解决IllegalArgumentException: At least one JPA metamodel must be present!问题

问题

SpringBoot2.0工程，启动报错，异常：nested exception is java.lang.IllegalArgumentException: At least one JPA metamodel must be present!。

详情如下：

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'jpaMappingContext': Invocation of init method failed; nested exception is java.lang.IllegalArgumentException: At least one JPA metamodel must be present!
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1710) ~[spring-beans-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:583) ~[spring-beans-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:502) ~[spring-beans-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:312) ~[spring-beans-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228) ~[spring-beans-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:310) ~[spring-beans-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:200) ~[spring-beans-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:741) ~[spring-beans-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:868) ~[spring-context-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:549) ~[spring-context-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:140) ~[spring-boot-2.0.0.RELEASE.jar:2.0.0.RELEASE]
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:752) [spring-boot-2.0.0.RELEASE.jar:2.0.0.RELEASE]
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:388) [spring-boot-2.0.0.RELEASE.jar:2.0.0.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:327) [spring-boot-2.0.0.RELEASE.jar:2.0.0.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1246) [spring-boot-2.0.0.RELEASE.jar:2.0.0.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1234) [spring-boot-2.0.0.RELEASE.jar:2.0.0.RELEASE]
	at com.example.demo.JtademoApplication.main(JtademoApplication.java:22) [classes/:na]
Caused by: java.lang.IllegalArgumentException: At least one JPA metamodel must be present!
	at org.springframework.util.Assert.notEmpty(Assert.java:450) ~[spring-core-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.data.jpa.mapping.JpaMetamodelMappingContext.<init>(JpaMetamodelMappingContext.java:54) ~[spring-data-jpa-2.0.5.RELEASE.jar:2.0.5.RELEASE]
	at org.springframework.data.jpa.repository.config.JpaMetamodelMappingContextFactoryBean.createInstance(JpaMetamodelMappingContextFactoryBean.java:88) ~[spring-data-jpa-2.0.5.RELEASE.jar:2.0.5.RELEASE]
	at org.springframework.data.jpa.repository.config.JpaMetamodelMappingContextFactoryBean.createInstance(JpaMetamodelMappingContextFactoryBean.java:43) ~[spring-data-jpa-2.0.5.RELEASE.jar:2.0.5.RELEASE]
	at org.springframework.beans.factory.config.AbstractFactoryBean.afterPropertiesSet(AbstractFactoryBean.java:141) ~[spring-beans-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1769) ~[spring-beans-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1706) ~[spring-beans-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	... 16 common frames omitted

分析

Spring Boot通过spring-boot-starter-data-jpa来提供对JPA的支持，Spring Boot默认的JPA实现者是Hibernate。
由于pom.xml文件中有定义spring-boot-starter-data-jpa。
<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>    

此时项目会自动开启如下两个自动配置类：
JpaRepositoriesAutoConfiguration
HibernateJpaAutoConfiguration

只要有此定义，JpaMetamodelMappingContext类在启动过程中会进行检查，至少得定义一个JPA metamode，否则就报At least one JPA metamodel must be present!错误。

JpaMetamodelMappingContext相关源代码如下：

/**
	 * Creates a new JPA {@link Metamodel} based {@link MappingContext}.
	 *
	 * @param models must not be {@literal null} or empty.
	 */
	public JpaMetamodelMappingContext(Set<Metamodel> models) {

		Assert.notNull(models, "JPA metamodel must not be null!");
		Assert.notEmpty(models, "At least one JPA metamodel must be present!");

		this.models = models;
		this.persistenceProvider = PersistenceProvider.fromMetamodel(models.iterator().next());
	}

解决

方法一：如果系统中未用到jpa，那去掉spring-boot-starter-data-jpa依赖即可。

方法二：在Application中添加@EnableAutoConfiguration(exclude = { JpaRepositoriesAutoConfiguration.class })注解。

方法二主要是用于自定义JPA配置实现。

文章结束。