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476 lines
15 KiB
476 lines
15 KiB
<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<title>Javassist Tutorial</title>
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<link rel="stylesheet" type="text/css" href="brown.css">
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</head>
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<body>
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<div align="right">Getting Started with Javassist</div>
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<div align="left"><a href="tutorial2.html">Previous page</a></div>
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<p>
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<a href="#intro">5. Bytecode level API</a>
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<ul>
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<li><a href="#classfile">Obtaining a <code>ClassFile</code> object</a>
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<br><li><a href="#member">Adding and removing a member</a>
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<br><li><a href="#traverse">Traversing a method body</a>
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<br><li><a href="#bytecode">Producing a bytecode sequence</a>
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<br><li><a href="#annotation">Annotations (Meta tags)</a>
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</ul>
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<p><a href="#generics">6. Generics</a>
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<p><a href="#varargs">7. Varargs</a>
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<p><a href="#j2me">8. J2ME</a>
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<p><a href="#boxing">9. Boxing/Unboxing
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<p><a href="#debug">10. Debug</a>
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<p><br>
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<a name="intro">
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<h2>5. Bytecode level API</h2>
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<p>
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Javassist also provides lower-level API for directly editing
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a class file. To use this level of API, you need detailed
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knowledge of the Java bytecode and the class file format
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while this level of API allows you any kind of modification
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of class files.
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<p>
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If you want to just produce a simple class file,
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<code>javassist.bytecode.ClassFileWriter</code> might provide
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the best API for you. It is much faster than
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<code>javassist.bytecode.ClassFile</code> although its API
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is minimum.
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<a name="classfile">
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<h3>5.1 Obtaining a <code>ClassFile</code> object</h3>
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<p>A <code>javassist.bytecode.ClassFile</code> object represents
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a class file. To obtian this object, <code>getClassFile()</code>
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in <code>CtClass</code> should be called.
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<p>Otherwise, you can construct a
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<code>javassist.bytecode.ClassFile</code> directly from a class file.
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For example,
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<ul><pre>
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BufferedInputStream fin
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= new BufferedInputStream(new FileInputStream("Point.class"));
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ClassFile cf = new ClassFile(new DataInputStream(fin));
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</pre></ul>
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<p>
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This code snippet creats a <code>ClassFile</code> object from
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<code>Point.class</code>.
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<p>
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A <code>ClassFile</code> object can be written back to a
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class file. <code>write()</code> in <code>ClassFile</code>
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writes the contents of the class file to a given
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<code>DataOutputStream</code>.
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<p>You can create a new class file from scratch. For example,
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<blockquote><pre>
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ClassFile cf = new ClassFile(false, "test.Foo", null);
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cf.setInterfaces(new String[] { "java.lang.Cloneable" });
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FieldInfo f = new FieldInfo(cf.getConstPool(), "width", "I");
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f.setAccessFlags(AccessFlag.PUBLIC);
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cf.addField(f);
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cf.write(new DataOutputStream(new FileOutputStream("Foo.class")));
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</pre></blockquote>
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<p>this code generates a class file <code>Foo.class</code> that contains
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the implementation of the following class:
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<blockquote><pre>
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package test;
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class Foo implements Cloneable {
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public int width;
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}
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</pre></blockquote>
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<p><br>
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<a name="member">
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<h3>5.2 Adding and removing a member</h3>
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<p>
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<code>ClassFile</code> provides <code>addField()</code> and
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<code>addMethod()</code> for adding a field or a method (note that
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a constructor is regarded as a method at the bytecode level).
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It also provides <code>addAttribute()</code> for adding an attribute
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to the class file.
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<p>
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Note that <code>FieldInfo</code>, <code>MethodInfo</code>, and
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<code>AttributeInfo</code> objects include a link to a
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<code>ConstPool</code> (constant pool table) object. The <code>ConstPool</code>
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object must be common to the <code>ClassFile</code> object and
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a <code>FieldInfo</code> (or <code>MethodInfo</code> etc.) object
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that is added to that <code>ClassFile</code> object.
