前言
上篇文章中分析了各个容器相关的 Pipeline 属性对象的 的 Valve 属性对象的 invoke 方法。在最后的 StandardWrapperValve 方法里调用 ApplicationFilterChain#doFilter 方法，本文分析一下这个方法。
1. ApplicationFilterChain#doFilter
/**
* Invoke the next filter in this chain, passing the specified request
* and response. If there are no more filters in this chain, invoke
* the service() method of the servlet itself.
*
* @param request The servlet request we are processing
* @param response The servlet response we are creating
*
* @exception IOException if an input/output error occurs
* @exception ServletException if a servlet exception occurs
*/
@Override
public void doFilter(ServletRequest request, ServletResponse response)
throws IOException, ServletException {
if( Globals.IS_SECURITY_ENABLED ) {
final ServletRequest req = request;
final ServletResponse res = response;
try {
java.security.AccessController.doPrivileged(
new java.security.PrivilegedExceptionAction() {
@Override
public Void run()
throws ServletException, IOException {
internalDoFilter(req,res);
return null;
}
}
);
} catch( PrivilegedActionException pe) {
Exception e = pe.getException();
if (e instanceof ServletException)
throw (ServletException) e;
else if (e instanceof IOException)
throw (IOException) e;
else if (e instanceof RuntimeException)
throw (RuntimeException) e;
else
throw new ServletException(e.getMessage(), e);
}
} else {
internalDoFilter(request,response);
}
} doFilter 方法就是简单调用 internalDoFilter 方法。
2. ApplicationFilterChain#internalDoFilter
/**
* The int which is used to maintain the current position
* in the filter chain.
*/
private int pos = 0;
/**
* The int which gives the current number of filters in the chain.
*/
private int n = 0;
private void internalDoFilter(ServletRequest request,
ServletResponse response)
throws IOException, ServletException {
// Call the next filter if there is one
if (pos < n) {
ApplicationFilterConfig filterConfig = filters[pos++];
try {
Filter filter = filterConfig.getFilter();
if (request.isAsyncSupported() && "false".equalsIgnoreCase(
filterConfig.getFilterDef().getAsyncSupported())) {
request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR, Boolean.FALSE);
}
if( Globals.IS_SECURITY_ENABLED ) {
final ServletRequest req = request;
final ServletResponse res = response;
Principal principal =
((HttpServletRequest) req).getUserPrincipal();
Object[] args = new Object[]{req, res, this};
SecurityUtil.doAsPrivilege ("doFilter", filter, classType, args, principal);
} else {
filter.doFilter(request, response, this);
}
} catch (IOException | ServletException | RuntimeException e) {
throw e;
} catch (Throwable e) {
e = ExceptionUtils.unwrapInvocationTargetException(e);
ExceptionUtils.handleThrowable(e);
throw new ServletException(sm.getString("filterChain.filter"), e);
}
return;
}
// We fell off the end of the chain -- call the servlet instance
try {
if (ApplicationDispatcher.WRAP_SAME_OBJECT) {
lastServicedRequest.set(request);
lastServicedResponse.set(response);
}
if (request.isAsyncSupported() && !servletSupportsAsync) {
request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR,
Boolean.FALSE);
}
// Use potentially wrapped request from this point
if ((request instanceof HttpServletRequest) &&
(response instanceof HttpServletResponse) &&
Globals.IS_SECURITY_ENABLED ) {
final ServletRequest req = request;
final ServletResponse res = response;
Principal principal =
((HttpServletRequest) req).getUserPrincipal();
Object[] args = new Object[]{req, res};
SecurityUtil.doAsPrivilege("service",
servlet,
classTypeUsedInService,
args,
principal);
} else {
servlet.service(request, response);
}
} catch (IOException | ServletException | RuntimeException e) {
throw e;
} catch (Throwable e) {
e = ExceptionUtils.unwrapInvocationTargetException(e);
ExceptionUtils.handleThrowable(e);
throw new ServletException(sm.getString("filterChain.servlet"), e);
} finally {
if (ApplicationDispatcher.WRAP_SAME_OBJECT) {
lastServicedRequest.set(null);
lastServicedResponse.set(null);
}
}
}internalDoFilter 方法里分了两部分，if (pos < n) 和 if 后面的 try-catch 语句。
- if (pos < n)
先从 filter 是数组里获取下一个 ApplicationFilterConfig 对象。
ApplicationFilterConfig filterConfig = filters[pos++];然后通过 filterConfig.getFilter() 获取一个 Filter 对象，并调用这个 Filter 的 doFilter 方法，并把 ApplicationFilterChain 对象传入 Filter#doFilter
在 if (pos < n) 里的 n 是这个 ApplicationFilterChain 里包含的 Filter 的总数。
每次调用 internalDoFilter 方法，pos 就加1，也就是获取下一个 ApplicationFilterConfig，直到 pos 等于 n 的时候才不执行 if 语句，为了达到这个效果，应用程序必须在自定义的在 Filter 里执行 FilterChain#doFilter 方法，才能再次进入 ApplicationFilterChain#internalDoFilter 方法，这样就完成了调用 ApplicationFilterChain 里所有的 Filter 的 doFilter 方法。
这种方式是责任链模式的一种体现。
- try-catch
internalDoFilter里的 try-catch 是在执行完所有的 Filter#doFilter 方法之后执行的。
try 语句块里先设置一下属性，然后执行
servlet.service(request, response);通过调用 Servlet 对象的 service 方法来处理请求。
到这里，tomcat 终于把请求交给了应用程序了，tomcat 整个接受请求、转发处理请求的流程就差不多完结了。
小结
本文分析了 ApplicationFilterChain#doFilter 方法，在这个方法里，tomcat 首先调用了 ApplicationFilterChain 里所有 Filter 的 doFilter 方法，然后调用 ApplicationFilterChain 里的 Servlet 对象的 service 方法把请求交给了应用程序。