设计模式在Spring中的应用

> 本文将会不断总结更新
## 工厂方法模式
Spring中提供了FactoryBean接口，用于创建各种不同的Bean。
![title](http://leanote.com/api/file/getImage?fileId=56ea88beab644175ec002d38)
开发人员也可以自己实现该接口，常用于框架集成。比如mybatis-spring-1.1.0.jar包中的SqlSessionFactoryBean就是如此。

## 模板方法模式
Spring针对JDBC,JMS,JPA等规范，都提供了相应的模板方法类，如[JdbcTemplate](http://static.springframework.org/spring/docs/2.5.x/reference/jdbc.html#jdbc-JdbcTemplate),[JmsTemplate](http://static.springframework.org/spring/docs/2.5.x/reference/jms.html#jms-jmstemplate), [JpaTemplate](http://static.springframework.org/spring/docs/2.5.x/reference/orm.html#orm-jpa-template)。
例如JdbcTemplate,它提供了很多常用的增加，删除，查询，修改方法模板。而JMSTemplate则提供了对于消息的发送，接收方法等。下面是JMSTemplate的部分方法图：
![title](http://leanote.com/api/file/getImage?fileId=56ea8a96ab644175ec002d4c)

## 代理模式
Spring中AOP，事务等都大量运用了代理模式。

## 观察者模式
Spring中提供了一种事件监听机制，即ApplicationListener，可以实现Spring容器内的事件监听。可以参考[这里](http://docs.spring.io/spring/docs/3.2.15.RELEASE/spring-framework-reference/htmlsingle/#context-functionality-events)
主要是以下两个接口：
发布消息
![title](http://leanote.com/api/file/getImage?fileId=56ea938eab644175ec002d9c)
监听消息
![title](http://leanote.com/api/file/getImage?fileId=56ea939eab6441777b003256)

## 单例模式
Spring默认的创建Bean的作用域就是单例，即每个Spring容器中只存在一个该类的实例。可以通过@Scope("prototype")来修改成prototype模式,prototype在设计模式中叫做原型模式，实际上，Spring中对于@Scope("prototype")标记的Bean的处理的确是原型模式。

## 原型模式
原型模式是创建型模式的一种,其特点在于通过“复制”一个已经存在的实例来返回新的实例,而不是新建实例。被复制的实例就是我们所称的“原型”，这个原型是可定制的。
原型模式多用于创建复杂的或者耗时的实例，因为这种情况下，复制一个已经存在的实例使程序运行更高效；或者创建值相等，只是命名不一样的同类数据。

Spring中，如果一个类被标记为"prototype",每一次请求（将其注入到另一个bean中，或者以程序的方式调用容器的getBean()方法）都会产生一个新的bean实例。
但是,Spring不能对一个prototype Bean的整个生命周期负责，容器在初始化、配置、装饰或者是装配完一个prototype实例后，将它交给客户端，随后就对该prototype实例不闻不问了。不管何种作用域，容器都会调用所有对象的初始化生命周期回调方法，而对prototype而言，任何配置好的析构生命周期回调方法都将不会被调用。清除prototype作用域的对象并释放任何prototype  bean所持有的昂贵资源，都是客户端代码的职责。

