Mybatis 的获取连接与回收连接：popConnection 与 pushConnection

PooledConnection

class PooledConnection implements InvocationHandler {

  private static final String CLOSE = "close";
  private static final Class<?>[] IFACES = new Class<?>[] { Connection.class };

  private final int hashCode;
  //记录当前连接所在的数据源对象，本次连接是有这个数据源创建的，关闭后也是回到这个数据源；
  private final PooledDataSource dataSource;
  //真正的连接对象
  private final Connection realConnection;
  //连接的代理对象
  private final Connection proxyConnection;
  //从数据源取出来连接的时间戳
  private long checkoutTimestamp;
  //连接创建的的时间戳
  private long createdTimestamp;
  //连接最后一次使用的时间戳
  private long lastUsedTimestamp;
  //根据数据库url、用户名、密码生成一个hash值，唯一标识一个连接池
  private int connectionTypeCode;
  //连接是否有效
  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);
  }

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    String methodName = method.getName();
    if (CLOSE.hashCode() == methodName.hashCode() && CLOSE.equals(methodName)) {//如果是调用连接的close方法，不是真正的关闭，而是回收到连接池
      dataSource.pushConnection(this);//通过pooled数据源来进行回收
      return null;
    } else {
      try {
    	  //使用前要检查当前连接是否有效
        if (!Object.class.equals(method.getDeclaringClass())) {
          // issue #579 toString() should never fail
          // throw an SQLException instead of a Runtime
          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.");
    }
  }
  public boolean isValid() {
    return valid && realConnection != null && dataSource.pingConnection(this);
  }
  //省略了一些方法
}

PollState

注意两个重要的容器 idleConnections 和 activeConnections

public class PoolState {

  protected PooledDataSource dataSource;
  //空闲的连接池资源集合
  protected final List<PooledConnection> idleConnections = new ArrayList<>();
  //活跃的连接池资源集合
  protected final List<PooledConnection> activeConnections = new ArrayList<>();
  //请求的次数
  protected long requestCount = 0;
  //累计的获得连接的时间
  protected long accumulatedRequestTime = 0;
  //累计的使用连接的时间。从连接取出到归还，算一次使用的时间；
  protected long accumulatedCheckoutTime = 0;
  //使用连接超时的次数
  protected long claimedOverdueConnectionCount = 0;
  //累计超时时间
  protected long accumulatedCheckoutTimeOfOverdueConnections = 0;
  //累计等待时间
  protected long accumulatedWaitTime = 0;
  //等待次数
  protected long hadToWaitCount = 0;
  //无效的连接次数
  protected long badConnectionCount = 0;

  public PoolState(PooledDataSource dataSource) {
    this.dataSource = dataSource;
  }

  public synchronized long getRequestCount() {
    return requestCount;
  }
  //略去了一些方法
}

PooledDataSource

popConnection

private PooledConnection popConnection(String username, String password) throws SQLException {
    boolean countedWait = false;
    PooledConnection conn = null;
    long t = System.currentTimeMillis();//记录尝试获取连接的起始时间戳
    int localBadConnectionCount = 0;//初始化获取到无效连接的次数

