java开发ServiceLoader实现机制及SPI应用

public static Driver getDriver(String url)
        throws SQLException {
    println("DriverManager.getDriver("" + url + "")");
    ensureDriversInitialized();
    ......
}
private static void ensureDriversInitialized() {
    ......
    AccessController.doPrivileged(new PrivilegedAction<Void>() {
                public Void run() {
                    // 核心代码ServiceLoader
                    ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
                    Iterator<Driver> driversIterator = loadedDrivers.iterator();
                    try {
                        while (driversIterator.hasNext()) {
                            driversIterator.next();
                        }
                    } catch (Throwable t) {
                        // Do nothing
                    }
                    return null;
                }
            });
    ......
}

    public static <S> ServiceLoader<S> load(Class<S> service) {
        // 从当前线程中获取ClassLoader
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        // 创建一个ServiceLoader对象
        return new ServiceLoader<>(Reflection.getCallerClass(), service, cl);
    }

       @Override
        public boolean hasNext() {
            if (acc == null) {
                return hasNextService();
            } else {
                PrivilegedAction<Boolean> action = new PrivilegedAction<>() {
                    public Boolean run() { return hasNextService(); }
                };
                return AccessController.doPrivileged(action, acc);
            }
        }

private boolean hasNextService() {
    // nextProvider默认为null，如果通过next()取出来了，nextProvider就会变成null
    while (nextProvider == null && nextError == null) {
        try {
            // 找到目标实现类
            Class<?> clazz = nextProviderClass();
            if (clazz == null)
                return false;
            if (clazz.getModule().isNamed()) {
                // ignore class if in named module
                continue;
            }
            // 判断service接口是否和clazz有父子关系
            if (service.isAssignableFrom(clazz)) {
                Class<? extends S> type = (Class<? extends S>) clazz;
                // 获取无参构造函数
                Constructor<? extends S> ctor
                            = (Constructor<? extends S>)getConstructor(clazz);
                // 包装成一个ProviderImpl对象
                ProviderImpl<S> p = new ProviderImpl<S>(service, type, ctor, acc);
                // 并赋值给nextProvider
                nextProvider = (ProviderImpl<T>) p;
            } else {
                fail(service, clazz.getName() + " not a subtype");
            }
        } catch (ServiceConfigurationError e) {
            nextError = e;
        }
    }
    return true;
}

private Class<?> nextProviderClass() {
    if (configs == null) {
        try {
            // 拼接文件名："META-INF/services/接口名称"
            // 比如接口名为：java.sql.Driver，
            // 那么文件路径就是："META-INF/services/java.sql.Driver"
            String fullName = PREFIX + service.getName();
            // 没有指定ClassLoader，就通过getSystemClassLoader()加载目标文件
            if (loader == null) {
                configs = ClassLoader.getSystemResources(fullName);
            } else if (loader == ClassLoaders.platformClassLoader()) {
                // 如果是platformClassLoader，它没有class path，那么看看BootLoader有没有class path
                if (BootLoader.hasClassPath()) {
                    configs = BootLoader.findResources(fullName);
                } else {
                    configs = Collections.emptyEnumeration();
                }
            } else {
                // 通过指定classLoader加载目标文件
                configs = loader.getResources(fullName);
            }
        } catch (IOException x) {
            fail(service, "Error locating configuration files", x);
        }
    }
    // 上面代码只会执行一次，这样configs就不会为null，下次进来直接取下一个实现类
    // 把configs内容解析成一个迭代器
    while ((pending == null) || !pending.hasNext()) {
        if (!configs.hasMoreElements()) {
            return null;
        }
        pending = parse(configs.nextElement());
    }
    // 通过迭代器获取下一个实现类名称
    String cn = pending.next();
    try {
        // 通过类名反射成Class对象
        return Class.forName(cn, false, loader);
    } catch (ClassNotFoundException x) {
        fail(service, "Provider " + cn + " not found");
        return null;
    }
}