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("{\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();
     }
 }

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