深入分析struts2的ognl表达式。前面的2篇文章已经详细的分析了ognl获取数据的过程和设置数据的过程。其中最重要的点是ognl的三要素：
（1）表达式（Expression）表达式是整个OGNL的核心，所有的OGNL操作都是针对表达式的解析后进行的。简介的概括“干什么”
（2）根对象（Root Object） 根对象可以理解为OGNL的操作对象。简介的概括“对谁干”或者“对谁操作”
（3）上下文环境（Context） OGNL的内部，所有的操作都会在一个特定的环境中运行，简介的概括“在哪干”或者“在哪操作”
如果明白了ognl的三要素，下面就好理解了。
《刨根问底-struts-serviceAction（）创建并执行action》中分析了执行action方法之前，循环执行拦截器，其中ParametersInterceptor拦截器主要工作：把jsp中传来的数值赋值到action类中的属性。现在就看看ParametersInterceptor类doIntercept（）方法
1、ParametersInterceptor类doIntercept（）代码：
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@Override public String doIntercept(ActionInvocation invocation) throws Exception { Object action = invocation.getAction(); if (!(action instanceof NoParameters)) { ActionContext ac = invocation.getInvocationContext(); final Map<String, Object> parameters = retrieveParameters(ac); if (LOG.isDebugEnabled()) { LOG.debug("Setting params " + getParameterLogMap(parameters)); } if (parameters != null) { Map<String, Object> contextMap = ac.getContextMap(); try { ReflectionContextState.setCreatingNullObjects(contextMap, true); ReflectionContextState.setDenyMethodExecution(contextMap, true); ReflectionContextState.setReportingConversionErrors(contextMap, true); ValueStack stack = ac.getValueStack(); setParameters(action, stack, parameters); } finally { ReflectionContextState.setCreatingNullObjects(contextMap, false); ReflectionContextState.setDenyMethodExecution(contextMap, false); ReflectionContextState.setReportingConversionErrors(contextMap, false); } } } return invocation.invoke(); } |
注释：（1）Object action = invocation.getAction()，获取action对象，在这里就是Login，其中属性userName和passwod都为空。
（2）判断action是否属于NoParameters接口，如果是什么不处理
（3）ActionContext ac = invocation.getInvocationContext()，获取上下文对象ActionContext ，作为struts2执行Action时的上下文，存储了action在执行时需要用到的对象。如我们需要关注的ServletContext，Session， HttpServletRequest ，parameters等。
ActionContext类中有Map context全局变量，把所有的数据都保存到其中。
现在贴出context中的值：
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{com.opensymphony.xwork2.dispatcher.HttpServletRequest=org.apache.struts2.dispatcher.StrutsRequestWrapper@e9c592, application={org.apache.catalina.resources=org.apache.naming.resources.ProxyDirContext@187b5ff, org.apache.AnnotationProcessor=org.apache.catalina.util.DefaultAnnotationProcessor@bdb6ae, org.apache.catalina.jsp_classpath=/E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/classes/; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/activemq-all-5.6.0.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/commons-beanutils-1.7.0.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/commons-dbutils-1.3.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/commons-io-1.4.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/commons-lang-2.1.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/commons-logging-1.0.4.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/freemarker-2.3.9.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/log4j_128.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/mysql-connector-java-5.1.9-bin.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/ognl-2.6.11.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/struts2-core-2.0.14.