基于数据源连接，动态构造JPA上下文EntityManager

因产品存在支持多种数据库平台的需求，采用JPA方式可以有效屏蔽不同数据库的语法差异，尤其是分页查询场景。

但访问异构数据源时，需要基于数据库连接信息动态构造JPA上下文，基于大模型查询了几种实现并进行验证可行的方式如下：

当前需求仅是写标准SQL使用JPA的Query对象以native方式执行即可满足，实际上JPA上下文也可以通过PersistenceUnitInfo 的entityClasses加载实体类，进而使用JPA实体方式访问数据库。

import javax.persistence.*;
import javax.persistence.spi.*;
import javax.sql.DataSource;
import java.net.URL;
import java.util.*;
import java.util.stream.Collectors;/*** 动态构造JPA上下文的工具类*/
public class DynamicJpaContext {// 线程安全的EntityManagerFactory缓存private static final Map<String, EntityManagerFactory> emfCache = new HashMap<>();public static void main(String[] args) {// 1. 数据库连接信息String driverClass = "com.mysql.cj.jdbc.Driver";String url = "jdbc:mysql://localhost:3306/testdb?useSSL=false&serverTimezone=UTC";String username = "tkk";String password = "tkk123A?";String dialect = "org.hibernate.dialect.MySQL8Dialect";EntityManager em = DynamicJpaContext2.createEntityManager("dynamic-unit-mysql", driverClass, url, username, password, dialect);try {Query query = em.createNativeQuery("select * from tb1");List list = query.setFirstResult(1).setMaxResults(10).getResultList();EntityTransaction transaction = em.getTransaction();try {transaction.begin();Query upd = em.createNativeQuery("update tb1 set vname = :v1 where oid = :o1");String p1 = "v1";String p2 = "o1";upd.setParameter(p1, "sss");upd.setParameter(p2, 2);int cnt = upd.executeUpdate();transaction.commit();} catch (Exception e) {if (transaction.isActive()) {transaction.rollback();}e.printStackTrace();}} catch (Exception e) {} finally {if (em != null && em.isOpen()) {em.close();}}// 应用关闭时清理资源//Runtime.getRuntime().addShutdownHook(new Thread(DynamicJpaContext::closeAll));
    }/*** 根据数据库连接信息创建EntityManager** @param driverClass 数据库驱动类* @param url 数据库连接URL* @param username 数据库用户名* @param password 数据库密码* @param dialect 数据库方言* @return EntityManager实例*/public static EntityManager createEntityManager(String persistenceUnitName,String driverClass,String url,String username,String password,String dialect) {// 创建唯一标识，用于缓存EntityManagerFactoryString cacheKey = generateCacheKey(url, username);// 检查缓存中是否已有对应的EntityManagerFactoryEntityManagerFactory emf = emfCache.get(cacheKey);if (emf == null || emf.isOpen() == false) {// 配置JPA属性Map<String, Object> properties = configureJpaProperties(driverClass, url, username, password, dialect);//// 创建EntityManagerFactory//emf = new HibernatePersistenceProvider()//        .createContainerEntityManagerFactory(//                new DynamicPersistenceUnitInfo("dynamic-unit"),//                properties);
PersistenceProviderResolver resolver = PersistenceProviderResolverHolder.getPersistenceProviderResolver();List<PersistenceProvider> providers = resolver.getPersistenceProviders();for (PersistenceProvider provider : providers) {emf = provider.createContainerEntityManagerFactory(new DefaultPersistenceUnitInfo(persistenceUnitName), properties);if (emf != null) {break;}}if (emf == null) {throw new PersistenceException("No Persistence provider for EntityManager named " + persistenceUnitName);}// 存入缓存
            emfCache.put(cacheKey, emf);}return emf.createEntityManager();}/*** 配置JPA属性*/private static Map<String, Object> configureJpaProperties(String driverClass, String url, String username, String password, String dialect) {Map<String, Object> properties = new HashMap<>();// 数据库连接配置properties.put("javax.persistence.jdbc.driver", driverClass);properties.put("javax.persistence.jdbc.url", url);properties.put("javax.persistence.jdbc.user", username);properties.put("javax.persistence.jdbc.password", password);// Hibernate配置properties.put("hibernate.dialect", dialect);return properties;}/*** 生成缓存键，基于URL和用户名*/private static String generateCacheKey(String url, String username) {return url + "_" + username;}/*** 关闭所有缓存的EntityManagerFactory*/public static void closeAll() {for (EntityManagerFactory emf : emfCache.values()) {if (emf.isOpen()) {emf.close();}}emfCache.clear();}// 内部类：默认实现private static class DefaultPersistenceUnitInfo implements PersistenceUnitInfo {private final String unitName;private final PersistenceUnitTransactionType transactionType = PersistenceUnitTransactionType.RESOURCE_LOCAL;private final List<Class<?>> entityClasses = new ArrayList<>();private final Properties properties= new Properties();public DefaultPersistenceUnitInfo(String unitName) {this.unitName = unitName;this.properties.put("hibernate.hbm2ddl.auto", "none"); // 自动更新表结构this.properties.put("hibernate.show_sql", "false"); // 显示SQL语句this.properties.put("hibernate.format_sql", "false"); // 格式化SQL语句this.properties.put("hibernate.connection.autocommit", "false"); // 关闭自动提交
        }@Overridepublic String getPersistenceUnitName() {return unitName;}@Overridepublic String getPersistenceProviderClassName() {return null;}@Overridepublic PersistenceUnitTransactionType getTransactionType() {return transactionType;}@Overridepublic DataSource getJtaDataSource() {return null;}@Overridepublic DataSource getNonJtaDataSource() {return null;}@Overridepublic List<String> getManagedClassNames() {return entityClasses.stream().map(Class::getName).collect(Collectors.toList());}// 以下为固定默认实现
        @Overridepublic List<String> getMappingFileNames() {return new ArrayList<>();}@Overridepublic List<URL> getJarFileUrls() {return new ArrayList<>();}@Overridepublic URL getPersistenceUnitRootUrl() {return null;}@Overridepublic boolean excludeUnlistedClasses() {return false;}@Overridepublic SharedCacheMode getSharedCacheMode() {return SharedCacheMode.UNSPECIFIED;}@Overridepublic ValidationMode getValidationMode() {return ValidationMode.AUTO;}@Overridepublic Properties getProperties() {return properties;}@Overridepublic String getPersistenceXMLSchemaVersion() {return "2.2";}@Overridepublic ClassLoader getClassLoader() {return Thread.currentThread().getContextClassLoader();}@Overridepublic void addTransformer(ClassTransformer transformer) {}@Overridepublic ClassLoader getNewTempClassLoader() {return null;}}
}