Hibernate get和load的区别

最新推荐文章于 2022-03-15 14:41:50 发布

Mooner_guo

最新推荐文章于 2022-03-15 14:41:50 发布

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分类专栏： Hibernate

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本文详细对比了Hibernate中load与get方法的行为差异，包括它们的加载时机、异常处理及缓存使用策略。

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hiernate.cfg.xml配置

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://www.hibernate.org/dtd/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.url">jdbc:mysql://localhost:3306/hibernate2</property>
        <property name="connection.username">root</property>
        <property name="connection.password">admin</property>

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

        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.MySQL5Dialect</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.internal.NoCacheProvider</property>

        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>

        <!-- Drop and re-create the database schema on startup -->
        <property name="hbm2ddl.auto">update</property>
        
        <property name="format_sql">true</property>
        
    </session-factory>

</hibernate-configuration>

测试代码：

@Test
  public void testGet() {
    Session session = factory.getCurrentSession();
    session.beginTransaction();
    Student s = (Student) session.get(Student.class, 1);
    System.out.println("testGet: " + s.getName());
    session.getTransaction().commit();
  }

  @Test
  public void testLoad() {
    Session session = factory.getCurrentSession();
    session.beginTransaction();
    Student s = (Student) session.load(Student.class, 1);
    System.out.println("testLoad: " + s.getName());
    session.getTransaction().commit();
  }

注意:目前数据库只有一条记录，id为1。调用的该对象的非关键字属性方法。

1.从上述代码，输出效果是一样的。

get方式：

15:08:51,585  INFO SchemaUpdate:182 - HHH000228: Running hbm2ddl schema update
15:08:51,595  INFO SchemaUpdate:193 - HHH000102: Fetching database metadata
15:08:51,595  INFO SchemaUpdate:205 - HHH000396: Updating schema
15:08:51,626  INFO TableMetadata:65 - HHH000261: Table found: hibernate2.csdn_student
15:08:51,626  INFO TableMetadata:66 - HHH000037: Columns: [id, sex, name]
15:08:51,626  INFO TableMetadata:68 - HHH000108: Foreign keys: []
15:08:51,626  INFO TableMetadata:69 - HHH000126: Indexes: [primary]
15:08:51,626  INFO SchemaUpdate:242 - HHH000232: Schema update complete
Hibernate: 
    select
        student0_.id as id1_0_0_,
        student0_.name as name2_0_0_,
        student0_.sex as sex3_0_0_ 
    from
        csdn_student student0_ 
    where
        student0_.id=?
testGet: Jack

load方式:

15:11:22,280  INFO SchemaUpdate:182 - HHH000228: Running hbm2ddl schema update
15:11:22,280  INFO SchemaUpdate:193 - HHH000102: Fetching database metadata
15:11:22,280  INFO SchemaUpdate:205 - HHH000396: Updating schema
15:11:22,311  INFO TableMetadata:65 - HHH000261: Table found: hibernate2.csdn_student
15:11:22,311  INFO TableMetadata:66 - HHH000037: Columns: [id, sex, name]
15:11:22,311  INFO TableMetadata:68 - HHH000108: Foreign keys: []
15:11:22,311  INFO TableMetadata:69 - HHH000126: Indexes: [primary]
15:11:22,311  INFO SchemaUpdate:242 - HHH000232: Schema update complete
Hibernate: 
    select
        student0_.id as id1_0_0_,
        student0_.name as name2_0_0_,
        student0_.sex as sex3_0_0_ 
    from
        csdn_student student0_ 
    where
        student0_.id=?
testLoad: Jack

2.但是修改java代码，

@Test
  public void testGet() {
    Session session = factory.getCurrentSession();
    session.beginTransaction();
    Student s = (Student) session.get(Student.class, 1);
    session.getTransaction().commit();
    System.out.println("testGet: " + s.getName());
  }

  @Test
  public void testLoad() {
    Session session = factory.getCurrentSession();
    session.beginTransaction();
    Student s = (Student) session.load(Student.class, 1);
    session.getTransaction().commit();
    System.out.println("testLoad: " + s.getName());
  }

get输出：

15:17:03,334  INFO SchemaUpdate:182 - HHH000228: Running hbm2ddl schema update
15:17:03,334  INFO SchemaUpdate:193 - HHH000102: Fetching database metadata
15:17:03,344  INFO SchemaUpdate:205 - HHH000396: Updating schema
15:17:03,365  INFO TableMetadata:65 - HHH000261: Table found: hibernate2.csdn_student
15:17:03,365  INFO TableMetadata:66 - HHH000037: Columns: [id, sex, name]
15:17:03,365  INFO TableMetadata:68 - HHH000108: Foreign keys: []
15:17:03,375  INFO TableMetadata:69 - HHH000126: Indexes: [primary]
15:17:03,375  INFO SchemaUpdate:242 - HHH000232: Schema update complete
Hibernate: 
    select
        student0_.id as id1_0_0_,
        student0_.name as name2_0_0_,
        student0_.sex as sex3_0_0_ 
    from
        csdn_student student0_ 
    where
        student0_.id=?
testGet: Jack

load输出：

15:18:23,699  INFO SchemaUpdate:182 - HHH000228: Running hbm2ddl schema update
15:18:23,699  INFO SchemaUpdate:193 - HHH000102: Fetching database metadata
15:18:23,709  INFO SchemaUpdate:205 - HHH000396: Updating schema
15:18:23,730  INFO TableMetadata:65 - HHH000261: Table found: hibernate2.csdn_student
15:18:23,730  INFO TableMetadata:66 - HHH000037: Columns: [id, sex, name]
15:18:23,730  INFO TableMetadata:68 - HHH000108: Foreign keys: []
15:18:23,730  INFO TableMetadata:69 - HHH000126: Indexes: [primary]
15:18:23,730  INFO SchemaUpdate:242 - HHH000232: Schema update complete

