Invisible java bytecode lines

Question

I am wondering why the line numbers of Java Bytecode are not consecutive.

What happens for example in the (not listed) lines 2 and 3 of the following getter?

public java.lang.String getX();

Code:
0:   aload_0
1:   getfield        #2; //Field x:Ljava/lang/String;
4:   areturn

I'm using the ASM framework to work on bytecode. When visiting the code of a method using the tree API I get these "hidden" instructions as well (however with opcode -1). I would like to know what they are for.

Answer 1

I don't think that 0, 1, 4 are linenumbers but byte offsets in the bytecode.

• aload_0 is one byte
• getfield is three bytes (one opcode, one "index" arg with two bytes"
• areturn is one byte

So 2 and 3 are simply part of the getfield operation.