MyBatis源码浅析



MyBatis源码浅析

posted on 2014-12-15 09:50 Tim-Tom

url: http://www.cnblogs.com/timlearn/p/4161567.html


什么是MyBatis      

      MyBatis是支持定制化SQL、存储过程以及高级映射的优秀的持久层框架。MyBatis 避免了几乎所有的 JDBC 代码和手工设置参数以及抽取结果集。MyBatis 使用简单的 XML 或注解来配置和映射基本体,将接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java对象)映射成数据库中的记录。

MyBatis简单示例

      虽然在使用MyBatis时一般都会使用XML文件,但是本文为了分析程序的简单性,简单的测试程序将不包含XML配置,该测试程序包含一个接口、一个启动类:

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public interface UserMapper {
   @Select ( "SELECT * FROM user WHERE id = #{id}" )
   User selectUser( int id);
}
 
public class Test2 {
     public static void main(String[] args) {
         SqlSessionFactory sqlSessionFactory = initSqlSessionFactory();
         SqlSession session = sqlSessionFactory.openSession();
         try {
             User user = (User) session.selectOne(
                     "org.mybatis.example.UserMapper.selectUser" , 1 );
             System.out.println(user.getUserAddress());
             System.out.println(user.getUserName());
         } finally {
             session.close();
         }
     }
 
     private static SqlSessionFactory initSqlSessionFactory() {
         DataSource dataSource = new PooledDataSource( "com.mysql.jdbc.Driver" ,
                 "jdbc:mysql://127.0.0.1:3306/jdbc" , "root" , "" );
         TransactionFactory transactionFactory = new JdbcTransactionFactory();
         Environment environment = new Environment( "development" ,
                 transactionFactory, dataSource);
         Configuration configuration = new Configuration(environment);
         configuration.addMapper(UserMapper. class );
         SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()
                 .build(configuration);
 
         return sqlSessionFactory;
     }
}

  UserMapper是一个接口,我们在构建sqlSessionFactory时通过configuration.addMapper(UserMapper.class)把该接口注册进了sqlSessionFactory中。从上面的代码中我们可以看出,要使用MyBatis,我们应该经过以下步骤:1、创建sqlSessionFactory(一次性操作);2、用sqlSessionFactory对象构造sqlSession对象;3、调用sqlSession的相应方法;4、关闭sqlSession对象。

      在main方法中,我们没有配置sql,也没有根据查询结果拼接对象,只需在调用sqlSession方法时传入一个命名空间以及方法参数参数即可,所有的操作都是面向对象的。在UserMapper接口中,我们定制了自己的sql,MyBatis把书写sql的权利给予了我们,方便我们进行sql优化及sql排错。

JDBC基础回顾

      直接使用JDBC是很痛苦的,JDBC连接数据库包含以下几个基本步骤:1、注册驱动 ;2、建立连接(Connection);3、创建SQL语句(Statement);4、执行语句;5、处理执行结果(ResultSet);6、释放资源,示例代码如下:

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public static void test() throws SQLException{
     // 1.注册驱动
     Class.forName( "com.mysql.jdbc.Driver" );
  
     // 2.建立连接  url格式 - JDBC:子协议:子名称//主机名:端口/数据库名?属性名=属性值&…
     Connection conn = DriverManager.getConnection( "jdbc:mysql://localhost:3306/jdbc" , "root" , "" );
  
     // 3.创建语句
     Statement st = conn.createStatement();
  
     // 4.执行语句
     ResultSet rs = st.executeQuery( "select * from user" );
  
     // 5.处理结果
     while (rs.next()) {<br>      User user = new User(rs.getObject( 1 ), rs.getObject( 2 ));
     }
  
     // 6.释放资源
     rs.close();
     st.close();
     conn.close();
}

  可以看到与直接使用JDBC相比,MyBatis为我们简化了很多工作:

      1、把创建连接相关工作抽象成一个sqlSessionFactory对象,一次创建多次使用;

      2、把sql语句从业务层剥离,代码逻辑更加清晰,增加可维护性;

      3、自动完成结果集处理,不需要我们编写重复代码。

      但是,我们应该知道的是,框架虽然能够帮助我们简化工作,但是框架底层的代码肯定还是最基础的JDBC代码,因为这是Java平台连接数据库的通用方法,今天我将分析一下MyBatis源码,看看MyBatis是如何把这些基础代码封装成一个框架的。

