CGLib动态代理的介绍及用法（单回调、多回调、不处理、固定值、懒加载）
前面介绍了代理模式，讲了动态代理常见的实现方式，包含了JDK的动态代理和CGLib的动态代理。本文将介绍下CGLib动态代理及几种用法。CGLib（Code Generation Library）是一个高效的代码生成库，底层实现是使用asm来转换字节码生成类。在生成代理类的场景中，由于JDK动态代理必须要求源对象有实现接口，而实际场景中，并不是所有类都有实现接口，因此使用CGLib可以用在未实现接口的类上。
值得注意的几点是：
1）使用CGLib代理的类不能是final修饰的，因为代理类需要继承主题类；
2）final修饰的方法不会被切入；
3）如果主题类的构造函数不是默认空参数的，那么在使用Enhancer类create的时候，选择create(java.lang.Class[] argumentTypes, java.lang.Object[] arguments) 方法。
接下来认识实现动态代理最重要的一个接口 MethodInteceptor
package net.sf.cglib.proxy;
/**
* General-purpose {@link Enhancer} callback which provides for "around advice".
* @author Juozas Baliuka [email protected]
* @version $Id: MethodInterceptor.java,v 1.8 2004/06/24 21:15:20 herbyderby Exp $
*/
public interface MethodInterceptor
extends Callback
{
/**
* All generated proxied methods call this method instead of the original method.
* The original method may either be invoked by normal reflection using the Method object,
* or by using the MethodProxy (faster).
* @param obj "this", the enhanced object
* @param method intercepted Method
* @param args argument array; primitive types are wrapped
* @param proxy used to invoke super (non-intercepted method); may be called
* as many times as needed
* @throws Throwable any exception may be thrown; if so, super method will not be invoked
* @return any value compatible with the signature of the proxied method. Method returning void will ignore this value.
* @see MethodProxy
*/
public Object intercept(Object obj, java.lang.reflect.Method method, Object[] args,
MethodProxy proxy) throws Throwable;
}MethodInterceptor，从名字上方法拦截器，就是对方法做切入的。intercept方式的4个参数分别对应增强对象、调用方法、方法参数以及调用父类方法的 代理。使用MethodProxy速度会更快，所以后面将用
下面介绍几种用法，这里使用spring包中cglib，其实和引单独的cglib包是一样，只不过spring为了版本不冲突，将cglib包含在自己的包中。
先定义一个主题对象
/**
* Create by zxb on 2017/4/23
*/
public class DBQuery {
public DBQuery() {
}
public DBQuery(Integer i) {
System.out.println("Here's in DBQuery Constructor");
}
public String getElement(String id) {
return id + "_CGLib";
}
public List getAllElements() {
return Arrays.asList("Hello_CGLib1", "Hello_CGLib2");
}
public String methodForNoop() {
return "Hello_Noop";
}
public String methodForFixedValue(String param) {
return "Hello_" + param;
}
public final String sayHello() {
return "Hello Everyone！";
}
} （一）单回调
切入类：
/**
* Create by zxb on 2017/4/22
*/
public class DBQueryProxy implements MethodInterceptor {
public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
System.out.println("Here in interceptor ！");
return methodProxy.invokeSuper(o, objects);
}
}测试类：
public class TestCGLibProxy {
public static void main(String[] args) {
DBQueryProxy dbQueryProxy = new DBQueryProxy();
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(DBQuery.class);
enhancer.setCallback(dbQueryProxy);
// DBQuery dbQuery = (DBQuery)enhancer.create(new Class[]{Integer.class}, new Object[]{1});
DBQuery dbQuery = (DBQuery) enhancer.create();
System.out.println(dbQuery.getElement("Hello"));
System.out.println();
System.out.println(dbQuery.getAllElements());
System.out.println();
System.out.println(dbQuery.sayHello());
}
}执行结果：
（二）多回调
在前面的基础上，加个切入类，并通过CallbackFilter来决定是使用哪个切入类
/**
* Create by zxb on 2017/4/22
*/
public class DBQueryProxy2 implements MethodInterceptor {
public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
System.out.println("Here in interceptor 2！");
return methodProxy.invokeSuper(o, objects);
}
}
测试类：
/**
* Create by zxb on 2017/4/22
*/
public class TestCGLibProxy {
public static void main(String[] args) {
DBQueryProxy dbQueryProxy = new DBQueryProxy();
