Spring AOP

AOP的实现原理

• 编译期：切面在编译期织入，不过需要重新定义新的语法关键字并由特殊的编译器编译来实现   AspectJ
• 类装载器：目标类被装载到Java虚拟机时，有一个特殊的类装载器对目标类的字节码进行增强
• 运行期：目标对象和切面都是标准的Java类，通过Java虚拟机的动态代理功能或CGLib事项的运行期动态织入

public class AopProxyFactory {

	public static Object createProxy(final Object target, 
	        final MethodBeforeAdvice methodBeforeAdvice) {
		return Proxy.newProxyInstance(target.getClass().getClassLoader(), 
				target.getClass().getInterfaces(),
				new InvocationHandler() {
					public Object invoke(Object proxy, Method method, Object[] args)
						throws Throwable {
						methodBeforeAdvice.before(method, args, target);
						return method.invoke(target, args);
					}
				});
	}
}


public class Main {
	public static void main(String[] args) {
		BookDao bookDao = new BookDao();
		BookServiceImpl target = new BookServiceImpl();
		target.setBookDao(bookDao);
		MethodBeforeAdvice log = new MethodBeforeAdvice() {
			public void before(Method arg0, Object[] arg1, Object arg2) throws Throwable {
				System.out.println("call method" + arg0.getName());
			}
		};
		BookService bookService = (BookService) AopProxyFactory.createProxy(target, log);
		bookService.query();
	}
}

术语解释

• Aspect:切面
切面是一个横跨多个核心逻辑的功能，或者称之为系统关注点，例如日志记录，事务处理，安全检查。以前我们在每个方法中调用重复编写这些代码，现在我们将这些散落的代码集中起来，放在一个模块中，这个模块就是切面
• Joinpoint：连接点
连接点就是应用程序流程的何处插入切面，（一个方法调用时，访问一个字段时，特定的异常抛出时）
• Pointcut：切入点
切入点是一组连接点的集合，比如一个切面要在调用，所有以delete开头的方法时执行，就可以用通配符delete*来表示这一组连接点，我们将其称为切面
• Advice：增强
在特定连接点上执行的动作，执行这个动作就相当于对原始对象的功能做了增强
• Introduction：引介
为一个已有的Java对象，动态的添加新的接口
• Weaving：织入
织入是将切面整合到程序中的过程
• Interceptor：拦截器
拦截器是一种实现增强的方式
• Target Object：目标对象
真正执行核心逻辑的对象。
• Advisor：一个Advice与Pointcut的结合
定义如何将advice织入目标对象

常用Advice
1.MethodBeforeAdvice：在一个方法执行前增强
2.AfterReturningAdvice：在一个方法执行后增强
3.ThrowsAdvice：在一个方法执行前后增强
4.MethodInterceptor：在一个方法执行前后增强  可以控制整个方法执行与否，修改返回值，功能最强大。能使用上述三种简单的实现，就不要使用MethodInterceptor

public interface UserService {
	void login(String username, String password);
	boolean create(String username, String password);
}

@Service("userServiceTarget")
public class UserServiceImpl implements UserService {
	public void login(String username, String password) {
		if ("zhang".equals(username) && "zhang".equals(password)) {
			
		} else {
			throw new RuntimeException();
		}
	}
	public boolean create(String username, String password) {
		return "admin".equals(username) ? false : true;
	}
}

@Component
public class LoginMethodBeforeAdvice implements MethodBeforeAdvice {
	public void before(Method method, Object[] args, Object target) 
	        throws Throwable {
		System.out.println("[LoginMethodBeforeAdvice] User " + args[0] + " try to login....");
	}
}

@Component
public class LoginMethodAfterAdvice implements AfterReturningAdvice {
	public void afterReturning(Object returnValue, Method method, Object[] args, 
	        Object target) throws Throwable {
		System.out.println("[LoginMethodAfterAdvice] User " + args[0] + " is login...");
	}
}