MyBatis调用流程

      我们最终调用的是sqlSession对象上的方法,所以我们先跟踪sqlSession的创建方法:sqlSessionFactory.openSession(),最终这个方法会调用到DefaultSqlSessionFactory的以下方法:

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private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
     Transaction tx = null ;
     try {
       final Environment environment = configuration.getEnvironment();
       final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
       tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
       final Executor executor = configuration.newExecutor(tx, execType);
       return new DefaultSqlSession(configuration, executor, autoCommit);
     } catch (Exception e) {
       closeTransaction(tx); // may have fetched a connection so lets call close()
       throw ExceptionFactory.wrapException( "Error opening session.  Cause: " + e, e);
     } finally {
       ErrorContext.instance().reset();
     }
   }

  最终返回的对象是一个DefaultSqlSession对象,在调试模式下,我们看到autoCommit为false,executor为CachingExecutor类型,在CachingExecutor里面有属性delegate,其类型为simpleExecutor:

MyBatis源码浅析_第1张图片

      现在,我们跟进DefaultSqlSession的selectOne()方法,查看该方法的调用流程,selectOne()方法又会调用selectList()方法:

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public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
   try {
     MappedStatement ms = configuration.getMappedStatement(statement);
     List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
     return result;
   } catch (Exception e) {
     throw ExceptionFactory.wrapException( "Error querying database.  Cause: " + e, e);
   } finally {
     ErrorContext.instance().reset();
   }
}

  可以看到要得到查询结果,最终还是要调用executor上的query方法,这里的executor是CachingExecutor实例,跟进程序得到如下代码:

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public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
   BoundSql boundSql = ms.getBoundSql(parameterObject);
   CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
   return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
 
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
     throws SQLException {
   Cache cache = ms.getCache();
   if (cache != null ) {
     flushCacheIfRequired(ms);
     if (ms.isUseCache() && resultHandler == null ) {
       ensureNoOutParams(ms, parameterObject, boundSql);
       @SuppressWarnings ( "unchecked" )
       List<E> list = (List<E>) tcm.getObject(cache, key);
       if (list == null ) {
         list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
         tcm.putObject(cache, key, list); // issue #578. Query must be not synchronized to prevent deadlocks
       }
       return list;
     }
   }
   return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

  MyBatis框架首先生成了一个boundSql和CacheKey,在boundSql中包含有我们传入的sql语句:

      生成boundSql和CacheKey后会调用一个重载函数,在重载函数中,我们会检测是否有缓存,这个缓存是MyBatis的二级缓存,我们没有配置,那么直接调用最后一句delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql),前面说过这个delagate其实就是simpleExecutor,跟进去查看一下:

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public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
     ErrorContext.instance().resource(ms.getResource()).activity( "executing a query" ).object(ms.getId());
     if (closed) throw new ExecutorException( "Executor was closed." );
     if (queryStack == 0 && ms.isFlushCacheRequired()) {
       clearLocalCache();
     }
     List<E> list;
     try {
       queryStack++;
       list = resultHandler == null ? (List<E>) localCache.getObject(key) : null ;
       if (list != null ) {
         handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
       } else {
         list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
       }
     } finally {
       queryStack--;
     }
     if (queryStack == 0 ) {
       for (DeferredLoad deferredLoad : deferredLoads) {
         deferredLoad.load();
       }
       deferredLoads.clear(); // issue #601
       if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
         clearLocalCache(); // issue #482
       }
     }
     return list;
   }

  关键代码是以下三行:

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list = resultHandler == null ? (List<E>) localCache.getObject(key) : null ;
if (list != null ) {
   handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
   list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}

  首先尝试从localCache中根据key得到List,这里的localCache是MyBatis的一级缓存,如果得不到则调用queryFromDatabase()从数据库中查询:

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private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
   List<E> list;
   localCache.putObject(key, EXECUTION_PLACEHOLDER);
   try {
     list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
   } finally {
     localCache.removeObject(key);
   }
   localCache.putObject(key, list);
   if (ms.getStatementType() == StatementType.CALLABLE) {
     localOutputParameterCache.putObject(key, parameter);
   }
   return list;
}

      其中关键代码是调用doQuery()代码,SimpleExecutor的doQuery()方法如下:

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public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
   Statement stmt = null ;
   try {
     Configuration configuration = ms.getConfiguration();
     StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
     stmt = prepareStatement(handler, ms.getStatementLog());
     return handler.<E>query(stmt, resultHandler);
   } finally {
     closeStatement(stmt);
   }
}

  调用了prepareStatement方法,该方法如下:

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private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
   Statement stmt;
   Connection connection = getConnection(statementLog);
   stmt = handler.prepare(connection);
   handler.parameterize(stmt);
   return stmt;
}

  终于,我们看到熟悉的代码了,首先得到Connection,然后从Connection中得到Statement,同时在调试模式下我们看到,我们的sql语句已经被设置到stmt中了:

  现在Statement对象有了,sql也设置进去了,就只差执行以及对象映射了,继续跟进代码,我们会跟踪到org.apache.ibatis.executor.statement.

