URL類的setURLStreamHandlerFactory支持多個factory的方法

之前碰到一個問題，需要支持hadoop的url，而java的URL類的setURLStreamHandlerFactory方法只能set一個factory

    public static void setURLStreamHandlerFactory(URLStreamHandlerFactory fac) {
        synchronized (streamHandlerLock) {
            if (factory != null) {
                throw new Error("factory already defined");
            }
            SecurityManager security = System.getSecurityManager();
            if (security != null) {
                security.checkSetFactory();
            }
            handlers.clear();
            factory = fac;
        }
    }

爲了能夠支持，網上搜了一下（https://stackoverflow.com/questions/41696088/register-custom-urlstreamhandler-in-spring-web-application-tomcat），可以使用反射的方式將原有的factory設置成null，然後再用裝飾器模式設置一個新的factory，用於兼容新的url類型，示例如下：

/**
     * 如果已經存在factory，則加一個裝飾器，將原來的factory和用來讀取hdfs的factory都封裝進去，按需使用
     *
     * @param fsUrlStreamHandlerFactory
     * @throws Exception
     */
    private static void registerFactory(final FsUrlStreamHandlerFactory fsUrlStreamHandlerFactory)
            throws Exception {
        log.info("registerFactory : " + fsUrlStreamHandlerFactory.getClass().getName());
        final Field factoryField = URL.class.getDeclaredField("factory");
        factoryField.setAccessible(true);
        final Field lockField = URL.class.getDeclaredField("streamHandlerLock");
        lockField.setAccessible(true);
        // use same lock as in java.net.URL.setURLStreamHandlerFactory
        synchronized (lockField.get(null)) {
            final URLStreamHandlerFactory originalUrlStreamHandlerFactory = (URLStreamHandlerFactory) factoryField.get(null);
            // Reset the value to prevent Error due to a factory already defined
            factoryField.set(null, null);
            URL.setURLStreamHandlerFactory(protocol -> {
                if (protocol.equals("hdfs")) {
                    return fsUrlStreamHandlerFactory.createURLStreamHandler(protocol);
                } else {
                    return originalUrlStreamHandlerFactory.createURLStreamHandler(protocol);
                }
            });
        }
    }