spring之DelegatingFilterProxy

DelegatingFilterProxy是一個標准servlet Filter的代理，代理實現了Filter接口的spring管理的Bean。支持一個在web.xml的init-param定義的"targetBeanName" filter，在spring applicationContext中指定了target bean的名稱。

web.xml通常包含一個DelegatingFilterProxy的定義，它在root springContext中指定了filter-name及對應的bean。在springcontext中實現了標准servlet filter接口的bean在調用時都將被代理。

   注意：Servlet Filter接口默認定義的lifecycle方法不會被代理到target bean，它依賴spring applicationContext管理target bean的lifecycle。在filter的屬性targetFilterLifecycle的init-param中指定為true將強制執行Filter.init()和Filter.destory()方法，讓Filter來管理filter的lifecycle。

　從spring 3.1開始，DelegatingFilterProxy更新到可以支持servlet 3.0基於實例的filter注冊方法來構建，通常和spring 3.1的 org.springframework.web.WebApplicationInitializer spi一起使用。這個類由spring security中Ben Alex 寫的FilterToBeanProxy靈感觸發而寫的。

具體實現如下：

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. 觸發代理方法

    @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);
    }

觸發代理

    /**
     * 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.銷毀方法

    /**
     * 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();
        }
    }