java.lang.NullPointerException com.action.StudentAction.list(StudentAction.java:20) sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) java.lang.reflect.Method.invoke(Method.java:498) ognl.OgnlRuntime.invokeMethod(OgnlRuntime.java:899) ognl.OgnlRuntime.callAppropriateMethod(OgnlRuntime.java:1544) ognl.ObjectMethodAccessor.callMethod(ObjectMethodAccessor.java:68) com.opensymphony.xwork2.ognl.accessor.XWorkMethodAccessor.callMethodWithDebugInfo(XWorkMethodAccessor.java:98) com.opensymphony.xwork2.ognl.accessor.XWorkMethodAccessor.callMethod(XWorkMethodAccessor.java:90) ognl.OgnlRuntime.callMethod(OgnlRuntime.java:1620) ognl.ASTMethod.getValueBody(ASTMethod.java:91) ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:212) ognl.SimpleNode.getValue(SimpleNode.java:258) ognl.Ognl.getValue(Ognl.java:470) ognl.Ognl.getValue(Ognl.java:434) com.opensymphony.xwork2.ognl.OgnlUtil$3.execute(OgnlUtil.java:374) com.opensymphony.xwork2.ognl.OgnlUtil.compileAndExecuteMethod(OgnlUtil.java:426) com.opensymphony.xwork2.ognl.OgnlUtil.callMethod(OgnlUtil.java:372) com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:438) com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:293) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:254) org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:250) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:179) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:99) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:263) org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:49) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:99) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.doIntercept(ConversionErrorInterceptor.java:142) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:99) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:137) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:99) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:137) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:99) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:201) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:67) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) org.apache.struts2.interceptor.DateTextFieldInterceptor.intercept(DateTextFieldInterceptor.java:133) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:89) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:243) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:101) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:142) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:160) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:175) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:99) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) org.apache.struts2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:121) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:167) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:203) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:196) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249) org.apache.struts2.factory.StrutsActionProxy.execute(StrutsActionProxy.java:48) org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:574) org.apache.struts2.dispatcher.ExecuteOperations.executeAction(ExecuteOperations.java:79) org.apache.struts2.dispatcher.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:141)
这个错误，网上搜了一些，原因有几种：
1、struts.xml里action类名有没有拼写错误，
2、struts.xml 里边extends="struts-default"有没有写错
3、你的method是不是public的？属性是private的？（常识）
4、是页面表单的属性与后台属性的不一致导致的
我的错误原因，是ApplicationContext.xml里出问题了
<bean id="sd" class="com.dao.impl.StudentDaoImpl">
<property name="sessionFactory" ref="sessionFactory"/>
</bean>
<bean id="stubiz" class="com.biz.impl.StudentBizImpl">
<property name="sd" ref="sd"/>
</bean>
看似没问题，但id="stubiz"里的stubiz错了，因为在struts2-spring-plugin-2.5.16.jar里的struts-plugin.xml文件中有配置
<struts>
<bean type="com.opensymphony.xwork2.ObjectFactory" name="spring"
class="org.apache.struts2.spring.StrutsSpringObjectFactory" />
<!-- Make the Spring object factory the automatic default -->
<constant name="struts.objectFactory" value="spring" />
……
</struts>
意思就是默认的action里的biz属性注入是spring注入，但这意味着需要和action里定义的biz属性一致。
而我定义的biz属性名为：
public class StudentAction extends BaseAction{
private StudentBiz sb;
……（略）
}
一个红红的“sb”，所以报错。如果我在后面加上一个专门给action注入值的配置：
<bean id="studentAction" class="com.action.StudentAction">
<property name="sb" ref="stubiz"/>
</bean>
就没错了。
调了老半天，找了好久百度，也没看出个所以然，然后自己突然悟出来的
顺便吐槽一下下面这个链接里的家伙，自己问题解决了也不说一下怎么解决的，害的我把整个帖子都认认真真看完了也没看到解决方法
。。。