问题与分析
某日忽然发现在用postman测试数据时报错如下：
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：
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：
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：
@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：
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：
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：
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：
@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：
@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
- springboot jpa 解决延迟加载问题
- No serializer found for class org.hibernate.proxy.pojo.bytebuddy.ByteBuddyInterceptor
- springboot集成jpa返回Json报错 com.fasterxml.jackson.databind.exc.InvalidDefinitionException:
- Hibernate和Spring整合出现懒加载异常：org.hibernate.LazyInitializationException: could not initialize proxy - no Session