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/xwork-2.0.7.jar; /E:/javahome/apache-tomcat-6.0.29/lib/; /E:/javahome/apache-tomcat-6.0.29/lib/annotations-api.jar; /E:/javahome/apache-tomcat-6.0.29/lib/catalina-ant.jar; /E:/javahome/apache-tomcat-6.0.29/lib/catalina-ha.jar; /E:/javahome/apache-tomcat-6.0.29/lib/catalina-tribes.jar; /E:/javahome/apache-tomcat-6.0.29/lib/catalina.jar; /E:/javahome/apache-tomcat-6.0.29/lib/el-api.jar; /E:/javahome/apache-tomcat-6.0.29/lib/jasper-el.jar; /E:/javahome/apache-tomcat-6.0.29/lib/jasper-jdt.jar; /E:/javahome/apache-tomcat-6.0.29/lib/jasper.jar; /E:/javahome/apache-tomcat-6.0.29/lib/jsp-api.jar; /E:/javahome/apache-tomcat-6.0.29/lib/servlet-api.jar; /E:/javahome/apache-tomcat-6.0.29/lib/tomcat-coyote.jar; /E:/javahome/apache-tomcat-6.0.29/lib/tomcat-dbcp.jar; /E:/javahome/apache-tomcat-6.0.29/lib/tomcat-i18n-es.jar; /E:/javahome/apache-tomcat-6.0.29/lib/tomcat-i18n-fr.jar; /E:/javahome/apache-tomcat-6.0.29/lib/tomcat-i18n-ja.jar; /E:/javahome/apache-tomcat-6.0.29/bin/bootstrap.jar; /E:/javahome/jdk/lib/tools.jar; /E:/javahome/jdk/jre/lib/ext/dnsns.jar; /E:/javahome/jdk/jre/lib/ext/localedata.jar; /E:/javahome/jdk/jre/lib/ext/sunjce_provider.jar; /E:/javahome/jdk/jre/lib/ext/sunmscapi.jar; /E:/javahome/jdk/jre/lib/ext/sunpkcs11.jar, org.apache.jasper.runtime.JspApplicationContextImpl=org.apache.jasper.runtime.JspApplicationContextImpl@f97d27,org.apache.catalina.WELCOME_FILES=[Ljava.lang.String;@17b6074, javax.servlet.context.tempdir=E:\javahome\workspace\.metadata\.plugins\org.eclipse.wst.server.core\tmp0\work\Catalina\localhost\strutsDemo}, com.opensymphony.xwork2.ActionContext.locale=zh_CN,com.opensymphony.xwork2.dispatcher.HttpServletResponse=org.apache.catalina.connector.ResponseFacade@12478a9,com.opensymphony.xwork2.ActionContext.name=doLogin, current.property.path=null,com.opensymphony.xwork2.ActionContext.application={org.apache.catalina.resources=org.apache.naming.resources.ProxyDirContext@187b5ff,org.apache.AnnotationProcessor=org.apache.catalina.util.DefaultAnnotationProcessor@bdb6ae, org.apache.catalina.jsp_classpath=/E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/classes/; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/activemq-all-5.6.0.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/commons-beanutils-1.7.0.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/commons-dbutils-1.3.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/commons-io-1.4.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/commons-lang-2.1.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/commons-logging-1.0.4.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/freemarker-2.3.9.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/log4j_128.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/mysql-connector-java-5.1.9-bin.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/ognl-2.6.11.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/struts2-core-2.0.14.jar; /E:/javahome/workspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/strutsDemo/WEB-INF/lib/xwork-2.0.7.jar; /E:/javahome/apache-tomcat-6.0.29/lib/;/E:/javahome/apache-tomcat-6.0.29/lib/annotations-api.jar; /E:/javahome/apache-tomcat-6.0.29/lib/catalina-ant.jar;/E:/javahome/apache-tomcat-6.0.29/lib/catalina-ha.jar; /E:/javahome/apache-tomcat-6.0.29/lib/catalina-tribes.jar;/E:/javahome/apache-tomcat-6.0.29/lib/catalina.jar; /E:/javahome/apache-tomcat-6.0.29/lib/el-api.jar;/E:/javahome/apache-tomcat-6.0.29/lib/jasper-el.jar; /E:/javahome/apache-tomcat-6.0.29/lib/jasper-jdt.jar;/E:/javahome/apache-tomcat-6.0.29/lib/jasper.jar; /E:/javahome/apache-tomcat-6.0.29/lib/jsp-api.jar; /E:/javahome/apache-tomcat-6.0.29/lib/servlet-api.jar; /E:/javahome/apache-tomcat-6.0.29/lib/tomcat-coyote.jar; /E:/javahome/apache-tomcat-6.0.29/lib/tomcat-dbcp.jar; /E:/javahome/apache-tomcat-6.0.29/lib/tomcat-i18n-es.jar; /E:/javahome/apache-tomcat-6.0.29/lib/tomcat-i18n-fr.jar; /E:/javahome/apache-tomcat-6.0.29/lib/tomcat-i18n-ja.jar; /