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In other words, a <code>FieldInfo</code> (or <code>MethodInfo</code> etc.) object
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must not be shared among different <code>ClassFile</code> objects.
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<p>
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To remove a field or a method from a <code>ClassFile</code> object,
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you must first obtain a <code>java.util.List</code>
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object containing all the fields of the class. <code>getFields()</code>
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and <code>getMethods()</code> return the lists. A field or a method can
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be removed by calling <code>remove()</code> on the <code>List</code> object.
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An attribute can be removed in a similar way.
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Call <code>getAttributes()</code> in <code>FieldInfo</code> or
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<code>MethodInfo</code> to obtain the list of attributes,
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and remove one from the list.
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<p><br>
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<a name="traverse">
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<h3>5.3 Traversing a method body</h3>
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<p>
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To examine every bytecode instruction in a method body,
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<code>CodeIterator</code> is useful. To otbain this object,
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do as follows:
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<ul><pre>
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ClassFile cf = ... ;
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MethodInfo minfo = cf.getMethod("move"); // we assume move is not overloaded.
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CodeAttribute ca = minfo.getCodeAttribute();
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CodeIterator i = ca.iterator();
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</pre></ul>
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<p>
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A <code>CodeIterator</code> object allows you to visit every
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bytecode instruction one by one from the beginning to the end.
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The following methods are part of the methods declared in
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<code>CodeIterator</code>:
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<ul>
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<li><code>void begin()</code><br>
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Move to the first instruction.<br>
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<li><code>void move(int index)</code><br>
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Move to the instruction specified by the given index.<br>
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<li><code>boolean hasNext()</code><br>
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Returns true if there is more instructions.<br>
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<li><code>int next()</code><br>
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Returns the index of the next instruction.<br>
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<em>Note that it does not return the opcode of the next
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instruction.</em><br>
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<li><code>int byteAt(int index)</code><br>
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Returns the unsigned 8bit value at the index.<br>
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<li><code>int u16bitAt(int index)</code><br>
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Returns the unsigned 16bit value at the index.<br>
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<li><code>int write(byte[] code, int index)</code><br>
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Writes a byte array at the index.<br>
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<li><code>void insert(int index, byte[] code)</code><br>
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Inserts a byte array at the index.
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Branch offsets etc. are automatically adjusted.<br>
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</ul>
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<p>The following code snippet displays all the instructions included
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in a method body:
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<ul><pre>
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CodeIterator ci = ... ;
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while (ci.hasNext()) {
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int index = ci.next();
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int op = ci.byteAt(index);
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System.out.println(Mnemonic.OPCODE[op]);
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}
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</pre></ul>
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<p><br>
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<a name="bytecode">
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<h3>5.4 Producing a bytecode sequence</h3>
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<p>
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A <code>Bytecode</code> object represents a sequence of bytecode
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instructions. It is a growable array of bytecode.
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Here is a sample code snippet:
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<ul><pre>
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ConstPool cp = ...; // constant pool table
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Bytecode b = new Bytecode(cp, 1, 0);
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b.addIconst(3);
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b.addReturn(CtClass.intType);
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CodeAttribute ca = b.toCodeAttribute();
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</pre></ul>
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<p>
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This produces the code attribute representing the following sequence:
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<ul><pre>
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iconst_3
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ireturn
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</pre></ul>
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<p>
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You can also obtain a byte array containing this sequence by
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calling <code>get()</code> in <code>Bytecode</code>. The
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obtained array can be inserted in another code attribute.
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<p>
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While <code>Bytecode</code> provides a number of methods for adding a
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specific instruction to the sequence, it provides
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<code>addOpcode()</code> for adding an 8bit opcode and
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<code>addIndex()</code> for adding an index.
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The 8bit value of each opcode is defined in the <code>Opcode</code>
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interface.
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<p>
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<code>addOpcode()</code> and other methods for adding a specific
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instruction are automatically maintain the maximum stack depth
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unless the control flow does not include a branch.
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This value can be obtained by calling <code>getMaxStack()</code>
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on the <code>Bytecode</code> object.
