Struts2是一种基于MVC模式的javaWeb框架，本质上相当于Servlet。
所谓MVC，就是模型-视图-控制器。
Model（模型）表示应用程序核心（比如数据库记录列表）。
View（视图）显示数据（数据库记录）。
Controller（控制器）处理输入（写入数据库记录）
而Struts的作用实际上是作为控制器，建立模型层和视图层的数据交互（就是通常所说控制model和jsp之间的数据交互）
上面都是废话，你只要知道struts2能控制model和jsp之间的数据交互即可。
web程序想要使用struts2框架，除了要引用相关jar包外，还要在web.xml中对struts2进行配置，只有配置后，struts2才能对浏览器的请求进行一系列处理。
怎么在web应用中配置struts2?
首先在web.xml中添加struts2的使用，代码如下：
<!-- 配置Struts2 核心 Filter -->
<filter>
<filter-name>action2</filter-name>
<filter-class>org.apache.struts2.dispatcher.filter.StrutsPrepareAndExecuteFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>action2</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
上述代码使用了Filter，那我们就回顾一下Filter的作用。
回顾：Filter，中文名为过滤器，通过Filter可以对web服务器的资源进行管理，例如Jsp,Servlet, 静态图片文件等进行拦截，从而实现一些特殊的功能。
实现URL级别的权限访问控制、过滤敏感词汇、压缩响应信息等一些高级功能
从代码层面来讲，只要创建一个实现了Filter接口的实现类，然后将这个类在web.xml进行配置，那么这个类就可以让用户在访问某个目标资源之前，对访问的请求和响应进行拦截。
更多介绍可以看我的另外一篇文章：http://blog.csdn.net/u012605477/article/details/75258358
注意：Struts2在web.xml中看来就是一个Filter，只不过其实现类StrutsPrepareAndExecuteFilter不是我们写的，是Struts2的。
为了验证StrutsPrepareAndExecuteFilter也实现了Filter接口，我们将其和我们自写的过滤器进行对比：
下面是自写的过滤器FilterA配置：
<filter>
<filter-name>FilterA</filter-name>
<filter-class>com.strategy.jpa.FilterA</filter-class>
</filter>
<filter-mapping>
<filter-name>FilterA</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
自写FilterA的代码：
package com.strategy.jpa;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
public class FilterA implements Filter {
public void init(FilterConfig filterConfig) throws ServletException {
}
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
chain.doFilter(request, response);
response.reset();
}
public void destroy() {
}
}
web中配置Struts2的代码：
* $Id: DefaultActionSupport.java 651946 2008-04-27 13:41:38Z apetrelli $
package org.apache.struts2.dispatcher.filter;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.struts2.RequestUtils;
import org.apache.struts2.StrutsStatics;
import org.apache.struts2.dispatcher.Dispatcher;
import org.apache.struts2.dispatcher.mapper.ActionMapping;
import org.apache.struts2.dispatcher.ExecuteOperations;
import org.apache.struts2.dispatcher.InitOperations;
import org.apache.struts2.dispatcher.PrepareOperations;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.List;
import java.util.regex.Pattern;
/**
* Handles both the preparation and execution phases of the Struts dispatching process. This filter is better to use
* when you don't have another filter that needs access to action context information, such as Sitemesh.
