Java -- JDK中SPI机制

3、在reload中创建了一个懒加载迭代器

public void reload() {
    providers.clear();
    // 懒加载迭代器
    lookupIterator = new LazyIterator(service, loader);
}

4、当这个迭代器被调用hasNext时，跟着代码走下去，会发现有个PREFIX

// 迭代方法
public boolean hasNext() {
    if (acc == null) {
        return hasNextService();
    } else {
        PrivilegedAction action = new PrivilegedAction() {
            public Boolean run() { return hasNextService(); }
        };
        return AccessController.doPrivileged(action, acc);
    }
}
 
private boolean hasNextService() {
    if (nextName != null) {
        return true;
    }
    if (configs == null) {
        try {
            // 由PREFIX + name获取了文件全路径
            String fullName = PREFIX + service.getName();
            if (loader == null)
                configs = ClassLoader.getSystemResources(fullName);
            else
                configs = loader.getResources(fullName);
        } catch (IOException x) {
            fail(service, "Error locating configuration files", x);
        }
    }
    while ((pending == null) || !pending.hasNext()) {
        if (!configs.hasMoreElements()) {
            return false;
        }
        pending = parse(service, configs.nextElement());
    }
    nextName = pending.next();
    return true;
}

5、指向了相对路径下的META-INF/services/

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

6、在Iterator遍历时，通过PREFIX + service.getName()，使用ClassLoader.getSystemResources(fullName)获取了配置里的内容，然后通过读文件的方式加载到自己的缓存，然后再调用迭代器的next()时，就会将其实例化

public S next() {
    if (acc == null) {
        return nextService();
    } else {
        PrivilegedAction action = new PrivilegedAction() {
            public S run() { return nextService(); }
        };
        return AccessController.doPrivileged(action, acc);
    }
}
 
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 {
        // 实例化
        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
}