十四、NHibernate之一级缓存

什么是NHibernate一级缓存
NHibernate一级缓存即ISession缓存，ISession缓存属于事务级缓存，是NHibernate内置的。ISession缓存中的数据只在本ISession周期内使用。
ISession实例创建后即可使用ISession缓存。此后，ISession实例操作数据时，首先查询内置缓存，如果ISession缓存中存在相应数据，则直接使用缓存数据。如果不存在，则查询数据库并把其结果存在缓存中。
 
一、一级缓存就在身边
1)       当查询条件相同，且是同一个ISession时由NHibernate自动调用缓存
a)       在DAL.Test中编写代码如下：
        [Test]
        public void
        {
            Console.WriteLine("第一次加载");
            Customer customer_1 = session.Get(1);
            Console.WriteLine("第二次加载");
            Customer customer_2 = session.Get(1);
          }
 
b)       跟踪NHibernate生成SQL的结果
------ Test started: Assembly: DAL.Test.dll ------
 
第一次加载
NHibernate: SELECT customer0_.CustomerId as CustomerId4_0_, customer0_.Firstname as Firstname4_0_, customer0_.Lastname as Lastname4_0_ FROM Customer customer0_ WHERE customer0_.CustomerId=@p0;@p0 = 1
第二次加载
 
1 passed, 0 failed, 0 skipped, took 4.92 seconds (NUnit 2.5.1).
我们可以看到，NHibernate只在第一次加载Customer对象时查询了数据库，而第二次加载时并没有生成相应的SQL语句，这是因为SessionTest()函数中的session在第一次加载后没有被销毁的情况下，对Customer进行第二次加载，那么NHibernate就会先到未被销毁的session中去查询，如果存在则直接从session中返回结果，不存在时，才生成SQL语句并到数据库中查询。
 
2)       这里是同一个ISession，不同的查询条件
a)       为了证明，修改程序如下：
        [Test]
        public void
        {
            Console.WriteLine("第一次加载");
            Customer customer_1 = session.Get(1);
            Console.WriteLine("第二次加载");
            Customer customer_2 = session.Get(2);
        } 
 
b)       跟踪NHibernate生成SQL的结果
------ Test started: Assembly: DAL.Test.dll ------
 
第一次加载
NHibernate: SELECT customer0_.CustomerId as CustomerId4_0_, customer0_.Firstname as Firstname4_0_, customer0_.Lastname as Lastname4_0_ FROM Customer customer0_ WHERE customer0_.CustomerId=@p0;@p0 = 1
第二次加载
NHibernate: SELECT customer0_.CustomerId as CustomerId4_0_, customer0_.Firstname as Firstname4_0_, customer0_.Lastname as Lastname4_0_ FROM Customer customer0_ WHERE customer0_.CustomerId=@p0;@p0 = 2
 
1 passed, 0 failed, 0 skipped, took 5.47 seconds (NUnit 2.5.1).
           这里果然生成了两个SQL语句，因为CustomerId为2的对象或者叫结果集在session中不存在，所以必须再到数据库中去查询。
 
3)       这里是同样的查询条件，不同的ISession
a)       为了证明，修改程序如下：
        [Test]
        public void
        {
            using (ISession
            {
                Console.WriteLine("第一次加载");
                Customer customer_1 = _session.Get(1);
            }
 
            using (ISession
            {
                Console.WriteLine("第二次加载");
                Customer customer_2 = _session.Get(1);
            }
        } 
 
b)       跟踪NHibernate生成SQL的结果
------ Test started: Assembly: DAL.Test.dll ------
 
第一次加载
NHibernate: SELECT customer0_.CustomerId as CustomerId4_0_, customer0_.Firstname as Firstname4_0_, customer0_.Lastname as Lastname4_0_ FROM Customer customer0_ WHERE customer0_.CustomerId=@p0;@p0 = 1
第二次加载
NHibernate: SELECT customer0_.CustomerId as CustomerId4_0_, customer0_.Firstname as Firstname4_0_, customer0_.Lastname as Lastname4_0_ FROM Customer customer0_ WHERE customer0_.CustomerId=@p0;@p0 = 1
 
1 passed, 0 failed, 0 skipped, took 4.95 seconds (NUnit 2.5.1).
 
