tutorial/tutorial3.html - platform/external/javassist - Git at Google

 <html>
 <head>
    <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
    <title>Javassist Tutorial</title>
    <link rel="stylesheet" type="text/css" href="brown.css">
 </head>

 <body>

 <div align="right">Getting Started with Javassist</div>

 <div align="left"><a href="tutorial2.html">Previous page</a></div>

 <p>
 <a href="#intro">5. Bytecode level API</a>
 <ul>
 <li><a href="#classfile">Obtaining a <code>ClassFile</code> object</a>
 <br><li><a href="#member">Adding and removing a member</a>
 <br><li><a href="#traverse">Traversing a method body</a>
 <br><li><a href="#bytecode">Producing a bytecode sequence</a>
 <br><li><a href="#annotation">Annotations (Meta tags)</a>

 </ul>

 <p><a href="#generics">6. Generics</a>

 <p><a href="#varargs">7. Varargs</a>

 <p><a href="#j2me">8. J2ME</a>

 <p><br>

 <a name="intro">
 <h2>5. Bytecode level API</h2>

 <p>
 Javassist also provides lower-level API for directly editing
 a class file.  To use this level of API, you need detailed
 knowledge of the Java bytecode and the class file format
 while this level of API allows you any kind of modification
 of class files.

 <p>
 If you want to just produce a simple class file,
 <code>javassist.bytecode.ClassFileWriter</code> might provide
 the best API for you.  It is much faster than
 <code>javassist.bytecode.ClassFile</code> although its API
 is minimum.

 <a name="classfile">
 <h3>5.1 Obtaining a <code>ClassFile</code> object</h3>

 <p>A <code>javassist.bytecode.ClassFile</code> object represents
 a class file.  To obtian this object, <code>getClassFile()</code>
 in <code>CtClass</code> should be called.

 <p>Otherwise, you can construct a
 <code>javassist.bytecode.ClassFile</code> directly from a class file.
 For example,

 <ul><pre>
 BufferedInputStream fin
     = new BufferedInputStream(new FileInputStream("Point.class"));
 ClassFile cf = new ClassFile(new DataInputStream(fin));
 </pre></ul>

 <p>
 This code snippet creats a <code>ClassFile</code> object from
 <code>Point.class</code>.

 <p>
 A <code>ClassFile</code> object can be written back to a
 class file.  <code>write()</code> in <code>ClassFile</code>
 writes the contents of the class file to a given
 <code>DataOutputStream</code>.

 <p><br>

 <a name="member">
 <h3>5.2 Adding and removing a member</h3>

 <p>
 <code>ClassFile</code> provides <code>addField()</code> and
 <code>addMethod()</code> for adding a field or a method (note that
 a constructor is regarded as a method at the bytecode level).
 It also provides <code>addAttribute()</code> for adding an attribute
 to the class file.

 <p>
 Note that <code>FieldInfo</code>, <code>MethodInfo</code>, and
 <code>AttributeInfo</code> objects include a link to a
 <code>ConstPool</code> (constant pool table) object.  The <code>ConstPool</code>
 object must be common to the <code>ClassFile</code> object and
 a <code>FieldInfo</code> (or <code>MethodInfo</code> etc.) object
 that is added to that <code>ClassFile</code> object.
 In other words, a <code>FieldInfo</code> (or <code>MethodInfo</code> etc.) object
 must not be shared among different <code>ClassFile</code> objects.

 <p>
 To remove a field or a method from a <code>ClassFile</code> object,
 you must first obtain a <code>java.util.List</code>
 object containing all the fields of the class.  <code>getFields()</code>
 and <code>getMethods()</code> return the lists.  A field or a method can
 be removed by calling <code>remove()</code> on the <code>List</code> object.
 An attribute can be removed in a similar way.
 Call <code>getAttributes()</code> in <code>FieldInfo</code> or
 <code>MethodInfo</code> to obtain the list of attributes,
 and remove one from the list.