@Component
public class LoginMethodThrowsAdvice implements ThrowsAdvice {
	public void afterThrowing(Throwable subClass) {
		System.out.println("[LoginMethodThrowsAdvice] An exception occur: " 
		    + subClass.getClass().getSimpleName());
	}
}

public class LoginMethodAroundAdvice implements MethodInterceptor {
	public Object invoke(MethodInvocation invocation) throws Throwable {
		// 这里实现MethodBeforeAdvice的功能
		Object ret = null;
		try {
			ret = invocation.proceed();
		} catch (Exception e) {
			// 这里实现ThrowsAdvice的功能
		}
		// 这里实现AfterReturningAdvice的功能
		return ret;
	}
}
	 
public class Drive {
	public static void main(String[] args) throws InterruptedException {

		ApplicationContext context = new ClassPathXmlApplicationContext("application.xml");
		UserService userService = (UserService) context.getBean("userService");

		try {
			userService.login("zhang", "zhang");
			Thread.sleep(500);
			userService.login("xxxx", "invaild-password");
		} catch (RuntimeException e) {
			//System.err.println("invalid user...");
		}
	}
}

输出
[LoginMethodBeforeAdvice] User zhang try to login....
[LoginMethodAfterAdvice] User zhang is login...
[LoginMethodBeforeAdvice] User xxxx try to login....
[LoginMethodThrowsAdvice] An exception occur: RuntimeException

<bean id="userService" class="org.springframework.aop.framework.ProxyFactoryBean">
	<property name="target" ref="userServiceTarget"/>
	<property name="interceptorNames">
		<list>
			<value>loginMethodBeforeAdvice</value>
			<value>loginMethodAfterAdvice</value>
			<value>loginMethodThrowsAdvice</value>
		</list>
	</property>
</bean>

三种Advice执行的位置不冲突，所以先后顺序无关，若加入MethodInterceptor，并且放置在最前面，则出现异常时，不执行invocation.proceed();其他三种Advice将不会被执行



Advisor 控制只在某些方法上执行
1.NameMatchMethodPointcutAdvisor
2.RegexpMethodPointcutAdvisor

@Component
public class CreateAfterReturningAdvice implements AfterReturningAdvice {
	public void afterReturning(Object returnValue, Method method, Object[] args, 
	    Object target) throws Throwable {
		System.out.println("[CreateAfterReturningAdvice] create user " + args[0]);
	}
}
	 
public class Drive {
	public static void main(String[] args) throws InterruptedException {
		ApplicationContext context = new ClassPathXmlApplicationContext("application.xml");
		UserService userService = (UserService) context.getBean("userService");

		try {
			userService.create("zhang", "zhang");
			userService.login("zhang", "zhang");
			Thread.sleep(500);
			userService.create("admin", "invaild-password");
			userService.login("admin", "invaild-password");
		} catch (RuntimeException e) {
			//System.err.println("invalid user...");
		}
	}
}

输出
[CreateAfterReturningAdvice] create user zhang
[LoginMethodAfterAdvice] User zhang is login...
[LoginMethodBeforeAdvice] User zhang try to login....
[LoginMethodAfterAdvice] User zhang is login...
[LoginMethodThrowsAdvice] An exception occur: RuntimeException

<bean id="createAfterReturningAdvisor" 
	    class="org.springframework.aop.support.NameMatchMethodPointcutAdvisor">
	<property name="advice" ref="createAfterReturningAdvice"/>
	<property name="mappedName" value="create"/>
</bean>
 
<bean id="loginMethodBeforeAdvisor" 
		class="org.springframework.aop.support.RegexpMethodPointcutAdvisor">
	<property name="advice" ref="loginMethodBeforeAdvice"/>
	<property name="pattern" value=".*login.*"/>
</bean>
 
