5.1 javassist基本使用

假设要使用javassist创建一个类：

package com.alibaba.dubbo.demo.test;

public class Emp {
    //属性
    private int    account;
    private String name;

    //构造方法
    public Emp() {
        this.account = 1;
        this.name = "xiaona";
    }

    //getter
    public int getAccount() {
        return account;
    }

    //setter
    public void setAccount(int account) {
        this.account = account;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    //普通方法
    public void commonMethod() {
        System.out.println("haha");
    }
}

该类包含属性／构造方法／getter／setter／普通方法。

使用javassist来创建类：

package com.alibaba.dubbo.demo.test;

import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtConstructor;
import javassist.CtField;
import javassist.CtMethod;
import javassist.CtNewMethod;
import javassist.Modifier;

import java.io.File;
import java.io.FileOutputStream;

public class TestJavassitCompiler {
    public static void main(String[] args) throws Exception {
        ClassPool pool = ClassPool.getDefault();
        CtClass ctClass = pool.makeClass("com.alibaba.dubbo.demo.test.Emp");
        //添加属性:private String name
        CtField nameField = new CtField(pool.getCtClass("java.lang.String"), "name", ctClass);
        nameField.setModifiers(Modifier.PRIVATE);
        ctClass.addField(nameField);
        //添加属性:private int account
        CtField accountField = new CtField(pool.getCtClass("int"), "account", ctClass);
        accountField.setModifiers(Modifier.PRIVATE);
        ctClass.addField(accountField);
        //getter和setter
        ctClass.addMethod(CtNewMethod.getter("getName", nameField));
        ctClass.addMethod(CtNewMethod.setter("setName", nameField));
        ctClass.addMethod(CtNewMethod.getter("getAccount", accountField));
        ctClass.addMethod(CtNewMethod.setter("setAccount", accountField));
        //创建构造器
        CtConstructor ctConstructor = new CtConstructor(new CtClass[] {}, ctClass);
        String body = new StringBuilder("{
this.account = 1;
this.name = "xiaona";
}").toString();
        ctConstructor.setBody(body);
        ctClass.addConstructor(ctConstructor);
        //普通方法
        CtMethod ctMethod = new CtMethod(CtClass.voidType, "commonMethod", new CtClass[] {}, ctClass);
        ctMethod.setModifiers(Modifier.PUBLIC);
        ctMethod.setBody(new StringBuilder("{
 System.out.println("haha"); 
}").toString());
        ctClass.addMethod(ctMethod);

        Class<?> clazz = ctClass.toClass();
        Object obj = clazz.newInstance();
        obj.getClass().getMethod("commonMethod", new Class[] {}).invoke(obj, new Object[] {});//方法调用

        /**
         * 将字节码输出到文件中
         */
        byte[] codeByteArray = ctClass.toBytecode();
        FileOutputStream fos = new FileOutputStream(new File("/Users/jigangzhao/Desktop/Emp.class"));
        fos.write(codeByteArray);
        fos.close();
    }
}

之后可以使用javap命令来看一下/Users/jigangzhao/Desktop/Emp.class文件。

关于javassit的API请查看：

https://github.com/jboss-javassist/javassist/wiki/Tutorial-1