LockMode.UPGRADE 引发的对于Hibernate事务的思考

应用场景：现有一个表Article（帖子），其中有一个字段visit_amount表示这个帖子被浏览过多少次。
现在做一个实验，开两个线程做同一件事情，就是：取出原有的浏览次数，然后加一，再存回数据库。每个线程把这个动作重复10次。

设想的结果：如果浏览次数的初始值是0的话，那么程序运行完毕后，浏览次数应该增长到20。

package com.javaye;

import org.hibernate.class=hilite1>LockMode;
import org.hibernate.Session;
import org.hibernate.Transaction;

import com.javaye.models.Article;

public class Main {


    private  static void update(){
        Session session = HibernateSessionFactory.getSession();
        Transaction tx =  session.beginTransaction();
        Article art = (Article) session.load(Article.class, 1);
        art.setVisitAmount(art.getVisitAmount()+1);
        session.save(art);
        tx.commit();


    }

    private static void work(){
        for(int i=0;i<10;i++){
            System.out.println(Thread.currentThread().getName()+":"+(i+1)+"times.");
            update();
        }
    }

    public static void main(String[] args) throws Exception{
        Thread t1 = new Thread(
            new Runnable(){
                public void run(){
                    work();
                }
            }
        );

        Thread t2 = new Thread(
                new Runnable(){
                    public void run(){
                        work();
                    }
                }
        );
        t1.setName("Thread1");
        t2.setName("Thread2");
        t1.setDaemon(true);
        t2.setDaemon(true);
        t1.start();
        t2.start();
        t1.join();
        t2.join();

    }
}

package com.javaye;

import org.hibernate.LockMode;
import org.hibernate.Session;
import org.hibernate.Transaction;

import com.javaye.models.Article;

public class Main {
	

	private  static void update(){
		Session session = HibernateSessionFactory.getSession();
		Transaction tx =  session.beginTransaction();
		Article art = (Article) session.load(Article.class, 1);
		art.setVisitAmount(art.getVisitAmount()+1);
		session.save(art);
		tx.commit();
		
		
	}
	
	private static void work(){
		for(int i=0;i<10;i++){
			System.out.println(Thread.currentThread().getName()+":"+(i+1)+"times.");
			update();
		}
	}
	
	public static void main(String[] args) throws Exception{
		Thread t1 = new Thread(
			new Runnable(){
				public void run(){
					work();
				}
			}
		);
		
		Thread t2 = new Thread(
				new Runnable(){
					public void run(){
						work();
					}
				}
		);
		t1.setName("Thread1");
		t2.setName("Thread2");
		t1.setDaemon(true);
		t2.setDaemon(true);
		t1.start();
		t2.start();
		t1.join();
		t2.join();
		
	}
}

上面是第一个版本，结果测试的结果发现没有实现事务的隔离性，每次的结果是随机的，要么是10要么是11或12。

然后，我进行了修改，加上了session.evict(art)来清空缓存，然后发现结果变得好一些了，但是结果并不总是20，有时候是19或18。真奇怪。

再后来，把load语句改为:

Article art = (Article) session.load(Article.class, 1,class=hilite1>LockMode.UPGRADE);

Article art = (Article) session.load(Article.class, 1,LockMode.UPGRADE);

经过多次测试，结果总为20，实现了事务的隔离性。

最终work的代码如下：

package com.javaye;

import org.hibernate.class=hilite1>LockMode;
import org.hibernate.Session;
import org.hibernate.Transaction;

import com.javaye.models.Article;

public class Main {

    private static void insert(){
        Session session = HibernateSessionFactory.getSession();
        Transaction tx = session.beginTransaction();
        Article art = new Article();
        art.setTitle("AAA");
        art.setVisitAmount(0);
        session.saveOrUpdate(art);
        tx.commit();
    }

    private  static void update(){
        Session session = HibernateSessionFactory.getSession();
        Transaction tx =  session.beginTransaction();
        Article art = (Article) session.load(Article.class, 1,class=hilite1>LockMode.UPGRADE);
        art.setVisitAmount(art.getVisitAmount()+1);
        session.save(art);
        tx.commit();
        session.evict(art);

    }

    private static void work(){
        for(int i=0;i<10;i++){
            System.out.println(Thread.currentThread().getName()+":"+(i+1)+"times.");
            update();
        }
    }

    public static void main(String[] args) throws Exception{
        Thread t1 = new Thread(
            new Runnable(){
                public void run(){
                    work();
                }
            }
        );

        Thread t2 = new Thread(
                new Runnable(){
                    public void run(){
                        work();
                    }
                }
        );
        t1.setName("Thread1");
        t2.setName("Thread2");
        t1.setDaemon(true);
        t2.setDaemon(true);
        t1.start();
        t2.start();
        t1.join();
        t2.join();

    }
}

package com.javaye;

import org.hibernate.LockMode;
import org.hibernate.Session;
import org.hibernate.Transaction;

import com.javaye.models.Article;

public class Main {
	
	private static void insert(){
		Session session = HibernateSessionFactory.getSession();
		Transaction tx = session.beginTransaction();
		Article art = new Article();
		art.setTitle("AAA");
		art.setVisitAmount(0);
		session.saveOrUpdate(art);
		tx.commit();
	}
	
	private  static void update(){
		Session session = HibernateSessionFactory.getSession();
		Transaction tx =  session.beginTransaction();
		Article art = (Article) session.load(Article.class, 1,LockMode.UPGRADE);
		art.setVisitAmount(art.getVisitAmount()+1);
		session.save(art);
		tx.commit();
		session.evict(art);
		
	}
	
	private static void work(){
		for(int i=0;i<10;i++){
			System.out.println(Thread.currentThread().getName()+":"+(i+1)+"times.");
			update();
		}
	}
	
	public static void main(String[] args) throws Exception{
		Thread t1 = new Thread(
			new Runnable(){
				public void run(){
					work();
				}
			}
		);
		
		Thread t2 = new Thread(
				new Runnable(){
					public void run(){
						work();
					}
				}
		);
		t1.setName("Thread1");
		t2.setName("Thread2");
		t1.setDaemon(true);
		t2.setDaemon(true);
		t1.start();
		t2.start();
		t1.join();
		t2.join();
		
	}
}

查了一下文档，LockMode.UPGRADE是最高的锁级别了，如果换用其他的级别呢？我测试了LocMode.READ，发现还是不正确，用LockMode.WRITE的时候竟然抛异常。。