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CGLIB 实现代理对象API

發布時間：2023/12/3 编程问答 35 豆豆

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轉載自?CGLIB 實現代理對象API

1. 加入庫	cglib庫 cglib-2.2.jar asm庫(cglib 需要asm庫,如果沒有加入asm的jar文件,就會報asm錯誤) asm-3.1.jar
?2. 定義CGLIB操作類	package com.machome.cglibtest; import java.lang.reflect.Method; import com.machome.model.StuService; import net.sf.cglib.proxy.Enhancer; import net.sf.cglib.proxy.MethodInterceptor; import net.sf.cglib.proxy.MethodProxy; public class CGlibProxyFactory implements MethodInterceptor { ??? private Object targetObject; ??? ??? public Object createProxyInstance(Object targetObject){ ??? ?? this.targetObject = targetObject;??? //傳入用戶類 ??? ?? ??? ?? Enhancer enhancer = new Enhancer();????????? //Enhancer是cglib的核心類 ??? ?? ??? ???? // 將用戶類設為 Enhancer對象的superclass屬性,,即設為 Enhancer對象的父類 ??? ?? enhancer.setSuperclass(this.targetObject.getClass());????? ??? ???? // 設 Enhancer對象的Callbacks屬性,要求必須是Callback接口類型 ??? ?? enhancer.setCallback(this); ??? ?? ??? ?? return enhancer.create();? //生成代理對象 ??? } ??? ??? ??? @Override ??? public Object intercept(Object arg0, Method arg1, Object[] arg2, ??? ??? ??? MethodProxy arg3) throws Throwable { ??? ??? StuService bean = (StuService)this.targetObject; ??? ??? Object result = null;??? ??? ??? ??? if(!arg1.getName().equals("save")&&(bean.findAll().size()==0)) ??? ??? {? // 如果方法名不是save(即是get,update,delete方法),而同時實例的列表為空,就不執行方法,而是告警 ??? ??? ??? System.out.println("list is null,method is stoped");??? ??? ??? ??? ??? }else{ ??????? //執行代理方法,傳入實例和方法參數 ??? ??? result = arg3.invoke(targetObject, arg2);????? ?? ? ? ? ? ??? ??? } ??? ??? return result;?? ??? ??? } }
3.測試代碼	測試代碼: ??? ??? CGlibProxyFactory factory = new CGlibProxyFactory(); ??? ??? StuService bean = (StuService)factory.createProxyInstance( ??? ??? ??? ??? ??????????? new StuService()); ??? ??? ??? ??? List<Stu> stuList = bean.findAll(); ??? ??? if(stuList!=null){ ??? ??? for(Stu temp:stuList) ??? ??? ??? System.out.println(temp.getId()+":"+temp.getName()); ??? ??? System.out.println("finished"); 執行結果: list is null,method is stoped??????????? //bean.findAll()下邊的語句都沒執行)

cglib 執行intercept()的原理:

自定義類應該實現MethodInterceptor接口,,覆寫 intercept()方法

public interface MethodInterceptor extends Callback
{
??? public abstract Object intercept(Object obj, Method method, Object aobj[], MethodProxy methodproxy)
??????? throws Throwable;
}

Enhancer對象設其Callback對象(實際是Callbacks對象,一個數組,但我們經常只設一個) ???

?enhancer.setCallback(Callback callback);

這樣當Enhancer對象建立代理對象的時候,就會在執行用戶調用的方法前,先執行intercept()方法

常見的兩種不同的實現MethodInterceptor接口的cglib編程

1.自定義類直接實現MethodInterceptor接口,覆寫 intercept()方法

public class CGlibProxyFactory?implements MethodInterceptor?{
??? public Object createProxyInstance(Object targetObject){
??? ? ??? ...
??? ?? enhancer.setCallback(this);
????? ...
???? }
???
??? @Override
??? public Object?intercept(Object arg0, Method arg1, Object[] arg2,
??? ??? ??? MethodProxy arg3) throws Throwable {
??? ??? ??? ...
??? ??? ??? }???
}

2.另一種方式,定義一個內部類,該類實現MethodInterceptor接口,覆寫intercept()方法

public class CGlibProxyFactory1 {

??? public Object createProxyInstance(Object targetObject){
???? ...
???? enhancer.setCallback(new MethodInterceptorImpl());
???? ...
???? }

??? ?? private class MethodInterceptorImpl?implements MethodInterceptor?{

??? ??? @Override
??? ??? public Object?intercept(Object obj, Method method, Object[] aobj,
??? ??? ??? ??? MethodProxy methodproxy) throws Throwable {
??? ??? ??? ??? ...
??? ??? ??? ??? }
?? }
?}?

