责任链模式在Tomcat中的应用
转自： 感觉作者没有说到点子上，没有说出这个模式的思想，和应用的场景中带来哪些好处。
众所周知Tomcat中的Filter就是使用了责任链模式，创建一个Filter除了要在web.xml文件中做相应配置外，还需要实现javax.servlet.Filter接口。
public class TestFilter implements Filter{ public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { chain.doFilter(request, response); } public void destroy() { } public void init(FilterConfig filterConfig) throws ServletException { }}
使用DEBUG模式所看到的结果如下
其实在真正执行到TestFilter类之前，会经过很多Tomcat内部的类。顺带提一下其实Tomcat的容器设置也是责任链模式，注意被红色方框所圈中的类，从Engine到Host再到Context一直到Wrapper都是通过一个链传递请求。被绿色方框所圈中的地方有一个名为ApplicationFilterChain的类，ApplicationFilterChain类所扮演的就是抽象处理者角色，而具体处理者角色由各个Filter扮演。
第一个疑问是ApplicationFilterChain将所有的Filter存放在哪里？
答案是保存在ApplicationFilterChain类中的一个ApplicationFilterConfig对象的数组中。
/** * Filters. */ private ApplicationFilterConfig[] filters = new ApplicationFilterConfig[0];
那ApplicationFilterConfig对象又是什么呢？
ApplicationFilterConfig是一个Filter容器。以下是ApplicationFilterConfig类的声明：
/** * Implementation of a javax.servlet.FilterConfig useful in * managing the filter instances instantiated when a web application * is first started. * * @author Craig R. McClanahan * @version $Id: ApplicationFilterConfig.java 1201569 2011-11-14 01:36:07Z kkolinko $ */
当一个web应用首次启动时ApplicationFilterConfig会自动实例化，它会从该web应用的web.xml文件中读取配置的Filter的信息，然后装进该容器。
刚刚看到在ApplicationFilterChain类中所创建的ApplicationFilterConfig数组长度为零，那它是在什么时候被重新赋值的呢？
private ApplicationFilterConfig[] filters = new ApplicationFilterConfig[0];
是在调用ApplicationFilterChain类的addFilter()方法时。
/** * The int which gives the current number of filters in the chain. */ private int n = 0;
public static final int INCREMENT = 10;
void addFilter(ApplicationFilterConfig filterConfig) { // Prevent the same filter being added multiple times for(ApplicationFilterConfig filter:filters) if(filter==filterConfig) return; if (n == filters.length) { ApplicationFilterConfig[] newFilters = new ApplicationFilterConfig[n + INCREMENT]; System.arraycopy(filters, 0, newFilters, 0, n); filters = newFilters; } filters[n++] = filterConfig; }
变量n用来记录当前过滤器链里面拥有的过滤器数目，默认情况下n等于0，ApplicationFilterConfig对象数组的长度也等于0，所以当第一次调用addFilter()方法时，if (n == filters.length)的条件成立，ApplicationFilterConfig数组长度被改变。之后filters[n++] = filterConfig;将变量filterConfig放入ApplicationFilterConfig数组中并将当前过滤器链里面拥有的过滤器数目+1。
那ApplicationFilterChain的addFilter()方法又是在什么地方被调用的呢？
是在ApplicationFilterFactory类的createFilterChain()方法中。
1 public ApplicationFilterChain createFilterChain 2 (ServletRequest request, Wrapper wrapper, Servlet servlet) { 3 4 // get the dispatcher type 5 DispatcherType dispatcher = null; 6 if (request.getAttribute(DISPATCHER_TYPE_ATTR) != null) { 7 dispatcher = (DispatcherType) request.getAttribute(DISPATCHER_TYPE_ATTR); 8 } 9 String requestPath = null; 10 Object attribute = request.getAttribute(DISPATCHER_REQUEST_PATH_ATTR); 11 12 if (attribute != null){ 13 requestPath = attribute.toString(); 14 } 15 16 // If there is no servlet to execute, return null 17 if (servlet == null) 18 return (null); 19 20 boolean comet = false; 21 22 // Create and initialize a filter chain object 23 ApplicationFilterChain filterChain = null; 24 if (request instanceof Request) { 25 Request req = (Request) request; 26 comet = req.isComet(); 27 if (Globals.IS_SECURITY_ENABLED) { 28 // Security: Do not recycle 29 filterChain = new ApplicationFilterChain(); 30 if (comet) { 31 req.setFilterChain(filterChain); 32 } 33 } else { 34 filterChain = (ApplicationFilterChain) req.getFilterChain(); 35 if (filterChain == null) { 36 filterChain = new ApplicationFilterChain(); 37 req.setFilterChain(filterChain); 38 } 39 } 40 } else { 41 // Request dispatcher in use 42 filterChain = new ApplicationFilterChain(); 43 } 44 45 filterChain.setServlet(servlet); 46 47 filterChain.setSupport 48 (((StandardWrapper)wrapper).getInstanceSupport()); 49 50 // Acquire the filter mappings for this Context 51 StandardContext context = (StandardContext) wrapper.getParent(); 52 FilterMap filterMaps[] = context.findFilterMaps(); 53 54 // If there are no filter mappings, we are done 55 if ((filterMaps == null) || (filterMaps.length == 