mybatis默认的数据源连接池（PooledDataSource和UnPooledDataSource）

一般情况下我们操作数据库都是通过connection，但是频繁创建和删除connection会严重影响效率，因此在这种情况下我们一般会用到连接池，因为项目中用到的是mybatis，所以了解一下mybatis的默认的数据库连接池（大多数情况下我们会使用Durid连接池）

代码路径：

很明显看到datasource目录下有pooled和unpooled两个目录，分别是使用到了连接池和没有用到

1、unpooled目录下的UnpooledDatasource.java

/**
 * @author Clinton Begin
 * @author Eduardo Macarron
 */
public class UnpooledDataSource implements DataSource {
  
  private ClassLoader driverClassLoader;
  private Properties driverProperties;
  private static Map<String, Driver> registeredDrivers = new ConcurrentHashMap<String, Driver>();
　//常规配置
  private String driver;
  private String url;
  private String username;
  private String password;
　//是否自动commit    事务的隔离级别
  private Boolean autoCommit;
  private Integer defaultTransactionIsolationLevel;

　//注册驱动
  static {
    Enumeration<Driver> drivers = DriverManager.getDrivers();
    while (drivers.hasMoreElements()) {
      Driver driver = drivers.nextElement();
      registeredDrivers.put(driver.getClass().getName(), driver);
    }
  }

  public UnpooledDataSource() {
  }

  public UnpooledDataSource(String driver, String url, String username, String password) {
    this.driver = driver;
    this.url = url;
    this.username = username;
    this.password = password;
  }

　//重点getconnnection  获取数据库连接，调用doGetConnection方法
  @Override
  public Connection getConnection() throws SQLException {
    return doGetConnection(username, password);
  }

  @Override
  public Connection getConnection(String username, String password) throws SQLException {
    return doGetConnection(username, password);
  }

　//用户名  密码   设置配置
  private Connection doGetConnection(String username, String password) throws SQLException {
    Properties props = new Properties();
    if (driverProperties != null) {
      props.putAll(driverProperties);
    }
    if (username != null) {
      props.setProperty("user", username);
    }
    if (password != null) {
      props.setProperty("password", password);
    }
    return doGetConnection(props);
  }
　//真正获取连接的方法，由此可见，每次调用getConnection方法是，都会新建一个Connection对象，可想当并发量高是，数据库的连接就会被占满
  private Connection doGetConnection(Properties properties) throws SQLException {
    initializeDriver();
    Connection connection = DriverManager.getConnection(url, properties);
    configureConnection(connection);
    return connection;
  }
　//初始化  在调用doGetConnection方法中，会先进行一下判断，判断mybatis的configuration中是否已经有数据库驱动，没有的话会通过反射加载驱动
  private synchronized void initializeDriver() throws SQLException {
    if (!registeredDrivers.containsKey(driver)) {
      Class<?> driverType;
      try {
        if (driverClassLoader != null) {
          driverType = Class.forName(driver, true, driverClassLoader);
        } else {
          driverType = Resources.classForName(driver);
        }
        // DriverManager requires the driver to be loaded via the system ClassLoader.
        // http://www.kfu.com/~nsayer/Java/dyn-jdbc.html
        Driver driverInstance = (Driver)driverType.newInstance();
        DriverManager.registerDriver(new DriverProxy(driverInstance));
        registeredDrivers.put(driver, driverInstance);
      } catch (Exception e) {
        throw new SQLException("Error setting driver on UnpooledDataSource. Cause: " + e);
      }
    }
  }
　//设置是否自动提交   事务的隔离级别
  private void configureConnection(Connection conn) throws SQLException {
    if (autoCommit != null && autoCommit != conn.getAutoCommit()) {
      conn.setAutoCommit(autoCommit);
    }
    if (defaultTransactionIsolationLevel != null) {
      conn.setTransactionIsolation(defaultTransactionIsolationLevel);
    }
  }


}

2、PolledDataSource.java类，其内部的datasource其实也是通过unpolledDataSource对象来实现的，只是在获取连接connection的时候进行了处理（重点关注getConnection方法）

public class PooledDataSource implements DataSource {

  private static final Log log = LogFactory.getLog(PooledDataSource.class);
　//PollState 充当锁对象，这个对象中也有例如  空闲连接集合，活跃链接集合，请求的数量，等一些变量
  private final PoolState state = new PoolState(this);
　//PolledDataSource中的数据源也是使用的UnpolledDataSource对象来实现的
  private final UnpooledDataSource dataSource;

  // OPTIONAL CONFIGURATION FIELDS 线程池的最大活跃数量，最大空闲数量，等待时间等等。。。
  protected int poolMaximumActiveConnections = 10;
  protected int poolMaximumIdleConnections = 5;
  protected int poolMaximumCheckoutTime = 20000;
  protected int poolTimeToWait = 20000;
  protected int poolMaximumLocalBadConnectionTolerance = 3;
  protected String poolPingQuery = "NO PING QUERY SET";
  protected boolean poolPingEnabled;
  protected int poolPingConnectionsNotUsedFor;
　//url+username+password连接起来的字符串的hashcode值
  private int expectedConnectionTypeCode;

  public PooledDataSource() {
    dataSource = new UnpooledDataSource();
  }

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

  public PooledDataSource(String driver, String url, String username, String password) {
    dataSource = new UnpooledDataSource(driver, url, username, password);
    expectedConnectionTypeCode = assembleConnectionTypeCode(dataSource.getUrl(), dataSource.getUsername(), dataSource.getPassword());
  }

