JAVA SPI机制概述

SPI概述

1.SPI概念

①全称：Service Provider Interface
②是什么：是Java提供的一套用来被第三方实现或者扩展的接口，从JDBC4.0,JDK6开始Java实现了SPI机制
③作用：用来启用框架扩展和替换组件，为这些被扩展的API寻找服务实现。

2.SPI和API区别

概念	实现方	调用方	使用人员
API	制定接口并完成对接口的实现	仅仅依赖接口调用，且无权选择不同实现	应用开发人员
SPI		制定接口规范，提供给外部来实现，在调用时则选择自己需要的外部实现	框架扩展人员使用

3.SPI简单示例

①定义一个接口

package com.lemon.spi;/*** @author lemon* @Desc* @date 2020-09-22 11:14*/
public interface RenShu {void renShu();
}

②定义两个接口实现类

package com.lemon.spi;/*** @author lemon* @Desc* @date 2020-09-22 11:18*/
public class ShenLuoTianZheng implements RenShu{@Overridepublic void renShu() {System.out.println("神罗天征1！");}
}
package com.lemon.spi;/*** @author lemon* @Desc* @date 2020-09-22 11:19*/
public class WanXiangTianYin implements RenShu{@Overridepublic void renShu() {System.out.println("万象天引1！");}
}

③在resources下(非固定，只要相对路径是META-INF/services/接口全路径名)新建package META_INF和services，在services下新建和接口名相同的文件

④新建测试类

package com.lemon.spi;import java.util.ServiceLoader;/*** @author lemon* @Desc* @date 2020-09-22 11:10*/
public class Test {public static void main(String[] args) {ServiceLoader<RenShu> renShus = ServiceLoader.load(RenShu.class);for(RenShu renShu:renShus){renShu.renShu();}}
}

4.SPI源码分析

①ServiceLoader类实现流程

核心代码
①：实例化ServiceLoader及私有内部类LazyIteratorpublic void reload() {providers.clear();lookupIterator = new LazyIterator(service, loader);}private ServiceLoader(Class<S> svc, ClassLoader cl) {service = Objects.requireNonNull(svc, "Service interface cannot be null");loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;reload();}
②进行迭代，实际调用的是LazyIterator的hasNextService与nextService方法
private boolean hasNextService() {if (nextName != null) {return true;}if (configs == null) {try {//获取目录下所有的类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;}private S nextService() {if (!hasNextService())throw new NoSuchElementException();String cn = nextName;nextName = null;Class<?> c = null;try {//反射加载类c = Class.forName(cn, false, loader);} catch (ClassNotFoundException x) {fail(service,"Provider " + cn + " not found");}if (!service.isAssignableFrom(c)) {fail(service,"Provider " + cn  + " not a subtype");}try {//实例化S p = service.cast(c.newInstance());//放进缓存providers.put(cn, p);return p;} catch (Throwable x) {fail(service,"Provider " + cn + " could not be instantiated",x);}throw new Error();          // This cannot happen}

5.JDBC的SPI

com.mysql.cj.jdbc.Driver是其默认的实现类

public class DriverManager {// List of registered JDBC driversprivate final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();private static volatile int loginTimeout = 0;private static volatile java.io.PrintWriter logWriter = null;private static volatile java.io.PrintStream logStream = null;// Used in println() to synchronize logWriterprivate final static  Object logSync = new Object();/* 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 {//此处进行spi机制调用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() {//此处进行迭代扫描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;}});

6.SPI优缺点

优点：
1.不需要改动源码就可以实现扩展，解耦。
2.实现扩展对原来的代码几乎没有侵入性。
3.只需要添加配置就可以实现扩展，符合开闭原则。

缺点：
1.不能按需加载，需要遍历所有的实。如果不想用某些实现类，或者某些类实例化很耗时，它也被载入并实例化了，这就造成了浪费。
2.ServiceLoader 线程不安全。

参考链接： https://www.cnblogs.com/jy107600/p/11464985.html