问题描述

(to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: com.zstu.metrocity.entity.User_$$_jvstba5_0["handler"])

或者

(to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: com.example.demo.Boy$HibernateProxy$CfQZBtEe["hibernateLazyInitializer"])

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class org.apache.ibatis.executor.loader.javassist.JavassistProxyFactory$EnhancedResultObjectProxyImpl and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: com.zstu.metrocity.entity.User_$$_jvstba5_0["handler"])at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77) ~[jackson-databind-2.10.2.jar:2.10.2]at com.fasterxml.jackson.databind.SerializerProvider.reportBadDefinition(SerializerProvider.java:1191) ~[jackson-databind-2.10.2.jar:2.10.2]at com.fasterxml.jackson.databind.DatabindContext.reportBadDefinition(DatabindContext.java:404) ~[jackson-databind-2.10.2.jar:2.10.2]at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.failForEmpty(UnknownSerializer.java:71) ~[jackson-databind-2.10.2.jar:2.10.2]at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.serialize(UnknownSerializer.java:33) ~[jackson-databind-2.10.2.jar:2.10.2]at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727) ~[jackson-databind-2.10.2.jar:2.10.2]at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:722) ~[jackson-databind-2.10.2.jar:2.10.2]at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:166) ~[jackson-databind-2.10.2.jar:2.10.2]at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480) ~[jackson-databind-2.10.2.jar:2.10.2]at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319) ~[jackson-databind-2.10.2.jar:2.10.2]at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1433) ~[jackson-databind-2.10.2.jar:2.10.2]at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:921) ~[jackson-databind-2.10.2.jar:2.10.2]at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:287) ~[spring-web-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.http.converter.AbstractGenericHttpMessageConverter.write(AbstractGenericHttpMessageConverter.java:104) ~[spring-web-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:295) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:181) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:82) ~[spring-web-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:124) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:888) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:793) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1040) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]at javax.servlet.http.HttpServlet.service(HttpServlet.java:634) ~[tomcat-embed-core-9.0.30.jar:9.0.30]at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]at javax.servlet.http.HttpServlet.service(HttpServlet.java:741) ~[tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.30.jar:9.0.30]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.30.jar:9.0.30]at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.2.3.RELEASE.jar:5.2.3.RELEASE]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) ~[tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541) [tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139) [tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) [tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) [tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) [tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:367) [tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) [tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:860) [tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1598) [tomcat-embed-core-9.0.30.jar:9.0.30]at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.30.jar:9.0.30]at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_181]at java.util.concurrent.ThreadPoolExecutor$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-9.0.30.jar:9.0.30]at java.lang.Thread.run(Thread.java:748) [na:1.8.0_181]

问题分析

MyBatis级联查询，配置了懒加载模式，通过Spring MVC返回json序列化时报错。

通过java的反射机制将pojo转换成json的。

因为json plugin用的是java的内审机制.被管理的pojo会加入["hibernateLazyInitializer","handler","fieldHandler"]等属性,json plugin会对这些属性拿出来操作,并读取里面一个不能被反射操作的属性就产生了异常。

实体类中有的字段值为null，所以在json化的时候，fasterxml.jackson将对象转换为json报错。

问题关键

在MappingJackson2HttpMessageConverter通过aop转化为json的时候不处理相关属性

API

@JsonIgnoreProperties

此注解用于类类，作用是json序列化时将java bean中的一些属性忽略掉，序列化和反序列化都受影响。

@JsonIgnore

此注解用于属性或者方法上（最好是属性上），作用和上面的@JsonIgnoreProperties一样。

解决方案

方法一：@JsonIgnoreProperties

在实体类上面加上注解

 @JsonIgnoreProperties(value = { "hibernateLazyInitializer", "handler" })

package cn.edu.aynu.Bean;import com.fasterxml.jackson.annotation.JsonIgnoreProperties;import java.io.Serializable;
@JsonIgnoreProperties(value = { "hibernateLazyInitializer", "handler" })
public class Book implements Serializable {private String  bookName;private String author;public Book(){}public Book(String bookName, String author) {this.bookName = bookName;this.author = author;}
}

@JsonIgnoreProperties(value={"hibernateLazyInitializer"}) （此时只是忽略hibernateLazyInitializer属性）要加载被lazy的，也就是many-to-one的one端的pojo上

这行代码的作用在于告诉你的json plugin组件，在将你的代理对象转换为json对象时，忽略value对应的数组中的属性.

如果你想在转换的时候继续忽略其他属性，可以在数组中继续加入

例如："hibernateLazyInitializer","handler","fieldHandler"

方法二：@JsonIgnore

对象的属性上面增加 @JsonIgnore 注解


@JsonIgnore
public Set<User> getUsers() {
return this.users;
}

注意：引入的类是 com.fasterxml.jackson.annotation.JsonIgnore，如果使用 org.codehaus.jackson.annotate.JsonIgnore 则不能生效，见 Spring @JsonIgnore not working 。

方法三：关闭LAZY加载

fetch= FetchType.LAZY改为fetch= FetchType.EAGER

但这样会导致每次查询数据库都要立即提取 OneToMany 的所有对象，所以非常不推荐。

方法四：OpenSessionInViewFilter

通过 Hibernate 的 OpenSessionInViewFilter 使得 FetchType 为 LAZY 的属性在序列化时为空，在 web.xml 中添加代码如下：

<filter> <filter-name>openSession</filter-name> <filter-class>org.springframework.orm.hibernate4.support.OpenSessionInViewFilter</filter-class>             <init-param> <param-name>singleSession</param-name> <param-value>false</param-value> </init-param>
</filter>
<filter-mapping> <filter-name>openSession</filter-name> <url-pattern>/*</url-pattern>
</filter-mapping>

Spring MVC配置详情

https://www.cnblogs.com/blogonfly/articles/3991782.html

Spring Boot 配置详情

https://my.oschina.net/wangxinxx/blog/864336

尽管 users 为空，但字段依然保留，对应输出：

{...,"users":[],...}

参考文章

https://blog.csdn.net/wgp15732622312/article/details/79951977

https://www.cnblogs.com/gugia/p/5117735.html

https://blog.csdn.net/u014086054/article/details/89046847

https://www.cnblogs.com/xuwenjin/p/8832522.html

https://www.cnblogs.com/zhulina-917/p/10557487.html

https://ask.csdn.net/questions/344738

https://www.cnblogs.com/cbread/p/4017987.html

https://www.cnblogs.com/softidea/p/5668697.html

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