代理过滤器 - 掘金

DelegatingFilterProxy这个东西太熟悉了，在类似shiro和springsecurity的权限认证框架都有见到它的身影，但又陌生，印象似乎只停留在DelegatingFilterProxy把servlet 容器中的filter同spring容器中的bean关联起来，交由spring容器管理。DelegatingFilterProxy代理类把filter和IOC容器进行了结合在一起，从spring上下文寻找要代理的过滤类，执行初始化，过滤和销毁过程。下面结合springsecurity就翻翻源码：

获取filter配置参数

DelegatingFilterProxy继承了GenericFilterBean ，父类实现了一堆接口：

public abstract class GenericFilterBean implements Filter, BeanNameAware, EnvironmentAware, EnvironmentCapable, ServletContextAware, InitializingBean, DisposableBean

BeanNameAware //返回当前的beanName

EnvironmentAware//读取配置文件

EnvironmentCapable//检查接受了BeanFactory接口的框架方法

ServletContextAware//获取ServletContext

InitializingBean//初始化bean对象

DisposableBean //销毁bean

父类中init方法将过滤器的参数配置传递给代理对象 源码如下：

public final void init(FilterConfig filterConfig) throws ServletException {
        Assert.notNull(filterConfig, "FilterConfig must not be null");
        this.filterConfig = filterConfig;
        PropertyValues pvs = new GenericFilterBean.FilterConfigPropertyValues(filterConfig, this.requiredProperties);
        if (!pvs.isEmpty()) {
            try {
                BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
                ResourceLoader resourceLoader = new ServletContextResourceLoader(filterConfig.getServletContext());
                Environment env = this.environment;
                if (env == null) {
                    env = new StandardServletEnvironment();
                }

                bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, (PropertyResolver)env));
                this.initBeanWrapper(bw);
                bw.setPropertyValues(pvs, true);
            } catch (BeansException var6) {
                String msg = "Failed to set bean properties on filter '" + filterConfig.getFilterName() + "': " + var6.getMessage();
                this.logger.error(msg, var6);
                throw new NestedServletException(msg, var6);
            }
        }

bean对象初始化

获取目标对象的name。

protected void initFilterBean() throws ServletException {
        synchronized(this.delegateMonitor) {
            if (this.delegate == null) {
                if (this.targetBeanName == null) {
                    this.targetBeanName = this.getFilterName();
                }

                WebApplicationContext wac = this.findWebApplicationContext();
                if (wac != null) {
                    this.delegate = this.initDelegate(wac);
                }
            }

        }
    }

初始化代理对象

在web.xml文件中配置的org.springframework.web.context.ContextLoaderListener实现加载Sping的上下文环境（由XmlWebApplicationContent来装载）。通过initDelegate(wac)得到代理过滤器

protected Filter initDelegate(WebApplicationContext wac) throws ServletException {
        String targetBeanName = this.getTargetBeanName();
        Assert.state(targetBeanName != null, "No target bean name set");
        Filter delegate = (Filter)wac.getBean(targetBeanName, Filter.class);
        if (this.isTargetFilterLifecycle()) {
            delegate.init(this.getFilterConfig());
        }

        return delegate;
    }

代理执行过滤方法

在执行过滤方法之前，通过invokeDelegate调用delegate执行过滤方法

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

过滤方法

public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws ServletException, IOException {
        Filter delegateToUse = this.delegate;
        if (delegateToUse == null) {
            synchronized(this.delegateMonitor) {
                delegateToUse = this.delegate;
                if (delegateToUse == null) {
                    WebApplicationContext wac = this.findWebApplicationContext();
                    if (wac == null) {
                        throw new IllegalStateException("No WebApplicationContext found: no ContextLoaderListener or DispatcherServlet registered?");
                    }

                    delegateToUse = this.initDelegate(wac);
                }

                this.delegate = delegateToUse;
            }
        }

销毁

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

    }

shiro的代理过滤器大致类似。