0)) 56 return (filterChain); 57 58 // Acquire the information we will need to match filter mappings 59 String servletName = wrapper.getName(); 60 61 // Add the relevant path-mapped filters to this filter chain 62 for (int i = 0; i < filterMaps.length; i++) { 63 if (!matchDispatcher(filterMaps[i] ,dispatcher)) { 64 continue; 65 } 66 if (!matchFiltersURL(filterMaps[i], requestPath)) 67 continue; 68 ApplicationFilterConfig filterConfig = (ApplicationFilterConfig) 69 context.findFilterConfig(filterMaps[i].getFilterName()); 70 if (filterConfig == null) { 71 // FIXME - log configuration problem 72 continue; 73 } 74 boolean isCometFilter = false; 75 if (comet) { 76 try { 77 isCometFilter = filterConfig.getFilter() instanceof CometFilter; 78 } catch (Exception e) { 79 // Note: The try catch is there because getFilter has a lot of 80 // declared exceptions. However, the filter is allocated much 81 // earlier 82 Throwable t = ExceptionUtils.unwrapInvocationTargetException(e); 83 ExceptionUtils.handleThrowable(t); 84 } 85 if (isCometFilter) { 86 filterChain.addFilter(filterConfig); 87 } 88 } else { 89 filterChain.addFilter(filterConfig); 90 } 91 } 92 93 // Add filters that match on servlet name second 94 for (int i = 0; i < filterMaps.length; i++) { 95 if (!matchDispatcher(filterMaps[i] ,dispatcher)) { 96 continue; 97 } 98 if (!matchFiltersServlet(filterMaps[i], servletName)) 99 continue;100 ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)101 context.findFilterConfig(filterMaps[i].getFilterName());102 if (filterConfig == null) {103 // FIXME - log configuration problem104 continue;105 }106 boolean isCometFilter = false;107 if (comet) {108 try {109 isCometFilter = filterConfig.getFilter() instanceof CometFilter;110 } catch (Exception e) {111 // Note: The try catch is there because getFilter has a lot of 112 // declared exceptions. However, the filter is allocated much113 // earlier114 }115 if (isCometFilter) {116 filterChain.addFilter(filterConfig);117 }118 } else {119 filterChain.addFilter(filterConfig);120 }121 }122 123 // Return the completed filter chain124 return (filterChain);125 126 }
可以将如上代码分为两段，51行之前为第一段，51行之后为第二段。
第一段的主要目的是创建ApplicationFilterChain对象以及一些参数设置。
第二段的主要目的是从上下文中获取所有Filter信息，之后使用for循环遍历并调用filterChain.addFilter(filterConfig);将filterConfig放入ApplicationFilterChain对象的ApplicationFilterConfig数组中。
那ApplicationFilterFactory类的createFilterChain()方法又是在什么地方被调用的呢？
是在StandardWrapperValue类的invoke()方法中被调用的。
由于invoke()方法较长，所以将很多地方省略。
public final void invoke(Request request, Response response) throws IOException, ServletException { ...省略中间代码 // Create the filter chain for this request ApplicationFilterFactory factory = ApplicationFilterFactory.getInstance(); ApplicationFilterChain filterChain = factory.createFilterChain(request, wrapper, servlet); ...省略中间代码 filterChain.doFilter(request.getRequest(), response.getResponse()); ...省略中间代码 }
那正常的流程应该是这样的：
在StandardWrapperValue类的invoke()方法中调用ApplicationFilterChai类的createFilterChain()方法———>在ApplicationFilterChai类的createFilterChain()方法中调用ApplicationFilterChain类的addFilter()方法———>在ApplicationFilterChain类的addFilter()方法中给ApplicationFilterConfig数组赋值。
根据上面的代码可以看出StandardWrapperValue类的invoke()方法在执行完createFilterChain()方法后，会继续执行ApplicationFilterChain类的doFilter()方法，然后在doFilter()方法中会调用internalDoFilter()方法。
以下是internalDoFilter()方法的部分代码
// Call the next filter if there is one if (pos < n) { //拿到下一个Filter，将指针向下移动一位 //pos它来标识当前ApplicationFilterChain（当前过滤器链）执行到哪个过滤器 ApplicationFilterConfig filterConfig = filters[pos++]; Filter filter = null; try { //获取当前指向的Filter的实例 filter = filterConfig.getFilter(); support.fireInstanceEvent(InstanceEvent.BEFORE_FILTER_EVENT, filter, request, response); 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 { //调用Filter的doFilter()方法 filter.doFilter(request, response, this); }
这里的filter.doFilter(request, response, this);就是调用我们前面创建的TestFilter中的doFilter()方法。而TestFilter中的doFilter()方法会继续调用chain.doFilter(request, response);方法，而这个chain其实就是ApplicationFilterChain,所以调用过程又回到了上面调用dofilter和调用internalDoFilter方法，这样执行直到里面的过滤器全部执行。
如果定义两个过滤器，则Debug结果如下：