PreparedStatementHandler类的executor方法:

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public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
   PreparedStatement ps = (PreparedStatement) statement;
   ps.execute();
   return resultSetHandler.<E> handleResultSets(ps);
}

  在这里,调用了ps.execute()方法执行sql,接下来调用的resultSetHandler.<E> handleResultSets(ps)方法明显是对结果集进行封装,我就不继续跟进了。      

MyBatis的数据库连接池

     上面一部分介绍了MyBatis执行的整体流程,这一部分打算讨论一个具体话题:MyBatis的数据库连接池。

     我们知道,每次连接数据库时都创建Connection是十分耗费性能的,所以我们在写JDBC代码时,一般都会使用数据库连接池,把用过的Connection不是直接关闭,而是放入数据库连接池中,方便下次复用,开源的数据库连接池有DBCP、C3P0等,MyBatis也实现了自己的数据库连接池,在这一节我将探索一下MyBatis实现的数据库连接池源码。

      跟进上一节的getConnection()方法,我们最终会进入JdbcTransaction的getConnection()方法,getConnection()方法又会调用openConnection()方法,而openConnection()又将调用dataSource的getConnection()方法:

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public Connection getConnection() throws SQLException {
     if (connection == null ) {
         openConnection();
     }
     return connection;
}
 
protected void openConnection() throws SQLException {
     if (log.isDebugEnabled()) {
         log.debug( "Opening JDBC Connection" );
     }
     connection = dataSource.getConnection();
     if (level != null ) {
         connection.setTransactionIsolation(level.getLevel());
     }
     setDesiredAutoCommit(autoCommmit);
}

  这里的dataSource是PooledDataSource类型,跟进查看源码如下:

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public Connection getConnection() throws SQLException {
   return popConnection(dataSource.getUsername(), dataSource.getPassword()).getProxyConnection();
}
 
private PooledConnection popConnection(String username, String password) throws SQLException {
   //暂不分析
}

      可以看到,在这里我们返回的对象其实已经不是原生的Connection对象了,而是一个动态代理对象,是PooledConnection的一个属性,所有对对Connection对象的操作都将被PooledConnection拦截,我们可以查看PooledConnection的定义如下:

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class PooledConnection implements InvocationHandler {
     private static final String CLOSE = "close" ;
     private static final Class<?>[] IFACES = new Class<?>[] { Connection. class };
     private int hashCode = 0 ;
     private PooledDataSource dataSource;
     private Connection realConnection;
     private Connection proxyConnection;
     private long checkoutTimestamp;
     private long createdTimestamp;
     private long lastUsedTimestamp;
     private int connectionTypeCode;
     private boolean valid;
 
     public PooledConnection(Connection connection, PooledDataSource dataSource) {
         this .hashCode = connection.hashCode();
         this .realConnection = connection;
         this .dataSource = dataSource;
         this .createdTimestamp = System.currentTimeMillis();
         this .lastUsedTimestamp = System.currentTimeMillis();
         this .valid = true ;
         this .proxyConnection = (Connection) Proxy.newProxyInstance(
                 Connection. class .getClassLoader(), IFACES, this );
     }
 
     public void invalidate() {
         valid = false ;
     }
 
     public boolean isValid() {
         return valid && realConnection != null
                 && dataSource.pingConnection( this );
     }
 
     public Connection getRealConnection() {
         return realConnection;
     }
 
     public Connection getProxyConnection() {
         return proxyConnection;
     }
 
     public int getRealHashCode() {
         if (realConnection == null ) {
             return 0 ;
         } else {
             return realConnection.hashCode();
         }
     }
 
     public int getConnectionTypeCode() {
         return connectionTypeCode;
     }
 
     public void setConnectionTypeCode( int connectionTypeCode) {
         this .connectionTypeCode = connectionTypeCode;
     }
 
