BasicDataSource.getConnection()

public Connection getConnection() throws SQLException {
        if (Utils.IS_SECURITY_ENABLED) {
            PrivilegedExceptionAction<Connection> action = new PaGetConnection();
            try {
                return AccessController.doPrivileged(action);
            } 
 
catch (PrivilegedActionException e) {
                Throwable cause = e.getCause();
                if (cause instanceof SQLException) {
                    throw (SQLException) cause;
                }
                throw new SQLException(e);
            }
        }
        return createDataSource().getConnection();
    }
 

createDataSource()

        }

        // Return the pool if we have already created it
        // This is double-checked locking. This is safe since dataSource is
        // volatile and the code is targeted at Java 5 onwards.
        if (dataSource != null) {
            return dataSource;
        }
        //同步方法，防止多線程重復創建數據源
        
 
synchronized (this) {
            if (dataSource != null) {
                return dataSource;
            }

            jmxRegister();

            // create factory which returns raw physical connections
　　　　　　　//創建JDBC連接工廠
            ConnectionFactory driverConnectionFactory = createConnectionFactory();

            // Set up the poolable connection factory
            boolean success = false;
　　　　　　　//創建連接池工廠
            PoolableConnectionFactory poolableConnectionFactory;
            try {
                poolableConnectionFactory = createPoolableConnectionFactory(
                        driverConnectionFactory);
                poolableConnectionFactory.setPoolStatements(
                        poolPreparedStatements);
                poolableConnectionFactory.setMaxOpenPrepatedStatements(
                        maxOpenPreparedStatements);
                success = true;
            } catch (SQLException se) {
                throw se;
            } catch (RuntimeException rte) {
                throw rte;
            } catch (Exception ex) {
                throw new SQLException("Error creating connection factory", ex);
            }

            if (success) {
                // create a pool for our connections
                createConnectionPool(poolableConnectionFactory);
            }

            // Create the pooling data source to manage connections
            DataSource newDataSource; 
            success = false;
            try {
                newDataSource = createDataSourceInstance();
                newDataSource.setLogWriter(logWriter);
                success = true;
            } catch (SQLException se) {
                throw se;
            } catch (RuntimeException rte) {
                throw rte;
            } catch (Exception ex) {
                throw new SQLException("Error creating datasource", ex);
            } finally {
                if (!success) {
                    closeConnectionPool();
                }
            }

            // If initialSize > 0, preload the pool
　　　　　　　//往連接池添加初始化大小的連接
            try {
                for (int i = 0 ; i < initialSize ; i++) {
                    connectionPool.addObject();
                }
            } catch (Exception e) {
                closeConnectionPool();
                throw new SQLException("Error preloading the connection pool", e);
            }

            // If timeBetweenEvictionRunsMillis > 0, start the pool‘s evictor task
            startPoolMaintenance();

            dataSource = newDataSource;
            return dataSource;
        }
    }

createConnectionFactory()：創建連接池工廠

protected ConnectionFactory createConnectionFactory() throws SQLException {
        // Load the JDBC driver class
　　　　 //加載數據庫驅動
        Driver driverToUse = this.driver;

        if (driverToUse == null) {
            Class<?> driverFromCCL = null;
            if (driverClassName != null) {
                try {
                    try {
                        if (driverClassLoader == null) {
                            driverFromCCL = Class.forName(driverClassName);
                        } else {
                            driverFromCCL = Class.forName(
                                    driverClassName, true, driverClassLoader);
                        }
                    } catch (ClassNotFoundException cnfe) {
                        driverFromCCL = Thread.currentThread(
                                ).getContextClassLoader().loadClass(
                                        driverClassName);
                    }
                } catch (Exception t) {
                    String message = "Cannot load JDBC driver class ‘" +
                        driverClassName + "‘";
                    logWriter.println(message);
                    t.printStackTrace(logWriter);
                    throw new SQLException(message, t);
                }
            }

            try {
                if (driverFromCCL == null) {
                    driverToUse = DriverManager.getDriver(url);
                } else {
                    // Usage of DriverManager is not possible, as it does not
                    // respect the ContextClassLoader
                    // N.B. This cast may cause ClassCastException which is handled below
                    driverToUse = (Driver) driverFromCCL.newInstance();
                    if (!driverToUse.acceptsURL(url)) {
                        throw new SQLException("No suitable driver", "08001");
                    }
                }
            } catch (Exception t) {
                String message = "Cannot create JDBC driver of class ‘" +
                    (driverClassName != null ? driverClassName : "") +
                    "‘ for connect URL ‘" + url + "‘";
                logWriter.println(message);
                t.printStackTrace(logWriter);
                throw new SQLException(message, t);
            }
        }

        // Set up the driver connection factory we will use
        String user = username;
        if (user != null) {
            connectionProperties.put("user", user);
        } else {
            log("DBCP DataSource configured without a ‘username‘");
        }

        String pwd = password;
        if (pwd != null) {
            connectionProperties.put("password", pwd);
        } else {
            log("DBCP DataSource configured without a ‘password‘");
        }
　　　　　//創建JDBC連接工廠
        ConnectionFactory driverConnectionFactory =
                new DriverConnectionFactory(driverToUse, url, connectionProperties);
        return driverConnectionFactory;
    }

createPoolableConnectionFactory()