E:/javahome/apache-tomcat-6.0.29/bin/bootstrap.jar; /E:/javahome/jdk/lib/tools.jar; /E:/javahome/jdk/jre/lib/ext/dnsns.jar; /E:/javahome/jdk/jre/lib/ext/localedata.jar; /E:/javahome/jdk/jre/lib/ext/sunjce_provider.jar; /E:/javahome/jdk/jre/lib/ext/sunmscapi.jar; /E:/javahome/jdk/jre/lib/ext/sunpkcs11.jar, org.apache.jasper.runtime.JspApplicationContextImpl=org.apache.jasper.runtime.JspApplicationContextImpl@f97d27, org.apache.catalina.WELCOME_FILES=[Ljava.lang.String;@17b6074, javax.servlet.context.tempdir=E:\javahome\workspace\.metadata\.plugins\org.eclipse.wst.server.core\tmp0\work\Catalina\localhost\strutsDemo}, last.property.accessed=null, attr=org.apache.struts2.util.AttributeMap@16dc5c9, com.opensymphony.xwork2.dispatcher.ServletContext=org.apache.catalina.core.ApplicationContextFacade@1d433c1, com.opensymphony.xwork2.ActionContext.session={}, com.opensymphony.xwork2.ActionContext.actionInvocation=com.opensymphony.xwork2.DefaultActionInvocation@92015d, session={},com.opensymphony.xwork2.util.ValueStack.ValueStack=com.opensymphony.xwork2.util.OgnlValueStack@197871d, last.bean.accessed=null, request={struts.valueStack=com.opensymphony.xwork2.util.OgnlValueStack@197871d},struts.actionMapping=org.apache.struts2.dispatcher.mapper.ActionMapping@898587, parameters={userName=[Ljava.lang.String;@9d9edd, password=[Ljava.lang.String;@28df48}, com.opensymphony.xwork2.ActionContext.parameters={userName=[Ljava.lang.String;@9d9edd,password=[Ljava.lang.String;@28df48}} |
上面的代码比较多，可以中重点看几个：
com.opensymphony.xwork2.dispatcher.HttpServletRequest=org.apache.struts2.dispatcher.StrutsRequestWrapper@e9c592
com.opensymphony.xwork2.dispatcher.HttpServletResponse=org.apache.catalina.connector.ResponseFacade@12478a9
com.opensymphony.xwork2.ActionContext.name=doLogin
com.opensymphony.xwork2.dispatcher.ServletContext=org.apache.catalina.core.ApplicationContextFacade@1d433c1,
com.opensymphony.xwork2.util.ValueStack.ValueStack=com.opensymphony.xwork2.util.OgnlValueStack@197871d,
parameters={userName=[Ljava.lang.String;@9d9edd, password=[Ljava.lang.String;@28df48},
这都是常用到的几个对象，尤其是parameters，其中保存了userName和password值
（4）Map parameters = ac.getParameters()，在ac对象的context中获取parameters 。
（5）Map contextMap = ac.getContextMap()，获取ac对象的context，在这是ognl.OgnlContext，如果这里为什么可以是OgnlContext有疑问，请看前面的几篇文章。简单的说一句OgnlContext 实现map接口。
（6） ValueStack stack = ac.getValueStack()，获得ValueStack ，这里是com.opensymphony.xwork2.util.OgnlValueStack
（7）setParameters设置参数
2、setParameters（）代码：
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protected void setParameters(Object action, ValueStack stack, final Map<String, Object> parameters) { ParameterNameAware parameterNameAware = (action instanceof ParameterNameAware) ? (ParameterNameAware) action : null; Map<String, Object> params; Map<String, Object> acceptableParameters; if (ordered) { params = new TreeMap<String, Object>(getOrderedComparator()); acceptableParameters = new TreeMap<String, Object>(getOrderedComparator()); params.putAll(parameters); } else { params = new TreeMap<String, Object>(parameters); acceptableParameters = new TreeMap<String, Object>(); } for (Map.Entry<String, Object> entry : params.entrySet()) { String name = entry.getKey(); boolean acceptableName = acceptableName(name) && (parameterNameAware == null || parameterNameAware.acceptableParameterName(name)); if (acceptableName) { acceptableParameters.put(name, entry.getValue()); } } ValueStack newStack = valueStackFactory.createValueStack(stack); boolean clearableStack = newStack instanceof ClearableValueStack; if (clearableStack) { //if the stack's context can