[druid 源码解析] 5 归还连接

我们在创建链接的时候会发现,返回给 Mybatis 的并不是一个简单的 connection 而是一个 DruidPooledConnection 这里是一个我们需要注意点的点,会到正题,我们拿到一个链接,假如使用完成后我们最重要就是 close 掉这个链接,那么 Mybatis 也是这样做的。我们可以看一下 SqlSessionInterceptor 这个类的 Invoke 方法,所有 Dao 都是通过这个拦截器生成代理对象。

private class SqlSessionInterceptor implements InvocationHandler {
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      SqlSession sqlSession = getSqlSession(SqlSessionTemplate.this.sqlSessionFactory,
          SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);
      try {
        Object result = method.invoke(sqlSession, args);
        if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
          // force commit even on non-dirty sessions because some databases require
          // a commit/rollback before calling close()
          sqlSession.commit(true);
        }
        return result;
      } catch (Throwable t) {
        Throwable unwrapped = unwrapThrowable(t);
        if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
          // release the connection to avoid a deadlock if the translator is no loaded. See issue #22
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
          sqlSession = null;
          Throwable translated = SqlSessionTemplate.this.exceptionTranslator
              .translateExceptionIfPossible((PersistenceException) unwrapped);
          if (translated != null) {
            unwrapped = translated;
          }
        }
        throw unwrapped;
      } finally {
        if (sqlSession != null) {
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
        }
      }
    }
  }

我们看 finally 方法最终调用了 closeSqlSession 方法,我们根据最后调用链路,最终跟踪到了DruidPooledConnection 的 recycle 方法,调用栈如下:

调用栈

我们先看一下他的 recycle 方法。

public void recycle() throws SQLException {
        if (this.disable) {
            return;
        }

        DruidConnectionHolder holder = this.holder;
        if (holder == null) {
            if (dupCloseLogEnable) {
                LOG.error("dup close");
            }
            return;
        }

        if (!this.abandoned) {
            DruidAbstractDataSource dataSource = holder.getDataSource();
            dataSource.recycle(this);
        }

        this.holder = null;
        conn = null;
        transactionInfo = null;
        closed = true;
    }

我们看一下主要的逻辑,就是调用 DruiddataSourcerecycle 方法, 兜兜转转终于回到了我们的 DruiddataSource

 protected void recycle(DruidPooledConnection pooledConnection) throws SQLException {
        final DruidConnectionHolder holder = pooledConnection.holder;

        if (holder == null) {
            LOG.warn("connectionHolder is null");
            return;
        }
        // 假如当前线程和获取链接不是同一个线程,这里会 warn 报错, 这里会涉及到事务相关的东西,我们后面详解。
        if (logDifferentThread //
            && (!isAsyncCloseConnectionEnable()) //
            && pooledConnection.ownerThread != Thread.currentThread()//
        ) {
            LOG.warn("get/close not same thread");
        }

        final Connection physicalConnection = holder.conn;
        // 从 activeConnections 中移除当前 connection
        if (pooledConnection.traceEnable) {
            Object oldInfo = null;
            activeConnectionLock.lock();
            try {
                if (pooledConnection.traceEnable) {
                    oldInfo = activeConnections.remove(pooledConnection);
                    pooledConnection.traceEnable = false;
                }
            } finally {
                activeConnectionLock.unlock();
            }
            if (oldInfo == null) {
                if (LOG.isWarnEnabled()) {
                    LOG.warn("remove abandonded failed. activeConnections.size " + activeConnections.size());
                }
            }
        }

        final boolean isAutoCommit = holder.underlyingAutoCommit;
        final boolean isReadOnly = holder.underlyingReadOnly;
        final boolean testOnReturn = this.testOnReturn;