JUnit提示:org.hibernate.LazyInitializationException: could not initialize proxy - no Session
  at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:164)
  at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:285)
  at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:185)
  at com.ksjl.eneity.Student_$$_javassist_0.getName(Student_$$_javassist_0.java)
  at com.ksjl.entity.StudentTest.testLoad(StudentTest.java:61)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
  at java.lang.reflect.Method.invoke(Method.java:597)
  at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
  at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
  at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
  at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
  at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
  at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
  at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
  at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
  at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
  at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
  at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
  at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
  at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
  at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
  at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
  at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
  at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
  at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
  at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

从上述可以看出，当session在关闭之前，get已经获取实体对象，而load没有获取实体对象。

原理：在调用get方法的时候，会先查session缓存，然后查询SessionFacotry二级缓存，如果依旧没得到对象，会查询数据库，发出SQL语句。

而调用load方法的时候，hibernate只是为其提供一个代理对象(proxy),返回一个代理对象。当该对象调用非关键字属性的时候，先查session缓存，然后查询SessionFacotry二级缓存，如果依旧没得到对象，会查询数据库，发出SQL语句。这就是所谓的延迟加载(lazy)。

3.修改java代码，

@Test
  public void testGet() {
    Session session = factory.getCurrentSession();
    session.beginTransaction();
    Student s = (Student) session.get(Student.class, 2);//表里只有id为1的记录
    System.out.println(s==null);
    session.getTransaction().commit();
    System.out.println(s.getClass());
  }

  @Test
  public void testLoad() {
    Session session = factory.getCurrentSession();
    session.beginTransaction();
    Student s = (Student) session.load(Student.class, 2);
    System.out.println(s==null);
    session.getTransaction().commit();
    System.out.println(s.getClass());
  }

get输出：

Juint输出：

java.lang.NullPointerException
  at com.ksjl.entity.StudentTest.testGet(StudentTest.java:52)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
  at java.lang.reflect.Method.invoke(Method.java:597)
  at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
  at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
  at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
  at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
  at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
  at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
  at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
  at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
  at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
  at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
  at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
  at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
  at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
  at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
  at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
  at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
  at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
  at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
  at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

控制台console输出：

15:37:16,135  INFO SchemaUpdate:182 - HHH000228: Running hbm2ddl schema update
15:37:16,135  INFO SchemaUpdate:193 - HHH000102: Fetching database metadata
15:37:16,145  INFO SchemaUpdate:205 - HHH000396: Updating schema
15:37:16,176  INFO TableMetadata:65 - HHH000261: Table found: hibernate2.csdn_student
15:37:16,176  INFO TableMetadata:66 - HHH000037: Columns: [id, sex, name]
15:37:16,176  INFO TableMetadata:68 - HHH000108: Foreign keys: []
15:37:16,176  INFO TableMetadata:69 - HHH000126: Indexes: [primary]
15:37:16,176  INFO SchemaUpdate:242 - HHH000232: Schema update complete
Hibernate: 
    select
        student0_.id as id1_0_0_,
        student0_.name as name2_0_0_,
        student0_.sex as sex3_0_0_ 
    from
        csdn_student student0_ 
    where
        student0_.id=?
True

load输出：

15:38:02,514  INFO SchemaUpdate:182 - HHH000228: Running hbm2ddl schema update
15:38:02,524  INFO SchemaUpdate:193 - HHH000102: Fetching database metadata
15:38:02,524  INFO SchemaUpdate:205 - HHH000396: Updating schema
15:38:02,555  INFO TableMetadata:65 - HHH000261: Table found: hibernate2.csdn_student
15:38:02,555  INFO TableMetadata:66 - HHH000037: Columns: [id, sex, name]
15:38:02,555  INFO TableMetadata:68 - HHH000108: Foreign keys: []
15:38:02,555  INFO TableMetadata:69 - HHH000126: Indexes: [primary]
15:38:02,555  INFO SchemaUpdate:242 - HHH000232: Schema update complete
false
class com.ksjl.eneity.Student_$$_javassist_0

从上述代码可以看出，如果调用get查询对象，数据库没有对应对象，那么立即报NullPointException异常，而load方式延时，只是获取代理xxx_$$javasist_0。

总结：

共同点:

都是通过实体ID获取对象。

区别：

1.get不延迟加载，load延迟加载。

2.调用get方法的时候，get先查session缓存，再查二级缓存，再查数据库，发出sql语句。

调用load方法，只是获取代理对象，当调用非关键字属性的时候，才去查询。

3.get不到对象，抛出NullPointException异常。

load不到对象，抛出ObjectNotFoundException异常