HTTP Status 500 - Could not write JSON document: Direct self-reference leading to cycle (through reference chain: java.util.ArrayList[0]->org.activiti.engine.impl.persistence.entity.ProcessDefinitionEntity["processDefinition"]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Direct self-reference leading to cycle (through reference chain: java.util.ArrayList[0]->org.activiti.engine.impl.persistence.entity.ProcessDefinitionEntity["processDefinition"])
type Exception report

message Could not write JSON document: Direct self-reference leading to cycle (through reference chain: java.util.ArrayList[0]->org.activiti.engine.impl.persistence.entity.ProcessDefinitionEntity["processDefinition"]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Direct self-reference leading to cycle (through reference chain: java.util.ArrayList[0]->org.activiti.engine.impl.persistence.entity.ProcessDefinitionEntity["processDefinition"])

description The server encountered an internal error that prevented it from fulfilling this request.

exception

org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON document: Direct self-reference leading to cycle (through reference chain: java.util.ArrayList[0]->org.activiti.engine.impl.persistence.entity.ProcessDefinitionEntity["processDefinition"]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Direct self-reference leading to cycle (through reference chain: java.util.ArrayList[0]->org.activiti.engine.impl.persistence.entity.ProcessDefinitionEntity["processDefinition"])
	org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:286)
	org.springframework.http.converter.AbstractGenericHttpMessageConverter.write(AbstractGenericHttpMessageConverter.java:106)
	org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:231)
	org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:174)
	org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:81)
	org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:113)
	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827)
	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738)
	org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
	org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963)
	org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897)
	org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
	org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
	org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
	org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197)
	org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
root cause

com.fasterxml.jackson.databind.JsonMappingException: Direct self-reference leading to cycle (through reference chain: java.util.ArrayList[0]->org.activiti.engine.impl.persistence.entity.ProcessDefinitionEntity["processDefinition"])
	com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:269)
	com.fasterxml.jackson.databind.ser.BeanPropertyWriter._handleSelfReference(BeanPropertyWriter.java:879)
	com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:666)
	com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:678)
	com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:157)
	com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:149)
	com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:112)
	com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25)
	com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:253)
	com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1425)
	com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:930)
	org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:279)
	org.springframework.http.converter.AbstractGenericHttpMessageConverter.write(AbstractGenericHttpMessageConverter.java:106)
	org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:231)
	org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:174)
	org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:81)
	org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:113)
	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827)
	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738)
	org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
	org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963)
	org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897)
	org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
	org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
	org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
	org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197)
	org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
note The full stack trace of the root cause is available in the Apache Tomcat/7.0.56 logs.
 

原因是：自己引用了自己， ProcessDefinition屬性裡面有一些屬性會導致迴圈引用

解決辦法：ProcessDefinition對應一個map 把需要顯示的屬性拿出來裝在一個集合中，然後返回這個集合就好了

       @ResponseBody
       @RequestMapping("/json")
	public List<Map<String,Object>> json() {
		//整合流程框架 查到所有流程資訊
		//ProcessDefinition屬性裡面有一些屬性會導致迴圈引用
		//ProcessDefinition對應一個map
		//將這個物件需要在頁面顯示的屬性提取出來
		List<Map<String, Object>> pd = new ArrayList<Map<String,Object>>();
		List<ProcessDefinition> list = repositoryService.createProcessDefinitionQuery().list();
		for(ProcessDefinition processDefinition:list) {
			Map<String, Object> values = new HashMap<String,Object>();
			values.put("id", processDefinition.getId());
			values.put("name", processDefinition.getName());
			values.put("key", processDefinition.getKey());
			values.put("version", processDefinition.getVersion());
			pd.add(values);
		}
		return pd;
	}