java spi（service provider interface）示例与原理解析

java spi（service provider interface）示例与原理解析

目的

提供服务注册与发现，让功能的使用方和提供方解耦

示例

1.提供接口

package com.yemin.service;

public interface Animals {
    public void run();
    public void eat();
}

2.实现接口

接口1

package com.yemin.serviceImpl;

import com.yemin.service.Animals;

public class Cat implements Animals {
    static {
        System.out.println("hello world cat");
    }
    @Override
    public void run() {
        System.out.println("cat is running");
    }
    @Override
    public void eat() {
        System.out.println("cat is eating");
    }
}

接口2

package com.yemin.serviceImpl;

import com.yemin.service.Animals;

public class Dog implements Animals {
    static {
        System.out.println("hello world dog");
    }
    @Override
    public void run() {
        System.out.println("dog is running");
    }
    @Override
    public void eat() {
        System.out.println("dog is eating");
    }
}

3.注册接口与实现

创建要注册接口的目录文件

resourcesMETA-INFservicescom.yemin.service.Animals

文件内放入具体实现,选择需要的实现

com.yemin.serviceImpl.Cat
com.yemin.serviceImpl.Dog

4.使用服务

在其他模块引用该模块依赖，或者打成jar包给其他项目使用

    public static void main(String[] args) {
        ServiceLoader<Animals> animalsIterator = ServiceLoader.load(Animals.class);
        for (Animals a:animalsIterator){
            System.out.println(a);
        }
    }

原理

1. ServiceLoader.load(Animals.class)
2. new ServiceLoader<>(service, loader)
3. reload()
4. lookupIterator = new LazyIterator(service, loader);

ServiceLoader寻找"META-INF/services/"下的接口实例

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

LazyIterator 每次获取下一个元素的时候，加载并实例化对象

private S nextService() {
    if (!hasNextService())
        throw new NoSuchElementException();
    String cn = nextName;
    nextName = null;
    Class<?> c = null;
    try {
        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
}