关于Hibernate基于version的乐观锁

刚刚接触SSH框架，虽然可能这个框架已经比较过时了，但是个人认为，SSH作为一个成熟的框架，作为框架的入门还是可以的。

马马虎虎学完了Hibernate的基础，总结一点心得之类的。

学习Hibernate的乐观锁时：

• 首先要知道为什么要用乐观锁。之所以要用乐观锁，就是为了避免脏数据。这很像数据库原理中的共享锁（读锁）和排它锁（写锁）。不管是乐观锁、共享锁、排它锁，其目的都是为了保证数据的一致性，也可以说是保证数据的正确性。所有锁的本质都是一样的。
• 其次要知道什么时候要用乐观锁。一般有多个事务要对同一数据进行操作时，就需要使用乐观锁。比如很经典的银行取钱问题，只有用锁来保证数据只能被一个事务所使用，在该事务结束使用之前，别的事务都不能对它做任何事，否则就可能出现丢失修改、读脏数据、数据不一致等问题。
• 第三要知道为什么可以使用乐观锁。Hibernate本身是对JDBC的轻量级封装，其目的是为了使开发人员可以像操作对象一样操作数据库，其本质就是数据库操作，所以数据库的锁机制是可以实现和使用的。
• 第四要知道乐观锁的实现机制。version元素是利用一个递增的整数来跟踪数据表中记录的版本的。在读取数据时，会将version一同读取出来，而在更新时，将version+1（使用hql在update时不校验version）。将提交数据的version与数据库库表中对应记录的version进行比较，如果提交的数据的version大于数据库库表中记录的version，则执行更新，否则便认为是过期或无效数据，不执行更新，并抛出异常。
• 最后要知道怎么使用乐观锁。
• 先要有一个entity class。在该类里面增加一个version属性，设为int类型，这个字段表示版本信息。

      【注：代码中有@的可以不用管，这个是注解方式。即直接在类上使用注解，来达到相同的配置效果。】

package hibernate;

import java.util.Set;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "product_")
public class Product {
	int id;
    String name;
    float price;
    Category category;
    Set users;
    int version;
    
    @Column(name = "version")
	public int getVersion() {
		return version;
	}
	public void setVersion(int version) {
		this.version = version;
	}
	
	@Column(name = "users")
	public Set getUsers() {
		return users;
	}
	public void setUsers(Set users) {
		this.users = users;
	}
	
	@Column(name = "category")
	public Category getCategory() {
		return category;
	}
	public void setCategory(Category category) {
		this.category = category;
	}
	
	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "id")
	public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    
    @Column(name = "name")
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    
    @Column(name = "price")
    public float getPrice() {
        return price;
    }
    public void setPrice(float price) {
        this.price = price;
    }
}

• 接下来就是进行修改entity class的配置文件。在"类名.hbm.xml"文件中，增加一个version字段，用于版本信息控制，这就是乐观锁的核心机制。

      【注：version标签必须跟在id标签后面，否则会有错，运行程序会报错，无法读取XML文件。 hibernate需要访问的属性一定要在"类名.hbm.xml"中定义】

• 还有在"包名.cfg.xml"文件中要配置好映射。

• 最后测试一下就好了。

public class TestHibernate {

    public static void main(String[] args) {
	SessionFactory sf = new Configuration().configure().buildSessionFactory();
		
        Session s1 = sf.openSession();
        Session s2 = sf.openSession();
 
        s1.beginTransaction();
        s2.beginTransaction();
 
        Product p1 = (Product) s1.get(Product.class, 1);
        System.out.println("产品原本价格是: " + p1.getPrice());
        p1.setPrice(p1.getPrice() + 1000);
 
        Product p2 = (Product) s2.get(Product.class, 1);
        p2.setPrice(p2.getPrice() + 1000);
 
        s1.update(p1);
        s2.update(p2);
 
        s1.getTransaction().commit();
        s2.getTransaction().commit();
 
        Product p = (Product) s1.get(Product.class, 1);
        System.out.println("经过两次价格增加后，价格变为: " + p.getPrice());

        sf.close();
	}

}

• 运行结果：在main线程中出现了报错，因为s1已经修改了数据，但是s2也想修改，但是version的值已经由1变为2了，所以s2不能执行，报错Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect)，意思是"行被另一事务更新或删除（或未保存的值映射不正确）"，即s1已经修改了该行，产品原本价格是: 10000.0，程序执行完之后为11000.0，s2没有执行。

