springSecurity源码分析——DelegatingFilterProxy类的作用

使用过springSecurity的朋友都知道，首先需要在web.xml进行以下配置，

<filter>
  <filter-name>springSecurityFilterChain</filter-name>
  <filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
 </filter>

<filter-mapping>
  <filter-name>springSecurityFilterChain</filter-name>
  <url-pattern>/*</url-pattern>

 </filter-mapping>

从这个配置中，可能会给我们造成一个错觉，以为 DelegatingFilterProxy类就是springSecurity的入口，但其实这个类位于spring-web- 3.0.5.RELEASE.jar这个jar下面，说明这个类本身是和springSecurity无关。DelegatingFilterProxy类继承于抽象类GenericFilterBean,间接地implement 了javax.servlet.Filter接口，Servlet容器在启动时，首先会调用Filter的init方法,GenericFilterBean的作用主要是可以把Filter的初始化参数自动地set到继承于GenericFilterBean类的Filter中去。在其init方法的如下代码就是做了这个事：

PropertyValues pvs = new FilterConfigPropertyValues(filterConfig, this.requiredProperties);
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
ResourceLoader resourceLoader = new ServletContextResourceLoader(filterConfig.getServletContext());
bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader));
initBeanWrapper(bw);
bw.setPropertyValues(pvs, true);

 另外在init方法中调用了initFilterBean()方法，该方法是GenericFilterBean类是特地留给子类扩展用的，

protected void initFilterBean() throws ServletException {
        // 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.
        synchronized (this.delegateMonitor) {
            WebApplicationContext wac = findWebApplicationContext();
            if (wac != null) {
                this.delegate = initDelegate(wac);
            }
        }
    }

 可以看出上述代码首先看Filter是否提供了targetBeanName初始化参数，如果没有提供则直接使用filter的name做为beanName,产生了beanName后，由于我们在web.xml的filter的name是springSecurityFilterChain,从spring的IOC容器中取出bean的代码是initDelegate方法，下面是该方法代码：

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

 通过跟踪代码，发现取出的bean是org.springframework.security.FilterChainProxy，该类也是继承于GenericFilterBean,取出bean后，判断targetFilterLifecycle属性是false还是true,决定是否调用该类的init方法。这个FilterChainProxy bean实例最终被保存在DelegatingFilterProxy类的delegate属性里,

下面看一下DelegatingFilterProxy类的doFilter方法

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

 真正要关注invokeDelegate(delegateToUse, request, response, filterChain);这句代码,在下面可以看出DelegatingFilterProxy类实际是用其delegate属性即org.springframework.security.FilterChainProxy实例的doFilter方法来响应请求。

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

以上就是DelegatingFilterProxy类的一些内部运行机制，其实主要作用就是一个代理模式的应用,可以把servlet 容器中的filter同spring容器中的bean关联起来。