我有两个类(class)，Test2 和 Test3。 Test2 有一个属性 test3，它是 Test3 的一个实例。换句话说，我有一个单向的 OneToOne 关联，其中 test2 引用了 test3。

当我从数据库中选择 Test2 时，我可以看到正在进行单独的选择以获取关联的 test3 类的详细信息。这就是著名的 1+N 选择问题。

要解决此问题以使用单个选择，我正在尝试使用 fetch=join 注释，据我所知是 @Fetch(FetchMode.JOIN)

但是，在 fetch 设置为 join 的情况下，我仍然看到单独的选择。这是我设置的相关部分..

hibernate .cfg.xml:

<property name="max_fetch_depth">2</property>

测试 2:

public class Test2 {
 @OneToOne (cascade=CascadeType.ALL , fetch=FetchType.EAGER)
 @JoinColumn (name="test3_id")
 @Fetch(FetchMode.JOIN)
 public Test3 getTest3() {
  return test3;
}

注意，出于绝望，我将 FetchType 设置为 EAGER，即使对于 OneToOne 映射它默认为 EAGER，但它没有任何区别。

感谢您的帮助!

编辑:我几乎已经放弃尝试使用 FetchMode.JOIN - 任何人都可以确认他们已经让它工作，即产生左外连接吗？ 在文档中我看到“通常，映射文档不用于自定义提取。相反，我们保留默认行为，并为特定事务覆盖它，使用 HQL 中的左连接提取”

如果我改用左连接获取:

query = session.createQuery("from Test2 t2 left join fetch t2.test3");

然后我确实得到了我想要的结果 - 即查询中的左外连接。

编辑编号 2:

伙计们，非常感谢您的回复。现在我想弄个水落石出。我通常发现，当我调查某事时，我最终学到的东西比我想象的要多得多。

有一件事我已经学会了——我在旧版本的 hibernate 上运行，因为我没有意识到 maven 存储库已经过时了。现在我也连接到 jboss 存储库，并且我拥有最新版本的 hibernate 和 hibernate 注释 - 两种情况下都是 3.5.1-Final。

我已经设置了一个尽可能简化它的小测试用例 - 我仍然在 3.5.1-Final 中看到这个问题，虽然我 99% 确定这只是一些愚蠢的东西我是设置不正确，尤其是 Ross，因为你让它工作了(感谢你花时间顺便尝试一下)

所以我有这些类(class)(这次是全文)

A级

package com.play.hibernate2;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToOne;

import org.hibernate.annotations.Fetch;
import org.hibernate.annotations.FetchMode;

@Entity
public class A {

    private Integer id;
    private B b;

    public A() {
        super();
    }

    @Id
    @GeneratedValue
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @OneToOne (cascade=CascadeType.ALL)
    @Fetch(FetchMode.JOIN)
    public B getB() {
        return b;
    }

    public void setB(B b) {
        this.b = b;
    }
}

B级

package com.play.hibernate2;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;

@Entity
public class B {

    private Integer id;

    public B() {
        super();
    }

    @Id
    @GeneratedValue
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }   
}

我的整个hibernate.cfg.xml

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

    <session-factory>

        <!-- Database connection settings -->
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <!-- <property name="connection.driver_class">com.p6spy.engine.spy.P6SpyDriver</property> -->
        <property name="connection.url">jdbc:mysql://localhost:3306/play</property>
        <property name="connection.username">play</property>
        <property name="connection.password">play</property>

        <!-- JDBC connection pool (use the built-in) -->
        <property name="connection.pool_size">1</property>

        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>

        <!-- Enable Hibernate's automatic session context management -->
        <property name="current_session_context_class">thread</property>

        <!-- Disable the second-level cache  -->
        <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>

        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>
        <property name="generate_statistics">true</property>
        <!--
        <property name="cache.use_structured_entries">true</property>
        <property name="cache.use_query_cache">true</property>
        -->
        <property name="format_sql">true</property>
        <property name="use_sql_comments">true</property>

        <!-- I think this may fix my individual requests for OneToOne problem -->
        <property name="max_fetch_depth">2</property>
        <!-- <property name="default_batch_fetch_size">10</property> -->

    </session-factory>    

</hibernate-configuration>

测试类

package com.play.hibernate2;

import java.util.List;
import java.util.Map;


import org.hibernate.FlushMode;
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.tool.hbm2ddl.SchemaExport;

public class RunTests4 {
    private SessionFactory sessionFactory;

    public static void main(String[] args){
        RunTests4 d = new RunTests4();
        d.run3();
    }
    public void run3(){