<bean id="userService" class="org.springframework.aop.framework.ProxyFactoryBean">
	<property name="target" ref="userServiceTarget"/>
	<property name="interceptorNames">
		<list>
			<value>loginMethodBeforeAdvisor</value>
			<value>createAfterReturningAdvisor</value>
			<!-- loginMethodAfterAdvice loginMethodThrowsAdvice 每个方法执行时都会被调用 -->
			<value>loginMethodAfterAdvice</value>
			<value>loginMethodThrowsAdvice</value>
		</list>
	</property>
 </bean>

spring aop代理实际上包含了一个有Advisor和Advice组成的拦截链，Advice总是增强所有方法，Advisor同时包含Advice和Pointcut，因此Advisor会通过Pointcut计算是否应该调用Advice对某个方法进行增强。

使用自动代理
1.自动为多个目标Bean实现Aop代理
2.避免客户端直接访问目标Bean
通过BeanPostProcessor实现，BeanNameAutoProxyCreator DefaultAdvisorAutoProxyCreator AspectJInvocationContextExposingAdvisorAutoProxyCreator AnnotationAwareAspectJAtuoProxyCreator

<!-- BeanNameAutoProxyCreator 所有以Service结尾的Bean -->
<bean id="beanNameAutoProxy" 
    class="org.springframework.aop.framework.autoproxy.BeanNameAutoProxyCreator">
	<property name="beanNames" ref="*Service"/>
	<property name="interceptorNames">
		<list>
			<value>loginMethodBeforeAdvisor</value>
			<value>loginMethodAfterAdvice</value>
			<value>loginMethodThrowsAdvice</value>
			<value>createAfterReturningAdvisor</value>
		</list>
	</property>
</bean>
	 
<!-- DefaultAdvisorAutoProxyCreator -->
<bean id="defaultAutoProxy"
	class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator" />

引介 Introduction
一种类型特殊的拦截器，不能作用于任何切入点，只能作用于类，而非方法级别，换句话说就是给类动态的添加接口

想让User类具有Mutable接口，通过isReadonly来判断是否可以更改User类的信息

public class User {

	private String username;
	private String password;
	private String email;
	public String getUsername() {
		return username;
	}
	public void setUsername(String username) {
		this.username = username;
	}
	public String getPassword() {
		return password;
	}
	public void setPassword(String password) {
		this.password = password;
	}
	public String getEmail() {
		return email;
	}
	public void setEmail(String email) {
		this.email = email;
	}
}

public interface Mutable {
	boolean isReadonly();
	void setReadonly(boolean isReadOnly);
}

public class MutableInterceptor extends DelegatingIntroductionInterceptor 
        implements Mutable {
	private boolean readonly;
	public boolean isReadonly() {
		return this.readonly;
	}
	public void setReadonly(boolean readonly) {
		this.readonly = readonly;
	}
}


public static void main(String[] args) {
	
	ApplicationContext context = new ClassPathXmlApplicationContext("application.xml");
	User user = (User) context.getBean("user");
	System.out.println(user.getEmail());
	((Mutable)user).setReadonly(true);
	System.out.println(((Mutable)user).isReadonly());
	user.setEmail("[e-mail]");
	System.out.println(user.getEmail());
}
输出 
default-email
false
[e-mail]	

<bean id="user" scope="prototype" 
    class="org.springframework.aop.framework.ProxyFactoryBean">
	<property name="target" ref="target"/>
	<!-- 对类代理，因此设置为true，同时需要CGlib -->
	<property name="proxyTargetClass" value="true"/>
	<property name="proxyInterfaces">
		<list>
			<value>demo.aop.introduction.Mutable</value>
		</list>
	</property>
	<property name="interceptorNames">
		<list>
			<value>introductionAdvisor</value>
		</list>
	</property>
</bean>

<bean id="introductionAdvisor" scope="prototype" 
	class="org.springframework.aop.support.DefaultIntroductionAdvisor">
	<constructor-arg>
		<bean class="demo.aop.introduction.MutableInterceptor"/>
	</constructor-arg>
</bean>

<bean id="target" class="demo.aop.introduction.User">
	<property name="username" value="default-username"/>
	<property name="password" value="default-password"/>
	<property name="email" value="default-email"/>
</bean>

