javassit(1) 基础概念

通过javassit操作类，是各种链路追踪系统最基本的知识。通过java agent机制实现代码不侵入的监控性能，实现主动监控，主动告警，代码时间消耗记录等等...

核心类

ClassPool：map结构 key为类名 value为CtClass

CtClass：拿到对应的操作类对象

CtField：构造类字段

CtMethod：操作方法，能赋到CtClass上 成为一个新方法

CtConstructor：操作构造方法 赋值到CtClass 成为一个新的构造方法

实践

生成class文件

ClassPool pool = ClassPool.getDefault();
CtClass ctClass = pool.makeClass("demo.generate.GenerateHello");
// 构造接口
ctClass.setInterfaces(new CtClass[]{pool.makeInterface("java.lang.Cloneable")});
// 构造字段
CtField ctField = new CtField(CtClass.intType, "id", ctClass);
ctField.setModifiers(AccessFlag.PUBLIC);

// 类上增加字段
ctClass.addField(ctField);

// 增加构造函数
CtConstructor constructor = CtNewConstructor.make("public GenerateHello(int pid) {this.id=pid;}", ctClass);
ctClass.addConstructor(constructor);

// 增加方法
CtMethod ctMethod = CtNewMethod.make("public void hello(String desc){System.out.println(desc);}", ctClass);
ctClass.addMethod(ctMethod);

ctClass.writeFile();
Field[] fields = ctClass.toClass().getFields();
System.out.println("属性名称: " + fields[0].getName() + "属性类型: " + fields[0].getType());

生成对应的class文件，通过idea进入查看

package demo.generate;

public class GenerateHello implements Cloneable {
    public int id;

    public GenerateHello(int var1) {
        this.id = var1;
    }

    public void hello(String var1) {
        System.out.println(var1);
    }
}

修改java文件

Point

package demo.extend;

/**
 * @author chenhao.ych
 * @date 2019-07-15
 */
public class Point {

    private int x;

    private int y;

    public Point() {
    }

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

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

修改文件

 public static void main(String[] args) throws NotFoundException, CannotCompileException, IOException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
    ClassPool classPool = ClassPool.getDefault();

    CtClass ctClass = classPool.get("demo.extend.Point");
    CtMethod method = ctClass.getDeclaredMethod("move");
    method.insertBefore("{System.out.print(\"dx:\" + $1);System.out.println(\"dy: \" + $2);}");
    method.insertAfter("{System.out.println(this.x);System.out.println(this.y);}");

    ctClass.writeFile();

    Class clazz = ctClass.toClass();

    Method move = clazz.getMethod("move", new Class[]{int.class, int.class});
    Constructor constructor = clazz.getConstructor(new Class[]{int.class, int.class});

    move.invoke(constructor.newInstance(1, 2), 1, 2);

}

修改后的class文件反编译

package demo.extend;

public class Point {
    private int x;
    private int y;

    public Point() {
    }

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public void move(int dx, int dy) {
        System.out.print("dx:" + dx);
        System.out.println("dy: " + dy);
        this.x += dx;
        this.y += dy;
        Object var4 = null;
        System.out.println(this.x);
        System.out.println(this.y);
    }
}