        Session session = getSession();
        session.beginTransaction();

        createEntities(session);

        session.getTransaction().commit();

        System.out.println("NOW WITH A NEW TRANSACTION");
        session = getSession();
        session.beginTransaction();

        Query query = session.createQuery("from A");
        List results = query.list();
        for (int i=0; i<results.size(); i++){
            System.out.println("Row "+i+" was:");
            A a = (A)results.get(i);
            System.out.println("Result "+i);
            System.out.println(a.toString());
        }

        session.getTransaction().commit();


    }
    public void createEntities(Session session){
        for (int i=0; i<2; i++){
            A a = new A();

            B b = new B();

            a.setB(b);

            session.save(a);

        }

    }
    public Session getSession(){
        if (sessionFactory == null){
            AnnotationConfiguration config = new AnnotationConfiguration();
            config.addAnnotatedClass(A.class);
            config.addAnnotatedClass(B.class);
            config.configure();
            new SchemaExport(config).create(true,true);

            sessionFactory = config.buildSessionFactory();
        }
        Session session = sessionFactory.getCurrentSession();

        return session;
    }

}

最后的日志输出显示了额外的选择以取回关联的类

2 [main] INFO org.hibernate.cfg.annotations.Version - Hibernate Annotations 3.5.1-Final
23 [main] INFO org.hibernate.cfg.Environment - Hibernate 3.5.1-Final
28 [main] INFO org.hibernate.cfg.Environment - hibernate.properties not found
32 [main] INFO org.hibernate.cfg.Environment - Bytecode provider name : javassist
37 [main] INFO org.hibernate.cfg.Environment - using JDK 1.4 java.sql.Timestamp handling
160 [main] INFO org.hibernate.annotations.common.Version - Hibernate Commons Annotations 3.2.0.Final
176 [main] INFO org.hibernate.cfg.Configuration - configuring from resource: /hibernate.cfg.xml
176 [main] INFO org.hibernate.cfg.Configuration - Configuration resource: /hibernate.cfg.xml
313 [main] INFO org.hibernate.cfg.Configuration - Configured SessionFactory: null
338 [main] INFO org.hibernate.dialect.Dialect - Using dialect: org.hibernate.dialect.MySQLDialect
462 [main] INFO org.hibernate.cfg.AnnotationBinder - Binding entity from annotated class: com.play.hibernate2.Test2
545 [main] INFO org.hibernate.cfg.annotations.EntityBinder - Bind entity com.play.hibernate2.Test2 on table Test2
649 [main] INFO org.hibernate.cfg.AnnotationBinder - Binding entity from annotated class: com.play.hibernate2.Test3
650 [main] INFO org.hibernate.cfg.annotations.EntityBinder - Bind entity com.play.hibernate2.Test3 on table Test3
651 [main] INFO org.hibernate.cfg.AnnotationBinder - Binding entity from annotated class: com.play.hibernate2.A
651 [main] INFO org.hibernate.cfg.annotations.EntityBinder - Bind entity com.play.hibernate2.A on table A
653 [main] INFO org.hibernate.cfg.AnnotationBinder - Binding entity from annotated class: com.play.hibernate2.B
653 [main] INFO org.hibernate.cfg.annotations.EntityBinder - Bind entity com.play.hibernate2.B on table B
678 [main] INFO org.hibernate.cfg.AnnotationConfiguration - Hibernate Validator not found: ignoring
687 [main] INFO org.hibernate.tool.hbm2ddl.SchemaExport - Running hbm2ddl schema export
688 [main] INFO org.hibernate.tool.hbm2ddl.SchemaExport - exporting generated schema to database
691 [main] INFO org.hibernate.connection.DriverManagerConnectionProvider - Using Hibernate built-in connection pool (not for production use!)
691 [main] INFO org.hibernate.connection.DriverManagerConnectionProvider - Hibernate connection pool size: 1
698 [main] INFO org.hibernate.connection.DriverManagerConnectionProvider - autocommit mode: false
711 [main] INFO org.hibernate.connection.DriverManagerConnectionProvider - using driver: com.mysql.jdbc.Driver at URL: jdbc:mysql://localhost:3306/play
711 [main] INFO org.hibernate.connection.DriverManagerConnectionProvider - connection properties: {user=play, password=****}

    alter table A 
        drop 
        foreign key FK412E010759

    alter table Test2 
        drop 
        foreign key FK4CF5DC04B7E1B79

    drop table if exists A

    drop table if exists B

    drop table if exists Test2

    drop table if exists Test3

    create table A (
        id integer not null auto_increment,
        b_id integer,
        primary key (id)
    )

