Mybatis数据源

在描述mybatis数据源以前，先抛出几个问题，这几个问题都能在本文获得解答

1.mybatis是如何获取到mysql链接的？

2.mybatis的Connection是怎么被建立的？

　　

1.Datasource的分类

---

咱们已一段mybatis的配置文件为例

<environments default="development">
  <environment id="development">
    <transactionManager type="JDBC">
      <property name="..." value="..."/>
    </transactionManager>
    <dataSource type="POOLED">
      <property name="driver" value="${driver}"/>
      <property name="url" value="${url}"/>
      <property name="username" value="${username}"/>
      <property name="password" value="${password}"/>
    </dataSource>
  </environment>
</environments>

datasource的type共有三个选项

UNPOOLED    不使用链接池的数据源

POOLED      使用链接池的数据源

JNDI        使用JNDI实现的数据源

 

2.Datasource的配置加载与建立

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 mybatis在项目启动阶段会加载配置文件，读取xml中的配置信息到Configuration中。咱们看下datasource是怎么加载进来的。这段代码在org.apache.ibatis.builder.xml.XMLConfigBuilder#parseConfiguration

 点进去，找到这个方法。

 

 这里经过工厂模式，反射生成一个DatasourceFactory。

这里DataSourceFactory有三个子类，也就是上述三个datasource的类型。

 

3.Connection的建立

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 当Datasource的配置加载到configuration后。每一次执行sql都须要Datasource对象建立Connection去链接数据库。

咱们以一次查询为例,看下这个connection究竟是怎么生成的。

transaction对象持有connection和datasource，咱们再点进去

 

终于看到了真正获取connection是datasource，那datasource是怎么获取connection的呢？

 

4.链接池

---

 datasource有三种类型，相对应的每种datasource建立connection各不相同。常见的是POOLEDA和UNPOOLED两种类型。

 UNPOOLED顾名思义就是不用链接池的方式，每次用到一个就生产一个

POOLEDA类型采用了链接池的方式，内部经过Poolstate对象来维护链接池对象

 Poolstate内部有两个数组，idleConnections用来存放空闲的connection，activeConnections用来存放链接中的connection。

具体的获取一个Connection链接以下

private final PoolState state = new PoolState(this);
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) {
      //这个对象锁锁的范围有点大，不过这也是由于PoolState内部的两个集合是ArrayList会产生并发问题
      synchronized (state) {
        //有空闲链接
        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;
  }

1.   先看是否有空闲(idle)状态下的PooledConnection对象，若是有，就直接返回一个可用的PooledConnection对象；不然进行第2步。

2.  查看活动状态的PooledConnection池activeConnections是否已满；若是没有满，则建立一个新的PooledConnection对象，而后放到activeConnections池中，而后返回此PooledConnection对象；不然进行第三步；

3.  看最早进入activeConnections池中的PooledConnection对象是否已通过期：若是已通过期，从activeConnections池中移除此对象，而后建立一个新的PooledConnection对象，添加到activeConnections中，而后将此对象返回；不然进行第4步。

4.  线程等待，循环2步

 

以上描述的是经过链接池获取connection，那在关闭connection后，参照获取链接的步骤，首先从activeConnections中移除，再判断idle数组是否已经满了，若是满了再判断下数组中第一个链接是否已经任然可用，若是可用再把这个这个链接销毁。

上文咱们提到了PoolState

 两个集合里面存放着PooledConnection对象，PooledConnection对象是Connection的代理类

PooledConnection相比Connection多了什么呢，答案就在invoke方法中，当connection对象调用close方法时，会调用datasource的pushConnection方法，pushConnection的方法大体和上面的猜测一致