从一个ConnectionPool的实现看design pattern的运用 (三)

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从一个ConnectionPool的实现看design pattern的运用 ()

根据上回对PooledConnection的分析,下面是对一个可重用PooledConnection的实现:

 

public class PooledConnection implements Connection{

           

            public interface Pool{

            //引入这个interface, 是因为我们的PooledConnection只需要知道如何返还Connection. 本着接口最小化原则,我们只定义我们需要的操作。

                        void closeConnection(Connection conn);

            }

 

    private interface ConnectionState{

            //state pattern的interface.

        ConnectionState close()

        throws SQLException;

            //close()方法是唯一引起状态转移的方法。

        boolean isClosed();

        Connection getOpenConnection()

        throws SQLException;

    }

 

    private static class ClosedConnection implements ConnectionState{

        public final ConnectionState close(){return this;}

            //当一个Connection已经closed了的时候,它实际上已经死了。所有对它的操作,除了isClosed()和close(), 只产生异常。所以,一个closed的Connection, 它已经不需要保存那个物理数据库连接和对出身ConnectionPool的连接。而且因为所有的 closed connection的状态都一样,所以可以用singleton来节省内存。

 

        public final Connection getOpenConnection()

        throws SQLException{

            throw new SQLException("Connection closed");

        }

        public final boolean isClosed(){return true;}

        private ClosedConnection(){}

        private static final ConnectionState _instance = new ClosedConnection();

        static ConnectionState instance(Connection conn, Pool pool){return _instance;}

    }

   

    private static class OpenConnection implements ConnectionState{

            private final Pool pool;

        private final Connection conn;

        public final ConnectionState close(){

            //对一个open connection的关闭,会把原始数据库连接返还到connection pool. 同时,该连接死亡。

                pool.closeConnection(conn);

                return ClosedConnection.instance(conn, pool);

        }

        public final Connection getOpenConnection()

        {return conn;}

        public final boolean isClosed(){return false;}

        OpenConnection(Connection conn, Pool pool){

                    this.conn = conn; this.pool = pool;

        }

        static ConnectionState instance(Connection conn, Pool pool){

            return new OpenConnection(conn, pool);

        }

    }

    private ConnectionState state;

 

  //用静态的工厂方法,可以隐藏我们的实现类,以后,根据需要,我们可以方便地修改实现类,比如用内部类取代。

//根据要修饰的Connection的状态,初始化PooledConnection

    public static Connection decorate(Connection conn, Pool pool)

    throws SQLException{

        if(conn.isClosed()){

            return new PooledConnection(ClosedConnection.instance(conn, pool));

        }

        else{

            return new PooledConnection(OpenConnection.instance(conn, pool));

        }

    }

   

 

    private PooledConnection(ConnectionState state){

            this.state = state;

    }      

    public final boolean isClosed(){

        return state.isClosed();

    }

 

    public final void close()

    throws SQLException{

        state = state.close();

    }

    private final Connection getOpenConnection()

    throws SQLException

    {return state.getOpenConnection();}

 

    /*****然后,做委托****/

    public final Statement createStatement()

    throws SQLException{

        return getOpenConnection().createStatement();

    }

    public final void rollback()throws SQLException{

        getOpenConnection().rollback();

    }

//等等等等

}

 

 

好,再来看看ConnectionPoolImpl怎样使用PooledConnection.

 

public class ConnectionPoolImpl implements ConnectionPool{

public synchronized Connection getConnection(){

            Connection ret;

                        如果pool里有Connection

从pool中去掉一个Connection conn;

clients++;

ret = conn;

                        否则,如果clients<maxClients

                                    生成一个新的连接conn

                                    clients++;

                                    ret = conn;

                        否则,wait(),直到pool中有空闲Connection     

 

                        //下面的这个匿名类实际上是个adapter pattern. J                    

                        return PooledConnection.decorate(ret,

new PooledConnection.Pool{

public void closeConnection(Connection conn){

ConnectionPoolImpl.this.closeConnection(conn);

}

}

            }

            //其他都和原来一样

}

 

这样,所有对ConnectionPool的实现,都可以在返回一个物理Connection之前,把它用PooledConnection封装一下。如此,代码得到了重用。ConnectionPool的实现者可以把主要精力放在怎样处理池的各种功能。而不是怎样包装Connection.

世界真美好!

 

 

不过。。。。。

 

万一,李四忘了用PooledConnection包装他的Connection怎么办?编译器不会报错,因为反正都是Connection类型。

 

“你也太杞人忧天了吧?他忘了揍他不就得了?”哎,保不齐呀!人不是机器,总有犯错的时候,到时候揍他有啥用?还手疼呢。

 

同学们,今天的家庭作业是:想办法让李四的健忘症不会影响我们的ConnectionPool大业。

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