 <p><br>

 <a name="traverse">
 <h3>5.3 Traversing a method body</h3>

 <p>
 To examine every bytecode instruction in a method body,
 <code>CodeIterator</code> is useful.  To otbain this object,
 do as follows:

 <ul><pre>
 ClassFile cf = ... ;
 MethodInfo minfo = cf.getMethod("move");    // we assume move is not overloaded.
 CodeAttribute ca = minfo.getCodeAttribute();
 CodeIterator i = ca.iterator();
 </pre></ul>

 <p>
 A <code>CodeIterator</code> object allows you to visit every
 bytecode instruction one by one from the beginning to the end.
 The following methods are part of the methods declared in
 <code>CodeIterator</code>:

 <ul>
 <li><code>void begin()</code><br>
 Move to the first instruction.<br>
 <li><code>void move(int index)</code><br>
 Move to the instruction specified by the given index.<br>
 <li><code>boolean hasNext()</code><br>
 Returns true if there is more instructions.<br>
 <li><code>int next()</code><br>
 Returns the index of the next instruction.<br>
 <em>Note that it does not return the opcode of the next
 instruction.</em><br>
 <li><code>int byteAt(int index)</code><br>
 Returns the unsigned 8bit value at the index.<br>
 <li><code>int u16bitAt(int index)</code><br>
 Returns the unsigned 16bit value at the index.<br>
 <li><code>int write(byte[] code, int index)</code><br>
 Writes a byte array at the index.<br>
 <li><code>void insert(int index, byte[] code)</code><br>
 Inserts a byte array at the index.
 Branch offsets etc. are automatically adjusted.<br>
 </ul>

 <p>The following code snippet displays all the instructions included
 in a method body:

 <ul><pre>
 CodeIterator ci = ... ;
 while (ci.hasNext()) {
     int index = ci.next();
     int op = ci.byteAt(index);
     System.out.println(Mnemonic.OPCODE[op]);
 }
 </pre></ul>

 <p><br>

 <a name="bytecode">
 <h3>5.4 Producing a bytecode sequence</h3>

 <p>
 A <code>Bytecode</code> object represents a sequence of bytecode
 instructions.  It is a growable array of bytecode.
 Here is a sample code snippet:

 <ul><pre>
 ConstPool cp = ...;    // constant pool table
 Bytecode b = new Bytecode(cp, 1, 0);
 b.addIconst(3);
 b.addReturn(CtClass.intType);
 CodeAttribute ca = b.toCodeAttribute();
 </pre></ul>

 <p>
 This produces the code attribute representing the following sequence:

 <ul><pre>
 iconst_3
 ireturn
 </pre></ul>

 <p>
 You can also obtain a byte array containing this sequence by
 calling <code>get()</code> in <code>Bytecode</code>.  The
 obtained array can be inserted in another code attribute.

 <p>
 While <code>Bytecode</code> provides a number of methods for adding a
 specific instruction to the sequence, it provides
 <code>addOpcode()</code> for adding an 8bit opcode and
 <code>addIndex()</code> for adding an index.
 The 8bit value of each opcode is defined in the <code>Opcode</code>
 interface.

 <p>
 <code>addOpcode()</code> and other methods for adding a specific
 instruction are automatically maintain the maximum stack depth
 unless the control flow does not include a branch.
 This value can be obtained by calling <code>getMaxStack()</code>
 on the <code>Bytecode</code> object.
 It is also reflected on the <code>CodeAttribute</code> object
 constructed from the <code>Bytecode</code> object.
 To recompute the maximum stack depth of a method body,
 call <code>computeMaxStack()</code> in <code>CodeAttribute</code>.

 <p><br>

 <a name="annotation">
 <h3>5.5 Annotations (Meta tags)</h3>

 <p>Annotations are stored in a class file
 as runtime invisible (or visible) annotations attribute.
 These attributes can be obtained from <code>ClassFile</code>,
 <code>MethodInfo</code>, or <code>FieldInfo</code> objects.
 Call <code>getAttribute(AnnotationsAttribute.invisibleTag)</code>
 on those objects.  For more details, see the javadoc manual
 of <code>javassist.bytecode.AnnotationsAttribute</code> class
 and the <code>javassist.bytecode.annotation</code> package.

 <p>Javassist also let you access annotations by the higher-level
 API.
 If you want to access annotations through <code>CtClass</code>,
 call <code>getAnnotations()</code> in <code>CtClass</code> or
 <code>CtBehavior</code>.

