浅谈Spring设计模式

工厂模式

BeanFactory，从xml或者注解加载BeanDefinition，然后实例化对象，其中AbstractAutowiredCapableBeanFactory实现了主要的逻辑

单例模式

spring创建的bean默认为singleton

适配器模式

比如说Springmvc的HandlerInterceptorAdapter就是个接口适配器，实现了AsyncHandlerInterceptor(HandlerInterceptor的子类)，ThemeChangeInterceptor继承至HandlerInterceptorAdapter，只需要重写关注的方法即可，不相关的方法完全可以忽略。

public abstract class HandlerInterceptorAdapter implements AsyncHandlerInterceptor {

    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {
        return true;
    }

    @Override
    public void postHandle(
            HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView)
            throws Exception {
    }

    @Override
    public void afterCompletion(
            HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)
            throws Exception {
    }

    @Override
    public void afterConcurrentHandlingStarted(
            HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {
    }

}

public class ThemeChangeInterceptor extends HandlerInterceptorAdapter {

    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws ServletException {
        // do somethings        
    }

}

个人认为用得最好的地方莫过于spring-jms的MessagingMessageListenerAdapter，在onMessage中将jsm的Message转换为message模块的Message对象(内部类的LazyResolutionMessage，重写了getPayload、getHeader)，并交给message模块的InvocableHandleMethod，这样一来便可以实现jms与spring message无缝适配对接了，在spring-websocket也是相同的套路

public class MessagingMessageListenerAdapter extends AbstractAdaptableMessageListener {

    private InvocableHandlerMethod handlerMethod;

    public void setHandlerMethod(InvocableHandlerMethod handlerMethod) {
        this.handlerMethod = handlerMethod;
    }

    @Override
    public void onMessage(javax.jms.Message jmsMessage, Session session) throws JMSException {
        Message<?> message = toMessagingMessage(jmsMessage);
        if (logger.isDebugEnabled()) {
            logger.debug("Processing [" + message + "]");
        }
        Object result = invokeHandler(jmsMessage, session, message);
        if (result != null) {
            handleResult(result, jmsMessage, session);
        }
        else {
            logger.trace("No result object given - no result to handle");
        }
    }

    protected Message<?> toMessagingMessage(javax.jms.Message jmsMessage) {
        try {
            return (Message<?>) getMessagingMessageConverter().fromMessage(jmsMessage);
        }
        catch (JMSException ex) {
            throw new MessageConversionException("Could not convert JMS message", ex);
        }
    }

    // 忽略部分代码
}

装饰模式

装饰模式是指在不影响其它类的情况下，动态透明的扩展一个对象的功能，比如TransactionAwareCacheDecorator增加了对事务的支持，在事务提交、回滚的时候分别对Cache的数据进行处理。不过，装饰模式和静态代理还是有区别的，装饰模式只是在调用前后加了些逻辑，但是最终还是需要调用父类或者装饰类；而代理模式是需要or不需要来完成对代理方法的调用，比如AOP可以有选择性地处理某些方法，并不一定会执行代理类的具体方法。

public class TransactionAwareCacheDecorator implements Cache {

    private final Cache targetCache;

    @Override
    public void put(final Object key, final Object value) {
        if (TransactionSynchronizationManager.isSynchronizationActive()) {
            TransactionSynchronizationManager.registerSynchronization(new TransactionSynchronizationAdapter() {
                @Override
                public void afterCommit() {
                    targetCache.put(key, value);
                }
            });
        }
        else {
            this.targetCache.put(key, value);
        }
    }

    // 忽略部分代码

}

观察者模式

在spring中我们借助ApplicationListener、ApplicationEventPublisher便可以完成简直的事件通知。当调用ApplicationEventPublisher#publishEvent()时，spring会查找实现了ApplicationListener接口、并且使用了指定泛型的bean，然后调用其onApplicationEvent。下面的代码示范了spring发出ContextRefreshedEvent事件。

ApplicationEventPublisher eventPublisher = xxx;
eventPublisher.publishEvent( new ContextRefreshedEvent( applicationContext ) );

public class TestListener implements ApplicationListener<ContextRefreshedEvent> {

    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        // invoke after spring context refreshed
    }

}

策略模式

比如SpringMVC的HandlerMethodArgumentResolver接口，使用HandlerMethodArgumentResolverComposite(实现HandlerMethodArgumentResolver接口)，通过遍历内部所有的HandlerMethodArgumentResolver(当然有缓存 机制)，选择support返回true的实例，并把接口的调用交给该实例处理