*/
public class StrutsPrepareAndExecuteFilter implements StrutsStatics, Filter {
private static final Logger LOG = LogManager.getLogger(StrutsPrepareAndExecuteFilter.class);
protected PrepareOperations prepare;
protected ExecuteOperations execute;
protected List<Pattern> excludedPatterns = null;
public void init(FilterConfig filterConfig) throws ServletException {
InitOperations init = new InitOperations();
Dispatcher dispatcher = null;
try {
FilterHostConfig config = new FilterHostConfig(filterConfig);
init.initLogging(config);
dispatcher = init.initDispatcher(config);
init.initStaticContentLoader(config, dispatcher);
prepare = new PrepareOperations(dispatcher);
execute = new ExecuteOperations(dispatcher);
this.excludedPatterns = init.buildExcludedPatternsList(dispatcher);
postInit(dispatcher, filterConfig);
} finally {
if (dispatcher != null) {
dispatcher.cleanUpAfterInit();
}
init.cleanup();
}
}
/**
* Callback for post initialization
*
* @param dispatcher the dispatcher
* @param filterConfig the filter config
*/
protected void postInit(Dispatcher dispatcher, FilterConfig filterConfig) {
}
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) res;
try {
String uri = RequestUtils.getUri(request);
if (excludedPatterns != null && prepare.isUrlExcluded(request, excludedPatterns)) {
LOG.trace("Request {} is excluded from handling by Struts, passing request to other filters", uri);
chain.doFilter(request, response);
} else {
LOG.trace("Checking if {} is a static resource", uri);
boolean handled = execute.executeStaticResourceRequest(request, response);
if (!handled) {
LOG.trace("Assuming uri {} as a normal action", uri);
prepare.setEncodingAndLocale(request, response);
prepare.createActionContext(request, response);
prepare.assignDispatcherToThread();
request = prepare.wrapRequest(request);
ActionMapping mapping = prepare.findActionMapping(request, response, true);
if (mapping == null) {
LOG.trace("Cannot find mapping for {}, passing to other filters", uri);
chain.doFilter(request, response);
} else {
LOG.trace("Found mapping {} for {}", mapping, uri);
execute.executeAction(request, response, mapping);
}
}
}
} finally {
prepare.cleanupRequest(request);
}
}
public void destroy() {
prepare.cleanupDispatcher();
}
}
对比可知，struts2也实现了Filter接口，只是他比我们自己写的FilterA多实现了一个接口StrutsStatics，其它的没有变化。
我们对上面的代码进行了解：
在Init方法里面，我们看到有一个config对象，该类对象的作用是对FilterConfig进行封装。
package org.apache.struts2.dispatcher.filter;
import org.apache.struts2.util.MakeIterator;
import javax.servlet.FilterConfig;
import javax.servlet.ServletContext;
import java.util.Iterator;
import org.apache.struts2.dispatcher.HostConfig;
/**
* Host configuration that wraps FilterConfig
*/
public class FilterHostConfig implements HostConfig {
private FilterConfig config;
public FilterHostConfig(FilterConfig config) {
this.config = config;
}
public String getInitParameter(String key) {
return config.getInitParameter(key);
}
public Iterator<String> getInitParameterNames() {
return MakeIterator.convert(config.getInitParameterNames());
}
public ServletContext getServletContext() {
return config.getServletContext();
}
}
然后又在Init方法里面通过创建的config对象，并调用initDispatcher方法创建了转发器对象dispatcher对象。以及通过initLogging方法初始化日志记录器。
private Dispatcher createDispatcher( HostConfig filterConfig ) {
Map<String, String> params = new HashMap<>();
for ( Iterator e = filterConfig.getInitParameterNames(); e.hasNext(); ) {
String name = (String) e.next();
String value = filterConfig.getInitParameter(name);
params.put(name, value);
}
return new Dispatcher(filterConfig.getServletContext(), params);
}
public void cleanup() {
ActionContext.setContext(null);
}
public void initLogging( HostConfig filterConfig ) {
String factoryName = filterConfig.getInitParameter("loggerFactory");
if (factoryName != null) {
try {
Class cls = ClassLoaderUtil.loadClass(factoryName, this.getClass());
LoggerFactory fac = (LoggerFactory) cls.newInstance();
LoggerFactory.setLoggerFactory(fac);
} catch ( InstantiationException e ) {
System.err.println("Unable to instantiate logger factory: " + factoryName + ", using default");
e.printStackTrace();
} catch ( IllegalAccessException e ) {
System.err.println("Unable to access logger factory: " + factoryName + ", using default");
e.printStackTrace();
} catch ( ClassNotFoundException e ) {
System.err.println("Unable to locate logger factory class: " + factoryName + ", using default");
e.printStackTrace();
}
}
}
接下来调用了init对象initStaticContentLoader(config, dispatcher);方法加载一些静态资源。
public StaticContentLoader initStaticContentLoader( HostConfig filterConfig, Dispatcher dispatcher ) {
StaticContentLoader loader = dispatcher.getContainer().getInstance(StaticContentLoader.class);
loader.setHostConfig(filterConfig);
return loader;
}
最重要的就是dispatcher，它主要将filter拦截到的请求转入struts2的请求处理模块，我们必须知道这一点。
其次是FilterConfig，它将我们在web.xml中的Filter的配置信息也保存到了dispatcher中。下面是该对象的构造方法。
public Dispatcher(ServletContext servletContext, Map<String, String> initParams) {
this.servletContext = servletContext;
this.initParams = initParams;
}
接下来就是prepare和execute对象，和InitOperations类似，也是进行了封装一些操作，都是截取一部分重要的代码。
public PrepareOperations(Dispatcher dispatcher) {
this.dispatcher = dispatcher;
}
public ActionContext createActionContext(HttpServletRequest request, HttpServletResponse response) {
ActionContext ctx;
Integer counter = 1;
Integer oldCounter = (Integer) request.getAttribute(CLEANUP_RECURSION_COUNTER);
if (oldCounter != null) {
counter = oldCounter + 1;
}
ActionContext oldContext = ActionContext.getContext();
if (oldContext != null) {
// detected existing context, so we are probably in a forward
ctx = new ActionContext(new HashMap<>(oldContext.getContextMap()));
} else {
ValueStack stack = dispatcher.getContainer().getInstance(ValueStackFactory.class).createValueStack();
stack.getContext().putAll(dispatcher.createContextMap(request, response, null));
ctx = new ActionContext(stack.getContext());
}
request.setAttribute(CLEANUP_RECURSION_COUNTER, counter);
ActionContext.setContext(ctx);
return ctx;
}
execute对象
public ExecuteOperations(Dispatcher dispatcher) {
this.dispatcher = dispatcher;
}
public boolean executeStaticResourceRequest(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
// there is no action in this request, should we look for a static resource?
