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：
1 //得到所有部门 2 //返回json对象字符串 3 public String getAllDep(){ 4 List list = deptDAO.findAll(); 5 JsonConfig config = new JsonConfig(); 6 config.setExcludes(new String[]{"emps"});//除去emps属性 7 String json = JSONArray.fromObject(list, config).toString(); 8 return json; 9 }10 11 //得到所有员工12 public String getAllEmp(int id){13 List list = empDAO.findByProperty("dept.deptId", id);14 JsonConfig config = new JsonConfig();15 config.setExcludes(new String[]{"dept"});//除去dept属性16 String json = JSONArray.fromObject(list, config).toString();17 return json;18 }
异常代码如下：
严重: Servlet.service() for servlet springMVC threw exceptionnet.sf.json.JSONException: There is a cycle in the hierarchy! at net.sf.json.util.CycleDetectionStrategy$StrictCycleDetectionStrategy.handleRepeatedReferenceAsObject(CycleDetectionStrategy.java:97) at net.sf.json.JSONObject._fromBean(JSONObject.java:857) at net.sf.json.JSONObject.fromObject(JSONObject.java:192) at net.sf.json.JSONObject._processValue(JSONObject.java:2774) at net.sf.json.JSONObject._setInternal(JSONObject.java:2798) at net.sf.json.JSONObject.setValue(JSONObject.java:1507) at net.sf.json.JSONObject._fromBean(JSONObject.java:940) at net.sf.json.JSONObject.fromObject(JSONObject.java:192) at net.sf.json.JSONArray._processValue(JSONArray.java:2557) at net.sf.json.JSONArray.processValue(JSONArray.java:2588) at net.sf.json.JSONArray.addValue(JSONArray.java:2575) at net.sf.json.JSONArray._fromCollection(JSONArray.java:1082) at net.sf.json.JSONArray.fromObject(JSONArray.java:145) at net.sf.json.JSONObject._processValue(JSONObject.java:2749) at net.sf.json.JSONObject._setInternal(JSONObject.java:2798) at net.sf.json.JSONObject.setValue(JSONObject.java:1507) at net.sf.json.JSONObject._fromBean(JSONObject.java:940) at net.sf.json.JSONObject.fromObject(JSONObject.java:192) at net.sf.json.JSONArray._processValue(JSONArray.java:2557) at net.sf.json.JSONArray.processValue(JSONArray.java:2588) at net.sf.json.JSONArray.addValue(JSONArray.java:2575) at net.sf.json.JSONArray._fromCollection(JSONArray.java:1082) at net.sf.json.JSONArray.fromObject(JSONArray.java:145) at com.service.EmpService.getAllDep(EmpService.java:31) at com.service.EmpService$$FastClassByCGLIB$$fef4bb53.invoke() at net.sf.cglib.proxy.MethodProxy.invoke(MethodProxy.java:149) at org.springframework.aop.framework.Cglib2AopProxy$CglibMethodInvocation.invokeJoinpoint(Cglib2AopProxy.java:700) at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:149) at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:106) at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171) at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:89) at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171) at org.springframework.aop.framework.Cglib2AopProxy$DynamicAdvisedInterceptor.intercept(Cglib2AopProxy.java:635) at com.service.EmpService$$EnhancerByCGLIB$$8fce1d77.getAllDep( ) at com.action.DepAction.getAllDept(DepAction.java:22) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:601) at org.springframework.web.bind.annotation.support.HandlerMethodInvoker.doInvokeMethod(HandlerMethodInvoker.java:413) at org.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:134) at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:310) at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:297) at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:875) at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:809) at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:571) at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:501) at javax.servlet.http.HttpServlet.service(HttpServlet.java:690) at javax.servlet.http.HttpServlet.service(HttpServlet.java:803) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) at org.springframework.orm.hibernate3.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:198) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261) at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844) at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581) at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447) at java.lang.Thread.run(Thread.java:722)