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SpringBoot JPA懒加载异常 - com.fasterxml.jackson.databind.JsonMappingException: could not initialize proxy

问题与分析

某日忽然发现在用postman测试数据时报错如下:

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com.fasterxml.jackson.databind.JsonMappingException: could not initialize proxy [com.cbxsoftware.cbx.attachment.entity.RefAttachment#c109ec36e60c4a89a10eabc72416d984] - no Session (through reference chain: com.cbxsoftware.cbx.sampletracker.elasticsearch.entity.SampleTrackerDetailEstc["sampleTracker"]->com.cbxsoftware.cbx.sampletracker.elasticsearch.entity.SampleTrackerEstc["sampleTracker"]->com.cbxsoftware.cbx.sampletracker.entity.SampleTracker["item"]->com.cbxsoftware.cbx.item.entity.RefItem["image"]->com.cbxsoftware.cbx.image.entity.RefImage["propFormat"]->com.cbxsoftware.cbx.attachment.entity.RefAttachment$HibernateProxy$uNA5RwMT["revision"])
at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:394)
at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:353)
at com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:316)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:727)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
at com.fasterxml.jackson.databind.ser.impl.UnwrappingBeanSerializer.serialize(UnwrappingBeanSerializer.java:120)
at com.fasterxml.jackson.databind.ser.impl.UnwrappingBeanPropertyWriter.serializeAsField(UnwrappingBeanPropertyWriter.java:127)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)
at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:3905)
at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:3219)
...
Caused by: org.hibernate.LazyInitializationException: could not initialize proxy [com.cbxsoftware.cbx.attachment.entity.RefAttachment#c109ec36e60c4a89a10eabc72416d984] - no Session
at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:169)
at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:309)
at org.hibernate.proxy.pojo.bytebuddy.ByteBuddyInterceptor.intercept(ByteBuddyInterceptor.java:45)
at org.hibernate.proxy.ProxyConfiguration$InterceptorDispatcher.intercept(ProxyConfiguration.java:95)
at com.cbxsoftware.cbx.attachment.entity.RefAttachment$HibernateProxy$uNA5RwMT.getRevision(Unknown Source)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:688)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
... 128 common frames omitted

报错很明显,是由于hibernate的懒加载引起的。项目使用的是SpringBoot框架,JPA默认使用的是hibernate的实现,而hibernate的懒加载机制其实就是延迟加载对象,如果没有在session关闭前使用到对象里除id以外的属性时,就只会返回一个没有初始化过的包含了id的代理类。很多时候,这个代理类会引发上述的异常。

简单说一下为什么会触发懒加载异常,首先hibernate开启一个session(会话),然后开启transaction(事务),接着发出sql找回数据并组装成pojo(或者说entity、model),这时候如果pojo里有懒加载的对象,并不会去发出sql查询db,而是直接返回一个懒加载的代理对象,这个对象里只有id。如果接下来没有其他的操作去访问这个代理对象除了id以外的属性,就不会去初始化这个代理对象,也就不会去发出sql查找db。接着事务提交,session关闭。如果这时候再去访问代理对象除了id以外的属性时,就会报上述的懒加载异常,原因是这时候已经没有session了,无法初始化懒加载的代理对象。

解决方法一

如果是spring继承的hibernate,根据上述的原因,可以延长session的生命周期,但是这里用的是SpringBoot的JPA,处理方法不同,需要在application.properties配置下懒加载相关的东西:

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spring.jpa.properties.hibernate.enable_lazy_load_no_trans=true

进行该配置后,可以在session关闭时也能另外开启一个新的session和事务来访问db以取回懒加载对象的数据。

解决方法二

因为该懒加载异常是缺少session导致的,那么可以通过在方法前添加事务注解@Transactional的方式来解决,只要事务没有提交,session就不会关闭,自然就不会出现上述的懒加载异常。不过由于该事务注解是用Spring AOP实现的,存在着一些坑,比如类内直接调用无效或者对非public方法无效等,需要多加注意。

当使用了上述两种方法后,发现不再触发LazyInitializationException,但是却发生了另一个新的异常InvalidDefinitionException