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It is also reflected on the <code>CodeAttribute</code> object
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constructed from the <code>Bytecode</code> object.
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To recompute the maximum stack depth of a method body,
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call <code>computeMaxStack()</code> in <code>CodeAttribute</code>.
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<p><code>Bytecode</code> can be used to construct a method.
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For example,
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<blockquote><pre>
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ClassFile cf = ...
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Bytecode code = new Bytecode(cf.getConstPool());
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code.addAload(0);
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code.addInvokespecial("java/lang/Object", MethodInfo.nameInit, "()V");
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code.addReturn(null);
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code.setMaxLocals(1);
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MethodInfo minfo = new MethodInfo(cf.getConstPool(), MethodInfo.nameInit, "()V");
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minfo.setCodeAttribute(code.toCodeAttribute());
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cf.addMethod(minfo);
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</pre></blockquote>
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<p>this code makes the default constructor and adds it to the class specified
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by <code>cf</code>. The <code>Bytecode</code> object is first converted into
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a <code>CodeAttribute</code> object and then added to the method specified
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by <code>minfo</code>. The method is finally added to a class file <code>cf</code>.
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<p><br>
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<a name="annotation">
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<h3>5.5 Annotations (Meta tags)</h3>
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<p>Annotations are stored in a class file
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as runtime invisible (or visible) annotations attribute.
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These attributes can be obtained from <code>ClassFile</code>,
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<code>MethodInfo</code>, or <code>FieldInfo</code> objects.
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Call <code>getAttribute(AnnotationsAttribute.invisibleTag)</code>
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on those objects. For more details, see the javadoc manual
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of <code>javassist.bytecode.AnnotationsAttribute</code> class
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and the <code>javassist.bytecode.annotation</code> package.
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<p>Javassist also let you access annotations by the higher-level
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API.
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If you want to access annotations through <code>CtClass</code>,
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call <code>getAnnotations()</code> in <code>CtClass</code> or
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<code>CtBehavior</code>.
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<p><br>
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<h2><a name="generics">6. Generics</a></h2>
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<p>The lower-level API of Javassist fully supports generics
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introduced by Java 5. On the other hand, the higher-level
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API such as <code>CtClass</code> does not directly support
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generics. However, this is not a serious problem for bytecode
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transformation.
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<p>The generics of Java is implemented by the erasure technique.
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After compilation, all type parameters are dropped off. For
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example, suppose that your source code declares a parameterized
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type <code>Vector<String></code>:
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<ul><pre>
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Vector<String> v = new Vector<String>();
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:
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String s = v.get(0);
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</pre></ul>
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<p>The compiled bytecode is equivalent to the following code:
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<ul><pre>
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Vector v = new Vector();
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:
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String s = (String)v.get(0);
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</pre></ul>
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<p>So when you write a bytecode transformer, you can just drop
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off all type parameters. Because the compiler embedded in Javassist
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does not support generics,
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you must insert an explicit type cast at the
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caller site if the source code is compiled by Javassist, for example,
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through <code>CtMethod.make()</code>. No type cast
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is necessary if the source code is compiled by a normal Java compiler
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such as <code>javac</code>.
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<p>For example, if you have a class:
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<ul><pre>
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public class Wrapper<T> {
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T value;
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public Wrapper(T t) { value = t; }
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}
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</pre></ul>
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<p>and want to add an interface <code>Getter<T></code> to the
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class <code>Wrapper<T></code>:
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<ul><pre>
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public interface Getter<T> {
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T get();
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}
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</pre></ul>
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<p>then the interface you really have to add is <code>Getter</code>
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(the type parameters <code><T></code> drops off)
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and the method you also have to add to the <code>Wrapper</code>
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class is this simple one:
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<ul><pre>
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public Object get() { return value; }
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</pre></ul>
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<p>Note that no type parameters are necessary.
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Since <code>get</code> returns an <code>Object</code>, an explicit type cast
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is needed at the caller site if the source code is compiled by Javassist.