 <p><br>

 <h2><a name="generics">6. Generics</a></h2>

 <p>The lower-level API of Javassist fully supports generics
 introduced by Java 5.  On the other hand, the higher-level
 API such as <code>CtClass</code> does not directly support
 generics.  However, this is not a serious problem for bytecode
 transformation.

 <p>The generics of Java is implemented by the erasure technique.
 After compilation, all type parameters are dropped off.  For
 example, suppose that your source code declares a parameterized
 type <code>Vector&lt;String&gt;</code>:

 <ul><pre>
 Vector&lt;String&gt; v = new Vector&lt;String&gt();
   :
 String s = v.get(0);
 </pre></ul>

 <p>The compiled bytecode is equivalent to the following code:

 <ul><pre>
 Vector v = new Vector();
   :
 String s = (String)v.get(0);
 </pre></ul>

 <p>So when you write a bytecode transformer, you can just drop
 off all type parameters.  For example, if you have a class:

 <ul><pre>
 public class Wrapper&lt;T&gt; {
   T value;
   public Wrapper(T t) { value = t; }
 }
 </pre></ul>

 <p>and want to add an interface <code>Getter&lt;T&gt;</code> to the
 class <code>Wrapper&lt;T&gt;</code>:

 <ul><pre>
 public interface Getter&lt;T&gt; {
   T get();
 }
 </pre></ul>

 <p>Then the interface you really have to add is <code>Getter</code>
 (the type parameters <code>&lt;T&gt;</code> drops off)
 and the method you also have to add to the <code>Wrapper</code>
 class is this simple one:

 <ul><pre>
 public Object get() { return value; }
 </pre></ul>

 <p>Note that no type parameters are necessary.

 <p><br>

 <h2><a name="varargs">7. Varargs</a></h2>

 <p>Currently, Javassist does not directly support varargs.  So to make a method with varargs,
 you must explicitly set a method modifier.  But this is easy.
 Suppose that now you want to make the following method:

 <ul><pre>
 public int length(int... args) { return args.length; }
 </pre></ul>

 <p>The following code using Javassist will make the method shown above:

 <ul><pre>
 CtClass cc = /* target class */;
 CtMethod m = CtMethod.make("public int length(int[] args) { return args.length; }", cc);
 m.setModifiers(m.getModifiers() | Modifier.VARARGS);
 cc.addMethod(m);
 <pre></ul>

 <p>The parameter type <code>int...</code> is changed into <code>int[]</code>
 and <code>Modifier.VARARGS</code> is added to the method modifiers.

 <p>To call this method, you must write:

 <ul><pre>
 length(new int[] { 1, 2, 3 });
 </pre></ul>

 <p>instead of this method call using the varargs mechanism:

 <ul><pre>
 length(1, 2, 3);
 </pre></ul>

 <p><br>

 <h2><a name="j2me">8. J2ME</a></h2>

 <p>If you modify a class file for the J2ME execution environment,
 you must perform <it>preverification</it>.  Preverifying is basically
 producing stack maps, which is similar to stack map tables introduced
 into J2SE at JDK 1.6.  Javassist maintains the stack maps for J2ME only if
 <code>javassist.bytecode.MethodInfo.doPreverify</code> is true.

 <p>You can also manually
 produce a stack map for a modified method.
 For a given method represented by a <code>CtMethod</code> object <code>m</code>,
 you can produce a stack map by calling the following methods:

 <ul><pre>
 m.getMethodInfo().rebuildStackMapForME(cpool);
 </pre></ul>

 <p>Here, <code>cpool</code> is a <code>ClassPool</code> object, which is
 available by calling <code>getClassPool()</code> on a <code>CtClass</code>
 object.  A <code>ClassPool</code> object is responsible for finding
 class files from given class pathes.  To obtain all the <code>CtMethod</code>
 objects, call the <code>getDeclaredMethods</code> method on a <code>CtClass</code> object.

