Mybatis深入之获取数据库连接

简介

主要记录Mybatis何时获取数据库连接以及获取数据库连接的过程。难点在于明白在使用Mybatis数据库连接池情况下的数据库连接的获取过程。

何时获取数据库连接

Mybatis只有在真正执行sql操作的时候才会去获取数据库连接。至于如何验证：

不深入源码

简单来讲就是有意将数据库配置信息写成、在一个sql执行过程中看哪一步抛数据库连接异常。

    public static void main(String[] args) throws Exception {        String mybatisConfigPath = "config/mybatis/mybatis.xml";        InputStream inputStream = Resources.getResourceAsStream(mybatisConfigPath);        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);        SqlSession sqlSession = sqlSessionFactory.openSession();        int count = (Integer)sqlSession.selectOne("org.alien.mybatis.samples.mapper.AuthorMapper.getAllAuthorsCount");        System.out.println(count);    }

• 上面是一段Mybatis执行代码
• 我们可以将Mybatis连接数据库的信息有意写错
• 再DEBUG模式下一步一步调试看哪一步会抛异常
• 抛异常的那一步就是真正获取数据库连接的一步

异常信息：

深入源码

这里简单提一下、具体后面会有。最有迷惑性的是觉得在openSession()的时候会获取数据库连接、其实不然：
openSession()最终只是返回一个操作数据库的会话、并不包含数据库连接，DefaultSqlSession（这个是Mybatis初始化的时候返回的一个SqlSession）中的方法：

  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();    }  }

• 主要装配DefaultSqlSession中执行Sql的Excutor、后面关于一个Sql完整的执行过程会有对其的详细分析

显然真正获取数据库连接的操作是在sqlSession.selectOne("org.alien.mybatis.samples.mapper.AuthorMapper.getAllAuthorsCount");进行的。

获取数据库连接

在真正的获取数据库连接代码之前、还有许多为sql执行而生的代码、这里暂时忽略或者一些必要的说明、主要重心放在如何获取数据库连接。
书接上回、从前面执行sql代码开始：

int count = (Integer)sqlSession.selectOne("org.alien.mybatis.samples.mapper.AuthorMapper.getAllAuthorsCount");

下图是上面代码一系列方法调用过程：

经过一系列调用到SimpleExecutor——》doQuery()：

  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();      //RoutingStatementHandler      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);      //MappedStatement、这一句是关键      stmt = prepareStatement(handler, ms.getStatementLog());      /*       *参数：       * stmt: PreparedStatementLogger       * resultHandler: null        */      return handler.<E>query(stmt, resultHandler);    } finally {      closeStatement(stmt);    }  }

• 上面经过一系列跳转之后各个引用的实例我加了注释、有兴趣的可以自己跟一下、有时候可能要多跟几遍
• 关键代码stmt=prepareStatement(handler,ms.getStatementLog());、这一句根据方法名就能猜测、是根据Connection来获取执行Sql的PrepareStatement
• 但是到现在为止我们都没有看到方法的参数中有关于数据库连接的、在此方法中

SimpleExecutor——》prepareStatement();

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {    Statement stmt;    //获取数据库连接。statementLog:org.apache.ibatis.loggin.slf4j.Slf4Impl    Connection connection = getConnection(statementLog);    //获取执行Sql的Statement——PrepareStatementLogger    //PrepareStatementLogger是PrepareStatement的代理、多了对使用Mybatis执行sql语句时记录sql语句的功能    stmt = handler.prepare(connection);    //将执行Sql需要的参数设置到PrepareStatement中。    handler.parameterize(stmt);    return stmt;  }

其他的不关注、这里只看数据库连接代码：Connection connection = getConnection(statementLog);BaseExecutor——》getConnection():

  protected Connection getConnection(Log statementLog) throws SQLException {    //如果关于数据库连接的日志记录级别是DEBUG级别、则为获取的Connection进行代理、新增日志记录功能、这里不是重点。    if (statementLog.isDebugEnabled()) {      return ConnectionLogger.newInstance(connection, statementLog, queryStack);    } else {      return connection;    }  }

• Mybatis中关于事务配置项的值是”JDBC”、所以从Mybatis深入之事务管理 知道这里的transaction其实是：JdbcTransaction
• 最终到JdbcTransaction获取连接的方法中

JdbcTransaction——》openConnection()

  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);  }

• 上面的dataSource从 Mybatis深入之DataSource实例化过程 知道当使用数据库连接池的时候实例化的是PooledDataSource

PooledDataSource——》getConnection()：

  public Connection getConnection() throws SQLException {/* * 为理解方便、将原来代码拆分如下： */    PooledConnection pooledConnection = popConnection(dataSource.getUsername(), dataSource.getPassword());    Connection connection = pooledConnection.getProxyConnection();    return connection ;//return popConnection(dataSource.getUsername(), dataSource.getPassword()).getProxyConnection();  }

• 从拆分代码看分两步
  
  • 获取数据库真正连接
  • 获取真正数据库连接的代理类作为最终返回结果、至于代理是做什么、后面继续

PooledDataSource——》popConnection():

  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;  }

• 先解释一下逻辑、再关注具体的数据库连接方法

1.  先看是否有空闲(idle)状态下的PooledConnection对象，如果有，就直接返回一个可用的PooledConnection对象；否则进行第2步。2.  查看活动状态的PooledConnection池activeConnections是否已满；如果没有满，则创建一个新的PooledConnection对象，然后放到activeConnections池中，然后返回此PooledConnection对象；否则进行第三步；3.  看最先进入activeConnections池中的PooledConnection对象是否已经过期：如果已经过期，从activeConnections池中移除此对象，然后创建一个新的PooledConnection对象，添加到activeConnections中，然后将此对象返回；否则进行第4步。4.  线程等待，循环2步

具体的创建数据库连接代码conn = new PooledConnection(dataSource.getConnection(), this);

• 上面代码中的dataSource为UnpooledDataSource、可以从 Mybatis深入之DataSource实例化过程 了解原因。

所以先要看UnpooledDataSource——getConnection() 经过一系列跳转到同类如下方法：

  private Connection doGetConnection(Properties properties) throws SQLException {    initializeDriver();    Connection connection = DriverManager.getConnection(url, properties);    configureConnection(connection);    return connection;  }

• 上面我们可以看到很熟悉的加载驱动、获取数据库连接

补充

其实关于数据库部分还有很多要写的、比如数据库连接池工作原理、数据库连接何时关闭。这里暂不准备一次将所有的东都放在一起。觉得分开点更容易说清楚理解起来不是那么费劲。
后面会有一篇Mybatis数据库连接池原理来分析它。

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