URLStreamHandlerFactory mockFactory2 = mock(URLStreamHandlerFactory.class);

        Handler.setURLStreamHandlerFactory(mockFactory1); // First call, should succeed.

        assertThrows(IllegalStateException.class, () -> {
            Handler.setURLStreamHandlerFactory(mockFactory2); // Second call, should fail.
        }, "Setting the factory a second time should throw IllegalStateException.");
    }

     * stream handlers.  This can be called once per JVM instance.
     *
     * @param factory The URL stream handler factory.
     */
    public static void setURLStreamHandlerFactory(final URLStreamHandlerFactory factory) {
        synchronized (PROTOCOL_HANDLERS) {
            if (Handler.factory != null) {
                throw new IllegalStateException("URLStreamHandlerFactory already set.");

    @BeforeAll
    static void registerSmbHandler() {
        if (!handlerRegistered) {
            try {
                // Try to register the handler factory
                URL.setURLStreamHandlerFactory(new URLStreamHandlerFactory() {
                    @Override
                    public URLStreamHandler createURLStreamHandler(String protocol) {
                        if ("smb".equals(protocol)) {

     * stream handlers. This can be called once per JVM instance.
     *
     * @param factory
     *            The URL stream handler factory.
     */
    public static void setURLStreamHandlerFactory(final URLStreamHandlerFactory factory) {
        synchronized (PROTOCOL_HANDLERS) {
            if (Handler.factory != null) {
                throw new IllegalStateException("URLStreamHandlerFactory already set.");