來源：http://www.bjsxt.com/
一、S02E216_01位元組碼操作_javassist庫、介紹、動態建立新類、屬性、方法、構造器

位元組碼操作

常見的位元組碼操作類庫

JAVAssist庫

package com.test.javassist;

import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtConstructor;
import javassist.CtField;
import
 
 javassist.CtMethod;

/**
 * 測試使用javassist生成一個新的類
 */
public class Demo01 {
    public static void main(String[] args) throws CannotCompileException, Exception {
        ClassPool pool = ClassPool.getDefault();
        CtClass cc = pool.makeClass("com.test.bean.Emp");

        //建立屬性
        CtField f1 = CtField.make("private int empno;"
 
,cc);
        CtField f2 = CtField.make("private String ename;",cc);
        cc.addField(f1);
        cc.addField(f2);

        //建立方法
        CtMethod m1 = CtMethod.make("public int getEmpno(){return empno;}", cc);
        CtMethod m2 = CtMethod.make("public void setEmpno(){this.empno = empno;}", cc);
        cc.addMethod(m1);
        cc.addMethod(m2);

        //新增構造器
 

        CtConstructor constructor = new CtConstructor(new CtClass[]{CtClass.intType, pool.get("java.lang.String")},cc);
        constructor.setBody("{this.empno = empno; this.ename = ename;}");
        cc.addConstructor(constructor);

        cc.writeFile("g:/java/test");//將上面構造好的類寫入到指定的工作空間中
        System.out.println("生成類，成功！");
    }
}

使用XJad反編譯後生成的.java檔案

// Decompiled by Jad v1.5.8e2. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://kpdus.tripod.com/jad.html
// Decompiler options: packimports(3) fieldsfirst ansi space 
// Source File Name:   Emp.java

package com.test.bean;


public class Emp
{

    private int empno;
    private String ename;

    public int getEmpno()
    {
        return empno;
    }

    public void setEmpno()
    {
        empno = empno;
    }

    public Emp(int i, String s)
    {
        empno = empno;
        ename = ename;
    }
}

一、S02E217_01位元組碼操作javaassist庫介紹_API詳解

package com.test.javassist;

import java.lang.reflect.Method;
import java.util.Arrays;

import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtConstructor;
import javassist.CtField;
import javassist.CtMethod;
import javassist.CtNewMethod;
import javassist.Modifier;
/**
 * 測試javassist的API
 */
public class Demo2 {
    public static void main(String[] args) throws Exception {
        test06();
    }
    /**
     * 處理類的基本用法
     * @throws Exception 
     */
    public static void test01() throws Exception{
        ClassPool pool = ClassPool.getDefault();
        CtClass cc = pool.get("com.test.javassist.Emp");//獲取已有的類

        byte[] bytes = cc.toBytecode();
        System.out.println(Arrays.toString(bytes));

        System.out.println(cc.getName());//獲取類名
        System.out.println(cc.getSimpleName());//獲取簡要類名
        System.out.println(cc.getSuperclass());//獲取父類
        System.out.println(cc.getInterfaces());//獲取介面
    }
    /**
     * 測試產生新的方法
     */
    public static void test02() throws Exception{
        ClassPool pool = ClassPool.getDefault();
        CtClass cc = pool.get("com.test.javassist.Emp");//獲取已有的類
        //CtMethod m = CtNewMethod.make("public int add(int a,int b){return a+b;}", cc);
        //另一種方式，引數：返回型別，方法名，可變引數型別，CtClass物件
        CtMethod m = new CtMethod(CtClass.intType, "add", 
                new CtClass[]{CtClass.intType,CtClass.intType}, cc);

        m.setModifiers(javassist.Modifier.PUBLIC);
        m.setBody("{System.out.println(\"返回方法體中的列印資訊\");return $1+$2;}");

        cc.addMethod(m);

        //通過反射呼叫新生成的方法
        Class clazz = cc.toClass();
        Object obj = clazz.newInstance();//呼叫Emp無參構造器，建立新的Emp物件
        Method method = clazz.getDeclaredMethod("add", int.class,int.class);
        Object result = method.invoke(obj, 200,300);
        System.out.println(result);
    }
    /**
     * 修改已有方法的方法體內容
     * @throws Exception 
     */
    public static void test03() throws Exception{
        ClassPool pool = ClassPool.getDefault();
        CtClass cc = pool.get("com.test.javassist.Emp");//獲取已有的類

        CtMethod cm = cc.getDeclaredMethod("sayHello", new CtClass[]{CtClass.intType});
        cm.insertBefore("System.out.println($1);System.out.println(\"start!!!\");");
        cm.insertAt(9, "int b=3;System.out.println(\"b=\"+b);");
        cm.insertAfter("System.out.println(\"end!!!\");");

        //通過反射呼叫新生成的方法
        Class clazz = cc.toClass();
        Object obj = clazz.newInstance();//呼叫Emp無參構造器，建立新的Emp物件
        Method method = clazz.getDeclaredMethod("sayHello", int.class);
        method.invoke(obj, 300);
    }
    /**
     * 測試產生新的屬性
     */
    public static void test04() throws Exception{
        ClassPool pool = ClassPool.getDefault();
        CtClass cc = pool.get("com.test.javassist.Emp");//獲取已有的類

        //CtField f1 = CtField.make("private int salary;", cc);
        //另一種方式
        CtField f1 = new CtField(CtClass.intType, "salary",cc);
        f1.setModifiers(Modifier.PRIVATE);
        cc.addField(f1);

        //cc.getDeclaredField("salary");//獲取指定的屬性
        //另一種方式
        cc.addMethod(CtNewMethod.getter("getSalary", f1));
        cc.addMethod(CtNewMethod.getter("setSalary", f1));

        //通過反射呼叫，省略。。。
    }
    /**
     * 測試構造器
     */
    public static void test05() throws Exception{
        ClassPool pool = ClassPool.getDefault();
        CtClass cc = pool.get("com.test.javassist.Emp");//獲取已有的類

        CtConstructor[] cs = cc.getConstructors();
        for (CtConstructor c : cs) {
            System.out.println(c.getLongName());
        }
    }
    /**
     * 測試註解
     */
    public static void test06() throws Exception{
        ClassPool pool = ClassPool.getDefault();
        CtClass cc = pool.get("com.test.javassist.Emp");//獲取已有的類

        Object[] all = cc.getAnnotations();
        Author a = (Author) all[0];
        String name = a.name();
        int year = a.year();
        System.out.println("name:" + name + ",year:" + year);
    }
}

javabean

package com.test.javassist;

@Author(name = "test",year= 2016)
public class Emp {
    private int empNo;
    private String empName;

    public Emp() {
    }
    public Emp(int empNo, String empName) {
        super();
        this.empNo = empNo;
        this.empName = empName;
    }

    public int getEmpNo() {
        return empNo;
    }
    public void setEmpNo(int empNo) {
        this.empNo = empNo;
    }
    public String getEmpName() {
        return empName;
    }
    public void setEmpName(String empName) {
        this.empName = empName;
    }

    public void sayHello(int a){
        System.out.println("sayHello," + a);
    }
}

註解

package com.test.javassist;

public @interface Author {
    String name();
    int year();
}