Java SPI机制详解

1、什么是SPI？

   SPI 全称为 (Service Provider Interface) ，是JDK内置的一种服务提供发现机制。SPI是一种动态替换发现的机制， 比如有个接口，想运行时动态的给它添加实现，你只需要添加一个实现。我们经常遇到的就是java.sql.Driver接口，其他不同厂商可以针对同一接口做出不同的实现，mysql和postgresql都有不同的实现提供给用户，而Java的SPI机制可以为某个接口寻找服务实现。

​ 如上图所示，接口对应的抽象SPI接口；实现方实现SPI接口；调用方依赖SPI接口。

​ SPI接口的定义在调用方，在概念上更依赖调用方；组织上位于调用方所在的包中，实现位于独立的包中。

​ 当服务的提供者提供了一种接口的实现之后，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。当其他的程序需要这个服务的时候，就可以通过查找这个jar包（一般都是以jar包做依赖）的META-INF/services/中的配置文件，配置文件中有接口的具体实现类名，可以根据这个类名进行加载实例化，就可以使用该服务了。JDK中查找服务实现的工具类是：java.util.ServiceLoader。

2、SPI的用途

​ 数据库DriverManager、Spring、ConfigurableBeanFactory等都用到了SPI机制，这里以数据库DriverManager为例，看一下其实现的内幕。

​ DriverManager是jdbc里管理和注册不同数据库driver的工具类。针对一个数据库，可能会存在着不同的数据库驱动实现。我们在使用特定的驱动实现时，不希望修改现有的代码，而希望通过一个简单的配置就可以达到效果。 在使用mysql驱动的时候，会有一个疑问，DriverManager是怎么获得某确定驱动类的？我们在运用Class.forName(“com.mysql.jdbc.Driver”)加载mysql驱动后，就会执行其中的静态代码把driver注册到DriverManager中，以便后续的使用。

Driver实现

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!");}}
}

驱动的类的静态代码块中，调用DriverManager的注册驱动方法new一个自己当参数传给驱动管理器。

Mysql DriverManager实现

    /*** 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");}

可以看到其内部的静态代码块中有一个loadInitialDrivers方法，loadInitialDrivers用法用到了上文提到的spi工具类ServiceLoader:

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);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();}} 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);}}

先查找jdbc.drivers属性的值，然后通过SPI机制查找驱动

public final class ServiceLoader<S>implements Iterable<S>
{private static final String PREFIX = "META-INF/services/";

private boolean hasNextService() {if (nextName != null) {return true;}if (configs == null) {try {String fullName = PREFIX + service.getName();if (loader == null)configs = ClassLoader.getSystemResources(fullName);elseconfigs = loader.getResources(fullName);} 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;}

可以看到加载META-INF/services/ 文件夹下类名为文件名（这里相当于Driver.class.getName()）的资源，然后将其加载到虚拟机。

注释有这么一句“Load these drivers, so that they can be instantiated.” 意思是加载SPI扫描到的驱动来触发他们的初始化。即触发他们的static代码块

/*** Registers the given driver with the {@code DriverManager}.* A newly-loaded driver class should call* the method {@code registerDriver} to make itself* known to the {@code DriverManager}. If the driver is currently* registered, no action is taken.** @param driver the new JDBC Driver that is to be registered with the*               {@code DriverManager}* @param da     the {@code DriverAction} implementation to be used when*               {@code DriverManager#deregisterDriver} is called* @exception SQLException if a database access error occurs* @exception NullPointerException if {@code driver} is null* @since 1.8*/public static synchronized void registerDriver(java.sql.Driver driver,DriverAction da)throws SQLException {/* Register the driver if it has not already been added to our list */if(driver != null) {registeredDrivers.addIfAbsent(new DriverInfo(driver, da));} else {// This is for compatibility with the original DriverManagerthrow new NullPointerException();}println("registerDriver: " + driver);}

将自己注册到驱动管理器的驱动列表中

public class DriverManager {// List of registered JDBC driversprivate final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();

当获取连接的时候调用驱动管理器的连接方法从列表中获取。

  @CallerSensitivepublic static Connection getConnection(String url,String user, String password) throws SQLException {java.util.Properties info = new java.util.Properties();if (user != null) {info.put("user", user);}if (password != null) {info.put("password", password);}return (getConnection(url, info, Reflection.getCallerClass()));}

 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;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)) {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");}

private static boolean isDriverAllowed(Driver driver, ClassLoader classLoader) {boolean result = false;if(driver != null) {Class<?> aClass = null;try {aClass =  Class.forName(driver.getClass().getName(), true, classLoader);} catch (Exception ex) {result = false;}result = ( aClass == driver.getClass() ) ? true : false;}return result;}