注意 public Object intercept(Object obj, Method method, Object[] aobj,MethodProxy methodproxy)方法的第一個參數obj有很大問題
intercept方法里的主要動作是執行invoke方法
有兩種執行方法:

法1:	將參數obj傳入invokeSuper()方法: return methodproxy.invokeSuper(obj, aobj); obj應該是傳入Enhancer 對象的用戶對象 (即上邊enhancer.setSuperclass(this.targetObject.getClass()))
法2:	或者將傳入Enhancer對象前的原始用戶對象傳入invoke()方法: return methodproxy.invoke(targetObject, aobj);

實際工作中發現,參數obj不態好用. 如果你想intercept()方法中調用對象執行一些方法的話,obj會出stackOverflow這樣的異常
如下面的例子:

??? public Object intercept(Object obj, Method method, Object[] aobj,
??? ??? ??? MethodProxy methodproxy) throws Throwable {
??? ??? StuService bean =?(StuService)obj;
??? ??? Object result = null;??? ???
??? ??? if(!method.getName().equals("save")&&(bean.findAll().size()==0))
??? ??? {? // 如果方法名不是save,而實例的列表為空,就不執行方法,而是告警
??? ??? ??? System.out.println("list is null,method is stoped");??? ??? ???
??? ??? }else{
??? ??? result = methodproxy.invokeSuper(obj, aobj);
??? ??? }
??? ??? return result;
? ? }

執行時會出線面的異常:
Exception in thread "main" java.lang.StackOverflowError

改成下面就好了:??? ???
??? public Object intercept(Object obj, Method method, Object[] aobj,
??? ??? ??? MethodProxy methodproxy) throws Throwable {
??? ??? StuService bean =?(StuService)this.targetObject;? //從外部傳入的原始用戶實例
??? ??? Object result = null;??? ???
??? ??? if(!method.getName().equals("save")&&(bean.findAll().size()==0))
??? ??? {? // 如果方法名不是save,而實例的列表為空,就不執行方法,而是告警
??? ??? ??? System.out.println("list is null,method is stoped");??? ??? ???
??? ??? }else{
??? ??? result = methodproxy.invoke(targetObject, aobj);
??? ??? }
??? ??? return result;
}

cglib 在 spring,hibernate中的應用
Hibernate主要是利用cglib生成pojo的子類并override get方法來實現lazy loading機制
Spring則是利用cglib來實現動態代理。

spring,hibernate同時支持proxy類動態代理和cglib,二者的區別
proxy動態代理只能對 實現了接口 的類生成代理，而不能針對類
CGLIB是針對類生成代理，主要是對用戶類生成一個子類

spring 下缺省是支持proxy 動態代理,如何強制使用CGLIB生成代理？
spring 主要在兩個地方使用代理

AOP：
<aop:aspectj-autoproxy?proxy-target-class="true"/>?

Transaction:
<tx:annotation-driven transaction-manager="txManager"?proxy-target-class="true"/>

1. 加入庫	cglib庫 cglib-2.2.jar asm庫(cglib 需要asm庫,如果沒有加入asm的jar文件,就會報asm錯誤) asm-3.1.jar
?2. 定義CGLIB操作類	package com.machome.cglibtest; import java.lang.reflect.Method; import com.machome.model.StuService; import net.sf.cglib.proxy.Enhancer; import net.sf.cglib.proxy.MethodInterceptor; import net.sf.cglib.proxy.MethodProxy; public class CGlibProxyFactory implements MethodInterceptor { ??? private Object targetObject; ??? ??? public Object createProxyInstance(Object targetObject){ ??? ?? this.targetObject = targetObject;??? //傳入用戶類 ??? ?? ??? ?? Enhancer enhancer = new Enhancer();????????? //Enhancer是cglib的核心類 ??? ?? ??? ???? // 將用戶類設為 Enhancer對象的superclass屬性,,即設為 Enhancer對象的父類 ??? ?? enhancer.setSuperclass(this.targetObject.getClass());????? ??? ???? // 設 Enhancer對象的Callbacks屬性,要求必須是Callback接口類型 ??? ?? enhancer.setCallback(this); ??? ?? ??? ?? return enhancer.create();? //生成代理對象 ??? } ??? ??? ??? @Override ??? public Object intercept(Object arg0, Method arg1, Object[] arg2, ??? ??? ??? MethodProxy arg3) throws Throwable { ??? ??? StuService bean = (StuService)this.targetObject; ??? ??? Object result = null;??? ??? ??? ??? if(!arg1.getName().equals("save")&&(bean.findAll().size()==0)) ??? ??? {? // 如果方法名不是save(即是get,update,delete方法),而同時實例的列表為空,就不執行方法,而是告警 ??? ??? ??? System.out.println("list is null,method is stoped");??? ??? ??? ??? ??? }else{ ??????? //執行代理方法,傳入實例和方法參數 ??? ??? result = arg3.invoke(targetObject, arg2);????? ?? ? ? ? ? ??? ??? } ??? ??? return result;?? ??? ??? } }
3.測試代碼	測試代碼: ??? ??? CGlibProxyFactory factory = new CGlibProxyFactory(); ??? ??? StuService bean = (StuService)factory.createProxyInstance( ??? ??? ??? ??? ??????????? new StuService()); ??? ??? ??? ??? List<Stu> stuList = bean.findAll(); ??? ??? if(stuList!=null){ ??? ??? for(Stu temp:stuList) ??? ??? ??? System.out.println(temp.getId()+":"+temp.getName()); ??? ??? System.out.println("finished"); 執行結果: list is null,method is stoped??????????? //bean.findAll()下邊的語句都沒執行)