be cleared, do that to prevent OGNL //from having access to objects in the stack, see XW-641 ((ClearableValueStack)newStack).clearContextValues(); Map<String, Object> context = newStack.getContext(); ReflectionContextState.setCreatingNullObjects(context, true); ReflectionContextState.setDenyMethodExecution(context, true); ReflectionContextState.setReportingConversionErrors(context, true); } boolean memberAccessStack = newStack instanceof MemberAccessValueStack; if (memberAccessStack) { //block or allow access to properties //see WW-2761 for more details MemberAccessValueStack accessValueStack = (MemberAccessValueStack) newStack; accessValueStack.setAcceptProperties(acceptParams); accessValueStack.setExcludeProperties(excludeParams); } for (Map.Entry<String, Object> entry : acceptableParameters.entrySet()) { String name = entry.getKey(); Object value = entry.getValue(); try { newStack.setValue(name, value); } catch (RuntimeException e) { if (devMode) { String developerNotification = LocalizedTextUtil.findText(ParametersInterceptor.class, "devmode.notification", ActionContext.getContext().getLocale(), "Developer Notification:\n{0}", new Object[]{ "Unexpected Exception caught setting '" + name + "' on '" + action.getClass() + ": " + e.getMessage() }); LOG.error(developerNotification); if (action instanceof ValidationAware) { ((ValidationAware) action).addActionMessage(developerNotification); } } } } if (clearableStack && (stack.getContext() != null) && (newStack.getContext() != null)) stack.getContext().put(ActionContext.CONVERSION_ERRORS, newStack.getContext().get(ActionContext.CONVERSION_ERRORS)); addParametersToContext(ActionContext.getContext(), acceptableParameters); } |
注释：（1）判断action是否实现ParameterNameAware接口，如果没有创建TreeMap，并且赋值给params
（2）ValueStack newStack = ValueStackFactory.getFactory().createValueStack(stack)，获得一个valueStack。 ValueStackFactory.getFactory()获得是OgnlValueStackFactory。
（3）前面代码略过，请看重要的一行 newStack.setValue(name, value)，为什么在OgnlValueStack设置呢？很奇怪！继续看
3、OgnlValueStack类setValue（）代码：
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public void setValue(String expr, Object value) { setValue(expr, value, devMode); } |
注释：很简单，继续看。
4、setValue（）代码：
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public void setValue(String expr, Object value, boolean throwExceptionOnFailure) { Map<String, Object> context = getContext(); try { context.put(XWorkConverter.CONVERSION_PROPERTY_FULLNAME, expr); context.put(REPORT_ERRORS_ON_NO_PROP, (throwExceptionOnFailure) ? Boolean.TRUE : Boolean.FALSE); ognlUtil.setValue(expr, context, root, value); } catch (OgnlException e) { if (throwExceptionOnFailure) { e.printStackTrace(System.out); System.out.println("expr: " + expr + " val: " + value + " context: " + context + " root:" + root + " value: " + value); String msg = "Error setting expression '" + expr + "' with value '" + value + "'"; throw new XWorkException(msg, e); } else { if (LOG.isWarnEnabled()) { LOG.warn("Error setting value", e); } } } catch (RuntimeException re) { //XW-281 if (throwExceptionOnFailure) { StringBuilder msg = new StringBuilder(); msg.append("Error setting expression '"); msg.append(expr); msg.append("' with value "); if (value instanceof Object[]) { Object[] valueArray = (Object[]) value; msg.append("["); for (int index = 0; index < valueArray.length; index++) { msg.append("'"); msg.append(valueArray[index]); msg.append("'"); if (index < (valueArray.length + 1)) msg.append(", "); } msg.append("]"); } else { msg.append("'"); msg.append(value); msg.append("'"); } throw new XWorkException(msg.toString(), re); } else { if (LOG.isWarnEnabled()) { LOG.warn("Error setting value", re); } } } finally { ReflectionContextState.clear(context); context.remove(XWorkConverter.CONVERSION_PROPERTY_FULLNAME); context.remove(REPORT_ERRORS_ON_NO_PROP); } } |
