https://segmentfault.com/a/1190000017517197?utm_source=tag-newest
一句话总结：
1 自动加载：DriverManager的静态代码块执行的时刻，使用该时刻当前线程类加载器加载java.sql.Driver文件，并用当前线程类加载器加载及、实例化、registerDriver
2 没有自动加载的，手动forName，传入一个类加载器及是否初始化；或loadClass➕实例化（load/find class 与 forname 在static代码块加载的不同 （二）非系统类jdbc）
3 getConncetion时，校验调用getConnection的类所在类加载器与driver实现所在类加载器（校验存放的driver是否属于调用者的Classloader），JDBC注册原理与自定义类加载器解决com.cloudera.hive.jdbc41.HS2Driver的加载【重点】 这个连接通过伪造一个中间人骗过校验
SPI概述
SPI全称为(Service Provider Interface) ，是JDK内置的一种服务提供发现机制；主要被框架的开发人员使用，比如java.sql.Driver接口，数据库厂商实现此接口即可，当然要想让系统知道具体实现类的存在，还需要使用固定的存放规则，需要在classpath下的META-INF/services/目录里创建一个以服务接口命名的文件，这个文件里的内容就是这个接口的具体的实现类；下面以JDBC为实例来进行具体的分析。
JDBC驱动
1.准备驱动包
<dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> <version>5.1.47</version> </dependency> <dependency> <groupId>org.postgresql</groupId> <artifactId>postgresql</artifactId> <version>42.2.2</version> </dependency> <dependency> <groupId>com.microsoft.sqlserver</groupId> <artifactId>mssql-jdbc</artifactId> <version>7.0.0.jre8</version> </dependency>分别准备了mysql，postgresql和sqlserver，可以打开jar，发现每个jar包的META-INF/services/都存在一个java.sql.Driver文件，文件里面存在一个或多个类名，比如mysql：
com.mysql.jdbc.Driver com.mysql.fabric.jdbc.FabricMySQLDriver提供的每个驱动类占据一行，解析的时候会按行读取，具体使用哪个会根据url来决定；
2.简单实例
String url = "jdbc:mysql://localhost:3306/db3"; String username = "root"; String password = "root"; String sql = "update travelrecord set name=\'bbb\' where id=1"; Connection con = DriverManager.getConnection(url, username, password);类路径下存在多个驱动包，具体在使用DriverManager.getConnection应该使用哪个驱动类会解析url来识别，不同的数据库有不同的url前缀；
3.驱动类加载分析
具体META-INF/services/下的驱动类是什么时候加载的，DriverManager有一个静态代码块：
static { loadInitialDrivers(); println("JDBC DriverManager initialized"); } private static void loadInitialDrivers() { String drivers; try { drivers = AccessController.doPrivileged(new PrivilegedAction<String>() { public String run() { return System.getProperty("jdbc.drivers"); } }); } catch (Exception ex) { drivers = null; } // If the driver is packaged as a Service Provider, load it. // Get all the drivers through the classloader // exposed as a java.sql.Driver.class service. // ServiceLoader.load() replaces the sun.misc.Providers() AccessController.doPrivileged(new PrivilegedAction<Void>() { public Void run() { ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);【会遍历DriverManage静态代码块执行时，当前线程类加载器下找】 Iterator<Driver> driversIterator = loadedDrivers.iterator(); /* Load these drivers, so that they can be instantiated. * It may be the case that the driver class may not be there * i.e. there may be a packaged driver with the service class * as implementation of java.sql.Driver but the actual class * may be missing. In that case a java.util.ServiceConfigurationError * will be thrown at runtime by the VM trying to locate * and load the service. * * Adding a try catch block to catch those runtime errors * if driver not available in classpath but it's * packaged as service and that service is there in classpath. */ try{ while(driversIterator.hasNext()) { driversIterator.next();【重要，在这一句forname和实例化】 } } catch(Throwable t) { // Do nothing } return null; } }); println("DriverManager.initialize: jdbc.drivers = " + drivers); 【第二种方式】 if (drivers == null || drivers.equals("")) { return; } String[] driversList = drivers.split(":"); println("number of Drivers:" + driversList.length); for (String aDriver : driversList) { try { println("DriverManager.Initialize: loading " + aDriver); Class.forName(aDriver, true, ClassLoader.getSystemClassLoader()); } catch (Exception ex) { println("DriverManager.Initialize: load failed: " + ex); } } }在加载DriverManager类的时候会执行loadInitialDrivers方法，方法内通过了两种加载驱动类的方式，分别是：使用系统变量方式和ServiceLoader加载方式；系统变量方式其实就是在变量jdbc.drivers中配置好驱动类，然后使用Class.forName进行加载；下面重点看一下ServiceLoader方式，此处调用了load方法但是并没有真正去加载驱动类，而是返回了一个LazyIterator，后面的代码就是循环变量迭代器：
