原因分析：在解析bean时，出现死循环调用,即：多个Bean之间出现了相互调用
解决办法：过滤去掉bean中引起死循环调用的属性：
- List<Project> projectList = projectServices.find(); //获取数据
- //自定义JsonConfig用于过滤Hibernate配置文件所产生的递归数据
- JsonConfig config = new JsonConfig();
- config.setExcludes(new String[]{"documentSet","milestoneSet","issuesSet","userSet"}); //只要设置这个数组，指定过滤哪些字段。
- //组成JSON数组
- JSONArray json = JSONArray.fromObject(projectList, config);
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 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)