`作者注:16年4月29日补充`
## 职责链模式
在SpringMVC中，我们会经常使用一些拦截器(`HandlerInterceptor`),当存在多个拦截器的时候，所有的拦截器就构成了一条拦截器链。SpringMVC中使用`HandlerExecutionChain`类来将所有的拦截器组装在一起。
`HandlerExecutionChain`类的源代码如下：
``` java
public class HandlerExecutionChain {

private static final Log logger = LogFactory.getLog(HandlerExecutionChain.class);

private final Object handler;

private HandlerInterceptor[] interceptors;

private List<HandlerInterceptor> interceptorList;

private int interceptorIndex = -1;

/**
	 * Create a new HandlerExecutionChain.
	 * @param handler the handler object to execute
	 */
	public HandlerExecutionChain(Object handler) {
		this(handler, null);
	}

/**
	 * Create a new HandlerExecutionChain.
	 * @param handler the handler object to execute
	 * @param interceptors the array of interceptors to apply
	 * (in the given order) before the handler itself executes
	 */
	public HandlerExecutionChain(Object handler, HandlerInterceptor[] interceptors) {
		if (handler instanceof HandlerExecutionChain) {
			HandlerExecutionChain originalChain = (HandlerExecutionChain) handler;
			this.handler = originalChain.getHandler();
			this.interceptorList = new ArrayList<HandlerInterceptor>();
			CollectionUtils.mergeArrayIntoCollection(originalChain.getInterceptors(), this.interceptorList);
			CollectionUtils.mergeArrayIntoCollection(interceptors, this.interceptorList);
		}
		else {
			this.handler = handler;
			this.interceptors = interceptors;
		}
	}

/**
	 * Return the handler object to execute.
	 * @return the handler object
	 */
	public Object getHandler() {
		return this.handler;
	}

public void addInterceptor(HandlerInterceptor interceptor) {
		initInterceptorList();
		this.interceptorList.add(interceptor);
	}

public void addInterceptors(HandlerInterceptor[] interceptors) {
		if (interceptors != null) {
			initInterceptorList();
			this.interceptorList.addAll(Arrays.asList(interceptors));
		}
	}

private void initInterceptorList() {
		if (this.interceptorList == null) {
			this.interceptorList = new ArrayList<HandlerInterceptor>();
		}
		if (this.interceptors != null) {
			this.interceptorList.addAll(Arrays.asList(this.interceptors));
			this.interceptors = null;
		}
	}

/**
	 * Return the array of interceptors to apply (in the given order).
	 * @return the array of HandlerInterceptors instances (may be {@code null})
	 */
	public HandlerInterceptor[] getInterceptors() {
		if (this.interceptors == null && this.interceptorList != null) {
			this.interceptors = this.interceptorList.toArray(new HandlerInterceptor[this.interceptorList.size()]);
		}
		return this.interceptors;
	}

/**
	 * Apply preHandle methods of registered interceptors.
	 * @return {@code true} if the execution chain should proceed with the
	 * next interceptor or the handler itself. Else, DispatcherServlet assumes
	 * that this interceptor has already dealt with the response itself.
	 */
	boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
		if (getInterceptors() != null) {
			for (int i = 0; i < getInterceptors().length; i++) {
				HandlerInterceptor interceptor = getInterceptors()[i];
				if (!interceptor.preHandle(request, response, this.handler)) {
					triggerAfterCompletion(request, response, null);
					return false;
				}
				this.interceptorIndex = i;
			}
		}
		return true;
	}

/**
	 * Apply postHandle methods of registered interceptors.
	 */
	void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {
		if (getInterceptors() == null) {
			return;
		}
		for (int i = getInterceptors().length - 1; i >= 0; i--) {
			HandlerInterceptor interceptor = getInterceptors()[i];
			interceptor.postHandle(request, response, this.handler, mv);
		}
	}

/**
	 * Trigger afterCompletion callbacks on the mapped HandlerInterceptors.
	 * Will just invoke afterCompletion for all interceptors whose preHandle invocation
	 * has successfully completed and returned true.
	 */
	void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex)
			throws Exception {

if (getInterceptors() == null) {
			return;
		}
		for (int i = this.interceptorIndex; i >= 0; i--) {
			HandlerInterceptor interceptor = getInterceptors()[i];
			try {
				interceptor.afterCompletion(request, response, this.handler, ex);
			}
			catch (Throwable ex2) {
				logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
			}
		}
	}

/**
	 * Apply afterConcurrentHandlerStarted callback on mapped AsyncHandlerInterceptors.
	 */
	void applyAfterConcurrentHandlingStarted(HttpServletRequest request, HttpServletResponse response) {
