MyBatis链接管理(2)

MyBatis链接管理(1)

MySQL链接管理

在调用SqlSessionFactory的openSession函数时,只是建立了一个DefaultSqlSession实例，并无真正去链接MySQL:

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
        Transaction tx = null;

        DefaultSqlSession var8;
        try {
            Environment environment = this.configuration.getEnvironment();
            TransactionFactory transactionFactory = this.getTransactionFactoryFromEnvironment(environment);
            tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
            Executor executor = this.configuration.newExecutor(tx, execType);
            var8 = new DefaultSqlSession(this.configuration, executor, autoCommit);
        } catch (Exception var12) {
            this.closeTransaction(tx);
            throw ExceptionFactory.wrapException("Error opening session.  Cause: " + var12, var12);
        } finally {
            ErrorContext.instance().reset();
        }

        return var8;
    }

只有当真正执行SQL语句时，好比query，才经过SimpleExecutor调用doQuery->prepareStatement进入父类BaseExecutor的getConnection,最终经过Transaction(JdbcTransaction)调用DataSource(PooledDataSource)的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) {
            PoolState var8 = this.state;
            synchronized(this.state) {
                if (!this.state.idleConnections.isEmpty()) {
                    conn = (PooledConnection)this.state.idleConnections.remove(0);
                    if (log.isDebugEnabled()) {
                        log.debug("Checked out connection " + conn.getRealHashCode() + " from pool.");
                    }
                } else if (this.state.activeConnections.size() < this.poolMaximumActiveConnections) {
                    conn = new PooledConnection(this.dataSource.getConnection(), this);
                    if (log.isDebugEnabled()) {
                        log.debug("Created connection " + conn.getRealHashCode() + ".");
                    }
                } else {
                    PooledConnection oldestActiveConnection = (PooledConnection)this.state.activeConnections.get(0);
                    long longestCheckoutTime = oldestActiveConnection.getCheckoutTime();
                    if (longestCheckoutTime > (long)this.poolMaximumCheckoutTime) {
                        ++this.state.claimedOverdueConnectionCount;
                        this.state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime;
                        this.state.accumulatedCheckoutTime += longestCheckoutTime;
                        this.state.activeConnections.remove(oldestActiveConnection);
                        if (!oldestActiveConnection.getRealConnection().getAutoCommit()) {
                            try {
                                oldestActiveConnection.getRealConnection().rollback();
                            } catch (SQLException var16) {
                                log.debug("Bad connection. Could not roll back");
                            }
                        }

                        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 {
                        try {
                            if (!countedWait) {
                                ++this.state.hadToWaitCount;
                                countedWait = true;
                            }

                            if (log.isDebugEnabled()) {
                                log.debug("Waiting as long as " + this.poolTimeToWait + " milliseconds for connection.");
                            }

                            long wt = System.currentTimeMillis();
                            this.state.wait((long)this.poolTimeToWait);
                            this.state.accumulatedWaitTime += System.currentTimeMillis() - wt;
                        } catch (InterruptedException var17) {
                            break;
                        }
                    }
                }

                if (conn != null) {
                    if (conn.isValid()) {
                        if (!conn.getRealConnection().getAutoCommit()) {
                            conn.getRealConnection().rollback();
                        }

                        conn.setConnectionTypeCode(this.assembleConnectionTypeCode(this.dataSource.getUrl(), username, password));
                        conn.setCheckoutTimestamp(System.currentTimeMillis());
                        conn.setLastUsedTimestamp(System.currentTimeMillis());
                        this.state.activeConnections.add(conn);
                        ++this.state.requestCount;
                        this.state.accumulatedRequestTime += System.currentTimeMillis() - t;
                    } else {
                        if (log.isDebugEnabled()) {
                            log.debug("A bad connection (" + conn.getRealHashCode() + ") was returned from the pool, getting another connection.");
                        }

                        ++this.state.badConnectionCount;
                        ++localBadConnectionCount;
                        conn = null;
                        if (localBadConnectionCount > this.poolMaximumIdleConnections + this.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.");
        } else {
            return conn;
        }
    }

流程图以下：

这里须要注意的是，最终返回的是利用JAVA动态代理建立的实际链接的代理链接PooledConnection（具体实现原理见动态代理实现原理）：

public Connection getConnection() throws SQLException {
        return this.popConnection(this.dataSource.getUsername(), this.dataSource.getPassword()).getProxyConnection();
    }

当调用实际链接的方法时，就会进入PooledConnection的invoke方法：

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        String methodName = method.getName();
        if ("close".hashCode() == methodName.hashCode() && "close".equals(methodName)) {
            this.dataSource.pushConnection(this);
            return null;
        } else {
            try {
                if (!Object.class.equals(method.getDeclaringClass())) {
                    this.checkConnection();
                }

                return method.invoke(this.realConnection, args);
            } catch (Throwable var6) {
                throw ExceptionUtil.unwrapThrowable(var6);
            }
        }
    }

该方法会拦截实际链接的close方法，实际调用DataSource的pushConnection方法：

protected void pushConnection(PooledConnection conn) throws SQLException {
        synchronized(this.state) {
            this.state.activeConnections.remove(conn);
            if (conn.isValid()) {
                PoolState var10000;
                if (this.state.idleConnections.size() < this.poolMaximumIdleConnections && conn.getConnectionTypeCode() == this.expectedConnectionTypeCode) {
                    var10000 = this.state;
                    var10000.accumulatedCheckoutTime += conn.getCheckoutTime();
                    if (!conn.getRealConnection().getAutoCommit()) {
                        conn.getRealConnection().rollback();
                    }

                    PooledConnection newConn = new PooledConnection(conn.getRealConnection(), this);
                    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.");
                    }

                    this.state.notifyAll();
                } else {
                    var10000 = this.state;
                    var10000.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.");
                }

                ++this.state.badConnectionCount;
            }

        }
    }

该方法判断若是空闲链接数小于上限则回收该链接(这里没有想明白为何要从新建一个PooledConnection而不是直接复用原来的实例，但愿有高手能够在留言里帮忙指点迷津)并唤醒等待获取链接的线程，不然关闭链接。
这里涉及到的主要类的UML图以下：

在JdbcTransaction中利用PooledDataSource的popConnection获取到链接后，会设置是否自动提交（Mysql默认是自动提交，但MyBatis默认是关闭自动提交）和事务隔离等级:

protected void openConnection() throws SQLException {
    if (log.isDebugEnabled()) {
        log.debug("Opening JDBC Connection");
    }

    this.connection = this.dataSource.getConnection();
    if (this.level != null) {
        this.connection.setTransactionIsolation(this.level.getLevel());
    }

    this.setDesiredAutoCommit(this.autoCommmit);
}