spring之DelegatingFilterProxy

DelegatingFilterProxy是一个标准servlet Filter的代理，代理实现了Filter接口的spring管理的Bean。支持一个在web.xml的init-param定义的"targetBeanName" filter，在spring applicationContext中指定了target bean的名称。

web.xml通常包含一个DelegatingFilterProxy的定义，它在root springContext中指定了filter-name及对应的bean。在springcontext中实现了标准servlet filter接口的bean在调用时都将被代理。

   注意：Servlet Filter接口默认定义的lifecycle方法不会被代理到target bean，它依赖spring applicationContext管理target bean的lifecycle。在filter的属性targetFilterLifecycle的init-param中指定为true将强制执行Filter.init()和Filter.destory()方法，让Filter来管理filter的lifecycle。

　从spring 3.1开始，DelegatingFilterProxy更新到可以支持servlet 3.0基于实例的filter注册方法来构建，通常和spring 3.1的 org.springframework.web.WebApplicationInitializer spi一起使用。这个类由spring security中Ben Alex 写的FilterToBeanProxy灵感触发而写的。

具体实现如下：

1.初始化方法

    @Override    protected void initFilterBean() throws ServletException {        synchronized (this.delegateMonitor) {            if (this.delegate == null) {                // If no target bean name specified, use filter name.                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) {                    this.delegate = initDelegate(wac);                }            }        }    }

初始化代理

    /**     * Initialize the Filter delegate, defined as bean the given Spring     * application context.     * <p>The default implementation fetches the bean from the application context     * and calls the standard {@code Filter.init} method on it, passing     * in the FilterConfig of this Filter proxy.     * @param wac the root application context     * @return the initialized delegate Filter     * @throws ServletException if thrown by the Filter     * @see #getTargetBeanName()     * @see #isTargetFilterLifecycle()     * @see #getFilterConfig()     * @see javax.servlet.Filter#init(javax.servlet.FilterConfig)     */    protected Filter initDelegate(WebApplicationContext wac) throws ServletException {        Filter delegate = wac.getBean(getTargetBeanName(), Filter.class);        if (isTargetFilterLifecycle()) {            delegate.init(getFilterConfig());        }        return delegate;    }

2. 触发代理方法

    @Override    public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain)            throws ServletException, IOException {        // Lazily initialize the delegate if necessary.        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);    }

触发代理

    /**     * Actually invoke the delegate Filter with the given request and response.     * @param delegate the delegate Filter     * @param request the current HTTP request     * @param response the current HTTP response     * @param filterChain the current FilterChain     * @throws ServletException if thrown by the Filter     * @throws IOException if thrown by the Filter     */    protected void invokeDelegate(            Filter delegate, ServletRequest request, ServletResponse response, FilterChain filterChain)            throws ServletException, IOException {        delegate.doFilter(request, response, filterChain);    }

3.销毁方法

    /**     * Destroy the Filter delegate.     * Default implementation simply calls {@code Filter.destroy} on it.     * @param delegate the Filter delegate (never {@code null})     * @see #isTargetFilterLifecycle()     * @see javax.servlet.Filter#destroy()     */    protected void destroyDelegate(Filter delegate) {        if (isTargetFilterLifecycle()) {            delegate.destroy();        }    }