Generating a 'Hello, World!' class with the Java ASM library

Question

I have started messing around with the ASM API for a compiler project I am working on. However, I am finding that the documentation is less than clear for a newcomer in many places and I thought having a good solid example of generating a class that simply prints "Hello, World!" would be a great example to have on here.

Currently, I can generate a class with a main() (using the ClassWriter, ClassVisitor and MethodVisitor classes) but I can't seem to work out how to generate main's body. Could anyone give me an example of generating a class file in ASM that:

• contains a main()
• creates a local String variable in main() with the value "Hello, World!"
• prints the variable

Answer 1

You can compile a class using java, then get asm to print out the calls it would take to generate an equivalent class,

FAQ

ASMifierClassVisitor

The ASMifierClassVisitor javadocs actually has the hello world code in it,

import org.objectweb.asm.*;

public class HelloDump implements Opcodes {

  public static byte[] dump() throws Exception {

     ClassWriter cw = new ClassWriter(0);
     FieldVisitor fv;
     MethodVisitor mv;
     AnnotationVisitor av0;

     cw.visit(49,
             ACC_PUBLIC + ACC_SUPER,
             "Hello",
             null,
             "java/lang/Object",
             null);

     cw.visitSource("Hello.java", null);

     {
         mv = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
         mv.visitVarInsn(ALOAD, 0);
         mv.visitMethodInsn(INVOKESPECIAL,
                 "java/lang/Object",
                 "<init>",
                 "()V");
         mv.visitInsn(RETURN);
         mv.visitMaxs(1, 1);
         mv.visitEnd();
     }
     {
         mv = cw.visitMethod(ACC_PUBLIC + ACC_STATIC,
                 "main",
                 "([Ljava/lang/String;)V",
                 null,
                 null);
         mv.visitFieldInsn(GETSTATIC,
                 "java/lang/System",
                 "out",
                 "Ljava/io/PrintStream;");
         mv.visitLdcInsn("hello");
         mv.visitMethodInsn(INVOKEVIRTUAL,
                 "java/io/PrintStream",
                 "println",
                 "(Ljava/lang/String;)V");
         mv.visitInsn(RETURN);
         mv.visitMaxs(2, 1);
         mv.visitEnd();
     }
     cw.visitEnd();

     return cw.toByteArray();
  }
}

Answer 2

If you are using Eclipse, there is a great ASM plugin that will aid your learning. It displays existing Java code as the actual ASM calls needed to instrument said code. It is quite usefully for learning as you can see the ASM calls needed to implement specific Java code.