insertBefore example

public class Point {
    int x, y;
    void move(int dx, int dy) { x += dx; y += dy; }
}

public static void main(String[] args) throws Exception{
        ClassPool pool = ClassPool.getDefault();
        CtClass cc = pool.get("com.kq.javassist.entity.Point");
        CtMethod m = cc.getDeclaredMethod("move");
        m.insertBefore("{ System.out.println($1); System.out.println($2); }");
        cc.writeFile("D:\\javassist\\");
}

生成class

package com.kq.javassist.entity;

import java.io.PrintStream;

public class Point
{
  int x;
  int y;

  void move(int dx, int dy)
  {
    System.out.println(dx);System.out.println(dy);this.x += dx;this.y += dy;
  }
}

$1 and $2 are replaced with dx and dy, respectively.

$1, $2, $3 ... are updatable. If a new value is assigend to one of those variables, then the value of the parameter represented by that variable is also updated