     public long getCreatedTimestamp() {
         return createdTimestamp;
     }
 
     public void setCreatedTimestamp( long createdTimestamp) {
         this .createdTimestamp = createdTimestamp;
     }
 
     public long getLastUsedTimestamp() {
         return lastUsedTimestamp;
     }
 
     public void setLastUsedTimestamp( long lastUsedTimestamp) {
         this .lastUsedTimestamp = lastUsedTimestamp;
     }
 
     public long getTimeElapsedSinceLastUse() {
         return System.currentTimeMillis() - lastUsedTimestamp;
     }
 
     public long getAge() {
         return System.currentTimeMillis() - createdTimestamp;
     }
 
     public long getCheckoutTimestamp() {
         return checkoutTimestamp;
     }
 
     public void setCheckoutTimestamp( long timestamp) {
         this .checkoutTimestamp = timestamp;
     }
 
     public long getCheckoutTime() {
         return System.currentTimeMillis() - checkoutTimestamp;
     }
 
     public int hashCode() {
         return hashCode;
     }
 
     public boolean equals(Object obj) {
         if (obj instanceof PooledConnection) {
             return realConnection.hashCode() == (((PooledConnection) obj).realConnection
                     .hashCode());
         } else if (obj instanceof Connection) {
             return hashCode == obj.hashCode();
         } else {
             return false ;
         }
     }
 
     public Object invoke(Object proxy, Method method, Object[] args)
             throws Throwable {
         String methodName = method.getName();
         if (CLOSE.hashCode() == methodName.hashCode()
                 && CLOSE.equals(methodName)) {
             dataSource.pushConnection( this );
             return null ;
         } else {
             try {
                 if (!Object. class .equals(method.getDeclaringClass())) {
                     checkConnection();
                 }
                 return method.invoke(realConnection, args);
             } catch (Throwable t) {
                 throw ExceptionUtil.unwrapThrowable(t);
             }
         }
     }
 
     private void checkConnection() throws SQLException {
         if (!valid) {
             throw new SQLException(
                     "Error accessing PooledConnection. Connection is invalid." );
         }
     }
}

  可以看到这个类暴露了很多接口检测Connection状态,例如连接是否有效,连接创建时间最近使用连接等:

MyBatis源码浅析_第2张图片

      这个类实现了InvocationHandler接口,最主要的一个方法如下:

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public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
   String methodName = method.getName();
   if (CLOSE.hashCode() == methodName.hashCode() && CLOSE.equals(methodName)) {
     dataSource.pushConnection( this );
     return null ;
   } else {
     try {
       if (!Object. class .equals(method.getDeclaringClass())) {
         checkConnection();
       }
       return method.invoke(realConnection, args);
     } catch (Throwable t) {
       throw ExceptionUtil.unwrapThrowable(t);
     }
   }
}

  可以看到,PooledConnection会拦截close方法,当客户端调用close()方法时,程序不会关闭Connection,而是会调用dataSource.pushConnection(this)方法,该方法的实现如下:

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protected void pushConnection(PooledConnection conn) throws SQLException {
   synchronized (state) {
     state.activeConnections.remove(conn);
     if (conn.isValid()) {
       if (state.idleConnections.size() < poolMaximumIdleConnections && conn.getConnectionTypeCode() == expectedConnectionTypeCode) {
         state.accumulatedCheckoutTime += conn.getCheckoutTime();
         if (!conn.getRealConnection().getAutoCommit()) {
           conn.getRealConnection().rollback();
         }
         PooledConnection newConn = new PooledConnection(conn.getRealConnection(), this );
         state.idleConnections.add(newConn);
         newConn.setCreatedTimestamp(conn.getCreatedTimestamp());
         newConn.setLastUsedTimestamp(conn.getLastUsedTimestamp());
         conn.invalidate();
         if (log.isDebugEnabled()) {
           log.debug( "Returned connection " + newConn.getRealHashCode() + " to pool." );
         }
         state.notifyAll();
       } else {
         state.accumulatedCheckoutTime += conn.getCheckoutTime();
         if (!conn.getRealConnection().getAutoCommit()) {
           conn.getRealConnection().rollback();
         }
         conn.getRealConnection().close();
         if (log.isDebugEnabled()) {
           log.debug( "Closed connection " + conn.getRealHashCode() + "." );
         }
         conn.invalidate();
       }
     } else {
       if (log.isDebugEnabled()) {
         log.debug( "A bad connection (" + conn.getRealHashCode() + ") attempted to return to the pool, discarding connection." );
       }
       state.badConnectionCount++;
     }
   }
}

