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<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><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.

<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><br>

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

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Java(TM) is a trademark of Sun Microsystems, Inc.<br>
Copyright (C) 2000-2005 by Shigeru Chiba, All rights reserved.
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