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Mybatis技术内幕（2.4）：数据源模块

在数据持久层中，数据源是一个非常重要的组件，期性能直接关系到整个数据持久层的性能。在实践中比较常见的第三方数据源组件有DBCP、C3P0、阿里druid和Springboot2推荐的HikariCP等，Mybatis不仅可以集成第三方数据源组件，还提供了自己的数据源实现。

Mybatis数据源模块包结构如下：

2.0 DataSource

常见的数据源组件都实现了 javax.sql.DataSource 接口，Mybatis自身实现的数据源实现也不例外。Mybatis提供了两个 javax.sql.DataSource 接口实现，分别是PooledDataSource和UnpooledDataSource。

2.1 UnpooledDataSource 非池化DataSource

org.apache.ibatis.datasource.unpooled.UnpooledDataSource 实现DataSource接口，非池化的DataSource对象。代码如下：

/**
 * 非池化DataSource
 * @author Clinton Begin
 * @author Eduardo Macarron
 */
public class UnpooledDataSource implements DataSource {
	// Driver 类加载器
	private ClassLoader driverClassLoader;
	// Driver 属性
	private Properties driverProperties;
	// 已注册的 Driver 映射
	private static Map<String, Driver> registeredDrivers = new ConcurrentHashMap<>();

	// Driver 类名
	private String driver;
	// 数据库 URL
	private String url;
	// 数据库用户名
	private String username;
	// 数据库密码
	private String password;

	// 是否自动提交事务
	private Boolean autoCommit;
	// 默认事务隔离级别
	private Integer defaultTransactionIsolationLevel;

	static {
		// 初始化 registeredDrivers
		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;
	}

	public UnpooledDataSource(String driver, String url, Properties driverProperties) {
		this.driver = driver;
		this.url = url;
		this.driverProperties = driverProperties;
	}

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

	public UnpooledDataSource(ClassLoader driverClassLoader, String driver, String url, Properties driverProperties) {
		this.driverClassLoader = driverClassLoader;
		this.driver = driver;
		this.url = url;
		this.driverProperties = driverProperties;
	}

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

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

	@Override
	public void setLoginTimeout(int loginTimeout) {
		DriverManager.setLoginTimeout(loginTimeout);
	}

	@Override
	public int getLoginTimeout() {
		return DriverManager.getLoginTimeout();
	}

	@Override
	public void setLogWriter(PrintWriter logWriter) {
		DriverManager.setLogWriter(logWriter);
	}

	@Override
	public PrintWriter getLogWriter() {
		return DriverManager.getLogWriter();
	}

	public ClassLoader getDriverClassLoader() {
		return driverClassLoader;
	}

	public void setDriverClassLoader(ClassLoader driverClassLoader) {
		this.driverClassLoader = driverClassLoader;
	}

	public Properties getDriverProperties() {
		return driverProperties;
	}

	public void setDriverProperties(Properties driverProperties) {
		this.driverProperties = driverProperties;
	}

	public String getDriver() {
		return driver;
	}

	public synchronized void setDriver(String driver) {
		this.driver = driver;
	}

	public String getUrl() {
		return url;
	}

	public void setUrl(String url) {
		this.url = url;
	}

	public String getUsername() {
		return username;
	}

	public void setUsername(String username) {
		this.username = username;
	}

	public String getPassword() {
		return password;
	}

	public void setPassword(String password) {
		this.password = password;
	}

	public Boolean isAutoCommit() {
		return autoCommit;
	}

	public void setAutoCommit(Boolean autoCommit) {
		this.autoCommit = autoCommit;
	}

	public Integer getDefaultTransactionIsolationLevel() {
		return defaultTransactionIsolationLevel;
	}

	public void setDefaultTransactionIsolationLevel(Integer defaultTransactionIsolationLevel) {
		this.defaultTransactionIsolationLevel = defaultTransactionIsolationLevel;
	}

	private Connection doGetConnection(String username, String password) throws SQLException {
		// 创建 Properties 对象
		Properties props = new Properties();
		if (driverProperties != null) {
			// 设置 driverProperties 到 props 中
			props.putAll(driverProperties);
		}
		// 设置 user和password
		if (username != null) {
			props.setProperty("user", username);
		}
		if (password != null) {
			props.setProperty("password", password);
		}
		return doGetConnection(props);
	}

	private Connection doGetConnection(Properties properties) throws SQLException {
		// 初始化 Driver
		initializeDriver();
		// 获得 Connection 对象
		Connection connection = DriverManager.getConnection(url, properties);
		// 配置 Connection 对象
		configureConnection(connection);
		return connection;
	}

	/**
	 * 初始化 Driver
	 * 
	 * @throws SQLException
	 */
	private synchronized void initializeDriver() throws SQLException {
		// 判断 registeredDrivers是否已经存在该 driver，若不存在，进行初始化
		if (!registeredDrivers.containsKey(driver)) {
			Class<?> driverType;
			try {
				// 获得 driver 类
				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
				// 创建 Driver 对象
				Driver driverInstance = (Driver) driverType.newInstance();
				// 创建 DriverProxy对象，并注册到 DriverManager中
				DriverManager.registerDriver(new DriverProxy(driverInstance));
				// 添加到registeredDrivers中
				registeredDrivers.put(driver, driverInstance);
			} catch (Exception e) {
				throw new SQLException("Error setting driver on UnpooledDataSource. Cause: " + e);
			}
		}
	}

	private void configureConnection(Connection conn) throws SQLException {
		// 设置自动提交
		if (autoCommit != null && autoCommit != conn.getAutoCommit()) {
			conn.setAutoCommit(autoCommit);
		}
		// 设置事务隔离级别
		if (defaultTransactionIsolationLevel != null) {
			conn.setTransactionIsolation(defaultTransactionIsolationLevel);
		}
	}

	/**
	 * 静态代理
	 * @ClassName: DriverProxy  
	 * @Description: 主要是针对getParentLogger方法，使用 MyBatis 自定义的 Logger 对象 
	 * @date 2019年4月3日  
	 *
	 */
	private static class DriverProxy implements Driver {
		private Driver driver;

		DriverProxy(Driver d) {
			this.driver = d;
		}

		@Override
		public boolean acceptsURL(String u) throws SQLException {
			return this.driver.acceptsURL(u);
		}

		@Override
		public Connection connect(String u, Properties p) throws SQLException {
			return this.driver.connect(u, p);
		}

		@Override
		public int getMajorVersion() {
			return this.driver.getMajorVersion();
		}

		@Override
		public int getMinorVersion() {
			return this.driver.getMinorVersion();
		}

		@Override
		public DriverPropertyInfo[] getPropertyInfo(String u, Properties p) throws SQLException {
			return this.driver.getPropertyInfo(u, p);
		}

		@Override
		public boolean jdbcCompliant() {
			return this.driver.jdbcCompliant();
		}

		// @Override only valid jdk7+
		public Logger getParentLogger() {
			return Logger.getLogger(Logger.GLOBAL_LOGGER_NAME);
		}
	}

	@Override
	public <T> T unwrap(Class<T> iface) throws SQLException {
		throw new SQLException(getClass().getName() + " is not a wrapper.");
	}

	@Override
	public boolean isWrapperFor(Class<?> iface) throws SQLException {
		return false;
	}

	// @Override only valid jdk7+
	public Logger getParentLogger() {
		// requires JDK version 1.6
		return Logger.getLogger(Logger.GLOBAL_LOGGER_NAME);
	}
}
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