Today I received the question how someone could see the Java code for a Java class file. You can disassemble the Java byte code via the command line tool javap.

Lets assume you have this tiny Java class Test.java

package test;

public class Test {

	int number = 5;

	public void sayHello() {
		System.out.println("Hello");
	}
}

Compile this class via javac Test. javaand you receive Java.class

If you you run javap Test you receive the attributes and method signatures.

C:\temp\javaptest>javap Test
Compiled from "Test.java"
public class test.Test extends java.lang.Object{
    int number;
    public test.Test();
    public void sayHello();
}

If you you run javap -C Test you receive the byte-code

C:\temp\javaptest>javap -c Test
Compiled from "Test.java"
public class test.Test extends java.lang.Object{
int number;

public test.Test();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   aload_0
   5:   iconst_5
   6:   putfield        #2; //Field number:I
   9:   return

public void sayHello();
  Code:
   0:   getstatic       #3; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc     #4; //String Hello
   5:   invokevirtual   #5; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   return

}

To get the full Java source code you can use the tool jad.

16.11.2008 Updated entry based on comments from Eric and Phil. Thanks!

Filed under: Java

You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.