        try {
            // check need to rollback?
            if ((!isAutoCommit) && (!isReadOnly)) {
                pooledConnection.rollback();
            }
            // 假如不是同个线程,需要开启锁后才能rest,rest 主要是做一些关于重置 holder setting 的事情。
            // reset holder, restore default settings, clear warnings
            boolean isSameThread = pooledConnection.ownerThread == Thread.currentThread();
            if (!isSameThread) {
                final ReentrantLock lock = pooledConnection.lock;
                lock.lock();
                try {
                    holder.reset();
                } finally {
                    lock.unlock();
                }
            } else {
                holder.reset();
            }

            if (holder.discard) {
                return;
            }
            // j检查是否大于最大连接数,是的话清除
            if (phyMaxUseCount > 0 && holder.useCount >= phyMaxUseCount) {
                discardConnection(holder);
                return;
            }
            // 检查线程是否关闭
            if (physicalConnection.isClosed()) {
                lock.lock();
                try {
                    if (holder.active) {
                        activeCount--;
                        holder.active = false;
                    }
                    closeCount++;
                } finally {
                    lock.unlock();
                }
                return;
            }
            // 检查线程是否存活
            if (testOnReturn) {
                boolean validate = testConnectionInternal(holder, physicalConnection);
                if (!validate) {
                    JdbcUtils.close(physicalConnection);

                    destroyCountUpdater.incrementAndGet(this);

                    lock.lock();
                    try {
                        if (holder.active) {
                            activeCount--;
                            holder.active = false;
                        }
                        closeCount++;
                    } finally {
                        lock.unlock();
                    }
                    return;
                }
            }
            if (holder.initSchema != null) {
                holder.conn.setSchema(holder.initSchema);
                holder.initSchema = null;
            }

            if (!enable) {
                discardConnection(holder);
                return;
            }

            boolean result;
            final long currentTimeMillis = System.currentTimeMillis();
            // 检查是否超过醉倒时长
            if (phyTimeoutMillis > 0) {
                long phyConnectTimeMillis = currentTimeMillis - holder.connectTimeMillis;
                if (phyConnectTimeMillis > phyTimeoutMillis) {
                    discardConnection(holder);
                    return;
                }
            }
            // 获取锁,最重要的逻辑开始了
            lock.lock();
            try {
                // 计时器操作
                if (holder.active) {
                    activeCount--;
                    holder.active = false;
                }
                closeCount++;
                // 将连接放回 connections
                result = putLast(holder, currentTimeMillis);
                recycleCount++;
            } finally {
                lock.unlock();
            }

            if (!result) {
                JdbcUtils.close(holder.conn);
                LOG.info("connection recyle failed.");
            }
        } catch (Throwable e) {
            holder.clearStatementCache();

            if (!holder.discard) {
                discardConnection(holder);
                holder.discard = true;
            }

            LOG.error("recyle error", e);
            recycleErrorCountUpdater.incrementAndGet(this);
        }
    }

这里的逻辑比较简单,主要如下:

  1. 检查当前线程和获取链接的线程是否同一个,这里可能涉及到并发问题和事务问题,我们后续进行讲解。
  2. 件当前线程从活跃线程组 activeConnections 中移除,主要是方便后面的丢弃或者回收的工作。
  3. 然后检查是否需要进行回滚,不需要继续往下走。
  4. reset 当前 connectionholder 的相关配置。
  5. 接下来是对各项信息进行检查,主要是看连接是否还可以重用。
  6. 锁住然后进行真正的回收工作,这里回收交给了 putLast 方法。
    最后我们来看一下 putLast 方法。
boolean putLast(DruidConnectionHolder e, long lastActiveTimeMillis) {
        if (poolingCount >= maxActive || e.discard || this.closed) {
            return false;
        }

        e.lastActiveTimeMillis = lastActiveTimeMillis;
        connections[poolingCount] = e;
        incrementPoolingCount();

        if (poolingCount > poolingPeak) {
            poolingPeak = poolingCount;
            poolingPeakTime = lastActiveTimeMillis;
        }

        notEmpty.signal();
        notEmptySignalCount++;

        return true;
    }

这里主要是将连接放回到 connections 空闲连接数组的最后面,这里我们还记得,获取链从头开始取,然后释放就放回到最后一位。最后发送信号,通知等待线程获取连接。

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