Hibernate session.get 对现有记录返回 Null

问题描述 投票：0回答：1

我在 Java 应用程序中使用 Hibernate 通过主键从数据库中检索 Alien 对象。但是，即使我知道数据库中存在该记录，检索到的对象仍为空。这是我的代码：

package com.hks.DemoHib;

import java.util.Collection;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;

public class App {
    public static void main(String[] args) {
        Configuration con = new Configuration().configure().addAnnotatedClass(Laptop.class).addAnnotatedClass(Alien.class);

        SessionFactory sf = con.buildSessionFactory();
        Session session = sf.openSession();
        session.beginTransaction();

        Alien a1 = session.get(Alien.class, 1);

        if (a1 == null) {
            System.out.println("Alien with ID 1 not found!");
        } else {
            a1.setAname("Harsh");
            System.out.println(a1.getAname());

            Collection<Laptop> laps = a1.getLaps();
            for (Laptop l : laps) {
                System.out.println(l);
            }
        }

        session.getTransaction().commit();
    }
}

这是我的实体类：

外星人.java

import javax.persistence.*;
import java.util.Collection;

@Entity
@Table(name = "Alien")
public class Alien {
    @Id
    private int aid;
    private String aname;

    @OneToMany(mappedBy = "alien")
    private Collection<Laptop> laps;

    // Getters and Setters
}

笔记本电脑.java

import javax.persistence.*;

@Entity
@Table(name = "Laptop")
public class Laptop {
    @Id
    private int lid;
    private String lname;

    @ManyToOne
    @JoinColumn(name = "aid")
    private Alien alien;

    // Getters and Setters
}

hibernate.cfg.xml

<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="hibernate.connection.driver_class">com.mysql.cj.jdbc.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/your_database</property>
        <property name="hibernate.connection.username">your_username</property>
        <property name="hibernate.connection.password">your_password</property>
        <property name="hibernate.hbm2ddl.auto">update</property>
        <property name="show_sql">true</property>
        
        <mapping class="com.hks.DemoHib.Alien"/>
        <mapping class="com.hks.DemoHib.Laptop"/>
    </session-factory>
</hibernate-configuration>

我检查了数据库，确认有一条主键为 1 的 Alien 记录。尽管如此，session.get 方法还是返回 null。

餐桌外星人

帮助名字 1 纳文 2 拉胡尔 3 玛雅克

可能的原因是什么？我该如何解决？

java hibernate orm hibernate-session
1个回答
0
投票

注意事项：

我无法生成问题中描述的错误。

以下代码是可以正常执行的代码。

项目树

├── pom.xml
└── src
    └── main
        ├── java
        │   └── com
        │       └── hks
        │           └── DemoHib
        │               ├── Alien.java
        │               ├── App.java
        │               └── Laptop.java
        └── resources
            └── hibernate.cfg.xml

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.hks.DemoHib</groupId>
    <artifactId>testapp</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>testapp</name>
    <description>hibernate</description>
    <properties>
            <maven.compiler.source>1.8</maven.compiler.source>
                <maven.compiler.target>1.8</maven.compiler.target>
                <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
                <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
                
                <hibernate.version>5.4.33.Final</hibernate.version>                
                <mysql.connector.version>8.0.33</mysql.connector.version>
        </properties>

    <dependencies>
        <!-- Hibernate Core -->
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-core</artifactId>
            <version>${hibernate.version}</version>
        </dependency>
        
        <!-- MySQL Connector -->
        <dependency>
            <groupId>com.mysql</groupId>
            <artifactId>mysql-connector-j</artifactId>
            <version>${mysql.connector.version}</version>
        </dependency>
    </dependencies>
</project>

hibernate.cfg.xml

我将

hibernate.dialect
修改为
org.hibernate.dialect.MySQL5Dialect

<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <!--
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
         -->
        <property name="hibernate.dialect">org.hibernate.dialect.MySQL5Dialect</property>
        <property name="hibernate.connection.driver_class">com.mysql.cj.jdbc.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/your_database</property>
        <property name="hibernate.connection.username">your_username</property>
        <property name="hibernate.connection.password">your_password</property>
        <property name="hibernate.hbm2ddl.auto">update</property>
        <property name="show_sql">true</property>
        <property name="hibernate.format_sql">true</property>
        