String resourcePath = RequestUtils.getServletPath(request);
if ("".equals(resourcePath) && null != request.getPathInfo()) {
resourcePath = request.getPathInfo();
}
StaticContentLoader staticResourceLoader = dispatcher.getContainer().getInstance(StaticContentLoader.class);
if (staticResourceLoader.canHandle(resourcePath)) {
staticResourceLoader.findStaticResource(resourcePath, request, response);
// The framework did its job here
return true;
} else {
// this is a normal request, let it pass through
return false;
}
}
从上面的代码可以看出，这两个对象封装了请求预处理和请求处理的操作，当处理请求时方法被调用
在上述的代码中有个很重要的方法未罗列出来，那就是Disptcher中得init方法，该方法初始读取一些配置文件，包含我们想要知道的读取struts2.xml的方法。
public void init() {
if (configurationManager == null) {
configurationManager = createConfigurationManager(DefaultBeanSelectionProvider.DEFAULT_BEAN_NAME);
}
try {
init_FileManager();
init_DefaultProperties(); // [1]
init_TraditionalXmlConfigurations(); // [2]
init_LegacyStrutsProperties(); // [3]
init_CustomConfigurationProviders(); // [5]
init_FilterInitParameters() ; // [6]
init_AliasStandardObjects() ; // [7]
Container container = init_PreloadConfiguration();
container.inject(this);
init_CheckWebLogicWorkaround(container);
if (!dispatcherListeners.isEmpty()) {
for (DispatcherListener l : dispatcherListeners) {
l.dispatcherInitialized(this);
}
}
errorHandler.init(servletContext);
} catch (Exception ex) {
LOG.error("Dispatcher initialization failed", ex);
throw new StrutsException(ex);
}
}
上面是init方法，里面调用了诸多读取配置文件的方法，包含我们想要知道的读取struts.xml的方法：
private void init_TraditionalXmlConfigurations() {
String configPaths = initParams.get("config");
if (configPaths == null) {
configPaths = DEFAULT_CONFIGURATION_PATHS;
}
String[] files = configPaths.split("\s*[,]\s*");
for (String file : files) {
if (file.endsWith(".xml")) {
if ("xwork.xml".equals(file)) {
configurationManager.addContainerProvider(createXmlConfigurationProvider(file, false));
} else {
configurationManager.addContainerProvider(createStrutsXmlConfigurationProvider(file, false, servletContext));
}
} else {
throw new IllegalArgumentException("Invalid configuration file name");
}
}
}
在上面的代码中，设定了读取struts.xml的默认路径，其内容如下：
private static final String DEFAULT_CONFIGURATION_PATHS = "struts-default.xml,struts-plugin.xml,struts.xml";
至此，web应用配置struts2框架，以及struts2如何将Filter封装成struts2的，甚至如何读取struts.xml以及其他配置，并进行相关struts2参数初始化都有了大概了解，接下来说一下Action。
回到StrutsPrepareAndExecuteFilter类的doFilter中，
//每次发送一个Request，StrutsPrepareAndExecuteFilter都会调用doFilter方法
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) res;
try {
//设置编码和国际化
prepare.setEncodingAndLocale(request, response);
//ActionContext创建
prepare.createActionContext(request, response);
prepare.assignDispatcherToThread();
if ( excludedPatterns != null && prepare.isUrlExcluded(request, excludedPatterns)) {
chain.doFilter(request, response);
} else {
request = prepare.wrapRequest(request);
ActionMapping mapping = prepare.findActionMapping(request, response, true);
//如果找不到对应的action配置
if (mapping == null) {
/*
* 就是如果path是以“/struts”开头，则到初始参数packages配置的包路径去查找对应的静态资源并输出到页面流中，
* 当然.class文件除外。如果再没有则跳转到404
*/
boolean handled = execute.executeStaticResourceRequest(request, response);
if (!handled) {
chain.doFilter(request, response);
}
} else {
/*
* 找到对应action配置文件后，调用ExecuteOperations类中executeAction，
* 开始谳用Action的方法。
*/
execute.executeAction(request, response, mapping);
}
}
} finally {
