Compilation - Introduction to Compiler Construction in a Java World - page 17

JMethodDeclaration s and JConstructorDeclaration s add themselves, using addMethod
() , and then delegate their code generation to their bodies. It is not rocket science.
The code for JMethodDeclaration.codegen() illustrates what these codegen() meth-
ods look like:
publicvoidcodegen(CLEmitteroutput){
output.addMethod(mods,name,descriptor,null,false);
if(body!=null){
body.codegen(output);
}
//AddimplicitRETURN
if(returnType==Type.VOID){
output.addNoArgInstruction(RETURN);
}
}
In general, we generate only the class headers, members, and their instructions and
operands. CLEmitter takes care of the rest. For example, here is the result of executing
>javapHelloWorld
where javap is a Java tool that disassembles HelloWorld.class :
publicclassHelloWorldextendsjava.lang.Object
{
publicHelloWorld();
Code:
Stack=1,Locals=1,Args_size=1
0:aload_0
1:invokespecial #8;//Methodjava/lang/Object."<init>":()V
4:return
publicstaticvoidmain(java.lang.String[]);
Code:
Stack=2,Locals=1,Args_size=1
0:getstatic #17;//Fieldjava/lang/System.out:
//Ljava/io/PrintStream;
3:ldc #19;//StringHello,World!
5:invokevirtual #25;//Methodjava/io/PrintStream.println:
//(Ljava/lang/String;)V
8:return
}
We have shown only the instructions; tables such as the constant table have been left
out of our illustration.
In general, CLEmitter does the following for us:
Builds the constant table and generates references that the JVM can use to reference
names and constants; one need only generate the instructions and their operands,
using names and literals.
Computes branch offsets and addresses; the user can use mnemonic labels.
Computes the argument and local variable counts and the stack space a method
requires to do computation.
Constructs the complete class file.
The CLEmitter is discussed in more detail in Appendix D. JVM code generation is
discussed more fully in Chapter 5.