How do I apply the AdviceAdapter I have written?

问题

I have the following class:

import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.commons.AdviceAdapter;

public class NotEmptyAdvice extends AdviceAdapter
{
    protected NotEmptyAdvice(final MethodVisitor visitor, final int i, final String s, final String s2)
    {
        super(visitor, i, s, s2);
    }

    @Override
    protected void onMethodEnter()
    {
        super.mv.visitVarInsn(ALOAD, 1);
        super.mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/String", "isEmpty", "()Z" );
        super.mv.visitVarInsn(IFEQ, 1);
        super.mv.visitTypeInsn(NEW, "java/lang/IllegalArgumentException");
        super.mv.visitTypeInsn(NEW, "java/lang/StringBuilder");
        super.mv.visitMethodInsn(INVOKESPECIAL, "java/lang/StringBuilder", "<init>", "()V");
        super.mv.visitVarInsn(ALOAD, 1);
        super.mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;" );
        super.mv.visitLdcInsn(" can not be empty!");
        super.mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;" );
        super.mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;" );
        super.mv.visitMethodInsn(INVOKESPECIAL, "java/lang/IllegalArgumentException", "<init>", "(Ljava/lang/String;)V");
        super.mv.visitInsn(ATHROW);
    }
}

The asm library documentation is terse and obfuscated to say the least. I figured this much out, but extensive googling has not revealed how what I am expected to do with this class now that I have created it.

How to get this code to be applied to my objects at runtime?

回答1:

i have put together a quick code example here: gist.github.com/VijayKrishna/1ca807c952187a7d8c4d, that shows how to use a method adaptor, via its corresponding class-visitor/class-adaptor.

In summary, you first "invoke" the class adapter (say from within a main method) as follows:

ClassReader cr = new ClassReader(in);
ClassWriter cw = new ClassWriter(ClassReader.EXPAND_FRAMES);
ReturnAdapter returnAdapter = new ReturnAdapter(cw, className);
cr.accept(returnAdapter, 0);

You then follow it up adapting the methods as follows in the class-adaptor's visitMethod method:

public MethodVisitor visitMethod(int access ... ... ... ) {
  MethodVisitor mv;
  mv = cv.visitMethod(access, name, desc, signature, exceptions);
  mv = new MethodReturnAdapter(Opcodes.ASM4, className, access, name, desc, mv);
  return mv;
}

---

Here is the entire code snippet in case the gist is inaccessible:

public class ReturnAdapter extends ClassVisitor {
  private String className;

  public ReturnAdapter(ClassVisitor cv, String className) {
    super(Opcodes.ASM4, cv);
  }

  @Override
  public MethodVisitor visitMethod(
    int access,
    String name,
    String desc,
    String signature,
    String[] exceptions) {
    MethodVisitor mv;
    mv = cv.visitMethod(access, name, desc, signature, exceptions);
    mv = new MethodReturnAdapter(Opcodes.ASM4, className, access, name, desc, mv);
    return mv;
  }

  public static void main(String[] args) throws IOException {
    String classFile = args[0];//path of the class file
    String className = args[1];//name of the class
    File inFile = new File(classFile);
    FileInputStream in = new FileInputStream(inFile);

    // adapting the class.
    ClassReader cr = new ClassReader(in);
    ClassWriter cw = new ClassWriter(ClassReader.EXPAND_FRAMES);
    ReturnAdapter returnAdapter = new ReturnAdapter(cw, className);
    cr.accept(returnAdapter, 0);
  }
}

/**
* Method Visitor that inserts code right before its return instruction(s),
* using the onMethodExit(int opcode) method of the AdviceAdapter class,
* from ASM(.ow2.org).
* @author vijay
*
*/
class MethodReturnAdapter extends AdviceAdapter {
  public MethodReturnAdapter(
      int api,
      String owner,
      int access,
      String name,
      String desc,
      MethodVisitor mv) {
    super(Opcodes.ASM4, mv, access, name, desc);
  }

  public MethodReturnAdapter(
      MethodVisitor mv,
      int access,
      String name,
      String desc) {
    super(Opcodes.ASM4, mv, access, name, desc);
  }

  @Override
  protected void onMethodEnter(int opcode) {
      mv.visitVarInsn(ALOAD, 1);
      // and/or any other visit instructions.
  }

  @Override
  protected void onMethodExit(int opcode) {
    if(opcode != Opcodes.ATHROW) {
      mv.visitVarInsn(Opcodes.ALOAD, 1);
      // and/or any other visit instructions.
    }
  }
}

来源：https://stackoverflow.com/questions/23873481/how-do-i-apply-the-adviceadapter-i-have-written