Spring Web学习--DelegatingFilterProxy 代理Filter

迷南。 2022-05-17 12:08 123阅读 0赞

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