3. Call Java from the page - fakeivchenko/lwjwae GitHub Wiki

A binding makes a Java function callable from JavaScript. In this step, the Save button hands the note to Java, which stores it and answers with a status message.

1. In Main.java, bind a function named saveNote:

   package com.example.notes;
   
   import dev.ivchenko.lwjwae.Application;
   import dev.ivchenko.lwjwae.Window;
   import dev.ivchenko.lwjwae.WindowParameters;
   import java.util.List;
   import java.util.concurrent.CopyOnWriteArrayList;
   
   public class Main {
     private static final List<String> NOTES = new CopyOnWriteArrayList<>();
   
     public static void main(String[] args) {
       Application.launch(WindowParameters.of("Notes", "app/index.html"), Main::setUp);
     }
   
     static void setUp(Window window) {
       window.bind(
           "saveNote",
           note -> {
             NOTES.add(note);
             return "Saved " + NOTES.size() + (NOTES.size() == 1 ? " note" : " notes");
           });
     }
   }

   setUp runs before the page loads, so the page finds saveNote from its first line of script. The handler runs on a virtual thread of its own, so it can block, read files, or call a server without freezing the window.

2. In src/main/resources/app/app.js, call the function when the user clicks Save:

   const note = document.querySelector("#note");
   const status = document.querySelector("#status");
   
   document.querySelector("#save").addEventListener("click", async () => {
     status.textContent = await window.saveNote(note.value);
     note.value = "";
   });

   Every binding is an async function on the page: it returns a promise that resolves to what the Java handler returns.

3. In index.html, load the script at the end of body:

       <script src="app.js"></script>
     </body>

4. Run the application, write a note, and click Save:

   

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