5.1 javassist基本使用

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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("{\nthis.account = 1;\nthis.name = \"xiaona\";\n}").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("{\n System.out.println(\"haha\"); \n}").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();

     }

 }