DEFAULT_WIDTH = 300;
15      private static final int DEFAULT_HEIGHT = 200;
16
17      private JPanel panel;
18      private JButton yellowButton;
19      private JButton blueButton;
20      private JButton redButton;
21
22      public ButtonFrame()
23      {
24         setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);
25
26         panel = new JPanel();
27         add(panel);
28
29         yellowButton = new JButton("Yellow");
30         blueButton = new JButton("Blue");
31         redButton = new JButton("Red");
32
33         panel.add(yellowButton);
34         panel.add(blueButton);
35         panel.add(redButton);
36
37         ActionListenerInstaller.processAnnotations(this);
38      }
39
40      @ActionListenerFor(source = "yellowButton")
41      public void yellowBackground()
42      {
43         panel.setBackground(Color.YELLOW);
44      }
45
46      @ActionListenerFor(source = "blueButton")
47      public void blueBackground()
48      {
49         panel.setBackground(Color.BLUE);
50      }
51
52      @ActionListenerFor(source = "redButton")
53      public void redBackground()
54      {
55         panel.setBackground(Color.RED);
56      }
57   }

Listing 8.11 runtimeAnnotations/ActionListenerFor.java

 1   package runtimeAnnotations;
 2
 3   import java.lang.annotation.*;
 4
 5   /**
 6    * @version 1.00 2004-08-17
 7    * @author Cay Horstmann
 8    */
 9
10   @Target(ElementType.METHOD)
11   @Retention(RetentionPolicy.RUNTIME)
12   public @interface ActionListenerFor
13   {
14      String source();
15   }
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