1.SPI簡(jiǎn)述

SPI(Service Provider Interface)，是JDK內(nèi)置的一種服務(wù)提供發(fā)現(xiàn)機(jī)制。SPI通過加載ClassPath路徑下的META-INF/services文件夾下的所有文件，自動(dòng)加載文件里所制定的接口實(shí)現(xiàn)類。在Dubbo、JDBC（Class.forName("com.mysql.jdbc.Driver")讀取MySQL的jar包中的META-INF/services/com.mysql.jdbc.Driver文件）中都使用到了SPI機(jī)制。

不足：
ServiceLoader加載META-INF/services/*使用了延遲加載，但只能將實(shí)現(xiàn)類全部加載并實(shí)例化，然后通過遍歷全部獲取。

2.代碼示例：

2.1 Maven項(xiàng)目結(jié)構(gòu)：

Maven項(xiàng)目結(jié)構(gòu).png

2.2 接口類SpiService.java：

package com.lance.spi.demo.iservice;

public interface SpiService {
    void execute();
}

2.3 實(shí)現(xiàn)類ComplexSpiServiceImpl.java：

package com.lance.spi.demo.service.impl;

import com.lance.spi.demo.iservice.SpiService;

public class ComplexSpiServiceImpl  implements SpiService {
    @Override
    public void execute() {
        System.out.println("ComplexSpiServiceImpl execute.");
    }
}

2.4 實(shí)現(xiàn)類SimpleSpiServiceImpl.java：

package com.lance.spi.demo.service.impl;

import com.lance.spi.demo.iservice.SpiService;
public class SimpleSpiServiceImpl implements SpiService {
    @Override
    public void execute() {
        System.out.println("SimpleSpiService execute.");
    }
}

2.5 com.lance.spi.demo.iservice.SpiService文本文件：

com.lance.spi.demo.service.impl.ComplexSpiServiceImpl
com.lance.spi.demo.service.impl.SimpleSpiServiceImpl

2.6 主類JavaSpiMain.java：

package com.lance.spi.demo.main;

import com.lance.spi.demo.iservice.SpiService;
import sun.misc.Service;

import java.util.Iterator;
import java.util.ServiceLoader;

public class JavaSpiMain {


    public static void main(String[] args) {


        System.out.println("----------- Service.providers --------------");

        Iterator<SpiService> providers = Service.providers(SpiService.class);


        while(providers.hasNext()) {
            SpiService spiService = providers.next();
            spiService.execute();
        }
        System.out.println("------------ ServiceLoader.load ----------------");


        ServiceLoader<SpiService> load = ServiceLoader.load(SpiService.class);
        Iterator<SpiService> iterator = load.iterator();
        while(iterator.hasNext()) {
            SpiService spiService = iterator.next();
            spiService.execute();
        }



    }
}

2.7 輸出結(jié)果：

----------- Service.providers --------------
ComplexSpiServiceImpl execute.
SimpleSpiService execute.
------------ ServiceLoader.load ----------------
ComplexSpiServiceImpl execute.
SimpleSpiService execute.