javaSPI机制

service provider interface

// 通过遍历即可获取到对应的class,那么原理是什么呢？
Iterator<ABC> iterator = ServiceLoader.load(ABC.class).iterator();
while (iterator.hasNext()) {
    ABC next = iterator.next();
}

原理

java规定spi的配置文件都在这个目录META-INF/services/
该目录下可以有多个文件，文件的名称必须以class全类名命名。通过ServiceLoader类，读取名字为spi类的全名称的文件
内容为多行，一行为一个class的全类名。该class为spi的实现类。

ServiceLoader入口


public final class ServiceLoader<S> implements Iterable<S> {

    private static final String PREFIX = "META-INF/services/"; // 扫描的目录

    private final Class<S> service; // spi 要加载的class

    private final ClassLoader loader;// classLoader，默认为当前线程的classLoader
    
    private LinkedHashMap<String,S> providers = new LinkedHashMap<>(); // 已加载过的SPI都会缓存

    private LazyIterator lookupIterator; // 真正加载的class
    
    private ServiceLoader(Class<S> svc, ClassLoader cl) {
        loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
        lookupIterator = new LazyIterator(service, loader);
    }
    ...
    // 入口
    public static <S> ServiceLoader<S> load(Class<S> service, ClassLoader loader) {
        return new ServiceLoader<>(service, loader);
    }

    public Iterator<S> iterator() {
        return new Iterator<S>() {

            Iterator<Map.Entry<String,S>> knownProviders = providers.entrySet().iterator(); // 缓存

            public boolean hasNext() {
                if (knownProviders.hasNext())// 先用缓存
                    return true;
                return lookupIterator.hasNext();
            }

            public S next() {
                if (knownProviders.hasNext())
                    return knownProviders.next().getValue(); // 先用缓存
                return lookupIterator.next();
            }
            ...
        };
    }
}

LazyIterator 真正的加载类

private class LazyIterator implements Iterator<S> {
    ...
    Class<S> service;
    ClassLoader loader;
    Enumeration<URL> configs = null;// 已扫描的url
    Iterator<String> pending = null; // 当前扫描的url资源中的spi文本（className为多行）
    String nextName = null; //下一个spi实现类的名称
    private LazyIterator(Class<S> service, ClassLoader loader) {
        this.service = service;
        this.loader = loader;
    }
    
    // 迭代器的实现
    public boolean hasNext() {
        ...
        configs = configs != null ? configs : loader.getResources("META-INF/services/" + service.getName());
        while ((pending == null) || !pending.hasNext()) {
            if (!configs.hasMoreElements()) {
                return false;
            }

            pending = parse(service, configs.nextElement());
        }
        // 直接返回
        nextName = pending.next();
        return true;
        ...
    }

    // 迭代器的实现
    public S next() {
        ...
        String cn = nextName;
        nextName = null;
        Class<?> c = null;
        try {
            c = Class.forName(cn, false, loader);
        } catch (ClassNotFoundException x) {
            fail(service, "Provider " + cn + " not found");
        }
        // 该不是spi接口的实现，乱配置的就直接抛出异常
        if (!service.isAssignableFrom(c)) {
            fail(service, "Provider " + cn  + " not a subtype");
        }
        try {
            S p = service.cast(c.newInstance());
            providers.put(cn, p);// 缓存一下
            return p;// 反射创建对象，直接返回
        } catch (Throwable x) {
            fail(service, "Provider " + cn + " could not be instantiated", x);
        }
        ...
    }
    ...
}