    create table B (
        id integer not null auto_increment,
        primary key (id)
    )

    create table Test2 (
        id integer not null auto_increment,
        name varchar(255),
        value integer not null,
        test3_id integer,
        primary key (id)
    )

    create table Test3 (
        id integer not null auto_increment,
        name varchar(255),
        value integer not null,
        primary key (id)
    )

    alter table A 
        add index FK412E010759 (b_id), 
        add constraint FK412E010759 
        foreign key (b_id) 
        references B (id)

    alter table Test2 
        add index FK4CF5DC04B7E1B79 (test3_id), 
        add constraint FK4CF5DC04B7E1B79 
        foreign key (test3_id) 
        references Test3 (id)
2562 [main] INFO org.hibernate.tool.hbm2ddl.SchemaExport - schema export complete
2564 [main] INFO org.hibernate.connection.DriverManagerConnectionProvider - cleaning up connection pool: jdbc:mysql://localhost:3306/play
2571 [main] INFO org.hibernate.cfg.search.HibernateSearchEventListenerRegister - Unable to find org.hibernate.search.event.FullTextIndexEventListener on the classpath. Hibernate Search is not enabled.
2575 [main] INFO org.hibernate.connection.DriverManagerConnectionProvider - Using Hibernate built-in connection pool (not for production use!)
2575 [main] INFO org.hibernate.connection.DriverManagerConnectionProvider - Hibernate connection pool size: 1
2575 [main] INFO org.hibernate.connection.DriverManagerConnectionProvider - autocommit mode: false
2575 [main] INFO org.hibernate.connection.DriverManagerConnectionProvider - using driver: com.mysql.jdbc.Driver at URL: jdbc:mysql://localhost:3306/play
2575 [main] INFO org.hibernate.connection.DriverManagerConnectionProvider - connection properties: {user=play, password=****}
2622 [main] INFO org.hibernate.cfg.SettingsFactory - RDBMS: MySQL, version: 5.1.30
2622 [main] INFO org.hibernate.cfg.SettingsFactory - JDBC driver: MySQL-AB JDBC Driver, version: mysql-connector-java-5.1.9 ( Revision: ${svn.Revision} )
2633 [main] INFO org.hibernate.dialect.Dialect - Using dialect: org.hibernate.dialect.MySQLDialect
2635 [main] INFO org.hibernate.engine.jdbc.JdbcSupportLoader - Disabling contextual LOB creation as JDBC driver reported JDBC version [3] less than 4
2636 [main] INFO org.hibernate.transaction.TransactionFactoryFactory - Using default transaction strategy (direct JDBC transactions)
2638 [main] INFO org.hibernate.transaction.TransactionManagerLookupFactory - No TransactionManagerLookup configured (in JTA environment, use of read-write or transactional second-level cache is not recommended)
2638 [main] INFO org.hibernate.cfg.SettingsFactory - Automatic flush during beforeCompletion(): disabled
2638 [main] INFO org.hibernate.cfg.SettingsFactory - Automatic session close at end of transaction: disabled
2638 [main] INFO org.hibernate.cfg.SettingsFactory - JDBC batch size: 15
2638 [main] INFO org.hibernate.cfg.SettingsFactory - JDBC batch updates for versioned data: disabled
2638 [main] INFO org.hibernate.cfg.SettingsFactory - Scrollable result sets: enabled
2638 [main] INFO org.hibernate.cfg.SettingsFactory - JDBC3 getGeneratedKeys(): enabled
2638 [main] INFO org.hibernate.cfg.SettingsFactory - Connection release mode: auto
2639 [main] INFO org.hibernate.cfg.SettingsFactory - Maximum outer join fetch depth: 2
2639 [main] INFO org.hibernate.cfg.SettingsFactory - Default batch fetch size: 1
2639 [main] INFO org.hibernate.cfg.SettingsFactory - Generate SQL with comments: enabled
2639 [main] INFO org.hibernate.cfg.SettingsFactory - Order SQL updates by primary key: disabled
2639 [main] INFO org.hibernate.cfg.SettingsFactory - Order SQL inserts for batching: disabled
2639 [main] INFO org.hibernate.cfg.SettingsFactory - Query translator: org.hibernate.hql.ast.ASTQueryTranslatorFactory
2641 [main] INFO org.hibernate.hql.ast.ASTQueryTranslatorFactory - Using ASTQueryTranslatorFactory
2641 [main] INFO org.hibernate.cfg.SettingsFactory - Query language substitutions: {}
2641 [main] INFO org.hibernate.cfg.SettingsFactory - JPA-QL strict compliance: disabled
2641 [main] INFO org.hibernate.cfg.SettingsFactory - Second-level cache: enabled
2641 [main] INFO org.hibernate.cfg.SettingsFactory - Query cache: disabled
2644 [main] INFO org.hibernate.cfg.SettingsFactory - Cache region factory : org.hibernate.cache.impl.bridge.RegionFactoryCacheProviderBridge
2644 [main] INFO org.hibernate.cache.impl.bridge.RegionFactoryCacheProviderBridge - Cache provider: org.hibernate.cache.NoCacheProvider
2644 [main] INFO org.hibernate.cfg.SettingsFactory - Optimize cache for minimal puts: disabled
2644 [main] INFO org.hibernate.cfg.SettingsFactory - Structured second-level cache entries: disabled
2648 [main] INFO org.hibernate.cfg.SettingsFactory - Echoing all SQL to stdout
2648 [main] INFO org.hibernate.cfg.SettingsFactory - Statistics: enabled
2649 [main] INFO org.hibernate.cfg.SettingsFactory - Deleted entity synthetic identifier rollback: disabled
2649 [main] INFO org.hibernate.cfg.SettingsFactory - Default entity-mode: pojo
2649 [main] INFO org.hibernate.cfg.SettingsFactory - Named query checking : enabled
2649 [main] INFO org.hibernate.cfg.SettingsFactory - Check Nullability in Core (should be disabled when Bean Validation is on): enabled
2697 [main] INFO org.hibernate.impl.SessionFactoryImpl - building session factory
2796 [Finalizer] INFO org.hibernate.connection.DriverManagerConnectionProvider - cleaning up connection pool: jdbc:mysql://localhost:3306/play
2929 [main] INFO org.hibernate.impl.SessionFactoryObjectFactory - Not binding factory to JNDI, no JNDI name configured
Hibernate: 
    /* insert com.play.hibernate2.B
        */ insert 
        into
            B

