在 How Tomcat Works（十四） 中，本人并没有对javax.servlet.Filter及javax.servlet.FilterChain做详细的描述，本文在这里做一下补充
FilterChain接口只有一个方法，方法声明如下：
public void doFilter ( ServletRequest request, ServletResponse response ) throws IOException, ServletException;
在tomcat中，org.apache.catalina.core.ApplicationFilterChain类实现了该接口
ApplicationFilterChain类采用一个私有成员变量private ArrayList filters = new ArrayList()保存ApplicationFilterConfig对象的集合，ApplicationFilterConfig类实现了javax.servlet.FilterConfig接口，封装了对Filter实例的管理
下面是向ArrayList filters集合添加ApplicationFilterConfig实例
void
addFilter(ApplicationFilterConfig filterConfig) {
this
.filters.add(filterConfig);
}
setServlet()方法用于向ApplicationFilterChain对象设置当前的servlet实例
void
setServlet(Servlet servlet) {
this
.servlet =
servlet;
}
在ApplicationFilterChain类的实现FilterChain接口方法doFilter()中，调用了其私有方法internalDoFilter()，下面是该方法的具体实现
private
void
internalDoFilter(ServletRequest request, ServletResponse response)
throws
IOException, ServletException {
//
Construct an iterator the first time this method is called
if
(
this
.iterator ==
null
)
this
.iterator =
filters.iterator();
//
Call the next filter if there is one
if
(
this
.iterator.hasNext()) {
ApplicationFilterConfig filterConfig
=
(ApplicationFilterConfig) iterator.next();
Filter filter
=
null
;
try
{
filter
=
filterConfig.getFilter();
support.fireInstanceEvent(InstanceEvent.BEFORE_FILTER_EVENT,
filter, request, response);
filter.doFilter(request, response,
this
);
support.fireInstanceEvent(InstanceEvent.AFTER_FILTER_EVENT,
filter, request, response);
}
catch
(IOException e) {
if
(filter !=
null
)
support.fireInstanceEvent(InstanceEvent.AFTER_FILTER_EVENT,
filter, request, response, e);
throw
e;
}
catch
(ServletException e) {
if
(filter !=
null
)
support.fireInstanceEvent(InstanceEvent.AFTER_FILTER_EVENT,
filter, request, response, e);
throw
e;
}
catch
(RuntimeException e) {
if
(filter !=
null
)
support.fireInstanceEvent(InstanceEvent.AFTER_FILTER_EVENT,
filter, request, response, e);
throw
e;
}
catch
(Throwable e) {
if
(filter !=
null
)
support.fireInstanceEvent(InstanceEvent.AFTER_FILTER_EVENT,
filter, request, response, e);
throw
new
ServletException
(sm.getString(
"filterChain.filter"
), e);
}
return
;
}
//
We fell off the end of the chain -- call the servlet instance
try
{
support.fireInstanceEvent(InstanceEvent.BEFORE_SERVICE_EVENT,
servlet, request, response);
if
((request
instanceof
HttpServletRequest) &&
(response
instanceof
HttpServletResponse)) {
servlet.service((HttpServletRequest) request,
(HttpServletResponse) response);
}
else
{
servlet.service(request, response);
}
support.fireInstanceEvent(InstanceEvent.AFTER_SERVICE_EVENT,
servlet, request, response);
}
catch
(IOException e) {
support.fireInstanceEvent(InstanceEvent.AFTER_SERVICE_EVENT,
servlet, request, response, e);
throw
e;
}
catch
(ServletException e) {
support.fireInstanceEvent(InstanceEvent.AFTER_SERVICE_EVENT,
servlet, request, response, e);
throw
e;
}
catch
(RuntimeException e) {
support.fireInstanceEvent(InstanceEvent.AFTER_SERVICE_EVENT,
servlet, request, response, e);
throw
e;
}
catch
(Throwable e) {
support.fireInstanceEvent(InstanceEvent.AFTER_SERVICE_EVENT,
servlet, request, response, e);
throw
new
ServletException
(sm.getString(
"filterChain.servlet"
), e);
}
}
在调用了filter.doFilter(request, response, this)方法之后，继续执行servlet的service()方法
这里的this是ApplicationFilterChain对象自身，在我们编写的Filter实现类里面同在在执行完我们实现的过滤方法之后会继续调用FilterChain对象的void doFilter(ServletRequest request, ServletResponse response)方法，我们自定义的过滤器通常会这样实现：
class
SampleFilter
implements
Filter {
........
public
void
doFilter(ServletRequest request, ServletResponse response,FilterChain chain)
throws
IOException, ServletException {
//
do something
.....
