Tomcat源码解析系列（十九）ApplicationFilterChain

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。

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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 <code>service()</code> 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<Void>() {
                    @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。

1. 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。

1. 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。

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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。