使用hibernate容易出现该问题，主要是由于pojo类属性存在级联关系。比如说员工和部门，在员工表里面有部门属性，而在部门表里面有个员工集合，这样就存在了嵌套引用的问题了，就会抛出这个异常。
解决方法很简单，在将每个对象转为json对象的时候用setExcludes函数将级联的属性去除掉就可以了，如下面：
//得到所有部门
//返回json对象字符串
publicStringgetAllDep(){
Listlist=deptDAO.findAll();
JsonConfigconfig=newJsonConfig();
config.setExcludes(newString[]{"emps"});//除去emps属性
Stringjson=JSONArray.fromObject(list,config).toString();
returnjson;
}
//得到所有员工
publicStringgetAllEmp(intid){
Listlist=empDAO.findByProperty("dept.deptId",id);
JsonConfigconfig=newJsonConfig();
config.setExcludes(newString[]{"dept"});//除去dept属性
Stringjson=JSONArray.fromObject(list,config).toString();
returnjson;
}
异常代码如下：
严重:Servlet.service()forservletspringMVCthrewexception net.sf.json.JSONException:Thereisacycleinthehierarchy! atnet.sf.json.util.CycleDetectionStrategy$StrictCycleDetectionStrategy.handleRepeatedReferenceAsObject(CycleDetectionStrategy.java:97) atnet.sf.json.JSONObject._fromBean(JSONObject.java:857) atnet.sf.json.JSONObject.fromObject(JSONObject.java:192) atnet.sf.json.JSONObject._processValue(JSONObject.java:2774) atnet.sf.json.JSONObject._setInternal(JSONObject.java:2798) atnet.sf.json.JSONObject.setValue(JSONObject.java:1507) atnet.sf.json.JSONObject._fromBean(JSONObject.java:940) atnet.sf.json.JSONObject.fromObject(JSONObject.java:192) atnet.sf.json.JSONArray._processValue(JSONArray.java:2557) atnet.sf.json.JSONArray.processValue(JSONArray.java:2588) atnet.sf.json.JSONArray.addValue(JSONArray.java:2575) atnet.sf.json.JSONArray._fromCollection(JSONArray.java:1082) atnet.sf.json.JSONArray.fromObject(JSONArray.java:145) atnet.sf.json.JSONObject._processValue(JSONObject.java:2749) atnet.sf.json.JSONObject._setInternal(JSONObject.java:2798) atnet.sf.json.JSONObject.setValue(JSONObject.java:1507) atnet.sf.json.JSONObject._fromBean(JSONObject.java:940) atnet.sf.json.JSONObject.fromObject(JSONObject.java:192) atnet.sf.json.JSONArray._processValue(JSONArray.java:2557) atnet.sf.json.JSONArray.processValue(JSONArray.java:2588) atnet.sf.json.JSONArray.addValue(JSONArray.java:2575) atnet.sf.json.JSONArray._fromCollection(JSONArray.java:1082) atnet.sf.json.JSONArray.fromObject(JSONArray.java:145) atcom.service.EmpService.getAllDep(EmpService.java:31) atcom.service.EmpService$$FastClassByCGLIB$$fef4bb53.invoke(<generated>) atnet.sf.cglib.proxy.MethodProxy.invoke(MethodProxy.java:149) atorg.springframework.aop.framework.Cglib2AopProxy$CglibMethodInvocation.invokeJoinpoint(Cglib2AopProxy.java:700) atorg.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:149) atorg.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:106) atorg.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171) atorg.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:89) atorg.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171) atorg.springframework.aop.framework.Cglib2AopProxy$DynamicAdvisedInterceptor.intercept(Cglib2AopProxy.java:635) atcom.service.EmpService$$EnhancerByCGLIB$$8fce1d77.getAllDep(<generated>) atcom.action.DepAction.getAllDept(DepAction.java:22) atsun.reflect.NativeMethodAccessorImpl.invoke0(NativeMethod) atsun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) atsun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) atjava.lang.reflect.Method.invoke(Method.java:601) atorg.springframework.web.bind.annotation.support.HandlerMethodInvoker.doInvokeMethod(HandlerMethodInvoker.java:413) atorg.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:134) atorg.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:310) atorg.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:297) atorg.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:875) atorg.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:809) atorg.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:571) atorg.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:501) atjavax.servlet.http.HttpServlet.service(HttpServlet.java:690) atjavax.servlet.http.HttpServlet.service(HttpServlet.java:803) atorg.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290) atorg.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) atorg.springframework.orm.hibernate3.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:198) atorg.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76) atorg.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235) atorg.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) atorg.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230) atorg.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175) atorg.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128) atorg.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104) atorg.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) atorg.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261) atorg.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844) atorg.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581) atorg.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447) atjava.lang.Thread.run(Thread.java:722)