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Interface WebSocket.Listener

Enclosing interface:: WebSocket

public static interface WebSocket.Listener

The receiving interface of WebSocket.

A WebSocket invokes methods of the associated listener passing itself as an argument. These methods are invoked in a thread-safe manner, such that the next invocation may start only after the previous one has finished.

When data has been received, the WebSocket invokes a receive method. Methods onText, onBinary, onPing and onPong must return a CompletionStage that completes once the message has been received by the listener. If a listener's method returns null rather than a CompletionStage, WebSocket will behave as if the listener returned a CompletionStage that is already completed normally.

An IOException raised in WebSocket will result in an invocation of onError with that exception (if the input is not closed). Unless otherwise stated if the listener's method throws an exception or a CompletionStage returned from a method completes exceptionally, the WebSocket will invoke onError with this exception.

API Note:

The strict sequential order of invocations from WebSocket to Listener means, in particular, that the Listener's methods are treated as non-reentrant. This means that Listener implementations do not need to be concerned with possible recursion or the order in which they invoke WebSocket.request in relation to their processing logic.

Careful attention may be required if a listener is associated with more than a single WebSocket. In this case invocations related to different instances of WebSocket may not be ordered and may even happen concurrently.

CompletionStages returned from the receive methods have nothing to do with the counter of invocations. Namely, a CompletionStage does not have to be completed in order to receive more invocations of the listener's methods. Here is an example of a listener that requests invocations, one at a time, until a complete message has been accumulated, then processes the result, and completes the CompletionStage:

    WebSocket.Listener listener = new WebSocket.Listener() {

    List<CharSequence> parts = new ArrayList<>();
    CompletableFuture<?> accumulatedMessage = new CompletableFuture<>();

    public CompletionStage<?> onText(WebSocket webSocket,
                                     CharSequence message,
                                     boolean last) {
        parts.add(message);
        webSocket.request(1);
        if (last) {
            processWholeText(parts);
            parts = new ArrayList<>();
            accumulatedMessage.complete(null);
            CompletionStage<?> cf = accumulatedMessage;
            accumulatedMessage = new CompletableFuture<>();
            return cf;
        }
        return accumulatedMessage;
    }
};

Since:

Method Summary

Modifier and Type	Method	Description
`default CompletionStage<?>`	`onBinary(WebSocket webSocket, ByteBuffer data, boolean last)`	A binary data has been received.
`default CompletionStage<?>`	`onClose(WebSocket webSocket, int statusCode, String reason)`	Receives a Close message indicating the WebSocket's input has been closed.
`default void`	`onError(WebSocket webSocket, Throwable error)`	An error has occurred.
`default void`	`onOpen(WebSocket webSocket)`	A `WebSocket` has been connected.
`default CompletionStage<?>`	`onPing(WebSocket webSocket, ByteBuffer message)`	A Ping message has been received.
`default CompletionStage<?>`	`onPong(WebSocket webSocket, ByteBuffer message)`	A Pong message has been received.
`default CompletionStage<?>`	`onText(WebSocket webSocket, CharSequence data, boolean last)`	A textual data has been received.

Method Details

onOpen

default void onOpen(WebSocket webSocket)

A WebSocket has been connected.

This is the initial invocation and it is made once. It is typically used to make a request for more invocations.

Implementation Requirements:

The default implementation is equivalent to:

webSocket.request(1);

Parameters:

webSocket - the WebSocket that has been connected

onText

default CompletionStage<?> onText(WebSocket webSocket, CharSequence data, boolean last)

A textual data has been received.

Return a CompletionStage which will be used by the WebSocket as an indication it may reclaim the CharSequence. Do not access the CharSequence after this CompletionStage has completed.

Implementation Requirements:

The default implementation is equivalent to:

webSocket.request(1);
return null;

Implementation Note:

The data is always a legal UTF-16 sequence.

Parameters:

webSocket - the WebSocket on which the data has been received

data - the data

last - whether this invocation completes the message

Returns:

a CompletionStage which completes when the CharSequence may be reclaimed; or null if it may be reclaimed immediately

onBinary

default CompletionStage<?> onBinary(WebSocket webSocket, ByteBuffer data, boolean last)

A binary data has been received.