        <mapping class="com.hks.DemoHib.Alien"/>
        <mapping class="com.hks.DemoHib.Laptop"/>
        
    </session-factory>
</hibernate-configuration>

外星人.java

我将

Collection
修改为
List
。

package com.hks.DemoHib;

import javax.persistence.*;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;

@Entity
@Table(name = "Alien")
public class Alien {
    @Id
    private int aid;
    private String aname;

    @OneToMany(mappedBy = "alien", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Laptop> laps = new ArrayList<>();

    public int getAid() {
        return aid;
    }

    public void setAid(int aid) {
        this.aid = aid;
    }

    public String getAname() {
        return aname;
    }

    public void setAname(String aname) {
        this.aname = aname;
    }

    public List<Laptop> getLaps() {
        return laps;
    }

    public void setLaps(List<Laptop> laps) {
        this.laps = laps;
    }

    public void addLaptop(Laptop laptop) {
        this.laps.add(laptop);
        laptop.setAlien(this); //IMPORTANT
    }

    public void removeLaptop(Laptop laptop) {
        this.laps.remove(laptop);
        laptop.setAlien(null);  //IMPORTANT
    }
}

笔记本电脑.java

package com.hks.DemoHib;

import javax.persistence.*;

@Entity
@Table(name = "Laptop")
public class Laptop {
    @Id
    private int lid;
    private String lname;

    @ManyToOne
    @JoinColumn(name = "aid")
    private Alien alien;

    public int getLid() {
        return lid;
    }

    public void setLid(int lid) {
        this.lid = lid;
    }

    public String getLname() {
        return lname;
    }

    public void setLname(String lname) {
        this.lname = lname;
    }

    public Alien getAlien() {
        return alien;
    }

    public void setAlien(Alien alien) {
        this.alien = alien;
    }
}

应用程序.java

package com.hks.DemoHib;

import java.util.List;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class App {
    public static void main(String[] args) {
        Configuration con = new Configuration().configure().addAnnotatedClass(Laptop.class).addAnnotatedClass(Alien.class);
        SessionFactory sf = con.buildSessionFactory();
        Session session = sf.openSession();

        session.beginTransaction();
        Alien alien1 = new Alien();
              alien1.setAid(1);
              alien1.setAname("Navin");
        Laptop laptop1 = new Laptop();
               laptop1.setLid(1);
               laptop1.setLname("AAA");
        Laptop laptop2 = new Laptop();
               laptop2.setLid(2);
               laptop2.setLname("BBB");
               alien1.addLaptop(laptop1);
               alien1.addLaptop(laptop2);
        session.save(alien1);

        Alien alien2 = new Alien();
              alien2.setAid(2);
              alien2.setAname("Rahul");

        Alien alien3 = new Alien();
              alien3.setAid(3);
              alien3.setAname("Mayank");
        session.save(alien2);
        session.save(alien3);
        session.getTransaction().commit();

        session.beginTransaction();
        Alien a1 = session.get(Alien.class, 1);

        if (a1 == null) {
            System.out.println("Alien with ID 1 not found!");
        } else {
            a1.setAname("Harsh");
            System.out.println(a1.getAname());

            List<Laptop> laps = a1.getLaps();
            for (Laptop l : laps) {
                //System.out.println(l);
                System.out.println(l.getLid() + " " + l.getLname() + " " + l.getAlien().getAid());
           }
        }
        session.save(a1);
        session.getTransaction().commit();
    }
}