protected PoolableConnectionFactory createPoolableConnectionFactory(
            ConnectionFactory driverConnectionFactory) throws SQLException {
        PoolableConnectionFactory connectionFactory = null;
        try {
            connectionFactory = new PoolableConnectionFactory(driverConnectionFactory, registeredJmxName);
            connectionFactory.setValidationQuery(validationQuery);
            connectionFactory.setValidationQueryTimeout(validationQueryTimeout);
            connectionFactory.setConnectionInitSql(connectionInitSqls);
            connectionFactory.setDefaultReadOnly(defaultReadOnly);
            connectionFactory.setDefaultAutoCommit(defaultAutoCommit);
            connectionFactory.setDefaultTransactionIsolation(defaultTransactionIsolation);
            connectionFactory.setDefaultCatalog(defaultCatalog);
            connectionFactory.setCacheState(cacheState);
            connectionFactory.setPoolStatements(poolPreparedStatements);
            connectionFactory.setMaxOpenPrepatedStatements(maxOpenPreparedStatements);
            connectionFactory.setMaxConnLifetimeMillis(maxConnLifetimeMillis);
            connectionFactory.setRollbackOnReturn(getRollbackOnReturn());
            connectionFactory.setEnableAutoCommitOnReturn(getEnableAutoCommitOnReturn());
            connectionFactory.setDefaultQueryTimeout(getDefaultQueryTimeout());
            connectionFactory.setFastFailValidation(fastFailValidation);
            connectionFactory.setDisconnectionSqlCodes(disconnectionSqlCodes);
            validateConnectionFactory(connectionFactory);
        } catch (RuntimeException e) {
            throw e;
        } catch (Exception e) {
            throw new SQLException("Cannot create PoolableConnectionFactory (" + e.getMessage() + ")", e);
        }
        return connectionFactory;
    }

createConnectionPool()：創建對象池保存活躍連接

protected void createConnectionPool(PoolableConnectionFactory factory) {
        // Create an object pool to contain our active connections
        GenericObjectPoolConfig config = new GenericObjectPoolConfig();
        updateJmxName(config);
        config.setJmxEnabled(registeredJmxName != null);  // Disable JMX on the underlying pool if the DS is not registered.
        GenericObjectPool<PoolableConnection> gop;
        if (abandonedConfig != null &&
                (abandonedConfig.getRemoveAbandonedOnBorrow() ||
                 abandonedConfig.getRemoveAbandonedOnMaintenance())) {
            gop = new GenericObjectPool<>(factory, config, abandonedConfig);
        }
        else {
            gop = new GenericObjectPool<>(factory, config);
        }
        gop.setMaxTotal(maxTotal);
        gop.setMaxIdle(maxIdle);
        gop.setMinIdle(minIdle);
        gop.setMaxWaitMillis(maxWaitMillis);
        gop.setTestOnCreate(testOnCreate);
        gop.setTestOnBorrow(testOnBorrow);
        gop.setTestOnReturn(testOnReturn);
        gop.setNumTestsPerEvictionRun(numTestsPerEvictionRun);
        gop.setMinEvictableIdleTimeMillis(minEvictableIdleTimeMillis);
        gop.setTestWhileIdle(testWhileIdle);
        gop.setLifo(lifo);
        gop.setSwallowedExceptionListener(new SwallowedExceptionLogger(log, logExpiredConnections));
        gop.setEvictionPolicyClassName(evictionPolicyClassName);
        factory.setPool(gop);
　　　　　//生成對象池
        connectionPool = gop;
    }

從圖中可以看出，由執行器Executor獲取數據庫連接進行數據庫操作的

PoolableConnection.getConnection()

public Connection getConnection() throws SQLException {
        try {
　　　　　　　//從連接池獲取連接
            C conn = _pool.borrowObject();
            if (conn == null) {
                return null;
            }
            return new PoolGuardConnectionWrapper<>(conn);
        } catch(SQLException e) {
            throw e;
        } catch(NoSuchElementException e) {
            throw new SQLException("Cannot get a connection, pool error " + e.getMessage(), e);
        } catch(RuntimeException e) {
            throw e;
        } catch(Exception e) {
            throw new SQLException("Cannot get a connection, general error", e);
        }
    }

PoolableConnection.close()：這裏的關閉連接只是把對象返回到連接池中，並非是真正的關閉

public synchronized void close() throws SQLException {
        if (isClosedInternal()) {
            // already closed
            return;
        }

        boolean isUnderlyingConectionClosed;
        try {
            isUnderlyingConectionClosed = getDelegateInternal().isClosed();
        } catch (SQLException e) {
            try {
                _pool.invalidateObject(this);
            } catch(IllegalStateException ise) {
                // pool is closed, so close the connection
                passivate();
                getInnermostDelegate().close();
            } catch (Exception ie) {
                // DO NOTHING the original exception will be rethrown
            }
            throw new SQLException("Cannot close connection (isClosed check failed)", e);
        }

        /* Can‘t set close before this code block since the connection needs to
         * be open when validation runs. Can‘t set close after this code block
         * since by then the connection will have been returned to the pool and
         * may have been borrowed by another thread. Therefore, the close flag
         * is set in passivate().
         */
        if (isUnderlyingConectionClosed) {
            // Abnormal close: underlying connection closed unexpectedly, so we
            // must destroy this proxy
            try {
                _pool.invalidateObject(this);
            } catch(IllegalStateException e) {
                // pool is closed, so close the connection
                passivate();
                getInnermostDelegate().close();
            } catch (Exception e) {
                throw new SQLException("Cannot close connection (invalidating pooled object failed)", e);
            }
        } else {
            // Normal close: underlying connection is still open, so we
            // simply need to return this proxy to the pool
            try {
　　　　　　　　　 //將連接返回到連接池中
                _pool.returnObject(this);
            } catch(IllegalStateException e) {
                // pool is closed, so close the connection
                passivate();
                getInnermostDelegate().close();
            } catch(SQLException e) {
                throw e;
            } catch(RuntimeException e) {
                throw e;
            } catch(Exception e) {
                throw new SQLException("Cannot close connection (return to pool failed)", e);
            }
        }
    }

數據庫連接池DBCP源碼剖析