ServiceLoader机制及实例

动态加载某个接口的实现类
全局扫描所有的Class，判断是否实现了该接口，效率低

JDK提供了 ServiceLoader 机制
SPI   Service Provider Interface 
服务提供发现机制
有不少框架用它来做服务的扩展发现  动态替换发现机制
jar包 META-INF/services/ 目录下建立一个文件 
文件名 接口的全限定名，文件内容可以有多行，每行是该接口实现类的全限定名


典型使用场景 slf4j  日志桥接

jcl-over-slf4j-1.7.22.jar
/META-INF/services/org.apache.commons.logging.LogFactory

org.apache.commons.logging.impl.SLF4JLogFactory


log4j-to-slf4j-2.10.0.jar
/META-INF/services/org.apache.logging.log4j.spi.Provider

org.apache.logging.slf4j.SLF4JProvider



实例

4个工程
intf   接口   dyyx.Pet
impl   实现1  dyyx.Cat
impl2  实现2  dyyx.Dog
test   ServiceLoader 测试

/service-load-impl/src/main/resources/META-INF/services/dyyx.Pet
dyyx.Cat
/service-load-impl2/src/main/resources/META-INF/services/dyyx.Pet
dyyx.Dog


测试代码 Hello.java

import java.net.URL;
import java.net.URLClassLoader;
import java.util.Iterator;
import java.util.ServiceLoader;

public class Hello {
	
	private static final String DIR = "/Users/dugang/fun/hellocode/demo/serviceload/";
	
	public static void main(String[] args) throws Exception {
		System.out.println("hello");
		
		URL url1 = getUrl("impl");
		URL url2 = getUrl("impl2");		
		
		System.out.println(url1);
		System.out.println(url2);
		
		// 自定义类加载器
		ClassLoader classLoader = new URLClassLoader(new URL[]{url1,url2});
		System.out.println(classLoader);	
		
		Pet cat = (Pet) classLoader.loadClass("dyyx.Cat").newInstance();
		cat.sayHello();
		// 不能在 classpath里加入实现jar包 或 实现工程impl，否则 classload 为 AppClassLoader
		System.out.println(cat.getClass().getClassLoader());
		
		System.out.println();

		
		ClassLoader cl = null;
		// 加载不到
		showServices(cl);
		
		// 加载不到
		cl = Pet.class.getClassLoader();
		showServices(cl);
		
		// 可以加载到
		cl = classLoader;
		showServices(cl);

	}
	
	
	private static void showServices(ClassLoader cl){		
		System.out.println("ClassLoader="+cl);	
		if(cl!=null){
			System.out.println(cl.getClass().toString());
		}
		// ClassLoader loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl
		ServiceLoader services = ServiceLoader.load(Pet.class, cl);
		int num = 0;
		Iterator it = services.iterator();
		while (it.hasNext()) {
			num++;
			Pet pet = it.next();
			pet.sayHello();
		}
		System.out.println("services.num="+num);
		System.out.println("\n\n");
		
	}
	
	
	private static URL getUrl(String type)throws Exception{
		String str = "file:%s%s/target/service-load-%s-1.0.jar";
		str = String.format(str, DIR,type,type);
		return new URL(str);
	}
	
}



file:/Users/dugang/fun/hellocode/demo/serviceload/impl/target/service-load-impl-1.0.jar
file:/Users/dugang/fun/hellocode/demo/serviceload/impl2/target/service-load-impl2-1.0.jar
java.net.URLClassLoader@7f31245a

Cat sayHello Thu Jun 12 12:55:45 CST 2019
java.net.URLClassLoader@7f31245a

ClassLoader=null
services.num=0

ClassLoader=sun.misc.Launcher$AppClassLoader@2a139a55
class sun.misc.Launcher$AppClassLoader
services.num=0

ClassLoader=java.net.URLClassLoader@7f31245a
class java.net.URLClassLoader
Cat sayHello Wed Jun 12 22:55:45 CST 2019
Dog sayHello Wed Jun 12 22:55:45 CST 2019
services.num=2




完整代码
https://gitee.com/dyyx/hellocode/tree/master/demo/serviceload
 
 

jar tvf service-load-impl-1.0.jar