构建

mvn clean package

下载依赖项

mvn dependency:copy-dependencies -DoutputDirectory=target/libs

奔跑

java -cp "target/libs/*:target/testapp-0.0.1-SNAPSHOT.jar" com.hks.DemoHib.App

结果 - 完整日志

$ java -cp "target/libs/*:target/testapp-0.0.1-SNAPSHOT.jar" com.hks.DemoHib.App
7月 25, 2024 2:24:57 下午 org.hibernate.Version logVersion
INFO: HHH000412: Hibernate ORM core version 5.4.33.Final
7月 25, 2024 2:24:57 下午 org.hibernate.boot.jaxb.internal.stax.LocalXmlResourceResolver resolveEntity
WARN: HHH90000012: Recognized obsolete hibernate namespace http://hibernate.sourceforge.net/hibernate-configuration. Use namespace http://www.hibernate.org/dtd/hibernate-configuration instead.  Support for obsolete DTD/XSD namespaces may be removed at any time.
7月 25, 2024 2:24:57 下午 org.hibernate.annotations.common.reflection.java.JavaReflectionManager <clinit>
INFO: HCANN000001: Hibernate Commons Annotations {5.1.2.Final}
7月 25, 2024 2:24:57 下午 org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl configure
WARN: HHH10001002: Using Hibernate built-in connection pool (not for production use!)
7月 25, 2024 2:24:57 下午 org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001005: using driver [com.mysql.cj.jdbc.Driver] at URL [jdbc:mysql://localhost:3306/your_database]
7月 25, 2024 2:24:57 下午 org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001001: Connection properties: {password=****, user=your_username}
7月 25, 2024 2:24:57 下午 org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001003: Autocommit mode: false
7月 25, 2024 2:24:57 下午 org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl$PooledConnections <init>
INFO: HHH000115: Hibernate connection pool size: 20 (min=1)
7月 25, 2024 2:24:57 下午 org.hibernate.dialect.Dialect <init>
INFO: HHH000400: Using dialect: org.hibernate.dialect.MySQL5Dialect
7月 25, 2024 2:24:58 下午 org.hibernate.resource.transaction.backend.jdbc.internal.DdlTransactionIsolatorNonJtaImpl getIsolatedConnection
INFO: HHH10001501: Connection obtained from JdbcConnectionAccess [org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess@df921b1] for (non-JTA) DDL execution was not in auto-commit mode; the Connection 'local transaction' will be committed and the Connection will be set into auto-commit mode.
Hibernate: 
    
    create table Alien (
       aid integer not null,
        aname varchar(255),
        primary key (aid)
    ) engine=MyISAM
Hibernate: 
    
    create table Laptop (
       lid integer not null,
        lname varchar(255),
        aid integer,
        primary key (lid)
    ) engine=MyISAM
Hibernate: 
    
    alter table Laptop 
       add constraint FK6ov7gtqxs02wr7p13bn8u6whh 
       foreign key (aid) 
       references Alien (aid)
7月 25, 2024 2:24:58 下午 org.hibernate.engine.transaction.jta.platform.internal.JtaPlatformInitiator initiateService
INFO: HHH000490: Using JtaPlatform implementation: [org.hibernate.engine.transaction.jta.platform.internal.NoJtaPlatform]
Hibernate: 
    select
        laptop_.lid,
        laptop_.aid as aid3_1_,
        laptop_.lname as lname2_1_ 
    from
        Laptop laptop_ 
    where
        laptop_.lid=?
Hibernate: 
    select
        laptop_.lid,
        laptop_.aid as aid3_1_,
        laptop_.lname as lname2_1_ 
    from
        Laptop laptop_ 
    where
        laptop_.lid=?
Hibernate: 
    insert 
    into
        Alien
        (aname, aid) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        Laptop
        (aid, lname, lid) 
    values
        (?, ?, ?)
Hibernate: 
    insert 
    into
        Laptop
        (aid, lname, lid) 
    values
        (?, ?, ?)
Hibernate: 
    insert 
    into
        Alien
        (aname, aid) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        Alien
        (aname, aid) 
    values
        (?, ?)
Harsh
1 AAA 1
2 BBB 1
Hibernate: 
    update
        Alien 
    set
        aname=? 
    where
        aid=?

应用程序输出

Harsh
1 AAA 1
2 BBB 1

桌子

表：外星人

001.png

桌子：笔记本电脑

002.png

  • MySQL：8.4.0
  • 休眠核心：5.4.33.Final
  • mysql-连接器-j：8.0.33
  • JDK：1.8
  • hibernate.dialect：MySQL5Dialect
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