　//获取连接connection
  @Override
  public Connection getConnection() throws SQLException {
    return popConnection(dataSource.getUsername(), dataSource.getPassword()).getProxyConnection();
  }

  @Override
  public Connection getConnection(String username, String password) throws SQLException {
    return popConnection(username, password).getProxyConnection();
  }

　//线程安全的获取连接的方法（供PooledConnection中调用）  synchronized
  protected void pushConnection(PooledConnection conn) throws SQLException {
　　//所对象为PoolState
    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++;
      }
    }
  }

  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) {
　　　　　//如果state中空闲的连接不为空时，直接从state中的idelConnections集合中获取
        if (!state.idleConnections.isEmpty()) {
          // Pool has available connection
          conn = state.idleConnections.remove(0);
          if (log.isDebugEnabled()) {
            log.debug("Checked out connection " + conn.getRealHashCode() + " from pool.");
          }
        } else {
　　　　　　//state中的idelConnections集合中没有连接connection了，判断活跃的connection是否小于最大活跃线程数，小于的话就新建一个connection
          // Pool does not have available connection 
          if (state.activeConnections.size() < poolMaximumActiveConnections) {
            // Can create new connection
            conn = new PooledConnection(dataSource.getConnection(), this);
            if (log.isDebugEnabled()) {
              log.debug("Created connection " + conn.getRealHashCode() + ".");
            }
          } else {
　　　　　　　//当前活跃的connection数大于最大活跃connection数是，表明不能新建connection，这种情况下，会判断是否已经超过设定的时间20s，如果超过，会根据是否自动commit，
　　　　　　　/不是自动提交，会进行rollback操作
            // Cannot create new connection
            PooledConnection oldestActiveConnection = state.activeConnections.get(0);
            long longestCheckoutTime = oldestActiveConnection.getCheckoutTime();
            if (longestCheckoutTime > poolMaximumCheckoutTime) {//判断是否超时
              // Can claim overdue connection
              state.claimedOverdueConnectionCount++;
              state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime;
              state.accumulatedCheckoutTime += longestCheckoutTime;
              state.activeConnections.remove(oldestActiveConnection);
              if (!oldestActiveConnection.getRealConnection().getAutoCommit()) {//判断是否设置了自动提交，不是自动提交的话，会进行rollback
                try {
                  oldestActiveConnection.getRealConnection().rollback();
                } catch (SQLException e) {
                  /*
                     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");
                }  
              }
　　　　　　　　//将之前rollback的connection进行处理，重新生成一个connection，并设置创建时间和最后使用时间
              conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this);
              conn.setCreatedTimestamp(oldestActiveConnection.getCreatedTimestamp());
              conn.setLastUsedTimestamp(oldestActiveConnection.getLastUsedTimestamp());
              oldestActiveConnection.invalidate();
              if (log.isDebugEnabled()) {
                log.debug("Claimed overdue connection " + conn.getRealHashCode() + ".");
              }
            } else {
　　　　　　　　　//当活跃connection的集合中第一个connection不符合超时，则进行等待wait
              // Must wait
              try {
                if (!countedWait) {
                  state.hadToWaitCount++;
                  countedWait = true;
                }
　　　　　　　　　　　//pooltimeTowait默认也是20秒
                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;
              }
            }
          }
        }
　　　　//经过以上处理，如果conn仍然没有值，则会循环走一行流程，第二次一定会获取到conn对象（默认的超时时间是20秒，第一次已经等待过20s了）
        if (conn != null) {
　　　　　　//判断conn是否能用   通过ping能否ping通
          // ping to server and check the connection is valid or not
          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 + 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;
  }
}

3、看下创建PooledConnection对象（从上边截出的一部分代码，新建PooledConnection对象）

 // Pool does not have available connection
          if (state.activeConnections.size() < poolMaximumActiveConnections) {
            // Can create new connection
            conn = new PooledConnection(dataSource.getConnection(), this);
            if (log.isDebugEnabled()) {
              log.debug("Created connection " + conn.getRealHashCode() + ".");
            }
          }

PooledConnection构造函数：PooledConnection函数实现了InvocationHandler接口（动态代理）

  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();
　　//判断是否close，如果是close的话，会调用PooledDataSource中的pushConnection方法，将该conn从activeConnections删除，同时将此conn添加到idelConnections中
    if (CLOSE.hashCode() == methodName.hashCode() && CLOSE.equals(methodName)) {
      dataSource.pushConnection(this);
      return null;
    } else {
      try {
        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);
      }
    }
  }

参考：https://blog.csdn.net/drxRose/article/details/85710850

https://www.jianshu.com/p/21cf094cb97b?open_source=weibo_search