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问如何从EntityManager中分离实体
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Stack Overflow用户

提问于 2018-08-14 05:06:27

回答 3查看 2.3K关注 0票数 1

我的环境

Java 7/JPA 2/Hibernate 5.1。

我的情景

我正在构建一个存储库模式实现。所有的代码都是编写的，当没有发生错误情况时，所有代码都可以正常工作。

但是，假设将三个实体实例添加到存储库中。第一列和第三列都可以，但第二列缺少强制(非空)列的值。保存存储库时将返回DB错误。

当面对这种情况时，批处理过程应该只写一个日志消息，跳过无效的对象，然后继续到其他的对象。为了做到这一点，这个无效的实体只应该从存储库中删除，从存储库使用的底层EntityManager中分离它意味着什么。

我的问题

repository.exclude(entity)方法调用(在内部将实体与EntityManager分离)似乎不起作用，第二次保存存储库的尝试再次失败。

我的(部分) AbstractRepository类

public abstract class AbstractRepository<T> {
    private static Map<String,EntityManagerFactory> entityManagerFactories = new HashMap<String,EntityManagerFactory>();
    private static Map<EntityManagerFactory,EntityManager> entityManagers = new HashMap<EntityManagerFactory,EntityManager>();
    private EntityManager entityManager;
    private List<T> addedEntities = new ArrayList<T>();
    private List<T> deletedEntities = new ArrayList<T>();
    private List<T> updatedEntities = new ArrayList<T>();

    protected Class<T> entityClass = getEntityClass();

    // Many other declarations

    protected EntityManager createEntityManager() throws Exception {
        String persistenceUnitName = getPersistenceUnitName(); // using Reflection
        EntityManagerFactory entityManagerFactory = getEntityManagerFactory(persistenceUnitName);
        EntityManager entityManager = entityManagerFactory.createEntityManager();
        return entityManager;
    }

    public T add(T entity) {
        addedEntities.add(entity);
        return entity;
    }

    public void save() throws Exception {
        EntityManager entityManager = getEntityManager();
        EntityTransaction transaction = entityManager.getTransaction(); 
        transaction.begin();
        try {
            for(T entity : addedEntities)
                entityManager.persist(entity);
            for(T entity : updatedEntities)
                entityManager.merge(entity);
            for(T entity : deletedEntities)
                entityManager.remove(entity);
            transaction.commit();
        } catch(Exception e) {
            if(transaction.isActive())
                transaction.rollback();
            throw e;
        }
        addedEntities.clear();
        updatedEntities.clear();
        deletedEntities.clear();
    }

    public T exclude(T entity) throws Exception {
        if(entity == null)
            return null;
        addedEntities.remove(entity);
        deletedEntities.remove(entity);
        updatedEntities.remove(entity);
        getEntityManager().detach(entity);
        return entity;
    }

    public EntityManager getEntityManager() throws Exception {
        if(entityManager == null)
            entityManager = createEntityManager();
        return entityManager;
    }
}

我的仓库声明

@PersistenceUnit(unitName = "my-ds")
public class MestreRepository extends AbstractRepository<Mestre, Long> {
    public List<Mestre> all() throws Exception {
        List<Mestre> result = getEntityManager().createQuery("from Mestre", Mestre.class).getResultList();
        return result;
    }
}

我的测试代码

public class Main {
    public static void main(String[] args) {
        MestreRepository allMestres = new MestreRepository();

        Mestre mestre1 = new Mestre();
        mestre1.setNome("Mestre 1");
        Mestre mestre2 = new Mestre(); // This one lacks Nome and will fail to be saved
        Mestre mestre3 = new Mestre();
        mestre3.setNome("Mestre 3");

        allMestres.add(mestre1);
        allMestres.add(mestre2);
        allMestres.add(mestre3);

        System.out.println("Saving 3 mestres");
        try {
            allMestres.save();
            System.out.println("All 3 mestres saved"); // never happens!
        } catch(Exception e) {
            System.out.println("Error when salving 3 mestres");
            try {
                System.out.println("Excluding mestre 2");
                allMestres.exclude(mestre2);
                System.out.println("Salving other 2 mestres");
                allMestres.save();
                System.out.println("All 2 mestres salved"); // never happens!
            } catch(Exception e2) {
                System.out.println("Still having errors");
            }
        }

        allMestres.close();
    }
}

我的persistence.xml

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1"
    xmlns="http://xmlns.jcp.org/xml/ns/persistence"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
    <persistence-unit name="my-ds" transaction-type="RESOURCE_LOCAL">
        <class>domain.Mestre</class>
        <class>domain.Detalhe</class>

