java spi及原理

目录

spi核心其实也很简单….spi全名即

源码

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public static <S> ServiceLoader<S> load(Class<S> service) {
    ClassLoader cl = Thread.currentThread().getContextClassLoader();
    return ServiceLoader.load(service, cl);
}
// load的时候调用此构造器
private ServiceLoader(Class<S> svc, ClassLoader cl) {
    // 传入的api
    service = Objects.requireNonNull(svc, "Service interface cannot be null");
    // 类加载器
    loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
    // 
    acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
    reload();
}

// 
public void reload() {
    // 清空缓存(provider是一个map)
    providers.clear();
    lookupIterator = new LazyIterator(service, loader);
}

lazy迭代器

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private class LazyIterator
    implements Iterator<S>
{

    Class<S> service;
    ClassLoader loader;
    Enumeration<URL> configs = null;
    Iterator<String> pending = null;
    // 存储每次遍历配置文件内的全路径
    String nextName = null;

    private LazyIterator(Class<S> service, ClassLoader loader) {
        this.service = service;
        this.loader = loader;
    }
    ...
    public S next() {
        if (acc == null) {
            return nextService();
        } else {
            PrivilegedAction<S> action = new PrivilegedAction<S>() {
                public S run() { return nextService(); }
            };
            return AccessController.doPrivileged(action, acc);
        }
    }
    ...
    private S nextService() {
        if (!hasNextService())
            throw new NoSuchElementException();
        String cn = nextName;
        // 清空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());
            // 放入缓存..
            providers.put(cn, p);
            return p;
        } catch (Throwable x) {
            fail(service,
                 "Provider " + cn + " could not be instantiated",
                 x);
        }
        throw new Error();          // This cannot happen
    }
}

dubbo的spi