Struts2的Action类执行Hibernate动态绑定查询报出NullPointerException

今天在用Struts2做东西，我的意图是要做一个查询所有信息的方法，这个方法是用hibernate中的动态绑定查询条件去做的。

方法如下:

//动态查询拍卖物品
public List<Product> queryProduct(Product p,int beginnum,int pagesize){
Criteria c = se.createCriteria(Product.class);
if(p.getUserlist().getUId()!=null && !p.getUserlist().getUId().equals("")){
c.add(Restrictions.eq("userlist.UId", p.getUserlist().getUId()));
}
c.addOrder(Order.desc("PBegintime"));
c.setFirstResult((beginnum-1)* pagesize);
c.setMaxResults(pagesize);
return c.list();
}

这里面有一个动态绑定查询条件，如果不存在的条件则不加入到查询条件中，在方法

public List<Product> queryProduct(Product p,int beginnum,int pagesize)中存入Product对象（而且也存入Product中的UserList的UId属性），两个int参数，这样的话是正确的，能够正常查询。

但是如果只存入一个Product对象和两个int参数，不再Product对象中存入UserList的UId属性就会报出如下NullPointerException异常：

HTTP Status 500 

---

type Exception report

message

description The server encountered an internal error that prevented it from fulfilling this request.

exception

java.lang.NullPointerExceptiondao.ProductDaoImpl.queryProductSize(ProductDaoImpl.java:45)action.BuyAction.accessToBuy(BuyAction.java:42)sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)java.lang.reflect.Method.invoke(Unknown Source)com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:440)com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:279)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:163)com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:249)org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:122)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195)com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195)com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:148)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:93)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:235)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:89)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:128)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)org.apache.struts2.interceptor.ProfilingActivationInterceptor.intercept(ProfilingActivationInterceptor.java:104)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:267)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:126)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:138)com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:148)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:128)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:176)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:52)org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:468)org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:76)filter.CharsetFilter.doFilter(CharsetFilter.java:37)

note The full stack trace of the root cause is available in the Apache Tomcat/7.0.53 logs.

个人理解：

在我们的Action类中声明一个实体对象A，如果这个实体对象A中还持有其他实体对象B（hibernate中通过对象或集合的持有表达主外键关系），而且我们对这个实体对象A中的实体对象B的相关属性去做操作，Struts2就映射并绑定了他们，就不能为空。这可能和Struts2的内在实现机制有关，因为无论是hibernate还是Struts2，这些框架的最核心就是通过映射方式去完成操作，了解内在实现会有助于我们更好的使用它们。