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com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class org.hibernate.proxy.pojo.bytebuddy.ByteBuddyInterceptor and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: com.cbxsoftware.cbx.item.elasticsearch.entity.ItemEstc["mainEntity"]->com.cbxsoftware.cbx.item.entity.Item["image"]->com.cbxsoftware.cbx.image.entity.RefImage["propFormat"]->com.cbxsoftware.cbx.attachment.entity.RefAttachment$HibernateProxy$vTKSYzrN["hibernateLazyInitializer"])
at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77)
at com.fasterxml.jackson.databind.SerializerProvider.reportBadDefinition(SerializerProvider.java:1191)
at com.fasterxml.jackson.databind.DatabindContext.reportBadDefinition(DatabindContext.java:313)
at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.failForEmpty(UnknownSerializer.java:71)
at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.serialize(UnknownSerializer.java:33)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
at com.fasterxml.jackson.databind.ser.impl.UnwrappingBeanSerializer.serialize(UnwrappingBeanSerializer.java:120)
at com.fasterxml.jackson.databind.ser.impl.UnwrappingBeanPropertyWriter.serializeAsField(UnwrappingBeanPropertyWriter.java:127)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)
at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:3905)
at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:3219)
...

这个异常是由于hibernate在代理类里添加了一个属性hibernateLazyInitializer,当该对象转换成json的时候就会报错。解决方法是将该属性过滤掉,可以在对应的类名或者公共类前加上如下注解:

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@JsonIgnoreProperties(value = { "hibernateLazyInitializer" })

源码分析

因为对懒加载异常的发生有些好奇,所以看了下hibernate的源码,这里简单分析下,另外我看的是两个源码包如下:

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spring-orm-5.1.5.RELEASE.jar
hibernate-core-5.3.7.Final.jar

首先是关于spring.jpa.properties.hibernate.enable_lazy_load_no_trans=true的配置,前面半截是因为JPA集成了hibernate的配置,所以在hibernate中,这个配置应该是hibernate.enable_lazy_load_no_trans=true

在hibernate的一个常量接口org.hibernate.cfg.AvailableSettings中定义了各种配置常量,其中就包括上述这个配置:

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String ENABLE_LAZY_LOAD_NO_TRANS = "hibernate.enable_lazy_load_no_trans";

在启动项目的时候会读取配置文件,将其解析为一个HashMap<K,V>,这些参数在newEntityManagerFactoryBuilderImpl的时候被使用到,上面的常量会在org.hibernate.boot.internal.SessionFactoryOptionsBuilder里被拿来初始化:

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this.initializeLazyStateOutsideTransactions = cfgService.getSetting( ENABLE_LAZY_LOAD_NO_TRANS, BOOLEAN, false );

因为在配置文件里配置了该变量的值为true,所以这里在初始化的时候就会把initializeLazyStateOutsideTransactions的值设置为true。该变量由一个方法来判断其值是否为true:

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@Override
public boolean isInitializeLazyStateOutsideTransactionsEnabled() {
return initializeLazyStateOutsideTransactions;
}

接着在组装pojo时,会为懒加载对象创建对应的代理对象,当需要获取该代理对象除id以外的属性时,就会调用AbstractLazyInitializer#initialize()进行初始化,逻辑如下:

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@Override
public final void initialize() throws HibernateException {
if ( !initialized ) {
if ( allowLoadOutsideTransaction ) {
permissiveInitialization();
}
else if ( session == null ) {
throw new LazyInitializationException( "could not initialize proxy [" + entityName + "#" + id + "] - no Session" );
}
else if ( !session.isOpen() ) {
throw new LazyInitializationException( "could not initialize proxy [" + entityName + "#" + id + "] - the owning Session was closed" );
}
else if ( !session.isConnected() ) {
throw new LazyInitializationException( "could not initialize proxy [" + entityName + "#" + id + "] - the owning Session is disconnected" );
}
else {
target = session.immediateLoad( entityName, id );
initialized = true;
checkTargetState(session);
}
}
else {
checkTargetState(session);
}
}

如果在配置文件中设置了spring.jpa.properties.hibernate.enable_lazy_load_no_trans=true,那么上述的allowLoadOutsideTransaction变量值就为true,则可以进入permissiveInitialization()方法另起session和事务,最终避免懒加载异常LazyInitializationException。如果没有配置该参数,那么就会由于session已关闭(即为null)而抛出LazyInitializationException

参考链接

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