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Provides an ASM visitor that constructs a tree representation of the
classes it visits. This class adapter can be useful to implement "complex"
class manipulation operations, i.e., operations that would be very hard to
implement without using a tree representation (such as optimizing the number
of local variables used by a method).
However, this class adapter has a cost: it makes ASM bigger and slower. Indeed
it requires more than twenty new classes, and multiplies the time needed to
transform a class by almost two (it is almost two times faster to read, "modify"
and write a class with a ClassVisitor than with a ClassNode). This is why
this package is bundled in an optional asm-tree.jar library that
is separated from (but requires) the asm.jar library, which contains
the core ASM framework. This is also why it is recommended
not to use this class adapter when it is possible.
The root class is the ClassNode, that can be created from existing bytecode. For example:
ClassReader cr = new ClassReader(source);
ClassNode cn = new ClassNode();
cr.accept(cn, true);
Now the content of ClassNode can be modified and then
serialized back into bytecode:
ClassWriter cw = new ClassWriter(true);
cn.accept(cw);
Using a simple ClassVisitor it is possible to create MethodNode instances per-method.
In this example MethodNode is acting as a buffer that is flushed out at visitEnd() call:
ClassReader cr = new ClassReader(source);
ClassWriter cw = new ClassWriter();
ClassVisitor cv = new ClassVisitor(cw) {
public MethodVisitor visitMethod(int access, String name,
String desc, String signature, String[] exceptions) {
final MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);
MethodNode mn = new MethodNode(access, name, desc, signature, exceptions) {
public void visitEnd() {
// transform or analyze method code using tree API
accept(mv);
}
};
}
};
cr.accept(cv, true);
Several strategies can be used to construct method code from scratch. The first
option is to create a MethodNode, and then create XxxInsnNode instances and
add them to the instructions list:
MethodNode m = new MethodNode(...);
m.instructions.add(new VarInsnNode(ALOAD, 0));
...
Alternatively, you can use the fact that MethodNode is a MethodVisitor, and use
that to create the XxxInsnNode and add them to the instructions list through
the standard MethodVisitor methods:
MethodNode m = new MethodNode(...);
m.visitVarInsn(ALOAD, 0);
...
If you cannot generate all the instructions in sequential order, i.e. if you
need to save some pointer in the instruction list and then insert instructions
at that place after other instructions have been generated, you can use InsnList
methods insert() and insertBefore() to insert instructions at a saved pointer.
MethodNode m = new MethodNode(...);
m.visitVarInsn(ALOAD, 0);
AbstractInsnNode ptr = m.instructions.getLast();
m.visitVarInsn(ALOAD, 1);
// inserts an instruction between ALOAD 0 and ALOAD 1
m.instructions.insert(ptr, new VarInsnNode(ALOAD, 0));
...
If you need to insert instructions while iterating over an existing instruction
list, you can also use several strategies. The first one is to use a
ListIterator over the instruction list:
ListIterator it = m.instructions.iterator();
while (it.hasNext()) {
AbstractInsnNode n = (AbstractInsnNode) it.next();
if (...) {
it.add(new VarInsnNode(ALOAD, 0));
}
}
It is also possible to convert an instruction list into an array and iterate trough
array elements:
AbstractInsnNode[] insns = m.instructions.toArray();
for(int i = 0; i<insns.length; i++) {
AbstractInsnNode n = insns[i];
if (...) {
m.instructions.insert(n, new VarInsnNode(ALOAD, 0));
}
}
If you want to insert these instructions through the MethodVisitor methods,
you can use another instance of MethodNode as a MethodVisitor and then
insert instructions collected by that instance into the instruction list.
For example:
AbstractInsnNode[] insns = m.instructions.toArray();
for(int i = 0; i<insns.length; i++) {
AbstractInsnNode n = insns[i];
if (...) {
MethodNode mn = new MethodNode();
mn.visitVarInsn(ALOAD, 0);
mn.visitVarInsn(ALOAD, 1);
m.instructions.insert(n, mn.instructions);
}
}
@since ASM 1.3.3