悲观锁和乐观锁是：在事务隔离机制中设置了ReadCommited的情况下，两种可以避免不可重复读的方式。

设置成读已提交是考虑到安全和处理速度，保证并发效率，但是在这个情况下仍然需要避免不可重复读这种情况，于是hibernate提供两种锁来解决这个问题。

悲观锁：自己事务完成之前别人不能动数据。依赖于数据库，“for update”

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

@Entity
public class Account {
    private int id;
    private int balance; //BigDecimal
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public int getBalance() {
        return balance;
    }
    public void setBalance(int balance) {
        this.balance = balance;
    }
}

测试类：

package com.bjsxt.hibernate;

import java.math.BigDecimal;

import org.hibernate.LockMode;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.tool.hbm2ddl.SchemaExport;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

public class HibernateCacheTest {
    private static SessionFactory sf;
   
    @BeforeClass
    public static void beforeClass() {
        sf = new AnnotationConfiguration().configure().buildSessionFactory();
    }
    @AfterClass
    public static void afterClass() {
        sf.close();
    }
   
    @Test
    public void testSchemaExport() {
        new SchemaExport(new AnnotationConfiguration().configure()).create(false, true);
    }
   
    @Test
    public void testSave() {
        Session session = sf.openSession();
        session.beginTransaction();
       
        Account a = new Account();
        a.setBalance(100);
        session.save(a);
           
        session.getTransaction().commit();
        session.close();
    }
   
    @Test
    public void testOperation1() {
        Session session = sf.openSession();
        session.beginTransaction();
       
        Account a = (Account)session.load(Account.class, 1);
        int balance = a.getBalance();
        //do some caculations
        balance = balance - 10;
        a.setBalance(balance);
        session.getTransaction().commit();
        session.close();
    }
   
    @Test
    public void testPessimisticLock() {
        Session session = sf.openSession();
        session.beginTransaction();
       
        Account a = (Account)session.load(Account.class, 1, LockMode.UPGRADE);//NOWAIT ORACL支持
        int balance = a.getBalance();
        //do some caculation
        balance = balance - 10;
        a.setBalance(balance);
        session.getTransaction().commit();
        session.close();
    }
   
    public static void main(String[] args) {
        beforeClass();
    }
}

乐观锁：依赖于表的一个字段，该字段每当该条数据被更新了，字段的内容就会变化而且不会和以前重复。将model中加上@Version即可标记该字段。

package com.bjsxt.hibernate;

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

@Entity
public class Account {
    private int id;
    private int balance;
    private int version;
    @Version
    public int getVersion() {
        return version;
    }
    public void setVersion(int version) {
        this.version = version;
    }
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public int getBalance() {
        return balance;
    }
    public void setBalance(int balance) {
        this.balance = balance;
    }
}

测试类：

package com.bjsxt.hibernate;

import java.math.BigDecimal;

import org.hibernate.LockMode;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.tool.hbm2ddl.SchemaExport;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

public class HibernateCacheTest {
    private static SessionFactory sf;

@BeforeClass
    public static void beforeClass() {
        sf = new AnnotationConfiguration().configure().buildSessionFactory();
    }

@AfterClass
    public static void afterClass() {
        sf.close();
    }

@Test
    public void testSchemaExport() {
        new SchemaExport(new AnnotationConfiguration().configure()).create(
                false, true);
    }

@Test
    public void testSave() {
        Session session = sf.openSession();
        session.beginTransaction();

Account a = new Account();
        a.setBalance(100);
        session.save(a);

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

@Test
    public void testOptimisticLock() {
        Session session = sf.openSession();

Session session2 = sf.openSession();

session.beginTransaction();
        Account a1 = (Account) session.load(Account.class, 1);

session2.beginTransaction();
        Account a2 = (Account) session2.load(Account.class, 1);
       
        a1.setBalance(900);
        a2.setBalance(1100);

        session.getTransaction().commit();
        System.out.println(a1.getVersion());

        session2.getTransaction().commit();
        System.out.println(a2.getVersion());

        session.close();
        session2.close();
    }

public static void main(String[] args) {
        beforeClass();
    }
}