 <p><br>

 <a href="tutorial2.html">Previous page</a>

 <hr>
 Java(TM) is a trademark of Sun Microsystems, Inc.<br>
 Copyright (C) 2000-2010 by Shigeru Chiba, All rights reserved.
 </body>
 </html>
	<html>
	<head>
	<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
	<title>Javassist Tutorial</title>
	<link rel="stylesheet" type="text/css" href="brown.css">
	</head>

	<body>

	<div align="right">Getting Started with Javassist</div>

	<div align="left"><a href="tutorial2.html">Previous page</a></div>

	<p>
	<a href="#intro">5. Bytecode level API</a>
	<ul>
	<li><a href="#classfile">Obtaining a <code>ClassFile</code> object</a>
	<br><li><a href="#member">Adding and removing a member</a>
	<br><li><a href="#traverse">Traversing a method body</a>
	<br><li><a href="#bytecode">Producing a bytecode sequence</a>
	<br><li><a href="#annotation">Annotations (Meta tags)</a>

	</ul>

	<p><a href="#generics">6. Generics</a>

	<p><a href="#varargs">7. Varargs</a>

	<p><a href="#j2me">8. J2ME</a>

	<p><br>

	<a name="intro">
	<h2>5. Bytecode level API</h2>

	<p>
	Javassist also provides lower-level API for directly editing
	a class file. To use this level of API, you need detailed
	knowledge of the Java bytecode and the class file format
	while this level of API allows you any kind of modification
	of class files.

	<p>
	If you want to just produce a simple class file,
	<code>javassist.bytecode.ClassFileWriter</code> might provide
	the best API for you. It is much faster than
	<code>javassist.bytecode.ClassFile</code> although its API
	is minimum.

	<a name="classfile">
	<h3>5.1 Obtaining a <code>ClassFile</code> object</h3>

	<p>A <code>javassist.bytecode.ClassFile</code> object represents
	a class file. To obtian this object, <code>getClassFile()</code>
	in <code>CtClass</code> should be called.

	<p>Otherwise, you can construct a
	<code>javassist.bytecode.ClassFile</code> directly from a class file.
	For example,

	<ul><pre>
	BufferedInputStream fin
	= new BufferedInputStream(new FileInputStream("Point.class"));
	ClassFile cf = new ClassFile(new DataInputStream(fin));
	</pre></ul>

	<p>
	This code snippet creats a <code>ClassFile</code> object from
	<code>Point.class</code>.

	<p>
	A <code>ClassFile</code> object can be written back to a
	class file. <code>write()</code> in <code>ClassFile</code>
	writes the contents of the class file to a given
	<code>DataOutputStream</code>.

	<p><br>

	<a name="member">
	<h3>5.2 Adding and removing a member</h3>

	<p>
	<code>ClassFile</code> provides <code>addField()</code> and
	<code>addMethod()</code> for adding a field or a method (note that
	a constructor is regarded as a method at the bytecode level).
	It also provides <code>addAttribute()</code> for adding an attribute
	to the class file.

	<p>
	Note that <code>FieldInfo</code>, <code>MethodInfo</code>, and
	<code>AttributeInfo</code> objects include a link to a
	<code>ConstPool</code> (constant pool table) object. The <code>ConstPool</code>
	object must be common to the <code>ClassFile</code> object and
	a <code>FieldInfo</code> (or <code>MethodInfo</code> etc.) object
	that is added to that <code>ClassFile</code> object.
	In other words, a <code>FieldInfo</code> (or <code>MethodInfo</code> etc.) object
	must not be shared among different <code>ClassFile</code> objects.

	<p>
	To remove a field or a method from a <code>ClassFile</code> object,
	you must first obtain a <code>java.util.List</code>
	object containing all the fields of the class. <code>getFields()</code>
	and <code>getMethods()</code> return the lists. A field or a method can
	be removed by calling <code>remove()</code> on the <code>List</code> object.
	An attribute can be removed in a similar way.
	Call <code>getAttributes()</code> in <code>FieldInfo</code> or
	<code>MethodInfo</code> to obtain the list of attributes,
	and remove one from the list.


