SpringFramework之DelegatingFilterProxy简析
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，如下
List-1
public class DelegatingFilterProxy extends GenericFilterBean {2.重要方法讲解
DelegatingFilterProxy覆写了父类中的如下几个方法
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，之后使用这个bean.
由于DelegatingFilterProxy间接实现了Filter，所以要实现doFilter方法，如下：
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。
调用的时序图如下:
图2 DelegatingFilterProxy的doFilter调用时序图
思考：
1. DelegatingFilterProxy中有个属性，如下List-4所示。为什么即使没有显示的指定webApplicationContext的值，DelegatingFilterProxy依然可以从web container中得到targetBeanName对应的bean，why? 这是因为DelegatingFilterProxy实现了Filter，而Filter接口中有个方法init，如下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，这个方法可以拿到绑定到ServletContext的值。如果了解过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. 如下List-7所示，在web.xml中配置DelegatingFilterProxy时，我们并没有指定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()，那么我们来看看getFilterName()是如何实现的，如下List-8所示，由于DelegatingFilterProxy间接实现了Filter，所以它可以拿到FilterConfig，从FilterCofig中可以拿到这个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);
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