how asm add line attributes for java class so that it can support debug

Question

I wrote a java class with asm.jar, and it would dump the bytecode file naming SimpleDynamicInvoker.class

No other 3rd dependencies, except jdk, is required. In the eclipse, the executing SimpleDynamicInvoker is the same as what i expected.

The problem is that when I want to debug SimpleDynamicInvoker.class(no source code for it because it is executing result of my class). I want to check how jdk library is called. Then I added breakpoint in the java.lang.MethodHandle.java. I am sure some of its functions would be invoked when SimpleDynamicInvoker is launched.

But when i debug the SimpleDynamicInvokerin eclipse, it popups message:

Unable to install breakpoint ..... due to missing line number attributes

This error message is common in eclipse and i found similar solution in stackoverflow, yet my case is different.

Therefore my question how to add line information with asm so that it can support debeg? thanks The generated SimpleDynamicInvoker bytecode is like:

Classfile /C:/sxu/workspace/DynamicExample/bin/sxu/xxx/test/SimpleDynamicInvoker.class
Last modified Dec 12, 2013; size 472 bytes
MD5 checksum c50eca7b8806f93154318a72a52ada22
public class sxu.xxx.test.SimpleDynamicInvoker
BootstrapMethods:
  0: #17 invokestatic sxu/xxx/test/invoke/DynamicLinkageExample.bootstrapDynamic:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
   Method arguments:
 minor version: 0
 major version: 51
 flags: ACC_PUBLIC, ACC_SUPER
 Constant pool:
    #1 = Utf8               sxu/xxx/test/SimpleDynamicInvoker
    #2 = Class              #1             //  sxu/xxx/test/SimpleDynamicInvoker
    #3 = Utf8               java/lang/Object
    #4 = Class              #3             //  java/lang/Object 
    #5 = Utf8               <init>
    #6 = Utf8               ()V
    #7 = NameAndType        #5:#6          //  "<init>":()V
    #8 = Methodref          #4.#7          //  java/lang/Object."<init>":()V
    #9 = Utf8               main
    #10 = Utf8               ([Ljava/lang/String;)V
    #11 = Utf8               sxu/xxx/test/invoke/DynamicLinkageExample
    #12 = Class              #11            //  sxu/xxx/test/invoke/DynamicLinkageExample
    #13 = Utf8               bootstrapDynamic
    #14 = Utf8               (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
    #15 = NameAndType        #13:#14        //  bootstrapDynamic:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
    #16 = Methodref          #12.#15        //  sxu/xxx/test/invoke/DynamicLinkageExample.bootstrapDynamic:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
    #17 = MethodHandle       #6:#16         //  invokestatic sxu/xxx/test/invoke/DynamicLinkageExample.bootstrapDynamic:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
    #18 = Utf8               helloWorld
    #19 = NameAndType        #18:#6         //  helloWorld:()V
    #20 = InvokeDynamic      #0:#19         //  #0:helloWorld:()V
    #21 = Utf8               Code
    #22 = Utf8               BootstrapMethods
   {
      public sxu.xxx.test.SimpleDynamicInvoker();
      flags: ACC_PUBLIC
Code:
  stack=1, locals=1, args_size=1
     0: aload_0
     1: invokespecial #8                  // Method java/lang/Object."<init>":()V
     4: return

   public static void main(java.lang.String[]);
     flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=0, locals=1, args_size=1
        0: invokedynamic #20,  0             // InvokeDynamic #0:helloWorld:()V
        5: return
   }

Answer 1

If this message shows it means that there exsits no reference to a line number in the target class. This means that when using ASM you either:

• did not add a line number at all
• added the wrong line number

With an example class Test

package test;
public class Test {
  public Test() {
    super();
  }
}

I get the following byte code

public <init>()V
  L0
    LINENUMBER 4 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
    RETURN
  L1
    LOCALVARIABLE this Ltest/Test; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

As you can see from the byte code, the specified line 4 of the super constructor call is explicitly referenced in the byte code which is why a debugger can identify you break point location.

Be aware that with ASM, you need to add lines manually, calling the MethodVisitor#visitLine method which requires a Label to mark the referenced code section.