	<p><br>

	<a name="traverse">
	<h3>5.3 Traversing a method body</h3>

	<p>
	To examine every bytecode instruction in a method body,
	<code>CodeIterator</code> is useful. To otbain this object,
	do as follows:

	<ul><pre>
	ClassFile cf = ... ;
	MethodInfo minfo = cf.getMethod("move"); // we assume move is not overloaded.
	CodeAttribute ca = minfo.getCodeAttribute();
	CodeIterator i = ca.iterator();
	</pre></ul>

	<p>
	A <code>CodeIterator</code> object allows you to visit every
	bytecode instruction one by one from the beginning to the end.
	The following methods are part of the methods declared in
	<code>CodeIterator</code>:

	<ul>
	<li><code>void begin()</code><br>
	Move to the first instruction.<br>
	<li><code>void move(int index)</code><br>
	Move to the instruction specified by the given index.<br>
	<li><code>boolean hasNext()</code><br>
	Returns true if there is more instructions.<br>
	<li><code>int next()</code><br>
	Returns the index of the next instruction.<br>
	<em>Note that it does not return the opcode of the next
	instruction.</em><br>
	<li><code>int byteAt(int index)</code><br>
	Returns the unsigned 8bit value at the index.<br>
	<li><code>int u16bitAt(int index)</code><br>
	Returns the unsigned 16bit value at the index.<br>
	<li><code>int write(byte[] code, int index)</code><br>
	Writes a byte array at the index.<br>
	<li><code>void insert(int index, byte[] code)</code><br>
	Inserts a byte array at the index.
	Branch offsets etc. are automatically adjusted.<br>
	</ul>

	<p>The following code snippet displays all the instructions included
	in a method body:

	<ul><pre>
	CodeIterator ci = ... ;
	while (ci.hasNext()) {
	int index = ci.next();
	int op = ci.byteAt(index);
	System.out.println(Mnemonic.OPCODE[op]);
	}
	</pre></ul>

	<p><br>

	<a name="bytecode">
	<h3>5.4 Producing a bytecode sequence</h3>

	<p>
	A <code>Bytecode</code> object represents a sequence of bytecode
	instructions. It is a growable array of bytecode.
	Here is a sample code snippet:

	<ul><pre>
	ConstPool cp = ...; // constant pool table
	Bytecode b = new Bytecode(cp, 1, 0);
	b.addIconst(3);
	b.addReturn(CtClass.intType);
	CodeAttribute ca = b.toCodeAttribute();
	</pre></ul>

	<p>
	This produces the code attribute representing the following sequence:

	<ul><pre>
	iconst_3
	ireturn
	</pre></ul>

	<p>
	You can also obtain a byte array containing this sequence by
	calling <code>get()</code> in <code>Bytecode</code>. The
	obtained array can be inserted in another code attribute.

	<p>
	While <code>Bytecode</code> provides a number of methods for adding a
	specific instruction to the sequence, it provides
	<code>addOpcode()</code> for adding an 8bit opcode and
	<code>addIndex()</code> for adding an index.
	The 8bit value of each opcode is defined in the <code>Opcode</code>
	interface.

	<p>
	<code>addOpcode()</code> and other methods for adding a specific
	instruction are automatically maintain the maximum stack depth
	unless the control flow does not include a branch.
	This value can be obtained by calling <code>getMaxStack()</code>
	on the <code>Bytecode</code> object.
	It is also reflected on the <code>CodeAttribute</code> object
	constructed from the <code>Bytecode</code> object.
	To recompute the maximum stack depth of a method body,
	call <code>computeMaxStack()</code> in <code>CodeAttribute</code>.

	<p><br>

	<a name="annotation">
	<h3>5.5 Annotations (Meta tags)</h3>

	<p>Annotations are stored in a class file
	as runtime invisible (or visible) annotations attribute.
	These attributes can be obtained from <code>ClassFile</code>,
	<code>MethodInfo</code>, or <code>FieldInfo</code> objects.
	Call <code>getAttribute(AnnotationsAttribute.invisibleTag)</code>
	on those objects. For more details, see the javadoc manual
	of <code>javassist.bytecode.AnnotationsAttribute</code> class
	and the <code>javassist.bytecode.annotation</code> package.