prepare.cleanupRequest(request);
}
}
相关准备（prepare）方法，设置编码的方法不再叙述（setEncodingAndLocale），我们主要说一下ActionContext。
prepare.createActionContext(request, response);
ActionContext是一个struts2容器，主要存储request、session、application、parameters等相关信息。
ActionContext是一个线程的本地变量，这意味着不同的action之间不会共享ActionContext，所以也不用考虑线程安全问题
public ActionContext createActionContext(HttpServletRequest request, HttpServletResponse response) {
ActionContext ctx;
Integer counter = 1;
Integer oldCounter = (Integer) request.getAttribute(CLEANUP_RECURSION_COUNTER);
if (oldCounter != null) {
counter = oldCounter + 1;
}
ActionContext oldContext = ActionContext.getContext();
if (oldContext != null) {
// detected existing context, so we are probably in a forward
ctx = new ActionContext(new HashMap<>(oldContext.getContextMap()));
} else {
ValueStack stack = dispatcher.getContainer().getInstance(ValueStackFactory.class).createValueStack();
stack.getContext().putAll(dispatcher.createContextMap(request, response, null));
ctx = new ActionContext(stack.getContext());
}
request.setAttribute(CLEANUP_RECURSION_COUNTER, counter);
ActionContext.setContext(ctx);
return ctx;
}
我们再看一下ActionContext是什么，取部分代码，由此可知ActionContext是一个键值对集合，跟Spring中的bean类似：
public class ActionContext implements Serializable {
static ThreadLocal<ActionContext> actionContext = new ThreadLocal<>();
private Map<String, Object> context;
public ActionContext(Map<String, Object> context) {
this.context = context;
}
有了创建Action的方法，肯定有使用Action的方法，我们现在去找使用的方法：
public void serviceAction(HttpServletRequest request, HttpServletResponse response, ActionMapping mapping)
throws ServletException {
Map<String, Object> extraContext = createContextMap(request, response, mapping);
// 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);
boolean nullStack = stack == null;
if (nullStack) {
ActionContext ctx = ActionContext.getContext();
if (ctx != null) {
stack = ctx.getValueStack();
}
}
if (stack != null) {
extraContext.put(ActionContext.VALUE_STACK, valueStackFactory.createValueStack(stack));
}
String timerKey = "Handling request from Dispatcher";
try {
UtilTimerStack.push(timerKey);
String namespace = mapping.getNamespace();
String name = mapping.getName();
String method = mapping.getMethod();
ActionProxy proxy = getContainer().getInstance(ActionProxyFactory.class).createActionProxy(
namespace, name, method, extraContext, true, false);
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 (!nullStack) {
request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, stack);
}
} catch (ConfigurationException e) {
logConfigurationException(request, e);
sendError(request, response, HttpServletResponse.SC_NOT_FOUND, e);
} catch (Exception e) {
if (handleException || devMode) {
sendError(request, response, HttpServletResponse.SC_INTERNAL_SERVER_ERROR, e);
} else {
throw new ServletException(e);
}
} finally {
UtilTimerStack.pop(timerKey);
}
}
在上面的try语句里面我们可以得知，struts是怎么通过命名空间，读取action的配置，以及action的method方法，
尤其是如何从容器中获得ActionProxyFactory代理工厂 ，是怎么创建ActionProxy来执行一个特定的命名空间和动作的。
以及怎么通过我们在struts2中设定的跳转方法，跳转到指定页面的。
//执行execute方法，并转向结果
if (mapping.getResult() != null) {
Result result = mapping.getResult();
result.execute(proxy.getInvocation());
} else {
proxy.execute();
}
至此，对于Struts2是如何工作的，怎么读取xml的，如何搭建在web应用中基本叙述完了，更详细的实在说不完，各位大佬可以看下下面这位大神的博客，很给力，就是看起来太繁琐
原文链接：https://blog.csdn.net/u012605477/article/details/76129675