二、ISession.Get和ISession.Load的区别​
1)       ISession.Get
a)       编写代码如下：
        [Test]
        public void
        {
            Console.WriteLine("获取持久化实例:");
            Customer customer = session.Get(1);
            Console.WriteLine("访问customer属性:customerId=" + customer.CustomerId);
            Console.WriteLine("访问customer属性:firstName=" + customer.Firstname);
        }
 
b)       结果如下：
------ Test started: Assembly: DAL.Test.dll ------
 
获取持久化实例:
NHibernate: SELECT customer0_.CustomerId as CustomerId4_0_, customer0_.Firstname as Firstname4_0_, customer0_.Lastname as Lastname4_0_ FROM Customer customer0_ WHERE customer0_.CustomerId=@p0;@p0 = 1
访问customer属性:customerId=1
访问customer属性:firstName=luo
 
1 passed, 0 failed, 0 skipped, took 4.92 seconds (NUnit 2.5.1).
 
2)       ISession.Load 
a)       修改代码如下： 
        [Test]
        public void
        {
            Console.WriteLine("获取持久化实例:");
            Customer customer = session.Load(1);
            Console.WriteLine("访问customer属性:customerId=" + customer.CustomerId);
            Console.WriteLine("访问customer属性:firstName=" + customer.Firstname);
        }
 
b)       结果如下：
------ Test started: Assembly: DAL.Test.dll ------
 
获取持久化实例:
访问customer属性:customerId=1
NHibernate: SELECT customer0_.CustomerId as CustomerId4_0_, customer0_.Firstname as Firstname4_0_, customer0_.Lastname as Lastname4_0_ FROM Customer customer0_ WHERE customer0_.CustomerId=@p0;@p0 = 1
访问customer属性:firstName=luo
 
1 passed, 0 failed, 0 skipped, took 5.19 seconds (NUnit 2.5.1).
 
注意：上面两个方法和结果的比较，Get方法一被调用，立即产生SQL语句去查询数据库，而Load方法却是到了需要去取查询结果时才产生SQL去查询数据库，这就是它们两个最大的区别吧！
 
三、一级缓存管理
1)       ISession缓存管理的相关方法
a)       ISession.Evict(object)：从缓存中删除指定实例。
b)       ISession.Clear()：清空缓存。
c)        ISession.Contains(object)：检查缓存中是否包含指定实例。
 
2)       编写代码如下：
        [Test]
        public void
        {
            Customer
            customer_1 = session.Get(1); 
            customer_2 = session.Get(2); 
            Console.WriteLine("customer_1:"
            Console.WriteLine("customer_2:" + session.Contains(customer_2).ToString() + "/r/n");
 
            session.Evict(customer_1);     //清除某个缓存
            Console.WriteLine("customer_1:"
            Console.WriteLine("customer_2:" + session.Contains(customer_2).ToString() + "/r/n");
 
            session.Clear();                //清除该ISession中的所有缓存
            Console.WriteLine("customer_1:"
            Console.WriteLine("customer_2:" + session.Contains(customer_2).ToString() + "/r/n");
        }
 
3)       结果如下：
------ Test started: Assembly: DAL.Test.dll ------
 
NHibernate: SELECT customer0_.CustomerId as CustomerId4_0_, customer0_.Firstname as Firstname4_0_, customer0_.Lastname as Lastname4_0_ FROM Customer customer0_ WHERE customer0_.CustomerId=@p0;@p0 = 1
NHibernate: SELECT customer0_.CustomerId as CustomerId4_0_, customer0_.Firstname as Firstname4_0_, customer0_.Lastname as Lastname4_0_ FROM Customer customer0_ WHERE customer0_.CustomerId=@p0;@p0 = 2
customer_1:True
customer_2:True
 
customer_1:False
customer_2:True
 
customer_1:False
customer_2:True
 
 
1 passed, 0 failed, 0 skipped, took 4.89 seconds (NUnit 2.5.1).