        <exclude-unlisted-classes>true</exclude-unlisted-classes>
        <properties>
            <!-- Hibernate properties -->
            <property name="hibernate.connection.driver_class" value="oracle.jdbc.OracleDriver"/>
            <property name="hibernate.connection.url" value="xxx"/>
            <property name="hibernate.connection.username" value="yyy"/>
            <property name="hibernate.connection.password" value="***"/>
            <property name="hibernate.dialect" value="org.hibernate.dialect.Oracle10gDialect"/>
            <property name="hibernate.show_sql" value="true"/>
            <property name="hibernate.format_sql" value="true"/>
            <property name="hibernate.use_sql_comments" value="true"/>
        </properties>
    </persistence-unit>
 </persistence>

更新了save()方法

下面是save()方法的一个新版本，它可以使事情正常工作。它需要在flush()之前调用commit()，对于那些没有造成任何问题的实体，不需要调用persist()，而是调用merge()，因为它们已经有了一个Id。

public void save() throws Exception {
    List<T> processedEntities = new ArrayList<T>();
    EntityManager entityManager = getEntityManager();
    EntityTransaction transaction = entityManager.getTransaction(); 
    transaction.begin();
    try {
        for(T entity : addedEntities) {
            entityManager.persist(entity);
            processedEntities.add(entity);
        }
        for(T entity : updatedEntities)
            entityManager.merge(entity);
        for(T entity : deletedEntities)
            entityManager.merge(entity);
        entityManager.flush();
        transaction.commit();
    } catch(Exception e) {
        updatedEntities.addAll(processedEntities);
        addedEntities.removeAll(processedEntities);

        if(transaction.isActive())
            transaction.rollback();
        throw e;
    }
    addedEntities.clear();
    updatedEntities.clear();
    deletedEntities.clear();
}

java

hibernate

jpa-2.0

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回答 3

Stack Overflow用户

回答已采纳

发布于 2018-08-15 02:53:21

将评论转换为回答：

根据本指南，在分离实体之前需要flush()

更新OP的代码：

public void save() throws Exception {
    List<T> processedEntities = new ArrayList<T>();
    EntityManager entityManager = getEntityManager();
    EntityTransaction transaction = entityManager.getTransaction(); 
    transaction.begin();
    try {
        for(T entity : addedEntities) {
            entityManager.persist(entity);
            processedEntities.add(entity);
        }
        for(T entity : updatedEntities)
            entityManager.merge(entity);
        for(T entity : deletedEntities)
            entityManager.merge(entity);
        entityManager.flush();
        transaction.commit();
    } catch(Exception e) {
        updatedEntities.addAll(processedEntities);
        addedEntities.removeAll(processedEntities);

        if(transaction.isActive())
            transaction.rollback();
        throw e;
    }
    addedEntities.clear();
    updatedEntities.clear();
    deletedEntities.clear();
}

票数 4

Stack Overflow用户

发布于 2022-04-11 11:12:51

警诫

显然，单独使用merge进行更新是不够的。

for(T entity : updatedEntities)
        entityManager.merge(entity);

合并将将分离的实体状态(源)复制到托管实体实例(目标)，因此，要从数据源更新实体，首先需要将它们带到托管状态，然后按您的意愿分离和静音detach实体，然后调用merge使其再次进入托管状态以与flush或commit同步。

因此，用于更新的适当代码如下所示

    public <T> BehindCacheDirector<R, V> update(Class<?> type, T t, Long v) {
            entityManager.detach(entityManager.find(type, v));
            entityManager.merge(t);
    ...        
    }

对于delete，您需要将要删除的实体更改为托管状态，然后将其状态更改为删除状态，然后调用同步命令，即

    public BehindCacheBuilder<R, V> remove(Class<?> type, Object object) {
        entityManager.remove(entityManager.find(type, object));
    ...
    }