Hello,
i'm working with javassist for quite some time now and I must say it is really impressive what this thing can do, and with what ease!
Anyways, I'm confronted with one problem:
My tool inserts an arbitrary series of conditional branches and some code afterwards. This means I have to precalculate the number of bytes inserted for all following conditions and the code in respect to set the branch offset. All works well unless I load a string constant using addLdc(String). In this case I am unable to precalculate whether there are 3 bytes (ldc) or 4 bytes (ldc_w) inserted, since this depends on the ConstTables state at insertion time.
I've developed a workaround for my case by extending the Bytecode class and altering its behaviour with
private ArrayList<Integer> m_branches;
...
public void addBranch(int code, int branchOffset) {
// branch opcode is inserted at offset getSize()
m_branches.add(getSize());
addOpcode(code);
add16bit(branchOffset);
}
private void adjustBranches(int end, int shift) {
for (Integer pos : m_branches) {
int branchOffset = read16bit(pos + 1);
if ((pos + branchOffset) <= end) continue;
write16bit(pos + 1, branchOffset + shift);
}
}
@Override
public void addLdc(int i) {
if (i > 0xFF) {
adjustBranches(getSize(), 1);
addOpcode(LDC_W);
addIndex(i);
} else {
addOpcode(LDC);
add(i);
}
}
I think this is could be extended to a general solution, if it would be well defined how many bytes one should asume to be inserted.
Is there a way around this problem that I've overseen or is this really a "bug"?
---
By the way, is there any reason there is no
public void add16bit(int value) {
value = value % 65536;
add(value >> 8);
add(value % 256);
}
defined in Bytecode? Since it is required for inserting branch offsets...
