002-原始jpa以及基本加载过程

一、概述

1.1、创建

    public void createMethod() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("");
        EntityManager manager = factory.createEntityManager();
        manager.getTransaction().begin();// 开始事务
        manager.persist(new Object());
        manager.getTransaction().commit();
        manager.close();
        factory.close();
    }

1.2、查询

    // 立即加载
    public void getMethod() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("");
        EntityManager manager = factory.createEntityManager();
        Object object = manager.find(Object.class, 1);
        System.out.println(object);// 如果Object是null，没有异常打印null
        manager.close();
        factory.close();
    }

    // 延迟加载
    public void getMethod2() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("");
        EntityManager manager = factory.createEntityManager();
        // 延迟加载，不立即返回，返回一个代理对象，只有读取对象具体属性时，才返回具体对象
        Object object = manager.getReference(Object.class, 1);
        // 如果数据库没有object,在读取属性是出现异常
        // object.getName();
        manager.close();
        // 关闭后再读取有问题
        // object.getName();
        factory.close();
    }

1.3、更新

    public void updateMethod() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("");
        EntityManager manager = factory.createEntityManager();
        manager.getTransaction().begin();// 开始事务
        Object object = manager.find(Object.class, 1);
        object,setName("测试");
        manager.getTransaction().commit();
        manager.close();
        factory.close();
        // 四种 实体状态：new新建状态； managed托管状态，能够被更新【托管状态，被事务管理】，放到jdbc批处理中，commit之后开始提交；游离状态【托管】；删除状态
    }
    public void updateMethod2() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("");
        EntityManager manager = factory.createEntityManager();
        manager.getTransaction().begin();// 开始事务
        Object object = manager.find(Object.class, 1);
        manager.clear();//把实体管理器中的所有实体变成游离状态，此时提交;不会被更新，需要做以下处理
        object,setName("测试");
        manager.merge(object);//把实体放入实体管理器中，再次提交可以被更新
        manager.getTransaction().commit();
        manager.close();
        factory.close();
        // 四种 实体状态：new新建状态； managed托管状态，能够被更新【托管状态，被事务管理】，放到jdbc批处理中，commit之后开始提交；游离状态【托管】；删除状态
    }

1.4、删除

    public void deleteMethod() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("");
        EntityManager manager = factory.createEntityManager();
        manager.getTransaction().begin();// 开始事务
        Object object = manager.find(Object.class, 1);
        manager.remove(object);
        manager.getTransaction().commit();
        manager.close();
        factory.close();
    }

二、基本加载过程

EntityManagerFactory factory = Persistence.createEntityManagerFactory("");

查看：Persistence，javax.persistence.Persistence，其中：createEntityManagerFactory代码如下

    public static EntityManagerFactory createEntityManagerFactory(String persistenceUnitName, Map properties) {
        EntityManagerFactory emf = null;
        List<PersistenceProvider> providers = getProviders();//1
        for ( PersistenceProvider provider : providers ) {
            emf = provider.createEntityManagerFactory( persistenceUnitName, properties );//2
            if ( emf != null ) {
                break;
            }
        }
        if ( emf == null ) {
            throw new PersistenceException( "No Persistence provider for EntityManager named " + persistenceUnitName );
        }
        return emf;
    }

对于1句，

    private static List<PersistenceProvider> getProviders() {
        return PersistenceProviderResolverHolder
                .getPersistenceProviderResolver()
                .getPersistenceProviders();
    }

查看代码，查看PersistenceProviderResolverHolder的私有静态类CachingPersistenceProviderResolver，其中读取

Enumeration<URL> resources = cl.getResources( "META-INF/services/" + PersistenceProvider.class.getName() );

对于第二句

provider.createEntityManagerFactory(

选择任意一个持久化接口创建

三、sql语句的使用【增删改查】

    public void sqlQueryMethod() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("");
        EntityManager manager = factory.createEntityManager();
        // 面向对象查询语句
        // 位置参数
        Query query = manager.createQuery("selet o from Person o where o.id=?1");
        query.setParameter(1, 222);
        // 类型参数
        // Query query2 = manager.createQuery("selet o from Person o where o.id=:id");
        // query2.setParameter("id", 222);

        List resultList = query.getResultList();

        //如果只有一个值
        //query.getSingleResult();
        manager.close();
        factory.close();
    }
    public void sqlDeleteMethod() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("");
        EntityManager manager = factory.createEntityManager();
        manager.getTransaction().begin();// 开始事务
        // 面向对象查询语句
        // 位置参数
        Query query = manager.createQuery("delete from Person o where o.id=?1");
        query.setParameter(1, 222);
        query.executeUpdate();
        manager.getTransaction().commit();
        manager.close();
        factory.close();
    }
    public void sqlUpdateMethod() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("");
        EntityManager manager = factory.createEntityManager();
        manager.getTransaction().begin();// 开始事务
        // 面向对象查询语句
        // 位置参数
        Query query = manager.createQuery("update Person 0 set o.name=:name where  o.id=:id");
        query.setParameter("name", "zahangsan");
        query.setParameter("id", 11);
        query.executeUpdate();
        manager.getTransaction().commit();
        manager.close();
        factory.close();
    }

份

原文地址：https://www.cnblogs.com/bjlhx/p/9123916.html