This data is located in bytes from the buffer's position to its limit.

Return a CompletionStage which will be used by the WebSocket as an indication it may reclaim the ByteBuffer. Do not access the ByteBuffer after this CompletionStage has completed.

Implementation Requirements:

The default implementation is equivalent to:

webSocket.request(1);
return null;

Parameters:

webSocket - the WebSocket on which the data has been received

data - the data

last - whether this invocation completes the message

Returns:

a CompletionStage which completes when the ByteBuffer may be reclaimed; or null if it may be reclaimed immediately

onPing

default CompletionStage<?> onPing(WebSocket webSocket, ByteBuffer message)

A Ping message has been received.

As guaranteed by the WebSocket Protocol, the message consists of not more than 125 bytes. These bytes are located from the buffer's position to its limit.

Given that the WebSocket implementation will automatically send a reciprocal pong when a ping is received, it is rarely required to send a pong message explicitly when a ping is received.

Return a CompletionStage which will be used by the WebSocket as a signal it may reclaim the ByteBuffer. Do not access the ByteBuffer after this CompletionStage has completed.

Implementation Requirements:

The default implementation is equivalent to:

webSocket.request(1);
return null;

Parameters:

webSocket - the WebSocket on which the message has been received

message - the message

Returns:

a CompletionStage which completes when the ByteBuffer may be reclaimed; or null if it may be reclaimed immediately

onPong

default CompletionStage<?> onPong(WebSocket webSocket, ByteBuffer message)

A Pong message has been received.

As guaranteed by the WebSocket Protocol, the message consists of not more than 125 bytes. These bytes are located from the buffer's position to its limit.

Return a CompletionStage which will be used by the WebSocket as a signal it may reclaim the ByteBuffer. Do not access the ByteBuffer after this CompletionStage has completed.

Implementation Requirements:

The default implementation is equivalent to:

webSocket.request(1);
return null;

Parameters:

webSocket - the WebSocket on which the message has been received

message - the message

Returns:

a CompletionStage which completes when the ByteBuffer may be reclaimed; or null if it may be reclaimed immediately

onClose

default CompletionStage<?> onClose(WebSocket webSocket, int statusCode, String reason)

Receives a Close message indicating the WebSocket's input has been closed.

This is the last invocation from the specified WebSocket. By the time this invocation begins the WebSocket's input will have been closed.

A Close message consists of a status code and a reason for closing. The status code is an integer from the range 1000 <= code <= 65535. The reason is a string which has a UTF-8 representation not longer than 123 bytes.

If the WebSocket's output is not already closed, the CompletionStage returned by this method will be used as an indication that the WebSocket's output may be closed. The WebSocket will close its output at the earliest of completion of the returned CompletionStage or invoking either of the sendClose or abort methods.

API Note:

Returning a CompletionStage that never completes, effectively disables the reciprocating closure of the output.

To specify a custom closure code or reason code the sendClose method may be invoked from inside the onClose invocation:

   public CompletionStage<?> onClose(WebSocket webSocket,
                        int statusCode,
                        String reason) {
    webSocket.sendClose(CUSTOM_STATUS_CODE, CUSTOM_REASON);
    return new CompletableFuture<Void>();
}

Implementation Requirements:

The default implementation of this method returns null, indicating that the output should be closed immediately.

Parameters:

webSocket - the WebSocket on which the message has been received

statusCode - the status code

reason - the reason

Returns:

a CompletionStage which completes when the WebSocket may be closed; or null if it may be closed immediately

onError

default void onError(WebSocket webSocket, Throwable error)

An error has occurred.

This is the last invocation from the specified WebSocket. By the time this invocation begins both the WebSocket's input and output will have been closed. A WebSocket may invoke this method on the associated listener at any time after it has invoked onOpen, regardless of whether or not any invocations have been requested from the WebSocket.

If an exception is thrown from this method, resulting behavior is undefined.

Parameters:: webSocket - the WebSocket on which the error has occurred; error - the error

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https://docs.oracle.com/en/java/javase/21/docs/api/java.net.http/java/net/http/WebSocket.Listener.html