其实，我们在写业务代码的时候，很多童鞋喜欢用Map保存策略实现类，其实这样并不好，不够灵活，应该像spring这样提供一个support方法，可以支持更加复杂的逻辑判断

public class HandlerMethodArgumentResolverComposite implements HandlerMethodArgumentResolver {

    protected final Log logger = LogFactory.getLog(getClass());

    private final List<HandlerMethodArgumentResolver> argumentResolvers =
            new LinkedList<HandlerMethodArgumentResolver>();

    private final Map<MethodParameter, HandlerMethodArgumentResolver> argumentResolverCache =
            new ConcurrentHashMap<MethodParameter, HandlerMethodArgumentResolver>(256);

    @Override
    public boolean supportsParameter(MethodParameter parameter) {
        return (getArgumentResolver(parameter) != null);
    }

    private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {
        HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);
        if (result == null) {
            for (HandlerMethodArgumentResolver methodArgumentResolver : this.argumentResolvers) {
                if (methodArgumentResolver.supportsParameter(parameter)) {
                    result = methodArgumentResolver;
                    this.argumentResolverCache.put(parameter, result);
                    break;
                }
            }
        }
        return result;
    }

    @Override
    public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
            NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {

        HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
        if (resolver == null) {
            throw new IllegalArgumentException("Unknown parameter type [" + parameter.getParameterType().getName() + "]");
        }
        return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
    }

}

代理模式

静态代理在Spring websocket里面非常常见，比如说WebsocketHandlerDecorator，因为它不方便直接操作开发者自定义的WebSocketHandler，因此使用了代理模式。另外，cglib、jdk动态代理在spring中也是非常常见

public class WebSocketHandlerDecorator implements WebSocketHandler {

    private final WebSocketHandler delegate;

    public WebSocketHandlerDecorator(WebSocketHandler delegate) {
        Assert.notNull(delegate, "Delegate must not be null");
        this.delegate = delegate;
    }

    @Override
    public void afterConnectionEstablished(WebSocketSession session) throws Exception {
        this.delegate.afterConnectionEstablished(session);
    }

    @Override
    public void handleMessage(WebSocketSession session, WebSocketMessage<?> message) throws Exception {
        this.delegate.handleMessage(session, message);
    }

    // 省略部分代码

}

责任链模式

比如SpringMVC的HandlerExecutionChain与HandlerInterceptor，以及ExceptionHandlerResolver

下面是HandlerExecutionChain的部分代码，由DispatcherServlet调用，内部维护了interceptorIndex，用于标记当前调用HandlerInterceptor的位置

public class HandlerExecutionChain {

    private final Object handler;

    private HandlerInterceptor[] interceptors;

    private int interceptorIndex = -1;

    boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HandlerInterceptor[] interceptors = getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            for (int i = 0; i < interceptors.length; i++) {
                HandlerInterceptor interceptor = interceptors[i];
                if (!interceptor.preHandle(request, response, this.handler)) {
                    triggerAfterCompletion(request, response, null);
                    return false;
                }
                this.interceptorIndex = i;
            }
        }
        return true;
    }

    void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {
        HandlerInterceptor[] interceptors = getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            for (int i = interceptors.length - 1; i >= 0; i--) {
                HandlerInterceptor interceptor = interceptors[i];
                interceptor.postHandle(request, response, this.handler, mv);
            }
        }
    }

}

再比如说springMVC对异常的处理，DispatcherServlet会保存所有的HandlerExceptionResolver，当出现异常时，挨个调用HandlerExceptionResolver实例的resolveException方法，直到返回ModelAndView

建设者模式(Builder)

为了简化对象的创建过程而使用的一种设计模式，比如BeanDefinitionBuilder是为了简化BeanDefinition的创建过程，每次setXXX都会返回BeanDefinitionBuilder实例，方便以链条编码的方式创建BeanDefinitionBuilder

public class BeanDefinitionBuilder {

    public static BeanDefinitionBuilder genericBeanDefinition(Class<?> beanClass) {
        BeanDefinitionBuilder builder = new BeanDefinitionBuilder();
        builder.beanDefinition = new GenericBeanDefinition();
        builder.beanDefinition.setBeanClass(beanClass);
        return builder;
    }

    private AbstractBeanDefinition beanDefinition;

    public BeanDefinitionBuilder addConstructorArgValue(Object value) {
        this.beanDefinition.getConstructorArgumentValues().addIndexedArgumentValue(
                this.constructorArgIndex++, value);
        return this;
    }

    public BeanDefinitionBuilder addPropertyValue(String name, Object value) {
        this.beanDefinition.getPropertyValues().add(name, value);
        return this;
    }

}

模板模式

spring中很多地方会这样做，在一个方法里面完成了一小部分逻辑，然后接着调用一个或多个抽象方法，而这个抽象方法需要由子类重写，比如AbstractApplicationContext里面的refresh()、getBeanFactory()等等，这样便可以很好的提高了程序的扩展性