可以看到User对象却是具有了Mutable接口，因为没有ClassCastException。但是我们
ProxyFactoryBean返回的是一个AOP代理对象，因此它可以截获所有方法的调用，并委托给MethodInterceptor处理，MethodInterceptor集成自DelegatingIntroductionInterceptor，DelegatingIntroductionInterceptor实现了MethodInterceptor接口，因此可以实现方法拦截。

DelegatingIntroductionInterceptorde的invoke方法

public Object invoke(MethodInvocation mi) throws Throwable {
        // 调用前是否是新的接口的方法？
		if (isMethodOnIntroducedInterface(mi)) {
		    // 调用delegate对象的方法
			Object retVal = AopUtils.invokeJoinpointUsingReflection(this.delegate,
    			mi.getMethod(), mi.getArguments());
			if (retVal == this.delegate && mi instanceof ProxyMethodInvocation) {
				Object proxy = ((ProxyMethodInvocation) mi).getProxy();
				if (mi.getMethod().getReturnType().isInstance(proxy)) {
					retVal = proxy;
				}
			}
			return retVal;
		}
		// 重写invoke方法，会拦截所有方法
		// 如果只希望拦截原始对象（User）的方法，就覆写doProceed方法
		return doProceed(mi);
	}
	
// 修改后的	MutableInterceptor
public class MutableInterceptor extends DelegatingIntroductionInterceptor implements Mutable {
	private boolean readonly;
	public boolean isReadonly() {
		return this.readonly;
	}
	public void setReadonly(boolean readonly) {
		this.readonly = readonly;
	}
	@Override
	protected Object doProceed(MethodInvocation mi) throws Throwable {
		if (readonly && mi.getMethod().getName().startsWith("set")) {
			throw new UnsupportedOperationException("Object is set to readonly");
		}
		return super.doProceed(mi);
	}
}

禁止在运行期改变AOP代理
<property name="frozen" value="true"/>

使用AspectJ实现AOP

// 1 声明Aspect
@Aspect
public class LoggerAspect {

}

// 2 声明Advice
@Before("execution(* demo.aop.impl.UserServiceImpl.login(..)) && args(username, ..)")
public void logBefore(String username){
	System.out.println("[logger] User "+ username +" try to login....");
}

@Around("execution(* demo.aop.impl.UserServiceImpl.login(..))")
public Object securityCheck(ProceedingJoinPoint pjp) throws Throwable {
	String username = (String) pjp.getArgs()[0];
	if ("admin".equals(username)) {
		System.out.println("[security check admin is forbidden]");
		throw new RuntimeException();
	}
	return pjp.proceed();
}

// 3 开启AspectJ支持
<aop:aspectj-autoproxy/>

executionde的完整表达式：execution(修饰符? 返回类型 声明类型? 方法名称(参数类型) 异常类型?)

声明Pointcut
上面的Advice中，我们直接把Pointcut的表达式卸载相应的注解中。可以通过@Pointcut注解提取出来单独定义

@Aspect
@Component
public class LoggerAspect {

	@Pointcut("execution(* demo.service.impl.UserServiceImpl.login(..))")
	public void loginMethod(){}
	
	@Before("loginMethod() && args(username, ..)")
	public void logBefore(String username) {
		System.out.println("[logger] User " + username + " try to login....");
	}

	@AfterReturning("loginMethod() && args(username, ..)")
	public void logSuccess(String username) {
		System.out.println("[logger] User " + username + " has login....");
	}

	@AfterThrowing(pointcut = "loginMethod()", throwing = "e")
	public void logFailure(RuntimeException e) {
		System.out.println("[logger] Exception: " + e.getMessage());
	}
	
	@Around("loginMethod()")
	public Object securityCheck(ProceedingJoinPoint pjp) throws Throwable {
		String username = (String) pjp.getArgs()[0];
		if ("admin".equals(username)) {
			System.out.println("[security check] admin is forbidden");
			throw new RuntimeException();
		}
		return pjp.proceed();
	}
}