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
八月 05, 2018 10:49:36 上午 com.mchange.v2.log.MLog 
信息: MLog clients using java 1.4+ standard logging.
八月 05, 2018 10:49:38 上午 com.mchange.v2.c3p0.C3P0Registry banner
信息: Initializing c3p0-0.9.1 [built 16-January-2007 14:46:42; debug? true; trace: 10]
八月 05, 2018 10:49:38 上午 com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource getPoolManager
信息: Initializing c3p0 pool... com.mchange.v2.c3p0.PoolBackedDataSource@7cf67e4e [ connectionPoolDataSource -> com.mchange.v2.c3p0.WrapperConnectionPoolDataSource@3c314b76 [ acquireIncrement -> 2, acquireRetryAttempts -> 30, acquireRetryDelay -> 1000, autoCommitOnClose -> false, automaticTestTable -> null, breakAfterAcquireFailure -> false, checkoutTimeout -> 0, connectionCustomizerClassName -> null, connectionTesterClassName -> com.mchange.v2.c3p0.impl.DefaultConnectionTester, debugUnreturnedConnectionStackTraces -> false, factoryClassLocation -> null, forceIgnoreUnresolvedTransactions -> false, identityToken -> 2x00zq9x27t6vc4ficmh|4d1b0d2a, idleConnectionTestPeriod -> 3000, initialPoolSize -> 5, maxAdministrativeTaskTime -> 0, maxConnectionAge -> 0, maxIdleTime -> 50000, maxIdleTimeExcessConnections -> 0, maxPoolSize -> 20, maxStatements -> 100, maxStatementsPerConnection -> 0, minPoolSize -> 5, nestedDataSource -> com.mchange.v2.c3p0.DriverManagerDataSource@ce96782d [ description -> null, driverClass -> null, factoryClassLocation -> null, identityToken -> 2x00zq9x27t6vc4ficmh|52feb982, jdbcUrl -> jdbc:mysql://localhost:3306/test?characterEncoding=UTF-8, properties -> {user=******, password=******} ], preferredTestQuery -> null, propertyCycle -> 0, testConnectionOnCheckin -> false, testConnectionOnCheckout -> false, unreturnedConnectionTimeout -> 0, usesTraditionalReflectiveProxies -> false; userOverrides: {} ], dataSourceName -> null, factoryClassLocation -> null, identityToken -> 2x00zq9x27t6vc4ficmh|4fcd19b3, numHelperThreads -> 3 ]
Hibernate: select product0_.id as id0_0_, product0_.ver as ver0_0_, product0_.name as name0_0_, product0_.price as price0_0_, product0_.cid as cid0_0_ from product_ product0_ where product0_.id=?
Hibernate: select users0_.pid as pid0_1_, users0_.uid as uid1_, user1_.id as id3_0_, user1_.name as name3_0_ from user_product users0_ inner join user_ user1_ on users0_.uid=user1_.id where users0_.pid=?
Hibernate: select products0_.uid as uid3_1_, products0_.pid as pid1_, product1_.id as id0_0_, product1_.ver as ver0_0_, product1_.name as name0_0_, product1_.price as price0_0_, product1_.cid as cid0_0_ from user_product products0_ inner join product_ product1_ on products0_.pid=product1_.id where products0_.uid=?
Hibernate: select products0_.uid as uid3_1_, products0_.pid as pid1_, product1_.id as id0_0_, product1_.ver as ver0_0_, product1_.name as name0_0_, product1_.price as price0_0_, product1_.cid as cid0_0_ from user_product products0_ inner join product_ product1_ on products0_.pid=product1_.id where products0_.uid=?
Hibernate: select products0_.uid as uid3_1_, products0_.pid as pid1_, product1_.id as id0_0_, product1_.ver as ver0_0_, product1_.name as name0_0_, product1_.price as price0_0_, product1_.cid as cid0_0_ from user_product products0_ inner join product_ product1_ on products0_.pid=product1_.id where products0_.uid=?
产品原本价格是: 10000.0
Hibernate: select product0_.id as id0_0_, product0_.ver as ver0_0_, product0_.name as name0_0_, product0_.price as price0_0_, product0_.cid as cid0_0_ from product_ product0_ where product0_.id=?
Hibernate: select users0_.pid as pid0_1_, users0_.uid as uid1_, user1_.id as id3_0_, user1_.name as name3_0_ from user_product users0_ inner join user_ user1_ on users0_.uid=user1_.id where users0_.pid=?
Hibernate: select products0_.uid as uid3_1_, products0_.pid as pid1_, product1_.id as id0_0_, product1_.ver as ver0_0_, product1_.name as name0_0_, product1_.price as price0_0_, product1_.cid as cid0_0_ from user_product products0_ inner join product_ product1_ on products0_.pid=product1_.id where products0_.uid=?
Hibernate: select products0_.uid as uid3_1_, products0_.pid as pid1_, product1_.id as id0_0_, product1_.ver as ver0_0_, product1_.name as name0_0_, product1_.price as price0_0_, product1_.cid as cid0_0_ from user_product products0_ inner join product_ product1_ on products0_.pid=product1_.id where products0_.uid=?
Hibernate: select products0_.uid as uid3_1_, products0_.pid as pid1_, product1_.id as id0_0_, product1_.ver as ver0_0_, product1_.name as name0_0_, product1_.price as price0_0_, product1_.cid as cid0_0_ from user_product products0_ inner join product_ product1_ on products0_.pid=product1_.id where products0_.uid=?
Hibernate: update product_ set ver=?, name=?, price=?, cid=? where id=? and ver=?
Hibernate: update product_ set ver=?, name=?, price=?, cid=? where id=? and ver=?
Exception in thread "main" org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [hibernate.Product#1]
	at org.hibernate.persister.entity.AbstractEntityPersister.check(AbstractEntityPersister.java:1950)
	at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:2594)
	at org.hibernate.persister.entity.AbstractEntityPersister.updateOrInsert(AbstractEntityPersister.java:2494)
	at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:2821)
	at org.hibernate.action.EntityUpdateAction.execute(EntityUpdateAction.java:113)
	at org.hibernate.engine.ActionQueue.execute(ActionQueue.java:273)
	at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:265)
	at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:185)
	at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:321)
	at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:51)
	at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1216)
	at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:383)
	at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:133)
	at hibernate.TestHibernate.main(TestHibernate.java:404)

• PS：除了乐观锁还有悲观锁，弄懂了乐观锁之后可以研究一下悲观锁。