DBQueryProxy2 dbQueryProxy2 = new DBQueryProxy2();
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(DBQuery.class);
enhancer.setCallbacks(new Callback[]{dbQueryProxy, dbQueryProxy2});
enhancer.setCallbackFilter(new CallbackFilter() {
public int accept(Method method) {
if (method.getName().equals("getElement")) {
return 0;
} else {
return 1;
}
}
});
DBQuery dbQuery = (DBQuery) enhancer.create();
System.out.println("========Inteceptor By DBQueryProxy ========");
System.out.println(dbQuery.getElement("Hello"));
System.out.println();
System.out.println("========Inteceptor By DBQueryProxy2 ========");
System.out.println(dbQuery.getAllElements());
}
}执行结果：
（三）不处理
利用枚举常量 Callback noopCb = NoOp.INSTANCE;
测试类：
public class TestCGLibProxy {
public static void main(String[] args) {
DBQueryProxy dbQueryProxy = new DBQueryProxy();
DBQueryProxy2 dbQueryProxy2 = new DBQueryProxy2();
Callback noopCb = NoOp.INSTANCE;
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(DBQuery.class);
enhancer.setCallbacks(new Callback[]{dbQueryProxy, dbQueryProxy2, noopCb});
enhancer.setCallbackFilter(new CallbackFilter() {
public int accept(Method method) {
if (method.getName().equals("getElement")) {
return 0;
} else if (method.getName().equals("getAllElements")) {
return 1;
} else {
return 2;
}
}
});
DBQuery dbQuery = (DBQuery) enhancer.create();
System.out.println("========Inteceptor By DBQueryProxy ========");
System.out.println(dbQuery.getElement("Hello"));
System.out.println();
System.out.println("========Inteceptor By DBQueryProxy2 ========");
System.out.println(dbQuery.getAllElements());
System.out.println();
System.out.println("========Return Original Value========");
System.out.println(dbQuery.methodForNoop());
}
}执行结果：
（四）固定值
需要实现FixedValue接口，会忽略原来函数的返回值，使用固定值来替换。
/**
* 返回固定的值
* Create by zxb on 2017/4/23
*/
public class DBQueryProxyFixedValue implements FixedValue {
public Object loadObject() throws Exception {
System.out.println("Here in DBQueryProxyFixedValue ! ");
return "Fixed Value";
}
}测试类：
public class TestCGLibProxy {
public static void main(String[] args) {
DBQueryProxy dbQueryProxy = new DBQueryProxy();
DBQueryProxy2 dbQueryProxy2 = new DBQueryProxy2();
Callback noopCb = NoOp.INSTANCE;
Callback fixedValue = new DBQueryProxyFixedValue();
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(DBQuery.class);
enhancer.setCallbacks(new Callback[]{dbQueryProxy, dbQueryProxy2, noopCb, fixedValue});
enhancer.setCallbackFilter(new CallbackFilter() {
public int accept(Method method) {
if (method.getName().equals("getElement")) {
return 0;
} else if (method.getName().equals("getAllElements")) {
return 1;
} else if (method.getName().equals("methodForNoop")) {
return 2;
} else if (method.getName().equals("methodForFixedValue")) {
return 3;
} else {
return 0;
}
}
});
DBQuery dbQuery = (DBQuery) enhancer.create();
System.out.println("========Inteceptor By DBQueryProxy ========");
System.out.println(dbQuery.getElement("Hello"));
System.out.println();
System.out.println("========Inteceptor By DBQueryProxy2 ========");
System.out.println(dbQuery.getAllElements());
System.out.println();
System.out.println("========Return Original Value========");
System.out.println(dbQuery.methodForNoop());
System.out.println();
System.out.println("========Return Fixed Value========");
System.out.println(dbQuery.methodForFixedValue("myvalue"));
}
}执行结果：
（五）懒加载
CGLib的懒加载，可以用在一些不需要立即加载完整对象实例的场景，比如说Hibernate中的查询对象，如果这个对象有关联其他对象，这个时候不会马上将关联对象一起查询出来关联，要等到调用到这个关联对象时才去做查询。利用CGLib的懒加载机制，可以很好的实现这个需求。需要了解2个接口，LazyLoader和Dispatcher。这两个接口的定义如下：
