ClassFile object
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.
If you want to just produce a simple class file,
A Otherwise, you can construct a
This code snippet creats a
A
Note that
To remove a field or a method from a
To examine every bytecode instruction in a method body,
A The following code snippet displays all the instructions included
in a method body:
A
This produces the code attribute representing the following sequence:
You can also obtain a byte array containing this sequence by
calling
While
Annotations are stored in a class file
as runtime invisible (or visible) annotations attribute.
These attributes can be obtained from Javassist also let you access annotations by the higher-level
API.
If you want to access annotations through The lower-level API of Javassist fully supports generics
introduced by Java 5. On the other hand, the higher-level
API such as 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 The compiled bytecode is equivalent to the following code:
So when you write a bytecode transformer, you can just drop
off all type parameters. For example, if you have a class:
and want to add an interface Then the interface you really have to add is Note that no type parameters are necessary.
If you modify a class file for the J2ME execution environment,
you must perform You can also manually
produce a stack map for a modified method.
For a given method represented by a Here,
5. Bytecode level API
javassist.bytecode.ClassFileWriter might provide
the best API for you. It is much faster than
javassist.bytecode.ClassFile although its API
is minimum.
5.1 Obtaining a
ClassFile objectjavassist.bytecode.ClassFile object represents
a class file. To obtian this object, getClassFile()
in CtClass should be called.
javassist.bytecode.ClassFile directly from a class file.
For example,
BufferedInputStream fin
= new BufferedInputStream(new FileInputStream("Point.class"));
ClassFile cf = new ClassFile(new DataInputStream(fin));
ClassFile object from
Point.class.
ClassFile object can be written back to a
class file. write() in ClassFile
writes the contents of the class file to a given
DataOutputStream.
5.2 Adding and removing a member
ClassFile provides addField() and
addMethod() for adding a field or a method (note that
a constructor is regarded as a method at the bytecode level).
It also provides addAttribute() for adding an attribute
to the class file.
FieldInfo, MethodInfo, and
AttributeInfo objects include a link to a
ConstPool (constant pool table) object. The ConstPool
object must be common to the ClassFile object and
a FieldInfo (or MethodInfo etc.) object
that is added to that ClassFile object.
In other words, a FieldInfo (or MethodInfo etc.) object
must not be shared among different ClassFile objects.
ClassFile object,
you must first obtain a java.util.List
object containing all the fields of the class. getFields()
and getMethods() return the lists. A field or a method can
be removed by calling remove() on the List object.
An attribute can be removed in a similar way.
Call getAttributes() in FieldInfo or
MethodInfo to obtain the list of attributes,
and remove one from the list.
5.3 Traversing a method body
CodeIterator is useful. To otbain this object,
do as follows:
ClassFile cf = ... ;
MethodInfo minfo = cf.getMethod("move"); // we assume move is not overloaded.
CodeAttribute ca = minfo.getCodeAttribute();
CodeIterator i = ca.iterator();
CodeIterator 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
CodeIterator:
void begin()
Move to the first instruction.
void move(int index)
Move to the instruction specified by the given index.
boolean hasNext()
Returns true if there is more instructions.
int next()
Returns the index of the next instruction.
Note that it does not return the opcode of the next
instruction.
int byteAt(int index)
Returns the unsigned 8bit value at the index.
int u16bitAt(int index)
Returns the unsigned 16bit value at the index.
int write(byte[] code, int index)
Writes a byte array at the index.
void insert(int index, byte[] code)
Inserts a byte array at the index.
Branch offsets etc. are automatically adjusted.
CodeIterator ci = ... ;
while (ci.hasNext()) {
int index = ci.next();
int op = ci.byteAt(index);
System.out.println(Mnemonic.OPCODE[op]);
}
5.4 Producing a bytecode sequence
Bytecode object represents a sequence of bytecode
instructions. It is a growable array of bytecode.
Here is a sample code snippet:
ConstPool cp = ...; // constant pool table
Bytecode b = new Bytecode(cp, 1, 0);
b.addIconst(3);
b.addReturn(CtClass.intType);
CodeAttribute ca = b.toCodeAttribute();
iconst_3
ireturn
get() in Bytecode. The
obtained array can be inserted in another code attribute.
Bytecode provides a number of methods for adding a
specific instruction to the sequence, it provides
addOpcode() for adding an 8bit opcode and
addIndex() for adding an index.
The 8bit value of each opcode is defined in the Opcode
interface.
addOpcode() 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 getMaxStack()
on the Bytecode object.
It is also reflected on the CodeAttribute object
constructed from the Bytecode object.
To recompute the maximum stack depth of a method body,
call computeMaxStack() in CodeAttribute.
5.5 Annotations (Meta tags)
ClassFile,
MethodInfo, or FieldInfo objects.
Call getAttribute(AnnotationsAttribute.invisibleTag)
on those objects. For more details, see the javadoc manual
of javassist.bytecode.AnnotationsAttribute class
and the javassist.bytecode.annotation package.
CtClass,
call getAnnotations() in CtClass or
CtBehavior.
6. Generics
CtClass does not directly support
generics. However, this is not a serious problem for bytecode
transformation.
Vector<String>:
Vector<String> v = new Vector<String>();
:
String s = v.get(0);
Vector v = new Vector();
:
String s = (String)v.get(0);
public class Wrapper<T> {
T value;
public Wrapper(T t) { value = t; }
}
Getter<T> to the
class Wrapper<T>:
public interface Getter<T> {
T get();
}
Getter
(the type parameters <T> drops off)
and the method you also have to add to the Wrapper
class is this simple one:
public Object get() { return value; }
7. J2ME
javassist.bytecode.MethodInfo.doPreverify is true.
CtMethod object m,
you can produce a stack map by calling the following methods:
m.getMethodInfo().rebuildStackMapForME(cpool);
cpool is a ClassPool object, which is
available by calling getClassPool() on a CtClass
object. A ClassPool object is responsible for finding
class files from given class pathes. To obtain all the CtMethod
objects, call the getDeclaredMethods method on a CtClass object.
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Copyright (C) 2000-2007 by Shigeru Chiba, All rights reserved.