Java|Java spi（service provider interface） Javaspi（serviceproviderinterface）

什么是spi，下面以mysql驱动为示例，直接贴源码。
1、jdbc在没有采用spi的情况下，是需要我们
Class.forName("com.mysql.jdbc.Driver");
2、看核心类java.sql.DriverManager.

public class DriverManager {/* Prevent the DriverManager class from being instantiated. */ private DriverManager(){}/** * 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"); }private static void loadInitialDrivers() { String drivers; try { drivers = AccessController.doPrivileged(new PrivilegedAction() { 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() { public Void run() {ServiceLoader loadedDrivers = ServiceLoader.load(Driver.class); Iterator 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(); } } catch(Throwable t) { // Do nothing } return null; } }); println("DriverManager.initialize: jdbc.drivers = " + drivers); 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); } } }

代码说明：重点看
ServiceLoader loadedDrivers = ServiceLoader.load(Driver.class);
以及

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); } }

3、看下mysql-connector-java-5.1.46.jar的spi实现

文章图片
20181118215424.png 【Java|Java spi（service provider interface）】META-INF/services/java.sql.Driver的内容是：
com.mysql.jdbc.Driver
com.mysql.fabric.jdbc.FabricMySQLDriver
4、最后贴出Driver实现类的源码：

package com.mysql.jdbc; import java.sql.DriverManager; import java.sql.SQLException; public class Driver extends NonRegisteringDriver implements java.sql.Driver { public Driver() throws SQLException { }static { try { DriverManager.registerDriver(new Driver()); } catch (SQLException var1) { throw new RuntimeException("Can't register driver!"); } } }

package com.mysql.fabric.jdbc; import com.mysql.jdbc.ExceptionInterceptor; import com.mysql.jdbc.NonRegisteringDriver; import com.mysql.jdbc.Util; import java.lang.reflect.Constructor; import java.sql.Connection; import java.sql.Driver; import java.sql.DriverManager; import java.sql.SQLException; import java.util.Properties; import java.util.logging.Logger; public class FabricMySQLDriver extends NonRegisteringDriver implements Driver { public static final String FABRIC_URL_PREFIX = "jdbc:mysql:fabric://"; public static final String FABRIC_SHARD_KEY_PROPERTY_KEY = "fabricShardKey"; public static final String FABRIC_SHARD_TABLE_PROPERTY_KEY = "fabricShardTable"; public static final String FABRIC_SERVER_GROUP_PROPERTY_KEY = "fabricServerGroup"; public static final String FABRIC_PROTOCOL_PROPERTY_KEY = "fabricProtocol"; public static final String FABRIC_USERNAME_PROPERTY_KEY = "fabricUsername"; public static final String FABRIC_PASSWORD_PROPERTY_KEY = "fabricPassword"; public static final String FABRIC_REPORT_ERRORS_PROPERTY_KEY = "fabricReportErrors"; public FabricMySQLDriver() throws SQLException { }public Connection connect(String url, Properties info) throws SQLException { Properties parsedProps = this.parseFabricURL(url, info); if (parsedProps == null) { return null; } else { parsedProps.setProperty("fabricProtocol", "http"); if (Util.isJdbc4()) { try { Constructor jdbc4proxy = Class.forName("com.mysql.fabric.jdbc.JDBC4FabricMySQLConnectionProxy").getConstructor(Properties.class); return (Connection)Util.handleNewInstance(jdbc4proxy, new Object[]{parsedProps}, (ExceptionInterceptor)null); } catch (Exception var5) { throw (SQLException)(new SQLException(var5.getMessage())).initCause(var5); } } else { return new FabricMySQLConnectionProxy(parsedProps); } } }public boolean acceptsURL(String url) throws SQLException { return this.parseFabricURL(url, (Properties)null) != null; }Properties parseFabricURL(String url, Properties defaults) throws SQLException { return !url.startsWith("jdbc:mysql:fabric://") ? null : super.parseURL(url.replaceAll("fabric:", ""), defaults); }public Logger getParentLogger() throws SQLException { throw new SQLException("no logging"); }static { try { DriverManager.registerDriver(new FabricMySQLDriver()); } catch (SQLException var1) { throw new RuntimeException("Can't register driver", var1); } } } }