public interface LazyLoader extends Callback {
Object loadObject() throws Exception;
}
public interface Dispatcher extends Callback {
Object loadObject() throws Exception;
}它们都继承了Callback接口，都有一个loadObject的方法，区别在于LazyLoader只有在第一次调用时，会执行loadObject获取对象，而Dispatcher会在每次调用时都触发loadObject方法，不理解？没关系，后面代码示例上可以看到明显的区别。假定有个学生类（Student），学术类包含2门课的课程表对象，分别是英语课程表（EnglishSchedule）和数学课程表（MathSchedule），它们都是课程表类的实例
/**
* 课程表
* Create by zxb on 2017/4/23
*/
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Schedule {
private String courseName;
private Date courseTime;
}
EnglishSchedule属性的加载依赖于ScheduleLazyLoader
/**
* Create by zxb on 2017/4/23
*/
public class ScheduleLazyLoader implements LazyLoader {
public Object loadObject() throws Exception {
System.out.println("before LazyLoader init...you can query from db...");
Schedule schedule = new Schedule();
schedule.setCourseName("English");
Calendar calendar = Calendar.getInstance();
calendar.set(2017,3,28);
schedule.setCourseTime(calendar.getTime());
System.out.println("after LazyLoader init...");
return schedule;
}
}MathSchedule属性的加载依赖于ScheduleDispatcher
/**
* Create by zxb on 2017/4/23
*/
public class ScheduleDispatcher implements Dispatcher {
public Object loadObject() throws Exception {
System.out.println("before Dispatcher init...you can query from db...");
Schedule schedule = new Schedule();
schedule.setCourseName("Math");
Calendar calendar = Calendar.getInstance();
calendar.set(2017,4,1);
schedule.setCourseTime(calendar.getTime());
System.out.println("after Dispatcher init...");
return schedule;
}
}
学生类：
定义时，需要对EnglishSchedule和MathSchedule先初始为动态代理的对象
package org.zheng.proxy.cglib.lazyload;
import lombok.Data;
import org.springframework.cglib.proxy.Enhancer;
/**
* Create by zxb on 2017/4/23
*/
@Data
public class Student {
private int id;
private String name;
/**
* 英语课时间表
*/
private Schedule EnglishSchedule;
/**
* 数学课时间表
*/
private Schedule MathSchedule;
public Student(int id, String name) {
this.id = id;
this.name = name;
this.EnglishSchedule = createEnglishSchedule();
this.MathSchedule = createMathSchedule();
}
private Schedule createEnglishSchedule() {
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(Schedule.class);
enhancer.setCallback(new ScheduleLazyLoader());
return (Schedule) enhancer.create();
}
private Schedule createMathSchedule() {
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(Schedule.class);
enhancer.setCallback(new ScheduleDispatcher());
return (Schedule) enhancer.create();
}
}
测试类：
/**
* 延迟加载属性
* Create by zxb on 2017/4/23
*/
public class LazyLoadTest {
public static void main(String[] args) {
Student student = new Student(666, "XiaoMing");
System.out.println("id=" + student.getId());
System.out.println("name=" + student.getName());
// LazyLoader 只有第一次，Dispatcher是每次都会进loadObject的方法
System.out.println("========First Get EnglishSchedule ========");
System.out.println(student.getEnglishSchedule());
System.out.println();
System.out.println("========First Get MathSchedule ========");
System.out.println(student.getMathSchedule());
System.out.println();
System.out.println("========Second Get EnglishSchedule ========");
System.out.println(student.getEnglishSchedule());
System.out.println();
System.out.println("========Second Get MathSchedule ========");
System.out.println(student.getMathSchedule());
}
}
执行结果：
可以看到第二次取懒加载对象的时候，实现LoadLazy接口不会重新执行loadObject，而实现Dispatcher的会重新执行LoadObject方法：）
以上，就是使用CGLib的几种常见用法。
项目完整代码：
https://github.com/difffate/JavaProject