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For example, if the type parameter <code>T</code>
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is <code>String</code>, then <code>(String)</code> must be inserted as follows:
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<ul><pre>
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Wrapper w = ...
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String s = (String)w.get();
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</pre></ul>
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<p>The type cast is not needed if the source code is compiled by a normal Java
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compiler because it will automatically insert a type cast.
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<p>If you need to make type parameters accessible through reflection
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during runtime, you have to add generic signatures to the class file.
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For more details, see the API documentation (javadoc) of the
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<code>setGenericSignature</code> method in the <code>CtClass</code>.
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<p><br>
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<h2><a name="varargs">7. Varargs</a></h2>
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<p>Currently, Javassist does not directly support varargs. So to make a method with varargs,
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you must explicitly set a method modifier. But this is easy.
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Suppose that now you want to make the following method:
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<ul><pre>
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public int length(int... args) { return args.length; }
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</pre></ul>
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<p>The following code using Javassist will make the method shown above:
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<ul><pre>
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CtClass cc = /* target class */;
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CtMethod m = CtMethod.make("public int length(int[] args) { return args.length; }", cc);
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m.setModifiers(m.getModifiers() | Modifier.VARARGS);
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cc.addMethod(m);
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<pre></ul>
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<p>The parameter type <code>int...</code> is changed into <code>int[]</code>
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and <code>Modifier.VARARGS</code> is added to the method modifiers.
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<p>To call this method in the source code compiled by the compiler embedded in Javassist,
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you must write:
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<ul><pre>
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length(new int[] { 1, 2, 3 });
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</pre></ul>
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<p>instead of this method call using the varargs mechanism:
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<ul><pre>
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length(1, 2, 3);
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</pre></ul>
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<p><br>
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<h2><a name="j2me">8. J2ME</a></h2>
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<p>If you modify a class file for the J2ME execution environment,
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you must perform <it>preverification</it>. Preverifying is basically
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producing stack maps, which is similar to stack map tables introduced
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into J2SE at JDK 1.6. Javassist maintains the stack maps for J2ME only if
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<code>javassist.bytecode.MethodInfo.doPreverify</code> is true.
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<p>You can also manually
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produce a stack map for a modified method.
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For a given method represented by a <code>CtMethod</code> object <code>m</code>,
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you can produce a stack map by calling the following methods:
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<ul><pre>
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m.getMethodInfo().rebuildStackMapForME(cpool);
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</pre></ul>
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<p>Here, <code>cpool</code> is a <code>ClassPool</code> object, which is
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available by calling <code>getClassPool()</code> on a <code>CtClass</code>
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object. A <code>ClassPool</code> object is responsible for finding
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class files from given class pathes. To obtain all the <code>CtMethod</code>
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objects, call the <code>getDeclaredMethods</code> method on a <code>CtClass</code> object.
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<p><br>
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<h2><a name="boxing">9. Boxing/Unboxing</h2>
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<p>Boxing and unboxing in Java are syntactic sugar. There is no bytecode for
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boxing or unboxing. So the compiler of Javassist does not support them.
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For example, the following statement is valid in Java:
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<ul><pre>
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Integer i = 3;
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</pre></ul>
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<p>since boxing is implicitly performed. For Javassist, however, you must explicitly
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convert a value type from <code>int</code> to <code>Integer</code>:
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<ul><pre>
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Integer i = new Integer(3);
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</pre></ul>
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<p><br>
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<h2><a name="debug">10. Debug</h2>
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<p>Set <code>CtClass.debugDump</code> to a directory name.
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Then all class files modified and generated by Javassist are saved in that
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directory. To stop this, set <code>CtClass.debugDump</code> to null.
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The default value is null.
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<p>For example,
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<ul><pre>
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CtClass.debugDump = "./dump";
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</pre></ul>
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<p>All modified class files are saved in <code>./dump</code>.
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<p><br>
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<a href="tutorial2.html">Previous page</a>
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<hr>
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Java(TM) is a trademark of Sun Microsystems, Inc.<br>
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Copyright (C) 2000-2015 by Shigeru Chiba, All rights reserved.
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</body>
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</html>
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