Spring Boot + JPA 因为 javassist 包出现 NullPointerException 问题的解决

Caused by: org.hibernate.MappingException: Could not get constructor for org.hibernate.persister.entity.SingleTableEntityPersister
	at org.hibernate.persister.internal.PersisterFactoryImpl.createEntityPersister(PersisterFactoryImpl.java:123) ~[hibernate-core-5.2.17.Final.jar:5.2.17.Final]
	at org.hibernate.persister.internal.PersisterFactoryImpl.createEntityPersister(PersisterFactoryImpl.java:77) ~[hibernate-core-5.2.17.Final.jar:5.2.17.Final]
	at org.hibernate.metamodel.internal.MetamodelImpl.initialize(MetamodelImpl.java:129) ~[hibernate-core-5.2.17.Final.jar:5.2.17.Final]
	at org.hibernate.internal.SessionFactoryImpl.<init>(SessionFactoryImpl.java:300) ~[hibernate-core-5.2.17.Final.jar:5.2.17.Final]
	at org.hibernate.boot.internal.SessionFactoryBuilderImpl.build(SessionFactoryBuilderImpl.java:462) ~[hibernate-core-5.2.17.Final.jar:5.2.17.Final]
	at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.build(EntityManagerFactoryBuilderImpl.java:892) ~[hibernate-core-5.2.17.Final.jar:5.2.17.Final]
	at org.springframework.orm.jpa.vendor.SpringHibernateJpaPersistenceProvider.createContainerEntityManagerFactory(SpringHibernateJpaPersistenceProvider.java:57) ~[spring-orm-5.0.10.RELEASE.jar:5.0.10.RELEASE]
	at org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.createNativeEntityManagerFactory(LocalContainerEntityManagerFactoryBean.java:365) ~[spring-orm-5.0.10.RELEASE.jar:5.0.10.RELEASE]
	at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.buildNativeEntityManagerFactory(AbstractEntityManagerFactoryBean.java:390) ~[spring-orm-5.0.10.RELEASE.jar:5.0.10.RELEASE]
	... 25 common frames omitted
Caused by: org.hibernate.HibernateException: Unable to instantiate default tuplizer [org.hibernate.tuple.entity.PojoEntityTuplizer]
	at org.hibernate.tuple.entity.EntityTuplizerFactory.constructTuplizer(EntityTuplizerFactory.java:91) ~[hibernate-core-5.2.17.Final.jar:5.2.17.Final]
	at org.hibernate.tuple.entity.EntityTuplizerFactory.constructDefaultTuplizer(EntityTuplizerFactory.java:116) ~[hibernate-core-5.2.17.Final.jar:5.2.17.Final]
	at org.hibernate.tuple.entity.EntityMetamodel.<init>(EntityMetamodel.java:382) ~[hibernate-core-5.2.17.Final.jar:5.2.17.Final]
	at org.hibernate.persister.entity.AbstractEntityPersister.<init>(AbstractEntityPersister.java:519) ~[hibernate-core-5.2.17.Final.jar:5.2.17.Final]
	at org.hibernate.persister.entity.SingleTableEntityPersister.<init>(SingleTableEntityPersister.java:124) ~[hibernate-core-5.2.17.Final.jar:5.2.17.Final]
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:na]
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:na]
	at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:na]
	at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490) ~[na:na]
	at org.hibernate.persister.internal.PersisterFactoryImpl.createEntityPersister(PersisterFactoryImpl.java:96) ~[hibernate-core-5.2.17.Final.jar:5.2.17.Final]
	... 33 common frames omitted
Caused by: java.lang.reflect.InvocationTargetException: null
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:na]
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:na]
	at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:na]
	at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490) ~[na:na]
	at org.hibernate.tuple.entity.EntityTuplizerFactory.constructTuplizer(EntityTuplizerFactory.java:88) ~[hibernate-core-5.2.17.Final.jar:5.2.17.Final]
	... 42 common frames omitted
Caused by: java.lang.NullPointerException: null

　　可以很轻易看到很多 javassist 相关的内容。这个 BUG 由低版本的 javassist 库引起，在 Spring Boot 2.1.0.RELEASE 中修复了这个问题，如果可以升级，推荐升级到最新版本的 Spring Boot 解决。如果无法升级版本，可以显式引入新版的 javassist 依赖即可

<dependency>
    <groupId>org.javassist</groupId>
    <artifactId>javassist</artifactId>
    <version>3.23.1-GA</version>
</dependency>