net.sf.json.JSONException: There is a cycle in the hierarchy!错误解决方案

　　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
    public String getAllDep(){
        List list = deptDAO.findAll();
        JsonConfig config = new JsonConfig();
        config.setExcludes(new String[]{"emps"});//除去emps属性
        String json = JSONArray.fromObject(list, config).toString();
        return json;
    }
    
    //得到所有员工
    public String getAllEmp(int id){
        List list = empDAO.findByProperty("dept.deptId", id);
        JsonConfig config = new JsonConfig();
        config.setExcludes(new String[]{"dept"});//除去dept属性
        String json = JSONArray.fromObject(list, config).toString();
        return json;
    }

异常代码如下：

严重: Servlet.service() for servlet springMVC threw exception
net.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(<generated>)
    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(<generated>)
    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)