        values
            ( )
Hibernate: 
    /* insert com.play.hibernate2.A
        */ insert 
        into
            A
            (b_id) 
        values
            (?)
Hibernate: 
    /* insert com.play.hibernate2.B
        */ insert 
        into
            B

        values
            ( )
Hibernate: 
    /* insert com.play.hibernate2.A
        */ insert 
        into
            A
            (b_id) 
        values
            (?)
NOW WITH A NEW TRANSACTION
Hibernate: 
    /* 
from
    A */ select
        a0_.id as id2_,
        a0_.b_id as b2_2_ 
    from
        A a0_
Hibernate: 
    /* load com.play.hibernate2.B */ select
        b0_.id as id3_0_ 
    from
        B b0_ 
    where
        b0_.id=?
Hibernate: 
    /* load com.play.hibernate2.B */ select
        b0_.id as id3_0_ 
    from
        B b0_ 
    where
        b0_.id=?
Row 0 was:
Result 0
com.play.hibernate2.A@351daa0e
Row 1 was:
Result 1
com.play.hibernate2.A@2e879860

编辑数字 3:

如果我按 Ross 的方式做事，在加载时，将创建一个左外连接。如果我用列表来做，会发出单独的选择。这是相关的代码。只有改变它才能重现行为上的差异:

    /* generates the left outer join
    A a = (A)session.load(A.class,1);
    System.out.println(a.getId()+" = "+a.getB().getName());
    */

    // Creates separate selects for each object b associated with each a
    Query query = session.createQuery("from A");
    List results = query.list();
    A a = (A)results.get(0);
    System.out.println(a.getId()+" = "+a.getB().getName());

我猜它可能被称为“错误”。正如我之前提到的，在文档中，他们说在 HQL 中而不是在映射中指定获取模式是“常见的”，我认为这可能意味着 HQL 方式有更多的人流量来解决它。 .?

(顺便说一下，我向 A 和 B 添加了一个额外的“名称”字段，否则 hibernate 会优化检索，因为它可以仅从 A 上的外键获取所有 B)

最佳答案

作为蒸馏:

如果您使用 Query 接口(interface)(例如:session.createQuery())，@Fetch(JOIN) 将被忽略，但如果您使用 Criteria 接口(interface)，它将被正确使用。

这实际上是 Hibernate 中的一个错误，从未被解决。不幸的是，很多应用程序使用 Query 接口(interface)，无法轻松迁移到 Criteria 接口(interface)。

如果您使用查询接口(interface)，您总是必须手动将 JOIN FETCH 语句添加到 HQL 中。

关于java - 在 Hibernate 中使用@Fetch(FetchMode.Join)，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/2931936/