There is a cycle in the hierarchy-蒲公英云

异常代码如下：

2019-02-09 12:14:10,221 [http-bio-8082-exec-1] ERROR freemarker.runtime - Template processing error: "Method public java.lang.String org.apache.commons.lang.exception.NestableRuntimeException.getMessage(int) threw an exception when invoked on net.sf.json.JSONException: There is a cycle in the hierarchy!"
Method public java.lang.String org.apache.commons.lang.exception.NestableRuntimeException.getMessage(int) threw an exception when invoked on net.sf.json.JSONException: There is a cycle in the hierarchy!
The problematic instruction:
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==> ${msgs[0][0]} [on line 76, column 25 in org/apache/struts2/dispatcher/error.ftl]
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Java backtrace for programmers:
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freemarker.template.TemplateModelException: Method public java.lang.String org.apache.commons.lang.exception.NestableRuntimeException.getMessage(int) threw an exception when invoked on net.sf.json.JSONException: There is a cycle in the hierarchy!
    at freemarker.ext.beans.SimpleMethodModel.exec(SimpleMethodModel.java:130)
    at freemarker.ext.beans.SimpleMethodModel.get(SimpleMethodModel.java:138)
    at freemarker.core.DynamicKeyName.dealWithNumericalKey(DynamicKeyName.java:111)
    at freemarker.core.DynamicKeyName._getAsTemplateModel(DynamicKeyName.java:90)
    at freemarker.core.Expression.getAsTemplateModel(Expression.java:89)
    at freemarker.core.Expression.getStringValue(Expression.java:93)
    at freemarker.core.DollarVariable.accept(DollarVariable.java:76)
    at freemarker.core.Environment.visit(Environment.java:221)
    at freemarker.core.MixedContent.accept(MixedContent.java:92)
    at freemarker.core.Environment.visit(Environment.java:221)
    at freemarker.core.IfBlock.accept(IfBlock.java:82)
    at freemarker.core.Environment.visit(Environment.java:221)
    at freemarker.core.IfBlock.accept(IfBlock.java:82)
    at freemarker.core.Environment.visit(Environment.java:221)
    at freemarker.core.MixedContent.accept(MixedContent.java:92)
    at freemarker.core.Environment.visit(Environment.java:221)
    at freemarker.core.Environment.process(Environment.java:199)
    at freemarker.template.Template.process(Template.java:259)
    at org.apache.struts2.dispatcher.Dispatcher.sendError(Dispatcher.java:845)
    at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:567)
    at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)
    at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.springframework.orm.hibernate3.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:233)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:421)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:744)
Caused by: java.lang.NullPointerException
    at freemarker.ext.beans.SimpleMemberModel.unwrapArguments(SimpleMemberModel.java:86)
    at freemarker.ext.beans.SimpleMethodModel.exec(SimpleMethodModel.java:106)
    ... 42 more

在使用JSONObject.fromObject的时候，出现“There is a cycle in the hierarchy”异常。

意思是：层次结构中有一个循环；也就是出现了死循环，也就是Model之间有循环包含关系；

解决办法：

使用setCycleDetectionStrategy防止自包含。

代码如下：

response.setContentType("text/html;charset=UTF-8");
         response.setCharacterEncoding("UTF-8");
         JsonConfig jc = new JsonConfig();
         jc.setCycleDetectionStrategy(CycleDetectionStrategy.LENIENT);
         jc.setExcludes(exclude);
         JSONArray jsonArray = JSONArray.fromObject(coll, jc);
         String result = jsonArray.toString();
         try {
            response.getWriter().write(result);
        } catch (IOException e) {            
            e.printStackTrace();
        }
     }