转自：http://www.cnblogs.com/hzhuxin/archive/2011/12/19/2293730.html
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>
<init-param>
<param-name>targetFilterLifecycle</param-name>
<param-value>true</param-value> <!-- 默认是false -->
</init-param>
</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：
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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
);
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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，
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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);
}
}
}
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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：
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protected
Filter initDelegate(WebApplicationContext wac)
throws
ServletException {
Filter delegate = wac.getBean(getTargetBeanName(), Filter.
class
);
if
(isTargetFilterLifecycle()) {
delegate.init(getFilterConfig());
}
return
delegate;
}
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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
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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);
}
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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。
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protected
void
invokeDelegate(
Filter delegate, ServletRequest request, ServletResponse response, FilterChain filterChain)
throws
ServletException, IOException {
delegate.doFilter(request, response, filterChain);
}
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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。
此外还要注意一个DelegatingFilterProxy的一个初始化参数：targetFilterLifecycle ,其默认值为false 。 但如果被其代理的filter的init()方法和destry()方法需要被调用时，需要设置targetFilterLifecycle为true。具体可见DelegatingFilterProxy中的如下代码：
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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);
}
}
}
}
protected
Filter initDelegate(WebApplicationContext wac)
throws
ServletException {
Filter delegate = wac.getBean(getTargetBeanName(), Filter.
class
);
if
(isTargetFilterLifecycle()) {
//注意这行
delegate.init(getFilterConfig());
}
return
delegate;
}
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