Spring Web学习--DelegatingFilterProxy 代理Filter

       Spring web在设计的时候考虑到某些功能的实现是通过Filter来拦截进行实现的，如果直接的简单的实现几个Filter好像也不是不可以（平时我们就是这么用的），但是Spring框架最核心的是IOC容器，和Spring框架最好的实现就是将要实现的Filter功能注册到IOC容器的一个Bean，这样就可以和Spring IOC容器进行完美的融合，所以Spring Web设计了DelegatingFilterProxy。本质上来说DelegatingFilterProxy就是一个Filter，其间接实现了Filter接口，但是在doFilter中其实调用的从Spring 容器中获取到的代理Filter的实现类delegate。

1、DelegatingFilterProxy根据targetBeanName从Spring 容器中获取被注入到Spring 容器的Filter实现类，在DelegatingFilterProxy配置时一般需要配置属性targetBeanName

@Override
protected void initFilterBean() throws ServletException {
	synchronized (this.delegateMonitor) {
		if (this.delegate == null) {
			// If no target bean name specified, use filter name.
                        //当Filter配置时如果没有设置targentBeanName属性，则直接根据Filter名称来查找
			if (this.targetBeanName == null) {
				this.targetBeanName = getFilterName();
			}
			// Fetch Spring root application context and initialize the delegate early,
			// if possible. If the root application context will be started after this
			// filter proxy, we'll have to resort to lazy initialization.
			WebApplicationContext wac = findWebApplicationContext();
			if (wac != null) {
                                //从Spring容器中获取注入的Filter的实现类
				this.delegate = initDelegate(wac);
			}
		}
	}
}

protected Filter initDelegate(WebApplicationContext wac) throws ServletException {
		//从Spring 容器中获取注入的Filter的实现类
		Filter delegate = wac.getBean(getTargetBeanName(), Filter.class);
		if (isTargetFilterLifecycle()) {
			delegate.init(getFilterConfig());
		}
		return delegate;
	}

2、在DelegatingFilterProxy的实现方法doFilter中，其实最终调用的是委派的类delegate

@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain)
			throws ServletException, IOException {

		// Lazily initialize the delegate if necessary.
		//获取委派Filter实现类
		Filter delegateToUse = this.delegate;
		if (delegateToUse == null) {
			synchronized (this.delegateMonitor) {
				if (this.delegate == null) {
					WebApplicationContext wac = findWebApplicationContext();
					if (wac == null) {
						throw new IllegalStateException("No WebApplicationContext found: " +
								"no ContextLoaderListener or DispatcherServlet registered?");
					}
					this.delegate = initDelegate(wac);
				}
				delegateToUse = this.delegate;
			}
		}

		// Let the delegate perform the actual doFilter operation.
		invokeDelegate(delegateToUse, request, response, filterChain);
	}
	
	//调用委派的Filter的doFilter方法
	protected void invokeDelegate(
			Filter delegate, ServletRequest request, ServletResponse response, FilterChain filterChain)
			throws ServletException, IOException {

		delegate.doFilter(request, response, filterChain);
	}

总结：Spring web通过提高DelegatingProxyFilter类给开发者提供了便利