		if (getInterceptors() == null) {
			return;
		}
		for (int i = getInterceptors().length - 1; i >= 0; i--) {
			if (interceptors[i] instanceof AsyncHandlerInterceptor) {
				try {
					AsyncHandlerInterceptor asyncInterceptor = (AsyncHandlerInterceptor) this.interceptors[i];
					asyncInterceptor.afterConcurrentHandlingStarted(request, response, this.handler);
				}
				catch (Throwable ex) {
					logger.error("Interceptor [" + interceptors[i] + "] failed in afterConcurrentHandlingStarted", ex);
				}
			}
		}
	}

/**
	 * Delegates to the handler's {@code toString()}.
	 */
	@Override
	public String toString() {
		if (this.handler == null) {
			return "HandlerExecutionChain with no handler";
		}
		StringBuilder sb = new StringBuilder();
		sb.append("HandlerExecutionChain with handler [").append(this.handler).append("]");
		if (!CollectionUtils.isEmpty(this.interceptorList)) {
			sb.append(" and ").append(this.interceptorList.size()).append(" interceptor");
			if (this.interceptorList.size() > 1) {
				sb.append("s");
			}
		}
		return sb.toString();
	}

}
```
`HandlerInterceptor`接口如下：
``` java
    public interface HandlerInterceptor {

/**
	 * Intercept the execution of a handler. Called after HandlerMapping determined
	 * an appropriate handler object, but before HandlerAdapter invokes the handler.
	 * <p>DispatcherServlet processes a handler in an execution chain, consisting
	 * of any number of interceptors, with the handler itself at the end.
	 * With this method, each interceptor can decide to abort the execution chain,
	 * typically sending a HTTP error or writing a custom response.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler chosen handler to execute, for type and/or instance evaluation
	 * @return {@code true} if the execution chain should proceed with the
	 * next interceptor or the handler itself. Else, DispatcherServlet assumes
	 * that this interceptor has already dealt with the response itself.
	 * @throws Exception in case of errors
	 */
	boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
	    throws Exception;

/**
	 * Intercept the execution of a handler. Called after HandlerAdapter actually
	 * invoked the handler, but before the DispatcherServlet renders the view.
	 * Can expose additional model objects to the view via the given ModelAndView.
	 * <p>DispatcherServlet processes a handler in an execution chain, consisting
	 * of any number of interceptors, with the handler itself at the end.
	 * With this method, each interceptor can post-process an execution,
	 * getting applied in inverse order of the execution chain.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler handler (or {@link HandlerMethod}) that started async
	 * execution, for type and/or instance examination
	 * @param modelAndView the {@code ModelAndView} that the handler returned
	 * (can also be {@code null})
	 * @throws Exception in case of errors
	 */
	void postHandle(
			HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView)
			throws Exception;

/**
	 * Callback after completion of request processing, that is, after rendering
	 * the view. Will be called on any outcome of handler execution, thus allows
	 * for proper resource cleanup.
	 * <p>Note: Will only be called if this interceptor's {@code preHandle}
	 * method has successfully completed and returned {@code true}!
	 * <p>As with the {@code postHandle} method, the method will be invoked on each
	 * interceptor in the chain in reverse order, so the first interceptor will be
	 * the last to be invoked.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler handler (or {@link HandlerMethod}) that started async
	 * execution, for type and/or instance examination
	 * @param ex exception thrown on handler execution, if any
	 * @throws Exception in case of errors
	 */
	void afterCompletion(
			HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)
			throws Exception;

}
```
需要注意的是`preHandle`方法的返回值是boolean类型，用于决定是否需要下一个拦截器继续处理。

## 参考资料
1. https://premaseem.wordpress.com/2013/02/09/spring-design-patterns-used-in-java-spring-framework/
2. http://stackoverflow.com/questions/755563/what-design-patterns-are-used-in-spring-framework
3. http://forum.spring.io/forum/other-spring-related/architecture/17610-a-reference-discussing-all-the-design-patterns-used-inside-spring
4. https://springframework.guru/gang-of-four-design-patterns/abstract-factory-design-pattern/
5. http://www.coin163.com/java/docs/201310/d_2850109002.html

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