Json序列化，有多对一和多对多关系时出现的问题

/**

     * 这是一张  单表中的双向一对多，多对一 关系。自己跟自己一对多多对一

*/

@Entity

@Table(name="tb_test")

public class CreditsEventBean implements Serializable{

    private static final long serialVersionUID = 1L;

    @Id

    @GeneratedValue(strategy = GenerationType.AUTO)

    private Integer id; //序号

    /**

     * 上级分类

     */ 

    @ManyToOne

    @JoinColumn(name = "pid")

    private CreditsEventBean pid;    

    /**

     * 下级分类

     */

    @OneToMany(mappedBy="pid")

    private List<CreditsEventBean>  creditsEventBean;

※ 上面的实体被使用时会出现死循环的问题如图：

java.lang.IllegalStateException: Cannot call sendError() after the response has been committed

    at org.apache.catalina.connector.ResponseFacade.sendError(ResponseFacade.java:478) ~[tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver.sendServerError(DefaultHandlerExceptionResolver.java:488) ~[spring-webmvc-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver.handleHttpMessageNotWritable(DefaultHandlerExceptionResolver.java:402) ~[spring-webmvc-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver.doResolveException(DefaultHandlerExceptionResolver.java:146) ~[spring-webmvc-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.servlet.handler.AbstractHandlerExceptionResolver.resolveException(AbstractHandlerExceptionResolver.java:137) [spring-webmvc-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.servlet.handler.HandlerExceptionResolverComposite.resolveException(HandlerExceptionResolverComposite.java:74) [spring-webmvc-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.servlet.DispatcherServlet.processHandlerException(DispatcherServlet.java:1182) [spring-webmvc-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1020) [spring-webmvc-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:971) [spring-webmvc-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:893) [spring-webmvc-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:969) [spring-webmvc-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:871) [spring-webmvc-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at javax.servlet.http.HttpServlet.service(HttpServlet.java:648) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:845) [spring-webmvc-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) [tomcat-embed-websocket-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.jasig.cas.client.util.AssertionThreadLocalFilter.doFilter(AssertionThreadLocalFilter.java:50) [cas-client-core-3.3.3.jar:3.3.3]

    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.jasig.cas.client.util.HttpServletRequestWrapperFilter.doFilter(HttpServletRequestWrapperFilter.java:70) [cas-client-core-3.3.3.jar:3.3.3]

    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.jasig.cas.client.validation.AbstractTicketValidationFilter.doFilter(AbstractTicketValidationFilter.java:238) [cas-client-core-3.3.3.jar:3.3.3]

    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.jasig.cas.client.authentication.AuthenticationFilter.doFilter(AuthenticationFilter.java:152) [cas-client-core-3.3.3.jar:3.3.3]

    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) [spring-web-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:87) [spring-web-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:77) [spring-web-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:121) [spring-web-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.2.4.RELEASE.jar:4.2.4.RELEASE]

    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:521) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1096) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:674) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1500) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1456) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_181]

    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_181]

    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.0.30.jar:8.0.30]

    at java.lang.Thread.run(Thread.java:748) [na:1.8.0_181]

解决方法：

@Entity

@Table(name="tb_test")

public class CreditsEventBean implements Serializable{

    private static final long serialVersionUID = 1L;

    @Id

    @GeneratedValue(strategy = GenerationType.AUTO)

    private Integer id; //序号

/**

@JsonBackReference和@JsonManagedReference：这两个标注通常配对使用，通常用在父子关系中。

*/

    /**

     * 上级分类

     */

    @JsonBackReference //标注的属性在序列化（serialization，即将对象转换为json数据）时，会被忽略（即结果中的json数据不包含该属性的内容）

    @ManyToOne

        @JoinColumn(name = "pid")

        private CreditsEventBean pid;    

    /**

     * 下级分类

     */

        @JsonManagedReference//标注的属性则会被序列化。

    @OneToMany(mappedBy="pid")

    private List<CreditsEventBean>  creditsEventBean;

如果想不忽略 pid字段（注意：被忽略的字段前端是拿不到值的）则：

@Entity

@Table(name="tb_test")

public class CreditsEventBean implements Serializable{

    private static final long serialVersionUID = 1L;

    @Id

    @GeneratedValue(strategy = GenerationType.AUTO)

    private Integer id; //序号

    /**

     * 上级分类

     */

    @ManyToOne

        @JoinColumn(name = "pid")

        private CreditsEventBean pid;    

    /**

     * 下级分类

     */  

    @OneToMany(mappedBy="pid")

    @JsonIgnoreProperties("pid") //指定字段不会被序列化

    private List<CreditsEventBean>  creditsEventBean;

除此之外还可以：
@JsonIgnore：直接忽略某个属性，以断开无限递归，序列化或反序列化均忽略。当然如果标注在get、set方法中，则可以分开控制，序列化对应的是get方法，反序列化对应的是set方法。

@JsonIgnore跟@JsonBackReference和@JsonManagedReference最大的区别：在父子关系中，当反序列化时，@JsonIgnore不会自动注入被忽略的属性值（父或子）

后记：在序列化时，@JsonBackReference的作用相当于@JsonIgnore，此时可以没有@JsonManagedReference。但在反序列化（deserialization，即json数据转换为对象）时，如果没有@JsonManagedReference，则不会自动注入@JsonBackReference标注的属性（被忽略的父或子）；如果有@JsonManagedReference，则会自动注入自动注入@JsonBackReference标注的属性。

@Entity
@Table(name="tb_credits_event")
public class CreditsEventBean implements Serializable{
   private static final long serialVersionUID = 1L;

   @Id
   @GeneratedValue(strategy = GenerationType.AUTO)
   private Integer id; //序号
   private String imagePath;//图片路径
   private String title;     //标题
   private String content; //内容
   private String eventRule; //活动规则
   private Integer credit; //积分值
   private Integer creditStatus;//事件类型 0消费，1获取
   private Integer activityShop;//属于活动的商品   0：常规商品    1：活动商品
   private Integer isShopEvent; //是商品兑换还是活动兑换 0商品兑换 1活动兑换
   private String activityStartCreditsDT;//开始时间
   private String activityEndCreditsDT;//结束时间
   private String userName; //创建者
   private String creteateTime;//创建时间（活动发布时间）
   private String cutOffTime;//活动报名截止日期
   private String address;//兑换(参与)地址
   private Integer maxPlayers; //活动限制人数
   private Integer remaining; //剩余人数
   private Integer sort = 1;//排序 -- 用于内容展示时过期内容的下沉（默认为1）
   @OneToOne
   @JoinColumn(name="role_id")//与权限表一对一
   private CreditsRoleBean roleBean;//活动有权限设置


    /**
     * 上级分类
     */
//   @JsonBackReference
   @ManyToOne
   @JoinColumn(name = "pid")
    private CreditsEventBean pid;

    /**
     * 下级分类
     */
//    @JsonManagedReference
   @OneToMany(mappedBy="pid")
//   @JsonIgnoreProperties("pid") //指定字段不会被序列化
   private List<CreditsEventBean> creditsEventBean;//活动跟商品之间关联

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Json序列化，有多对一和多对多关系时出现的问题

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