    while (conn == null) {
      synchronized (state) {//获取连接必须是同步的
        if (!state.idleConnections.isEmpty()) {//检测是否有空闲连接
          // Pool has available connection
          //有空闲连接直接使用
          conn = state.idleConnections.remove(0);
          if (log.isDebugEnabled()) {
            log.debug("Checked out connection " + conn.getRealHashCode() + " from pool.");
          }
        } else {// 没有空闲连接
          if (state.activeConnections.size() < poolMaximumActiveConnections) {//判断活跃连接池中的数量是否小于最大连接数
            // 小于则可创建新的连接
            conn = new PooledConnection(dataSource.getConnection(), this);
            if (log.isDebugEnabled()) {
              log.debug("Created connection " + conn.getRealHashCode() + ".");
            }
          } else {// 如果已经等于最大连接数，则不能创建新连接
            //获取最早创建的连接
            PooledConnection oldestActiveConnection = state.activeConnections.get(0);
            long longestCheckoutTime = oldestActiveConnection.getCheckoutTime();
            if (longestCheckoutTime > poolMaximumCheckoutTime) {//检测是否已经以及超过最长使用时间
              // 如果超时，对超时连接的信息进行统计
              state.claimedOverdueConnectionCount++;//超时连接次数+1
              state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime;//累计超时时间增加
              state.accumulatedCheckoutTime += longestCheckoutTime;//累计的使用连接的时间增加
              state.activeConnections.remove(oldestActiveConnection);//从活跃集合中删除
              if (!oldestActiveConnection.getRealConnection().getAutoCommit()) {//如果超时连接未提交，则手动回滚
                try {
                  oldestActiveConnection.getRealConnection().rollback();
                } catch (SQLException e) {//发生异常仅仅记录日志
                  /*
                     Just log a message for debug and continue to execute the following
                     statement like nothing happend.
                     Wrap the bad connection with a new PooledConnection, this will help
                     to not intterupt current executing thread and give current thread a
                     chance to join the next competion for another valid/good database
                     connection. At the end of this loop, bad {@link @conn} will be set as null.
                   */
                  log.debug("Bad connection. Could not roll back");
                }
              }
              //在连接池中创建新的连接，注意对于数据库来说，并没有创建新连接
              conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this);
              conn.setCreatedTimestamp(oldestActiveConnection.getCreatedTimestamp());
              conn.setLastUsedTimestamp(oldestActiveConnection.getLastUsedTimestamp());
              //让老连接失效
              oldestActiveConnection.invalidate();
              if (log.isDebugEnabled()) {
                log.debug("Claimed overdue connection " + conn.getRealHashCode() + ".");
              }
            } else {
              // 无空闲连接，最早创建的连接没有失效，无法创建新连接，只能阻塞
              try {
                if (!countedWait) {
                  state.hadToWaitCount++;//连接池累计等待次数加1
                  countedWait = true;
                }
                if (log.isDebugEnabled()) {
                  log.debug("Waiting as long as " + poolTimeToWait + " milliseconds for connection.");
                }
                long wt = System.currentTimeMillis();
                state.wait(poolTimeToWait);//阻塞等待指定时间
                state.accumulatedWaitTime += System.currentTimeMillis() - wt;//累计等待时间增加
              } catch (InterruptedException e) {
                break;
              }
            }
          }
        }
        if (conn != null) {//获取连接成功的，要测试连接是否有效，同时更新统计数据
          // ping to server and check the connection is valid or not
          if (conn.isValid()) {//检测连接是否有效
            if (!conn.getRealConnection().getAutoCommit()) {
              conn.getRealConnection().rollback();//如果遗留历史的事务，回滚
            }
            //连接池相关统计信息更新
            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 {//如果连接无效
            if (log.isDebugEnabled()) {
              log.debug("A bad connection (" + conn.getRealHashCode() + ") was returned from the pool, getting another connection.");
            }
            state.badConnectionCount++;//累计的获取无效连接次数+1
            localBadConnectionCount++;//当前获取无效连接次数+1
            conn = null;
            //拿到无效连接，但如果没有超过重试的次数，允许再次尝试获取连接，否则抛出异常
            if (localBadConnectionCount > (poolMaximumIdleConnections + poolMaximumLocalBadConnectionTolerance)) {
              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;
  }

获取连接流程图

最终要拿到的是 PooledConnection 对象，所谓创建连接，是创建 PooledConnection 对象。看前面的构造方法就知道，要创建 PooledConnection 对象，其实依赖于已有的 Connection 对象。这个 Connection 是 JDK 里的类，是真正从数据源里拿到的连接。

pushConnection

protected void pushConnection(PooledConnection conn) throws SQLException {

    synchronized (state) {//回收连接必须是同步的
      state.activeConnections.remove(conn);//从活跃连接池中删除此连接
      if (conn.isValid()) {
    	  //判断闲置连接池资源是否已经达到上限
        if (state.idleConnections.size() < poolMaximumIdleConnections && conn.getConnectionTypeCode() == expectedConnectionTypeCode) {
        	//没有达到上限，进行回收
          state.accumulatedCheckoutTime += conn.getCheckoutTime();
          if (!conn.getRealConnection().getAutoCommit()) {
            conn.getRealConnection().rollback();//如果还有事务没有提交，进行回滚操作
          }
          //基于该连接，创建一个新的连接资源，并刷新连接状态
          PooledConnection newConn = new PooledConnection(conn.getRealConnection(), this);
          state.idleConnections.add(newConn);
          newConn.setCreatedTimestamp(conn.getCreatedTimestamp());
          newConn.setLastUsedTimestamp(conn.getLastUsedTimestamp());
          //老连接失效
          conn.invalidate();
          if (log.isDebugEnabled()) {
            log.debug("Returned connection " + newConn.getRealHashCode() + " to pool.");
          }
          //唤醒其他被阻塞的线程
          state.notifyAll();
        } else {//如果闲置连接池已经达到上限了，将连接真实关闭
          state.accumulatedCheckoutTime += conn.getCheckoutTime();
          if (!conn.getRealConnection().getAutoCommit()) {
            conn.getRealConnection().rollback();
          }
          //关闭真的数据库连接
          conn.getRealConnection().close();
          if (log.isDebugEnabled()) {
            log.debug("Closed connection " + conn.getRealHashCode() + ".");
          }
          //将连接对象设置为无效
          conn.invalidate();
        }
      } else {
        if (log.isDebugEnabled()) {
          log.debug("A bad connection (" + conn.getRealHashCode() + ") attempted to return to the pool, discarding connection.");
        }
        state.badConnectionCount++;
      }
    }
  }

回收连接流程图