DelegatingFilterProxy干了什么？

org.springframework.web.filter.DelegatingFilterProxy

一般情况，创建一个Filter是交给自己来实现的。基于servlet规范，在web.xml中配置，自定义filter实现Filter接口：

public interface Filter {
void init(FilterConfig var1) throws ServletException;

void doFilter(ServletRequest var1, ServletResponse var2, FilterChain var3) throws IOException, ServletException;

void destroy();
}

但是从shiro的操作方式来看，这个反过来了：spring提供了DelegatingFilterProxy作为通用Filter代理类，shiro以factoryBean的形式来构造自己的filter。

那么这个DelegatingFilterProxy究竟干了些什么？

利用初始化方法获取被代理的对象delegate

protected Filter initDelegate(WebApplicationContext wac) throws ServletException {
Filter delegate = wac.getBean(getTargetBeanName(), Filter.class);
if (isTargetFilterLifecycle()) {
delegate.init(getFilterConfig());
}
return delegate;
}

通过doFilter方法来调用目标过滤器的doFilter方法

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 registered?");
}
this.delegate = initDelegate(wac);
}
delegateToUse = this.delegate;
}
}

// Let the delegate perform the actual doFilter operation.
invokeDelegate(delegateToUse, request, response, filterChain);
}

protected void invokeDelegate(
Filter delegate, ServletRequest request, ServletResponse response, FilterChain filterChain)
throws ServletException, IOException {

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

通过destory方法来释放一些资源

@Override
public void destroy() {
Filter delegateToUse = this.delegate;
if (delegateToUse != null) {
destroyDelegate(delegateToUse);
}
}

protected void destroyDelegate(Filter delegate) {
if (isTargetFilterLifecycle()) {
delegate.destroy();
}
}

这个三个操作就完全将详细的实现转移到被代理的Filter。因为spring是所有bean的容器，这个filter的实际创建就交给了其它框架，但是前提是，这些框架必须要实现FactoryBean接口。所以有了shiro的ShiroFilterFactoryBean(springSecurity原理应该跟这个差不多)。