private static final String PREFIX = "META-INF/services/"; private class LazyIterator implements Iterator<S> { Class<S> service; ClassLoader loader; Enumeration<URL> configs = null; Iterator<String> pending = null; String nextName = null; private LazyIterator(Class<S> service, ClassLoader loader) { this.service = service; this.loader = loader; } private boolean hasNextService() { if (nextName != null) { return true; } if (configs == null) { try { String fullName = PREFIX + service.getName(); service：java.sql.Driver if (loader == null) configs = ClassLoader.getSystemResources(fullName); else configs = loader.getResources(fullName);【重要，遍历META-INF/servises/java.sql.Driver资源文件】 } catch (IOException x) { fail(service, "Error locating configuration files", x); } } while ((pending == null) || !pending.hasNext()) { if (!configs.hasMoreElements()) { return false; } pending = parse(service, configs.nextElement()); } nextName = pending.next(); return true; } private S nextService() { if (!hasNextService()) throw new NoSuchElementException(); String cn = nextName; nextName = null; Class<?> c = null; try { c = Class.forName(cn, false, loader); 【重要，使用传入的当前线程类加载器加载】 } catch (ClassNotFoundException x) { fail(service, "Provider " + cn + " not found"); } if (!service.isAssignableFrom(c)) { fail(service, "Provider " + cn + " not a subtype"); } try { S p = service.cast(c.newInstance()); providers.put(cn, p); return p; } catch (Throwable x) { fail(service, "Provider " + cn + " could not be instantiated", x); } throw new Error(); // This cannot happen } ...... }类中指定了一个静态常量PREFIX = “META-INF/services/”，然后和java.sql.Driver拼接组成了fullName，然后通过类加载器(classloader getresource jar包资源冲突情况，父亲为大，兄弟之间，谁先加载谁牛逼（二）)去获取所有类路径（当前类加载）下java.sql.Driver文件，获取之后存放在configs中，里面的每个元素对应一个文件，每个文件中可能会存在多个驱动类，所以使用pending用来存放每个文件中的驱动信息，获取驱动信息之后在nextService中使用Class.forName加载类信息，并且指定不进行初始化；同时在下面使用newInstance 触发各驱动的静态代码块（load/find class 与 forname 在static代码块加载的不同）对驱动类进行了实例化操作；每个驱动类中都提供了一个静态注册代码块，比如mysql：
static { try { java.sql.DriverManager.registerDriver(new Driver()); } catch (SQLException E) { throw new RuntimeException("Can't register driver!"); } }这里又实例化了一个驱动类，同时注册到DriverManager；接下来就是调用DriverManager的getConnection方法，代码如下：
private static Connection getConnection( String url, java.util.Properties info, Class<?> caller) throws SQLException { /* * When callerCl is null, we should check the application's * (which is invoking this class indirectly) * classloader, so that the JDBC driver class outside rt.jar * can be loaded from here. */ ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;【调用方的类所在类加载器】 synchronized(DriverManager.class) { // synchronize loading of the correct classloader. if (callerCL == null) { callerCL = Thread.currentThread().getContextClassLoader(); } } if(url == null) { throw new SQLException("The url cannot be null", "08001"); } println("DriverManager.getConnection(\"" + url + "\")"); // Walk through the loaded registeredDrivers attempting to make a connection. // Remember the first exception that gets raised so we can reraise it. SQLException reason = null; for(DriverInfo aDriver : registeredDrivers) { // If the caller does not have permission to load the driver then // skip it. if(isDriverAllowed(aDriver.driver, callerCL)) {（JDBC注册原理与自定义类加载器解决com.cloudera.hive.jdbc41.HS2Driver的加载【重点】中用一个代理中间人骗过了） try { println(" trying " + aDriver.driver.getClass().getName()); Connection con = aDriver.driver.connect(url, info); if (con != null) { // Success! println("getConnection returning " + aDriver.driver.getClass().getName()); return (con); } } catch (SQLException ex) { if (reason == null) { reason = ex; } } } else { println(" skipping: " + aDriver.getClass().getName()); } } // if we got here nobody could connect. if (reason != null) { println("getConnection failed: " + reason); throw reason; } println("getConnection: no suitable driver found for "+ url); throw new SQLException("No suitable driver found for "+ url, "08001"); }此方法主要是遍历之前注册的DriverInfo，拿着url信息去每个驱动类中建立连接，当然每个驱动类中都会进行url匹配校验，成功之后返回Connection，如果中途有失败的连接并不影响尝试新的驱动连接，遍历完之后还是无法获取连接，则抛出异常；