Spring中的AOP

何為AOP

AOP，面向切面編程。

在不改動代碼的前提下，靈活的在現有代碼的執行順序前後，添加進新規機能。

來一個簡單的Sample:

目標類：

1. package com.hyron.tony;
3. public class CustomerService {
4. private String name;
5. private String url;
7. public void setName(String name) {
8. this.name = name;
9. }
11. public void setUrl(String url) {
12. this.url = url;
13. }
15. public void printName() {
16. System.out.println("Customer name : " + this.name);
17. }
19. public void printURL() {
20. System.out.println("Customer website : " + this.url);
21. }
23. public void printThrowException() {
24. throw new IllegalArgumentException();
25. }
27. }

advice：只以Around advice為例

1. import java.util.Arrays;
3. import org.aopalliance.intercept.MethodInterceptor;
4. import org.aopalliance.intercept.MethodInvocation;
6. public class HijackAroundMethod implements MethodInterceptor {
7. @Override
8. public Object invoke(MethodInvocation methodInvocation) throws Throwable {
10. System.out.println("Method name : "
11. + methodInvocation.getMethod().getName());
12. System.out.println("Method arguments : "
13. + Arrays.toString(methodInvocation.getArguments()));
15. // same with MethodBeforeAdvice
16. System.out.println("HijackAroundMethod : Before method hijacked!");
18. try {
19. // proceed to original method call
20. Object result = methodInvocation.proceed();
22. // same with AfterReturningAdvice
23. System.out.println("HijackAroundMethod : Before after hijacked!");
25. return result;
27. } catch (IllegalArgumentException e) {
28. // same with ThrowsAdvice
29. System.out
30. .println("HijackAroundMethod : Throw exception hijacked!");
31. throw e;
32. }
33. }
34. }

編織切入關系的配置文件：

1. <bean id="customerService" class="com.mkyong.customer.services.CustomerService">
2. <property name="name" value="Yong Mook Kim" />
3. <property name="url" value="http://www.mkyong.com" />
4. </bean>
6. <bean id="hijackAroundMethodBean" class="com.mkyong.aop.HijackAroundMethod" />
8. <bean id="customerServiceProxy" class="org.springframework.aop.framework.ProxyFactoryBean">
10. <property name="target" ref="customerService" />
12. <property name="interceptorNames">
13. <list>
14. <value>hijackAroundMethodBean</value>
15. </list>
16. </property>
17. </bean>

Sample的啟動：

1. import org.springframework.context.ApplicationContext;
2. import org.springframework.context.support.ClassPathXmlApplicationContext;
4. import com.mkyong.customer.services.CustomerService;
6. public class App {
7. public static void main(String[] args) {
8. ApplicationContext appContext = new ClassPathXmlApplicationContext(
9. new String[] { "Spring-Customer.xml" });
11. CustomerService cust = (CustomerService) appContext
12. .getBean("customerServiceProxy");
14. System.out.println("*************************");
15. cust.printName();
16. System.out.println("*************************");
17. cust.printURL();
18. System.out.println("*************************");
19. try {
20. cust.printThrowException();
21. } catch (Exception e) {
23. }
25. }
26. }

以上代碼，用customerServiceProxy代理CustomerService的執行

在customerServiceProxy的配置中，定義了用hijackAroundMethodBean作為方法攔截器，在hijackAroundMethodBean中利用invoke方法，攔截住所有的方法調用，塞入自己的邏輯業務。

AOP的兩種實現

上面看到的是Spring的Sample。

其實，Spring的AOP也是調用了其他開源技術實現。

比較常用的是JDK自己的Proxy，和開源的CGLIB

兩者的區別，Proxy需要Advice必須從接口繼承過來。如果切入的目標物是實體類，則無法使用。

CGLIB則可以用於直接覆蓋實體類的方法。

Spring對以上兩種都有支持。

Spring的底層實現

Spring在配置文件中，通過ProxyFactoryBean編織和實現了切面的構成。

我們在執行以下這行話的時候

CustomerService cust = (CustomerService) appContext

.getBean("customerServiceProxy");

其實是將動態對象的生成委托給了ProxyFactoryBean

當配置文件中 <bean>的class屬性配置的實現類是FactoryBean時，通過getBean方法返回的不是FactoryBean本身，而是 FactoryBean#getObject()方法所返回的對象，相當於FactoryBean#getObject()代理了getBean()方 法。

執行順序如下：

1. ProxyFactoryBean中的getObject

1. /**
2. * Return a proxy. Invoked when clients obtain beans from this factory bean.
3. * Create an instance of the AOP proxy to be returned by this factory.
4. * The instance will be cached for a singleton, and create on each call to
5. * <code>getObject()</code> for a proxy.
6. * @return a fresh AOP proxy reflecting the current state of this factory
7. */
8. public Object getObject() throws BeansException {
9. initializeAdvisorChain();
10. if (isSingleton()) {
11. return getSingletonInstance();
12. }
13. else {
14. if (this.targetName == null) {
15. logger.warn("Using non-singleton proxies with singleton targets is often undesirable. " +
16. "Enable prototype proxies by setting the 'targetName' property.");
17. }
18. return newPrototypeInstance();
19. }
20. }

2. ProxyFactoryBean中的initializeAdvisorChain

從配置文件中的advice list中取得interceptorNames,並將其加入advisorChain

1. for (String name : this.interceptorNames) {
2. if (logger.isTraceEnabled()) {
3. logger.trace("Configuring advisor or advice '" + name + "'");
4. }
6. if (name.endsWith(GLOBAL_SUFFIX)) {
7. if (!(this.beanFactory instanceof ListableBeanFactory)) {
8. throw new AopConfigException(
9. "Can only use global advisors or interceptors with a ListableBeanFactory");
10. }
11. addGlobalAdvisor((ListableBeanFactory) this.beanFactory,
12. name.substring(0, name.length() - GLOBAL_SUFFIX.length()));
13. }