JDK-SPI简介【一】

1.什么是SPI？

SPI：Service Provider Interface，是JDK提供的为某个接口寻找服务实现的机制。为了实现不对实现类进行硬编码，在程序里动态指明。

2.使用场景？

2.1 common-logging

apache最早提供的日志的门面接口。只有接口，没有实现。具体方案由各提供商实现， 发现日志提供商是通过扫描 META-INF/services/org.apache.commons.logging.LogFactory配置文件，通过读取该文件的内容找到日志提工商实现类。只要我们的日志实现里包含了这个文件，并在文件里制定 LogFactory工厂接口的实现类即可。

2.2 JDBC

META-INF.services包下有个java.sql.Driver文件：
com.mysql.jdbc.Driver
com.mysql.fabric.jdbc.FabricMySQLDriver

其中com.mysql.jdbc.Driver类是实现了java.sql.Driver接口，下面那com.mysql.fabric.jdbc.FabricMySQLDriver才是真正的实现类
 

3.例子：

3.1接口：

package com.test.spi;

public interface HelloInterface {

    public void sayHello();
}

3.2实现类：

public class ImageHello implements HelloInterface {

    @Override
    public void sayHello() {
        System.out.println("Image Hello");
    }
}

public class TextHello implements HelloInterface {

    @Override
    public void sayHello() {
        System.out.println("Text Hello.");
    }

}

3.3 文件

在src路径下的META-INF/services/com.test.spi.HelloInterface

com.test.spi.impl.TextHello
com.test.spi.impl.ImageHello

3.4 Main方法

public class SPIMain {
    public static void main(String[] args) {

        ServiceLoader loaders = ServiceLoader.load(HelloInterface.class);

        for (HelloInterface in : loaders) {
            in.sayHello();
        }
    }
}

3.5 运行结果：

Text Hello.
Image Hello

4.原理分析

4.1ServiceLoader实现了Iterable这个接口，因此可以在遍历的时候，重新进行lazy加载。

public final class ServiceLoader
    implements Iterable
{

    private static final String PREFIX = "META-INF/services/";

    // The class or interface representing the service being loaded
    private final Class service;

    // The class loader used to locate, load, and instantiate providers
    private final ClassLoader loader;

    // The access control context taken when the ServiceLoader is created
    private final AccessControlContext acc;

    // Cached providers, in instantiation order
    private LinkedHashMap providers = new LinkedHashMap<>();

    // The current lazy-lookup iterator
    private LazyIterator lookupIterator;
}

4.2

 private class LazyIterator
        implements Iterator
    {


        private S nextService() {
            if (!hasNextService())
                throw new NoSuchElementException();
            String cn = nextName;
            nextName = null;
            Class c = null;
            try {
                //延迟加载
                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 {
                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
        }

    }

参考：

1.http://www.cnblogs.com/zhongkaiuu/articles/5040971.html