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...
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