SpringFramework之DelegatingFilterProxy简析

注：分析的版本是SpringFramework-4.3.x，源码可自行到Github上下载

1.类继承图

先上一张图

                                            图1 DelegatingFilterProxy的类继承图

DelegatingFilterProxy只是直接继承了GenericFilterBean，以下

    List-1

public class DelegatingFilterProxy extends GenericFilterBean {

2.重要方法讲解

DelegatingFilterProxy覆写了父类中的以下几个方法

    List-2

@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);
				}
			}
		}
	}

	@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) {
				delegateToUse = this.delegate;
				if (delegateToUse == null) {
					WebApplicationContext wac = findWebApplicationContext();
					if (wac == null) {
						throw new IllegalStateException("No WebApplicationContext found: " +
								"no ContextLoaderListener or DispatcherServlet registered?");
					}
					delegateToUse = initDelegate(wac);
				}
				this.delegate = delegateToUse;
			}
		}

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

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

    使用Template pattern模式，DelegatingFilterProxy中的属性targetBeanName，它是个bean的名称，且这个bean要实现Filter。Spring会自动从web application中取出targetBeanName对应的bean，以后使用这个bean.

    

    因为DelegatingFilterProxy间接实现了Filter，因此要实现doFilter方法，以下：

    List-3

@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) {
				delegateToUse = this.delegate;
				if (delegateToUse == null) {
					WebApplicationContext wac = findWebApplicationContext();
					if (wac == null) {
						throw new IllegalStateException("No WebApplicationContext found: " +
								"no ContextLoaderListener or DispatcherServlet registered?");
					}
					delegateToUse = initDelegate(wac);
				}
				this.delegate = delegateToUse;
			}
		}

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

	/**
	 * 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);
	}

    在doFilter中调用了invokeDelegate方法，在invokeDelegate中，又调用了delegate的doFilter方法。

    调用的时序图以下:

 

                                        图2 DelegatingFilterProxy的doFilter调用时序图

 

思考：

    1. DelegatingFilterProxy中有个属性，以下List-4所示。为何即便没有显示的指定webApplicationContext的值，DelegatingFilterProxy依然能够从web container中获得targetBeanName对应的bean，why? 这是由于DelegatingFilterProxy实现了Filter，而Filter接口中有个方法init，以下List-5所示，init方法的参数FilterConfig中有个方法getServletContext，getServletContext方法返回一个ServletContext，ServletContext有getAttribute方法，这个方法能够拿到绑定到ServletContext的值。若是了解过ContextLoadListener和DispatcherServlet实现的，那么应该明白了。

    List-4

private WebApplicationContext webApplicationContext;

    List-5

public void init(FilterConfig filterConfig) throws ServletException;

    List-6

public ServletContext getServletContext();

    2. 以下List-7所示，在web.xml中配置DelegatingFilterProxy时，咱们并无指定DelegatingFilterProxy的targetBeanName，那么web container在处理DelegatingFilterProxy时，这个targetBeanName是什么呢，它的值是不能为null的。答案：在上面List-2中，方法initFilterBean()里面，有this.targetBeanName = getFilterName()，那么咱们来看看getFilterName()是如何实现的，以下List-8所示，因为DelegatingFilterProxy间接实现了Filter，因此它能够拿到FilterConfig，从FilterCofig中能够拿到这个Filter的filterName，从List-8中看出，当targetBeanName为null时，会用filterName进行赋值，按List-7中的配置的话，这个targetBeanName的值就是springSecurityFilterChain。注：问题2，我也是看了这篇博客后，才明白的。

    List-7 

<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>

    List-8

protected final String getFilterName() {
		return (this.filterConfig != null ? this.filterConfig.getFilterName() : this.beanName);
	}