CODEWITHSUNDEEP
javabytecodejava-bytecode-asm
Foreign
Foreign

Reputation: 405

What is the proper way to stack ASM visitors?

I have the following code:

for (Map.Entry<String, ClassReader> e : classes.entrySet())
{
    ClassReader reader = e.getValue();
    ClassWriter writer = new ClassWriter(Opcodes.ASM7);

    // Process all visitors
    reader.accept(new StringRemapper(writer, "String A", "String A!!"), ClassReader.EXPAND_FRAMES);
    reader.accept(new StringRemapper(writer, "String B", "String B!!"), ClassReader.EXPAND_FRAMES);

    // Update the class
    reader = new ClassReader(writer.toByteArray());
    e.setValue(reader);
}

The issue with the code above is that it is writing everything twice, because there are two visitors writing to the same writer (I guess).

To fix this issue, I need to add the code below after every reader.accept:

reader = new ClassReader(writer.toByteArray());
writer = new ClassWriter(Opcodes.ASM7);

The thing is, by doing so, am I misusing the visitor pattern? I mean, why do I need to create a new reader/writer and visit it only once? Shouldn't I be able to have multiple visitors?

I found this similar question Easy way to stack up a couple of ASM-Bytecode visitors? but couldn't understand the accepted answer.

I tried to pass the first visitor as a parameter to the second instead of the original ClassWriter, had the same result, duplicated code.

ClassVisitor last;
// Process all visitors
reader.accept(last = new StringRemapper(writer, "String A", "String A!!"), ClassReader.EXPAND_FRAMES);
reader.accept(new StringRemapper(last, "String B", "String B!!"), ClassReader.EXPAND_FRAMES);

Upvotes: 3

Views: 416

Answers (1)

Johannes Kuhn
Johannes Kuhn

Reputation: 15173

You are right, you did miss something: The ClassWriter is a ClassVisitor itself.

The right way to stack the visitors is to pass it's parent visitor to the constructor:

// The root visitor is the ClassWriter
ClassVisitor cv = writer;

// Add a visitor
cv = new StringRemapper(cv, "String A", "String A!!");
// Add another one
cv = new StringRemapper(cv, "String B", "String B!!");
// maybe add more?

// Process them
reader.accept(cv, ClassReader.EXPAND_FRAMES);
byte[] classFile = writer.toByteArray();

If your StringRemapper only takes a ClassWriter as parameter, then you should change it to accept ClassVisitors instread.

Another thing I noticed is your classes variable: It contains ClassReaders as values.
It might be an easier design to use ClassVisitors as value - with an initial ClassWriter value.
But this is a bigger design change.

Upvotes: 4

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