  可以看到,首先会把Connection从活跃列表中删除,然后检测空闲列表的长度有没有达到最大长度(默认为5),若没有达到,把Connection放入空闲链表,否则关闭连接。这里的state是一个PoolState对象,该对象定义如下:

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public class PoolState {
   protected PooledDataSource dataSource;
   protected final List<PooledConnection> idleConnections = new ArrayList<PooledConnection>();
   protected final List<PooledConnection> activeConnections = new ArrayList<PooledConnection>();
   protected long requestCount = 0 ;
   protected long accumulatedRequestTime = 0 ;
   protected long accumulatedCheckoutTime = 0 ;
   protected long claimedOverdueConnectionCount = 0 ;
   protected long accumulatedCheckoutTimeOfOverdueConnections = 0 ;
   protected long accumulatedWaitTime = 0 ;
   protected long hadToWaitCount = 0 ;
   protected long badConnectionCount = 0 ;
 
   public PoolState(PooledDataSource dataSource) {
     this .dataSource = dataSource;
   }
 
   public synchronized long getRequestCount() {
     return requestCount;
   }
 
   public synchronized long getAverageRequestTime() {
     return requestCount == 0 ? 0 : accumulatedRequestTime / requestCount;
   }
 
   public synchronized long getAverageWaitTime() {
     return hadToWaitCount == 0 ? 0 : accumulatedWaitTime / hadToWaitCount;
   }
 
   public synchronized long getHadToWaitCount() {
     return hadToWaitCount;
   }
 
   public synchronized long getBadConnectionCount() {
     return badConnectionCount;
   }
 
   public synchronized long getClaimedOverdueConnectionCount() {
     return claimedOverdueConnectionCount;
   }
 
   public synchronized long getAverageOverdueCheckoutTime() {
     return claimedOverdueConnectionCount == 0 ? 0 : accumulatedCheckoutTimeOfOverdueConnections / claimedOverdueConnectionCount;
   }
 
   public synchronized long getAverageCheckoutTime() {
     return requestCount == 0 ? 0 : accumulatedCheckoutTime / requestCount;
   }
 
   public synchronized int getIdleConnectionCount() {
     return idleConnections.size();
   }
 
   public synchronized int getActiveConnectionCount() {
     return activeConnections.size();
   }
}

  可以看到最终我们的Connection对象是放在ArrayList中的,该类还提供一些接口返回连接池基本信息。

      好了,现在我们可以回去看看PooledDataSource的popConnection方法了:

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private PooledConnection popConnection(String username, String password) throws SQLException {
     boolean countedWait = false ;
     PooledConnection conn = null ;
     long t = System.currentTimeMillis();
     int localBadConnectionCount = 0 ;
 
