SpringFramework之DelegatingFilterProxy簡析

注：分析的版本是SpringFramework-4.3.x，源碼可自行到Github上下載

1.類繼承圖

先上一張圖

                                            圖1 DelegatingFilterProxy的類繼承圖

DelegatingFilterProxy只是直接繼承了GenericFilterBean，以下

    List-1

public class DelegatingFilterProxy extends GenericFilterBean {

2.重要方法講解

DelegatingFilterProxy覆寫了父類中的以下幾個方法

    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中的屬性targetBeanName，它是個bean的名稱，且這個bean要實現Filter。Spring會自動從web application中取出targetBeanName對應的bean，以後使用這個bean.

    

    因爲DelegatingFilterProxy間接實現了Filter，因此要實現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中調用了invokeDelegate方法，在invokeDelegate中，又調用了delegate的doFilter方法。

    調用的時序圖以下:

 

                                        圖2 DelegatingFilterProxy的doFilter調用時序圖

 

思考：

    1. DelegatingFilterProxy中有個屬性，以下List-4所示。爲何即便沒有顯示的指定webApplicationContext的值，DelegatingFilterProxy依然能夠從web container中獲得targetBeanName對應的bean，why? 這是由於DelegatingFilterProxy實現了Filter，而Filter接口中有個方法init，以下List-5所示，init方法的參數FilterConfig中有個方法getServletContext，getServletContext方法返回一個ServletContext，ServletContext有getAttribute方法，這個方法能夠拿到綁定到ServletContext的值。若是瞭解過ContextLoadListener和DispatcherServlet實現的，那麼應該明白了。

    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時，咱們並無指定DelegatingFilterProxy的targetBeanName，那麼web container在處理DelegatingFilterProxy時，這個targetBeanName是什麼呢，它的值是不能爲null的。答案：在上面List-2中，方法initFilterBean()裏面，有this.targetBeanName = getFilterName()，那麼咱們來看看getFilterName()是如何實現的，以下List-8所示，因爲DelegatingFilterProxy間接實現了Filter，因此它能夠拿到FilterConfig，從FilterCofig中能夠拿到這個Filter的filterName，從List-8中看出，當targetBeanName爲null時，會用filterName進行賦值，按List-7中的配置的話，這個targetBeanName的值就是springSecurityFilterChain。注：問題2，我也是看了這篇博客後，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);
	}