waiting for an external response

Flow
1

Hello, i am trying to make a java application where the application sends a message to a database and then waits for a new item in the database. I tried doing a while(true) loop however that did not work. I have looked into threads but I am left very confused. How can I call a function that updates a message list when a function finds that a new item has been added to the database?
Thank you

2

Before I begin, I advise that you learn more about threads (and maybe Java itself) before delving into a framework such as Vaadin. There are probably a billion Java tutorials online, so I won’t link any here.

The following is a really bad approach to continuously query the DB for changes, so please allow others to offer you better solutions. Here’s a good place to start…

How to create a thread:

new Thread(() -> {
  SOME THREAD CODE
}).start();

Vaadin has it’s own UI thread, which must be accessed from within other threads with UI#access. You can read more about this here: https://vaadin.com/docs/latest/advanced/server-push. You will have to get the current UI instance from outside your thread:

UI ui = UI.getCurrent();

new Thread(() -> {
  ui.access(() -> {
    UPDATE UI
  });
}).start();

So, you could do something like the following:

UI ui = UI.getCurrent();

new Thread(() -> {
  while(true) {
    QUERY DB

    ui.access(() -> {
      UPDATE UI
    });

    try {
      Thread.sleep(5000);
    } catch(InterruptedException e) {
      e.printStackTrace();
      
      Thread.currentThread().interrupt();
    }
  }
}).start();
3

thank you so much :slightly_smiling_face:

4

For periodically executed tasks I would use a ScheduledExecutorService instead of manually create a thread and put it to sleep

5