注释：到这里OgnlUtil.setValue(expr, context, this.root, value)，在继续就是ognl框架执行的代码了，这里已经准备了ognl的三要素：expr表达式，context上下文，this.root 操作对象。但是这里为什么是root，为什么不是直接传递action对象呢？请继续看：
OgnlValueStack类中重要部分代码
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public class OgnlValueStack implements Serializable, ValueStack, ClearableValueStack, MemberAccessValueStack { CompoundRoot root; transient Map<String, Object> context; Map<Object, Object> overrides; private static CompoundRootAccessor accessor;/* */ public OgnlValueStack(ValueStack vs) {/* 104 */ setRoot(new CompoundRoot(vs.getRoot()));/* */ } public Map getContext()/* */ {/* 126 */ return this.context;/* */ } public CompoundRoot getRoot()/* */ {/* 159 */ return this.root;/* */ } |
注释：（1）OgnlValueStack中变量context就是上面解释的OgnlContext对象。
（2）ValueStack中的root对象是CompoundRoot，CompoundRoot继承了ArraryList，提供了额外的方法：push()和pop()方法，用来对root对象中所包含的数据进行存取！
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public class CompoundRoot extends ArrayList { public CompoundRoot() { } public CompoundRoot(List list) { super(list); } public CompoundRoot cutStack(int index) { return new CompoundRoot(subList(index, size())); } public Object peek() { return get(0); } public Object pop() { return remove(0); } public void push(Object o) { add(0, o); }} |
正是通过这两个方法，CompoundRoot变成了一个栈结构！压栈操作，将导致对象被放到CompoundRoot的第0个元素上（第0个元素是栈顶），其它对象被依次往后移动；出栈操作，将导致CompoundRoot的第0个元素被移除（即栈顶元素被弹出），其它对象被依次往前移动！
OGNL不支持多个root对象，而struts2能够支持多个root对象，它对OGNL做了扩展。
如果某个OGNL表达式被传递给ValueStack（即调用ValueStack的setValue或findValue方法），而表达式中包含有对root对象的访问操作，ValueStack将依次从栈顶往栈底搜索CompoundRoot对象中所包含的对象，看哪个对象具有相应的属性，找到之后，立刻返回。
在Struts2中，一个请求在最终到达Action的方法之前，Action对象本身会被压入ValueStack（实际上就是放到ValueStack的CompoundRoot中），所以Action对象是CompoundRoot中的一个元素。
（3）CompoundRoot保存了action对象，那什么时候取出来执行呢？CompoundRootAccessor就派上用场了。当问某个属性时，CompoundRootAccessor对象实例会负责在CompoundRoot对象中找到包含我们指定属性的对象。简单的看一下CompoundRootAccessor类中的setProperty（）方法：
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public void setProperty(Map context, Object target, Object name, Object value) throws OgnlException { CompoundRoot root = (CompoundRoot) target; OgnlContext ognlContext = (OgnlContext) context; for (Object o : root) { if (o == null) { continue; } try { if (OgnlRuntime.hasSetProperty(ognlContext, o, name)) { OgnlRuntime.setProperty(ognlContext, o, name, value); return; } else if (o instanceof Map) { Map<Object, Object> map = (Map) o; map.put(name, value); return; }// } catch (OgnlException e) {// if (e.getReason() != null) {// final String msg = "Caught an Ognl exception while setting property " + name;// log.error(msg, e);// throw new RuntimeException(msg, e.getReason());// } } catch (IntrospectionException e) { // this is OK if this happens, we'll just keep trying the next } } Boolean reportError = (Boolean) context.get(ValueStack.REPORT_ERRORS_ON_NO_PROP); final String msg = "No object in the CompoundRoot has a publicly accessible property named '" + name + "' (no setter could be found)."; if ((reportError != null) && (reportError.booleanValue())) { throw new XWorkException(msg); } else { if (devMode) { LOG.warn(msg); } } } |
首先会把target转换成CompoundRoot对象。
把context转换成OgnlContext对象。
遍历 类型为CompoundRoot的root 对象，上面解释道CompoundRoot保存里action对象，所以获得action对象，进行属性值设置，OgnlRuntime.setProperty(ognlContext, o, name, value)。
到这里整体分析完成了。
http://my.oschina.net/winHerson/blog/110152