In some scenarios, it can be useful to send some kind of health-check request when a connection is first opened. One such scenario might be when using a TCP Failover Client Connection Factory so that we can fail over if the selected server allowed a connection to be opened but reports that it is not healthy.spring-doc.cn

In order to support this feature, add a connectionTest to the client connection factory.spring-doc.cn

/**
 * Set a {@link Predicate} that will be invoked to test a new connection; return true
 * to accept the connection, false the reject.
 * @param connectionTest the predicate.
 * @since 5.3
 */
public void setConnectionTest(@Nullable Predicate<TcpConnectionSupport> connectionTest) {
    this.connectionTest = connectionTest;
}

To test the connection, attach a temporary listener to the connection within the test. If the test fails, the connection is closed and an exception thrown. When used with the TCP Failover Client Connection Factory this triggers trying the next server.spring-doc.cn

Only the first reply from the server will go to the test listener.
Only the first reply from the server will go to the test listener.

In the following example, the server is considered healthy if the server replies PONG when we send PING.spring-doc.cn

Message<String> ping = new GenericMessage<>("PING");
byte[] pong = "PONG".getBytes();
clientFactory.setConnectionTest(conn -> {
    CountDownLatch latch = new CountDownLatch(1);
    AtomicBoolean result = new AtomicBoolean();
    conn.registerTestListener(msg -> {
        if (Arrays.equals(pong, (byte[]) msg.getPayload())) {
            result.set(true);
        }
        latch.countDown();
        return false;
    });
    conn.send(ping);
    try {
        latch.await(10, TimeUnit.SECONDS);
    }
    catch (InterruptedException e) {
        Thread.currentThread().interrupt();
    }
    return result.get();
});