我们知道当我们连接MySQL数据库时，会使用如下代码:
1 try { 2 Class.forName("com.mysql.jdbc.Driver"); 3 connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "root", "123456"); 4 } catch (Exception e) { 5 e.printStackTrace(); 6 }
那么Class.forName()有什么作用呢？
首先我们知道Class.forName() 方法要求JVM查找并加载指定的类到内存中，此时将"com.mysql.jdbc.Driver" 当做参数传入，就是告诉JVM，去"com.mysql.jdbc"这个路径下找Driver类，将其加载到内存中。
由于加载类文件时会执行其中的静态代码块，其中Driver类的源码如下
public class Driver extends NonRegisteringDriver implements java.sql.Driver { public Driver() throws SQLException { } static { try { DriverManager.registerDriver(new Driver());//首先new一个Driver对象，并将它注册到DriverManage中 } catch (SQLException var1) { throw new RuntimeException("Can't register driver!"); } } }
接下来我们再看看这个DriverManager.registerDriver 方法：
public static synchronized void registerDriver(java.sql.Driver driver) throws SQLException { registerDriver(driver, null); }
继续看这个registerDriver(driver, null) 方法
private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();// registeredDrivers 是一个支持并发的arraylist ...... public static void registerDriver(java.sql.Driver driver, DriverAction da) throws SQLException { if (driver != null) { //如果该驱动尚未注册，那么将他添加到 registeredDrivers 中去。这是一个支持并发情况的特殊ArrayList registeredDrivers.addIfAbsent(new DriverInfo(driver, da)); } else { // This is for compatibility with the original DriverManager throw new NullPointerException(); } println("registerDriver: " + driver); }
此时，Class.forName(“com.mysql.jdbc.Driver”) 的工作就完成了，工作就是：将mysql驱动注册到DriverManager中去。接下来我们看是怎么进行调用的
2、DriverManager.getConnection方法分析
注册到DriverManager中之后，我们就可以通过DriverManager的getConnection方法获得mysql的连接了：
Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "root", "123456");
接下来我们在看看这个getConnection方法：
@CallerSensitive public static Connection getConnection(String url, String user, String password) throws SQLException { .... return (getConnection(url, info, Reflection.getCallerClass())); }
同样，调用了自身的 getConnection方法，继续往下看
1 private static Connection getConnection( 2 String url, java.util.Properties info, Class<?> caller) throws SQLException { 3 ClassLoader callerCL = caller != null ? caller.getClassLoader() : null; 4 synchronized(DriverManager.class) { 5 // synchronize loading of the correct classloader. 6 if (callerCL == null) { 7 callerCL = Thread.currentThread().getContextClassLoader(); 8 } 9 } 10 // Walk through the loaded registeredDrivers attempting to make a connection. 11 // Remember the first exception that gets raised so we can reraise it. 12 SQLException reason = null; 13 14 for(DriverInfo aDriver : registeredDrivers) { 15 // If the caller does not have permission to load the driver then skip it. 16 if(isDriverAllowed(aDriver.driver, callerCL)) { 17 try { 18 Connection con = aDriver.driver.connect(url, info); 19 if (con != null) { 20 // Success! 21 return (con); 22 } 23 } catch (SQLException ex) { 24 if (reason == null) { 25 reason = ex; 26 } 27 } 28 } else { 29 println("skipping: " + aDriver.getClass().getName()); 30 } 31 } 32 33 // if we got here nobody could connect. 34 if (reason != null) { 35 println("getConnection failed: " + reason); 36 throw reason; 37 } 38 throw new SQLException("No suitable driver found for "+ url, "08001"); 39 }
可以看到它对上文提到的静态变量 registeredDrivers 进行了遍历，调用了connect(url, info)方法，这是一个接口，由各个不同的驱动自己实现。
/** * Attempts to make a database connection to the given URL. * The driver should return "null" if it realizes it is the wrong kind * of driver to connect to the given URL. This will be common, as when * the JDBC driver manager is asked to connect to a given URL it passes * the URL to each loaded driver in turn. */ Connection connect(String url, java.util.Properties info) throws SQLException;
到此为止，我们就获得了connection对象，现在就可以对数据库进行操作了。
3、不手动注册驱动也能使用JDBC [ 去除class.forName ]
高版本的Oracle、MySql已经不需要写class.forName了，在DriverManager的源码中可以看到一个静态块
/** * Load the initial JDBC drivers by checking the System property * jdbc.properties and then use the {@code ServiceLoader} mechanism */ static { loadInitialDrivers(); println("JDBC DriverManager initialized"); }
进入loadInitialDrivers()方法中看到以下一段代码：
1 private static void loadInitialDrivers() { 2 String drivers; 3 try { 4 drivers = AccessController.doPrivileged(new PrivilegedAction<String>() { 5 public String run() { 6 return System.getProperty("jdbc.drivers"); 7 } 8 }); 9 } catch (Exception ex) { 10 drivers = null; 11 } 12 // If the driver is packaged as a Service Provider, load it. 13 // Get all the drivers through the classloader 14 // exposed as a java.sql.Driver.class service. 15 // ServiceLoader.load() replaces the sun.misc.Providers() 16 17 AccessController.doPrivileged(new PrivilegedAction<Void>() { 18 public Void run() { 19 20 ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class); 21 Iterator<Driver> driversIterator = loadedDrivers.iterator(); 22 23 /* Load these drivers, so that they can be instantiated. 24 * It may be the case that the driver class may not be there 25 * i.e. there may be a packaged driver with the service class 26 * as implementation of java.sql.Driver but the actual class 27 * may be missing. In that case a java.util.ServiceConfigurationError 28 * will be thrown at runtime by the VM trying to locate 29 * and load the service. 30 * 31 * Adding a try catch block to catch those runtime errors 32 * if driver not available in classpath but it's 33 * packaged as service and that service is there in classpath. 34 */ 35 try{ 36 while(driversIterator.hasNext()) { 37 driversIterator.next(); 38 } 39 } catch(Throwable t) { 40 // Do nothing 41 } 42 return null; 43 } 44 });
重点是第20行，ServiceLoader.load(Driver.class)
上面这行代码可以把类路径下所有jar包中META-INF/services/java.sql.Driver文件中定义的类加载上来，此类必须继承自java.sql.Driver。
Class.forName的主要目的是初始化数据库驱动，以执行驱动的静态块代码，其实主要是DriverManager.registerDriver(driver)方法。自己驱动包的驱动名当然开发商最清楚，所以就在发布驱动包的时候就直接配置了，所以不用再写Class.forName。转载链接： https://blog.csdn.net/zt928815211/article/details/83420828
https://www.zhihu.com/question/22925738/answer/23088255