     while (conn == null ) {
       synchronized (state) {
         if (state.idleConnections.size() > 0 ) {
           // Pool has available connection
           conn = state.idleConnections.remove( 0 );
           if (log.isDebugEnabled()) {
             log.debug( "Checked out connection " + conn.getRealHashCode() + " from pool." );
           }
         } else {
           // Pool does not have available connection
           if (state.activeConnections.size() < poolMaximumActiveConnections) {
             // Can create new connection
             conn = new PooledConnection(dataSource.getConnection(), this );
             @SuppressWarnings ( "unused" )
             //used in logging, if enabled
             Connection realConn = conn.getRealConnection();
             if (log.isDebugEnabled()) {
               log.debug( "Created connection " + conn.getRealHashCode() + "." );
             }
           } else {
             // Cannot create new connection
             PooledConnection oldestActiveConnection = state.activeConnections.get( 0 );
             long longestCheckoutTime = oldestActiveConnection.getCheckoutTime();
             if (longestCheckoutTime > poolMaximumCheckoutTime) {
               // Can claim overdue connection
               state.claimedOverdueConnectionCount++;
               state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime;
               state.accumulatedCheckoutTime += longestCheckoutTime;
               state.activeConnections.remove(oldestActiveConnection);
               if (!oldestActiveConnection.getRealConnection().getAutoCommit()) {
                 oldestActiveConnection.getRealConnection().rollback();
               }
               conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this );
               oldestActiveConnection.invalidate();
               if (log.isDebugEnabled()) {
                 log.debug( "Claimed overdue connection " + conn.getRealHashCode() + "." );
               }
             } else {
               // Must wait
               try {
                 if (!countedWait) {
                   state.hadToWaitCount++;
                   countedWait = true ;
                 }
                 if (log.isDebugEnabled()) {
                   log.debug( "Waiting as long as " + poolTimeToWait + " milliseconds for connection." );
                 }
                 long wt = System.currentTimeMillis();
                 state.wait(poolTimeToWait);
                 state.accumulatedWaitTime += System.currentTimeMillis() - wt;
               } catch (InterruptedException e) {
                 break ;
               }
             }
           }
         }
         if (conn != null ) {
           if (conn.isValid()) {
             if (!conn.getRealConnection().getAutoCommit()) {
               conn.getRealConnection().rollback();
             }
             conn.setConnectionTypeCode(assembleConnectionTypeCode(dataSource.getUrl(), username, password));
             conn.setCheckoutTimestamp(System.currentTimeMillis());
             conn.setLastUsedTimestamp(System.currentTimeMillis());
             state.activeConnections.add(conn);
             state.requestCount++;
             state.accumulatedRequestTime += System.currentTimeMillis() - t;
           } else {
             if (log.isDebugEnabled()) {
               log.debug( "A bad connection (" + conn.getRealHashCode() + ") was returned from the pool, getting another connection." );
             }
             state.badConnectionCount++;
             localBadConnectionCount++;
             conn = null ;
             if (localBadConnectionCount > (poolMaximumIdleConnections + 3 )) {
               if (log.isDebugEnabled()) {
                 log.debug( "PooledDataSource: Could not get a good connection to the database." );
               }
               throw new SQLException( "PooledDataSource: Could not get a good connection to the database." );
             }
           }
         }
       }
     }
 
     if (conn == null ) {
       if (log.isDebugEnabled()) {
         log.debug( "PooledDataSource: Unknown severe error condition.  The connection pool returned a null connection." );
       }
       throw new SQLException( "PooledDataSource: Unknown severe error condition.  The connection pool returned a null connection." );
     }
 
     return conn;
   }

  可以看到获取Connection一共分以下几种情况:1、如果有空闲Connection,那么直接使用空闲Connection,否则2;2、如果活跃Connection没有达到活跃Connection的上限,那么创建一个新Connection并返回,否则3;3、如果达到活跃上限,且被检出的Connection检出时间过长,那么把该Connection置为失效,新创建一个Connection,否则4;4、等待空闲Connection。

      至此,我们就把MyBatis的数据库连接池代码整理了一遍,其中有两个关键点:1、检出的Connection其实不是原生Connection,而是一个代理对象;2、存放Connection的容器是ArrayList,Connection的检出遵从先进先出原则。

MyBatis的缓存

      这篇博客讲的很好,mark一下:http://www.cnblogs.com/fangjian0423/p/mybatis-cache.html

MyBatis的事务

      首先回顾一下JDBC的事务知识。

      JDBC可以操作Connection的setAutoCommit()方法,给它false参数,提示数据库启动事务,在下达一连串的SQL命令后,自行调用Connection的commit()方法,提示数据库确认(Commit)操作。如果中间发生错误,则调用rollback(),提示数据库撤销(ROLLBACK)所有执行。同时,如果仅想要撤回某个SQL执行点,则可以设置存储点(SAVEPOINT)。一个示范的事务流程如下:

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Connection conn = ...;
Savepoint point = null ;
try {
     conn.setAutoCommit( false );
     Statement stmt = conn.createStatement();
     stmt.executeUpdate( "INSERT INTO ..." );
     ...
     point = conn.setSavepoint();
     stmt.executeUpdate( "INSERT INTO ..." );
     ...
     conn.commit();
} catch (SQLException e) {
     e.printStackTrace();
     if (conn != null ) {
         try {
             if (point == null ) {
                 conn.rollback();
             } else {
                 conn.rollback(point);
                 conn.releaseSavepoint(point);
             }
         } catch (SQLException ex) {
             ex.printStackTrace();
         }
     }
} finally {
     ...
     if (conn != null ) {
         try {
             conn.setAutoCommit( true );
             conn.close();
         } catch (SQLException ex) {
             ex.printStackTrace();
         }
     }
}

  在MyBatis调用流程一节就写过,在调试模式下,我们看到autoCommit为false,所以每个sqlSession其实都是一个事务,这也是为什么每次做删、改、查时都必须调用commit的原因。

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