	<p>Javassist also let you access annotations by the higher-level
	API.
	If you want to access annotations through <code>CtClass</code>,
	call <code>getAnnotations()</code> in <code>CtClass</code> or
	<code>CtBehavior</code>.

	<p><br>

	<h2><a name="generics">6. Generics</a></h2>

	<p>The lower-level API of Javassist fully supports generics
	introduced by Java 5. On the other hand, the higher-level
	API such as <code>CtClass</code> does not directly support
	generics. However, this is not a serious problem for bytecode
	transformation.

	<p>The generics of Java is implemented by the erasure technique.
	After compilation, all type parameters are dropped off. For
	example, suppose that your source code declares a parameterized
	type <code>Vector<String></code>:

	<ul><pre>
	Vector<String> v = new Vector<String&gt();
	:
	String s = v.get(0);
	</pre></ul>

	<p>The compiled bytecode is equivalent to the following code:

	<ul><pre>
	Vector v = new Vector();
	:
	String s = (String)v.get(0);
	</pre></ul>

	<p>So when you write a bytecode transformer, you can just drop
	off all type parameters. For example, if you have a class:

	<ul><pre>
	public class Wrapper<T> {
	T value;
	public Wrapper(T t) { value = t; }
	}
	</pre></ul>

	<p>and want to add an interface <code>Getter<T></code> to the
	class <code>Wrapper<T></code>:

	<ul><pre>
	public interface Getter<T> {
	T get();
	}
	</pre></ul>

	<p>Then the interface you really have to add is <code>Getter</code>
	(the type parameters <code><T></code> drops off)
	and the method you also have to add to the <code>Wrapper</code>
	class is this simple one:

	<ul><pre>
	public Object get() { return value; }
	</pre></ul>

	<p>Note that no type parameters are necessary.

	<p><br>

	<h2><a name="varargs">7. Varargs</a></h2>

	<p>Currently, Javassist does not directly support varargs. So to make a method with varargs,
	you must explicitly set a method modifier. But this is easy.
	Suppose that now you want to make the following method:

	<ul><pre>
	public int length(int... args) { return args.length; }
	</pre></ul>

	<p>The following code using Javassist will make the method shown above:

	<ul><pre>
	CtClass cc = /* target class */;
	CtMethod m = CtMethod.make("public int length(int[] args) { return args.length; }", cc);
	m.setModifiers(m.getModifiers() \| Modifier.VARARGS);
	cc.addMethod(m);
	<pre></ul>

	<p>The parameter type <code>int...</code> is changed into <code>int[]</code>
	and <code>Modifier.VARARGS</code> is added to the method modifiers.

	<p>To call this method, you must write:

	<ul><pre>
	length(new int[] { 1, 2, 3 });
	</pre></ul>

	<p>instead of this method call using the varargs mechanism:

	<ul><pre>
	length(1, 2, 3);
	</pre></ul>

	<p><br>

	<h2><a name="j2me">8. J2ME</a></h2>

	<p>If you modify a class file for the J2ME execution environment,
	you must perform <it>preverification</it>. Preverifying is basically
	producing stack maps, which is similar to stack map tables introduced
	into J2SE at JDK 1.6. Javassist maintains the stack maps for J2ME only if
	<code>javassist.bytecode.MethodInfo.doPreverify</code> is true.

	<p>You can also manually
	produce a stack map for a modified method.
	For a given method represented by a <code>CtMethod</code> object <code>m</code>,
	you can produce a stack map by calling the following methods:

	<ul><pre>
	m.getMethodInfo().rebuildStackMapForME(cpool);
	</pre></ul>

	<p>Here, <code>cpool</code> is a <code>ClassPool</code> object, which is
	available by calling <code>getClassPool()</code> on a <code>CtClass</code>
	object. A <code>ClassPool</code> object is responsible for finding
	class files from given class pathes. To obtain all the <code>CtMethod</code>
	objects, call the <code>getDeclaredMethods</code> method on a <code>CtClass</code> object.

	<p><br>

	<a href="tutorial2.html">Previous page</a>

	<hr>
	Java(TM) is a trademark of Sun Microsystems, Inc.<br>
	Copyright (C) 2000-2010 by Shigeru Chiba, All rights reserved.
	</body>
	</html>