JVM instruction ALOAD_0 in the 'main' method points to 'args' instead of 'this'?
I am trying to impleme开发者_如何转开发nt a subset of Java for an academic study. Well, I'm in the last stages (code generation) and I wrote a rather simple program to see how method arguments are handled:
class Main {
public static void main(String[] args) {
System.out.println(args.length);
}
}
Then I built it, and ran 'Main.class' through an online disassembler I found at: http://www.cs.cornell.edu/People/egs/kimera/disassembler.html
I get the following implementation for the 'main' method: (the disassembled output is in Jasmin)
.method public static main([Ljava/lang/String;)V
.limit locals 1
.limit stack 2
getstatic java/lang/System/out Ljava/io/PrintStream;
aload_0
arraylength
invokevirtual java/io/PrintStream.println(I)V
return
.end method
My problem with this is:
1.aload_0
is supposed to push 'this' on to the stack (thats what the JVM spec seems to say)
2. arraylength
is supposed to return the length of the array whose reference is on the top-of-stack
So according to me the combination of 1 & 2 should not even work.
How/why is it working? Or is the disassembler buggy and the actual bytecode is something else?
aload_0 is supposed to push 'this' on to the stack
Not quite … aload_0
reads the first reference argument (or, more generally, the first local reference variable) of the method and pushes it onto the stack.
In member functions, the first local variable happens to be the this
reference.
But main
is not a member function, it’s a static function so there is no this
argument, and the true first argument of the method is args
.
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