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();
}
}