这个例子是：计算一个方法的耗时，进入方法时记录时间，退出方法时计算时间差

public class MoonMethodVisitor extends AdviceAdapter {

    private int currentTimeVarIndex;//其实是一个编号，给后面的load和store用

    public MoonMethodVisitor(MethodVisitor methodVisitor, final int access,
                             final String name,
                             final String descriptor) {
        super(Opcodes.ASM6, methodVisitor, access, name, descriptor);
    }

    @Override
    protected void onMethodEnter() {
        super.onMethodEnter();
      
        //这里相当于 long currentTime = System.currentTimeMillis();
        currentTimeVarIndex = newLocal(Type.LONG_TYPE);//创建一个局部变量
        //INVOKESTATIC java/lang/System.currentTimeMillis ()J
        mv.visitMethodInsn(Opcodes.INVOKESTATIC, "java/lang/System", "currentTimeMillis", "()J", false);
        mv.visitVarInsn(LSTORE, time);
    }

    @Override
    protected void onMethodExit(int opcode) {
        super.onMethodExit(opcode);
        //这里相当于 System.out.println(System.currentTimeMillis() - currentTime);
        mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
        //INVOKESTATIC java/lang/System.currentTimeMillis ()J
        mv.visitMethodInsn(Opcodes.INVOKESTATIC, "java/lang/System", "currentTimeMillis", "()J", false);
        //LLOAD 0
        mv.visitVarInsn(Opcodes.LLOAD, currentTimeVarIndex);
        //LSUB
        mv.visitInsn(Opcodes.LSUB);
        //INVOKEVIRTUAL java/io/PrintStream.println (J)V
        mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(J)V", false);
    }
}

统计方法耗时