struts2 实现原理解析

转自macg

struts2 操作 action servlet,是通过在web.xml中配置一个filter实现的

<filter> <filter-name>struts2</filter-name> <filter-class>org.apache.struts2.dispatcher.FilterDispatcher</filter-class> </filter> <filter-mapping> <filter-name>struts2</filter-name> <url-pattern>/*</url-pattern> </filter-mapping>

注:现在新版本的struts2已经改成StrutsPrepareAndExecuteFilter,但原理大同小异,我们还按FilterDispatcher讲解

<filter-class>

org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter

</filter-class>

而这个Filter FilterDispatcher 实际做了下列工作

public
class FilterDispatcher implements StrutsStatics, Filter

1.filter的init()方法建立一个Dispatcher对象

以后struts2 都是围绕这个Dispatcher对象进行操作

init()方法将参数FilterConfig对象传给Dispatcher对象,实际就是将ServletContext对象传递给Dispatcher对象

而ServletContext包含了各种基本Servlet的运行环境,比如ServletContext提供了getAttribute(), setAttribute() 等方法

所以,实际是Dispatcher对象接管了基本Servlet的一切功能

public void init(FilterConfig filterConfig) throws ServletException { dispatcher = createDispatcher(filterConfig); ... }

protected Dispatcher createDispatcher(FilterConfig filterConfig) { ... return new Dispatcher(filterConfig.getServletContext(), params); }

2.filter的doFilter()方法继续操作前面init()方法建立的作Dispatcher对象,将HttpServletRequest对象和HttpServletResponse对象传入Dispatcher中

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException { HttpServletRequest request = (HttpServletRequest) req; HttpServletResponse response = (HttpServletResponse) res; ServletContext servletContext = getServletContext(); .... dispatcher.serviceAction(request, response, servletContext, mapping); }

Dispatcher对象是Struts2的核心操作对象,它主要实现实现了下面的工作

1.它是线程安全的,即Dispatcher对象支持多线程,且每线程一副本

public class Dispatcher { private static ThreadLocal<Dispatcher> instance = new ThreadLocal<Dispatcher>(); //Store the dispatcher instance for this thread. public static void setInstance(Dispatcher instance) { Dispatcher.instance.set(instance); // Tie the ObjectFactory threadlocal instance to this Dispatcher instance if (instance != null) { Container cont = instance.getContainer(); if (cont != null) { ObjectFactory.setObjectFactory(cont.getInstance(ObjectFactory.class)); } else { LOG.warn("This dispatcher instance doesn't have a container, so the object factory won't be set."); } } else { ObjectFactory.setObjectFactory(null); } } //Provide the dispatcher instance for the current thread. public static Dispatcher getInstance() { return instance.get(); }

2.上边说了,Dispatcher对象的建立/构造,是接受了参数FilterDispatcher过滤器的FilterConfig传来的ServletContext,这样才接管了基本Servlet的一切功能

private ServletContext servletContext; private Map<String, String> initParams; public Dispatcher(ServletContext servletContext, Map<String, String> initParams) { this.servletContext = servletContext; this.initParams = initParams; }

3. 上边说了,FilterDispatcher过滤器的doFilter()方法,调用了Dispatcher对象的serviceAction()方法,并把HttpServletRequest对象和HttpServletResponse对象传入

这个serviceAction()方法,就是整个Strtus2的主引擎.

serviceAction内部完成了多项功能:

配置文件加载,配置初始化

调用ActionProxy对象实现对Action类的执行

ActionProxy对象又会按照Struts2的Stack结构依次执行Inteceptor,action,method

public void serviceAction(HttpServletRequest request, HttpServletResponse response, ServletContext context,ActionMapping mapping) throws ServletException { Map<String, Object> extraContext = createContextMap(request, response, mapping, context); // If there was a previous value stack, then create a new copy and pass it in to be used by the new Action ValueStack stack = (ValueStack) request.getAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY); if (stack != null) { extraContext.put(ActionContext.VALUE_STACK, ValueStackFactory.getFactory().createValueStack(stack)); } try { String namespace = mapping.getNamespace(); String name = mapping.getName(); String method = mapping.getMethod(); Configuration config = configurationManager.getConfiguration(); ActionProxy proxy = config.getContainer().getInstance(ActionProxyFactory.class).createActionProxy( namespace, name, extraContext, true, false); proxy.setMethod(method); request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, proxy.getInvocation().getStack()); // if the ActionMapping says to go straight to a result, do it! if (mapping.getResult() != null) { Result result = mapping.getResult(); result.execute(proxy.getInvocation()); } else { proxy.execute(); } // If there was a previous value stack then set it back onto the request if (stack != null) { request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, stack); } } catch (ConfigurationException e) { } }