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open-in-view功能是指SpringBoot在请求开始前自动初始化一个数据库会话，一般是EntityManager。

当这个功能启用时，会输出一条日志：

2020-10-22 20:46:46.720  WARN 6484 --- [  restartedMain] JpaBaseConfiguration$JpaWebConfiguration : spring.jpa.open-in-view is enabled by default. Therefore, database queries may be performed during view rendering. Explicitly configure spring.jpa.open-in-view to disable this warning

-----open-in-view功能的启动条件是: 
1)  引用spring-boot-starter-jpa

2）或者引用spring-boot-starter-jdbc + hibernate 

3）或者，直接引用hibernate + spring-orm

----这个功能的启用过程：

1）在spring-boot-autoconfigure包中，META-INF/Spring.factories 文件配置了启动类：

org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\

HibernateJpaAutoConfiguration：

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ LocalContainerEntityManagerFactoryBean.class, EntityManager.class, SessionImplementor.class })
@EnableConfigurationProperties(JpaProperties.class)
@AutoConfigureAfter({ DataSourceAutoConfiguration.class })
@Import(HibernateJpaConfiguration.class)
public class HibernateJpaAutoConfiguration {

}

　从这个类的Condition条件可以看出其启用条件：‘

当前classpath下要有LocalContainerEntityManagerFactoryBean类，这个类在org.springframework.orm.jpa包下，所以要引用spring-orm

EntityManager在javax.persistence下，SessionImplementor 是Hibernate的类

2）HibernateJpaAutoConfiguration引入了HibernateJpaConfiguration，HibernateJpaConfiguration的启用条件是当前仅有一个DataSource bean

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties(HibernateProperties.class)
@ConditionalOnSingleCandidate(DataSource.class)
class HibernateJpaConfiguration extends JpaBaseConfiguration {

HibernateJpaConfiguration的基类JpaBaseConfiguration定义了一个内部类

@Configuration(proxyBeanMethods = false)
	@ConditionalOnWebApplication(type = Type.SERVLET)
	@ConditionalOnClass(WebMvcConfigurer.class)
	@ConditionalOnMissingBean({ OpenEntityManagerInViewInterceptor.class, OpenEntityManagerInViewFilter.class })
	@ConditionalOnMissingFilterBean(OpenEntityManagerInViewFilter.class)
	@ConditionalOnProperty(prefix = "spring.jpa", name = "open-in-view", havingValue = "true", matchIfMissing = true)
	protected static class JpaWebConfiguration {

		private static final Log logger = LogFactory.getLog(JpaWebConfiguration.class);

		private final JpaProperties jpaProperties;

		protected JpaWebConfiguration(JpaProperties jpaProperties) {
			this.jpaProperties = jpaProperties;
		}

		@Bean
		public OpenEntityManagerInViewInterceptor openEntityManagerInViewInterceptor() {
			if (this.jpaProperties.getOpenInView() == null) {
				logger.warn("spring.jpa.open-in-view is enabled by default. "
						+ "Therefore, database queries may be performed during view "
						+ "rendering. Explicitly configure spring.jpa.open-in-view to disable this warning");
			}
			return new OpenEntityManagerInViewInterceptor();
		}

		@Bean
		public WebMvcConfigurer openEntityManagerInViewInterceptorConfigurer(
				OpenEntityManagerInViewInterceptor interceptor) {
			return new WebMvcConfigurer() {

				@Override
				public void addInterceptors(InterceptorRegistry registry) {
					registry.addWebRequestInterceptor(interceptor);
				}

			};
		}

	}

这个类注册了OpenEntityManagerInViewInterceptor 拦截器。OpenEntityManagerInViewInterceptor 用来在请求启动前初始化EntityManager，在请求完成后关闭EntityManager

JpaBaseConfiguration 会自动创建一个事务管理器和EntityManagerFactory. OpenEntityManagerInViewInterceptor默认会采用这个EntityManagerFactory类创建EntityManager

@Bean
	@ConditionalOnMissingBean(TransactionManager.class)
	public PlatformTransactionManager transactionManager(
			ObjectProvider<TransactionManagerCustomizers> transactionManagerCustomizers) {
		JpaTransactionManager transactionManager = new JpaTransactionManager();
		transactionManagerCustomizers.ifAvailable((customizers) -> customizers.customize(transactionManager));
		return transactionManager;
	}

@Bean
	@Primary
	@ConditionalOnMissingBean({ LocalContainerEntityManagerFactoryBean.class, EntityManagerFactory.class })
	public LocalContainerEntityManagerFactoryBean entityManagerFactory(EntityManagerFactoryBuilder factoryBuilder) {
		Map<String, Object> vendorProperties = getVendorProperties();
		customizeVendorProperties(vendorProperties);
		return factoryBuilder.dataSource(this.dataSource).packages(getPackagesToScan()).properties(vendorProperties)
				.mappingResources(getMappingResources()).jta(isJta()).build();
	}

------------------------
LocalContainerEntityManagerFactoryBean 依赖DataSource，DataSourde的创建过程:

META-INF/Spring.factories 文件配置了启动类：org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ DataSource.class, EmbeddedDatabaseType.class })
@ConditionalOnMissingBean(type = "io.r2dbc.spi.ConnectionFactory")
@EnableConfigurationProperties(DataSourceProperties.class)
@Import({ DataSourcePoolMetadataProvidersConfiguration.class, DataSourceInitializationConfiguration.class })
public class DataSourceAutoConfiguration {

@Configuration(proxyBeanMethods = false)
@Conditional(EmbeddedDatabaseCondition.class)
@ConditionalOnMissingBean({ DataSource.class, XADataSource.class })
@Import(EmbeddedDataSourceConfiguration.class)
protected static class EmbeddedDatabaseConfiguration {
}
@Configuration(proxyBeanMethods = false)
@Conditional(PooledDataSourceCondition.class)
@ConditionalOnMissingBean({ DataSource.class, XADataSource.class })
@Import({ DataSourceConfiguration.Hikari.class, DataSourceConfiguration.Tomcat.class,
      DataSourceConfiguration.Dbcp2.class, DataSourceConfiguration.OracleUcp.class,
      DataSourceConfiguration.Generic.class, DataSourceJmxConfiguration.class })
protected static class PooledDataSourceConfiguration {
}

　DataSourceAutoConfiguration 的启用条件是，当前类路径下存在DataSource（框架类）和org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType，EmbeddedDatabaseType是spring-jdbc下的类

所以，只要引用spring-jdbc包，DataSource自动初始话过程就会启用。注意：spring-orm包引用了spring-jdbc。
DataSourceAutoConfiguration按照顺序，如果当前存在签入数据库引用，则创建签入数据源， 否侧尝试创建Hikari/Tomcat/Dbcp2/OracleUcp 等连接池类型的数据源

-----------

只要项目引用了hibernate，spring-orm， 并且当前容器中只有一个DataSource bean, 这个open-in-view默认会启用，如果需要禁用，则要显示设置 spring.jpa.open-in-view=false

-----------

可以在代码中设置spring.jpa.open-in-view默认值：

public static void main(String[] args) {

		new SpringApplicationBuilder(DemoApplication.class)
				.properties(props())
				.build()
				.run(args);

		//SpringApplication.run(DemoApplication.class, args);
	}

	private static Properties props() {
		Properties properties = new Properties();
		properties.setProperty("spring.jpa.open-in-view", "false");
		return properties;
	}