Lambda Does Not Work In Websocket Session

Question

I just encountered a peculiar little problem:

javax.websocket.Session session = //...
// this works
newSession.addMessageHandler(new MessageHandler.Whole<String>() {

    @Override
    public void onMessage(String message) {
        MyWebSocket.this.onMessage(message);
    }
});
// these don't work
newSession.addMessageHandler((MessageHandler.Whole<String>) MyWebSocket.this::onMessage);
newSession.addMessageHandler((MessageHandler.Whole<String>) message -> MyWebSocket.this.onMessage(message));


void onMessage(String message) {
    System.out.println(message);
}

Does anybody know why lambda expressions won't work in this instance? There is no compile error, no exception, no nothing. The method ''onMessage'' is just not called.

I use Java 1.8.0_65 and the Tyrus reference implementation 1.9.

Pavel Bucek · Accepted Answer · 2016-03-01T10:47:07.563

please see https://blogs.oracle.com/PavelBucek/entry/websocket_api_1_1_released

tldr; you have to use Session#addMessageHandler(Class<T> clazz, MessageHandler.Whole<T> handler)

/**
* Register to handle to incoming messages in this conversation. A maximum of one message handler per
* native websocket message type (text, binary, pong) may be added to each Session. I.e. a maximum
* of one message handler to handle incoming text messages a maximum of one message handler for
* handling incoming binary messages, and a maximum of one for handling incoming pong
* messages. For further details of which message handlers handle which of the native websocket
* message types please see {@link MessageHandler.Whole} and {@link MessageHandler.Partial}.
* Adding more than one of any one type will result in a runtime exception.
*
* @param clazz   type of the message processed by message handler to be registered.
* @param handler whole message handler to be added.
* @throws IllegalStateException if there is already a MessageHandler registered for the same native
*                               websocket message type as this handler.
*/
public void addMessageHandler(Class<T> clazz, MessageHandler.Whole<T> handler);

in order to use lambdas as message handers.

Gunther Rotsch · Answer 2 · 2016-02-29T11:26:59.833

-2

From my understanding MessageHandler would need to be a @FunctionalInterface to allow lambda expressions here, which isn't the case.

edited Feb 29 '16 at 11:26

answered Feb 29 '16 at 09:21

Gunther Rotsch

155
3

1

["However, the compiler will treat any interface meeting the definition of a functional interface as a functional interface regardless of whether or not a FunctionalInterface annotation is present on the interface declaration."](https://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html) You don't need the Annotation to use it as lambda – F. Lumnitz Feb 29 '16 at 09:27
1

Also there would be a compile error if that was the case. – Stefan S. Feb 29 '16 at 09:34

Lambda Does Not Work In Websocket Session

2 Answers2