How is the java "this" keyword implemented?

Question

How does the this pointer points to the object itself? Is it a java implementation or is it a compiler implementation?

Answer 1

In the JVM bytecode, local variable 0 (basically register 0) points to the current object when a method is invoked. The compiler simply uses this as an alias for local variable 0.

So I guess the answer is that the compiler implements this.

Answer 2

Sounds like a philosophical question. I am not sure that a Java implementation is.

this is defined in the JLS and is a keyword in Java and the compile has to comply with that standard. When you have a method like

object.method(args)

what is actually called in byte code is a method which looks like

method(object, args);

where this is the first argument.

At the JVM level, the parameters don't have names and the JIT could optimise the argument away if its not actually used.

Answer 3

Well if you are interested why not look at the byte code generated by the compiler

class HelloWorld
{
   private String hello = "Hello world!";

   private void printHello(){
   System.out.println (this.hello);
}

public static void main (String args[]){
  HelloWorld hello = new HelloWorld();
  hello.printHello();
}

}

Compile using

%JAVA_HOME%/bin/javac HelloWorld.java

Get bytecode using

javap -c HelloWorld

edit add output

enter code here

 HelloWorld();
 Code:
 0:   aload_0
 1:   invokespecial   #1; //Method java/lang/Object."<init>":()
 4:   aload_0
 5:   ldc     #2; //String Hello world!
 7:   putfield        #3; //Field hello:Ljava/lang/String;
 10:  return

 public static void main(java.lang.String[]);
 Code:
 0:   new     #6; //class HelloWorld
 3:   dup
 4:   invokespecial   #7; //Method "<init>":()V
 7:   astore_1
 8:   aload_1
 9:   invokespecial   #8; //Method printHello:()V
12:  return

}