//
request, response传递给下一个过滤器进行过滤
chain.doFilter(request, response);
}
}
ApplicationFilterConfig类实现了javax.servlet.FilterConfig接口，代表一个Filter容器，FilterConfig接口定义如下：
public
interface
FilterConfig {
public
String getFilterName();
public
ServletContext getServletContext();
public
String getInitParameter(String name);
public
Enumeration getInitParameterNames();
}
可以通过传入org.apache.catalina.Context对象和FilterDef对象（FilterDef对象用于对过滤器类的定义，包括过滤器类名、相关参数等）传给其构造函数构造一个ApplicationFilterConfig对象：
public
ApplicationFilterConfig(Context context, FilterDef filterDef)
throws
ClassCastException, ClassNotFoundException,
IllegalAccessException, InstantiationException,
ServletException {
super
();
this
.context =
context;
setFilterDef(filterDef);
}
ApplicationFilterConfig类的getFilter()方法返回一个javax.servlet.Filter对象，该方法负责载入并实例化一个过滤器类
Filter getFilter()
throws
ClassCastException, ClassNotFoundException,
IllegalAccessException, InstantiationException, ServletException {
//
Return the existing filter instance, if any
if
(
this
.filter !=
null
)
return
(
this
.filter);
//
Identify the class loader we will be using
String filterClass =
filterDef.getFilterClass();
ClassLoader classLoader
=
null
;
if
(filterClass.startsWith("org.apache.catalina."
))
classLoader
=
this
.getClass().getClassLoader();
else
classLoader
=
context.getLoader().getClassLoader();
ClassLoader oldCtxClassLoader
=
Thread.currentThread().getContextClassLoader();
//
Instantiate a new instance of this filter and return it
Class clazz =
classLoader.loadClass(filterClass);
this
.filter =
(Filter) clazz.newInstance();
filter.init(
this
);
return
(
this
.filter);
}
现在，也许我们更容易理解StandardWrapperValve类中创建过滤器链createFilterChain()方法了
private
ApplicationFilterChain createFilterChain(Request request,
Servlet servlet) {
//
If there is no servlet to execute, return null
if
(servlet ==
null
)
return
(
null
);
//
Create and initialize a filter chain object
ApplicationFilterChain filterChain =
new
ApplicationFilterChain();
filterChain.setServlet(servlet);
StandardWrapper wrapper
=
(StandardWrapper) getContainer();
filterChain.setSupport(wrapper.getInstanceSupport());
//
Acquire the filter mappings for this Context
StandardContext context =
(StandardContext) wrapper.getParent();
FilterMap filterMaps[]
=
context.findFilterMaps();
//
If there are no filter mappings, we are done
if
((filterMaps ==
null
) || (filterMaps.length == 0
))
return
(filterChain);
//
if (debug >= 1)
//
log("createFilterChain: Processing " + filterMaps.length +
//
" filter map entries");
//
Acquire the information we will need to match filter mappings
String requestPath =
null
;
if
(request
instanceof
HttpRequest) {
HttpServletRequest hreq
=
(HttpServletRequest) request.getRequest();
String contextPath
=
hreq.getContextPath();
if
(contextPath ==
null
)
contextPath
= ""
;
String requestURI
=
((HttpRequest) request).getDecodedRequestURI();
if
(requestURI.length() >=
contextPath.length())
requestPath
=
requestURI.substring(contextPath.length());
}
String servletName
=
wrapper.getName();
//
if (debug >= 1) {
//
log(" requestPath=" + requestPath);
//
log(" servletName=" + servletName);
//
}
int
n = 0
;
//
Add the relevant path-mapped filters to this filter chain
for
(
int
i = 0; i < filterMaps.length; i++
) {
//
if (debug >= 2)
//
log(" Checking path-mapped filter '" +
//
filterMaps[i] + "'");
if
(!
matchFiltersURL(filterMaps[i], requestPath))
continue
;
ApplicationFilterConfig filterConfig
=
(ApplicationFilterConfig)
context.findFilterConfig(filterMaps[i].getFilterName());
if
(filterConfig ==
null
) {
//
if (debug >= 2)
//
log(" Missing path-mapped filter '" +
//
filterMaps[i] + "'");
;
//
FIXME - log configuration problem
continue
;
}
//
if (debug >= 2)
//
log(" Adding path-mapped filter '" +
//
filterConfig.getFilterName() + "'");
filterChain.addFilter(filterConfig);
n
++
;
}
//
Add filters that match on servlet name second
for
(
int
i = 0; i < filterMaps.length; i++
) {
//
if (debug >= 2)
//
log(" Checking servlet-mapped filter '" +
//
filterMaps[i] + "'");
if
(!
matchFiltersServlet(filterMaps[i], servletName))
continue
;
ApplicationFilterConfig filterConfig
=
(ApplicationFilterConfig)
context.findFilterConfig(filterMaps[i].getFilterName());
if
(filterConfig ==
null
) {
//
if (debug >= 2)
//
log(" Missing servlet-mapped filter '" +
//
filterMaps[i] + "'");
;
//
FIXME - log configuration problem
continue
;
}
//
if (debug >= 2)
//
log(" Adding servlet-mapped filter '" +
//
filterMaps[i] + "'");
filterChain.addFilter(filterConfig);
n
++
;
}
//
Return the completed filter chain
//
if (debug >= 2)
//
log(" Returning chain with " + n + " filters");
return
(filterChain);
}
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