Getting variable from Java and using it on Javascript on JSP

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In this post we will learn how to use Java's variable on Javascript on JSP

Hello there,

I’m working with a project using JSP nowadays.
That project make uses of a lot of Javascripts calls.
I don't understand why people insists put on JSP: scriptlets, CSS and Javascript at the same page.
I, really, sure… I don't uderstand.

I going to show how we can use Java's variable and use it on Javascript method… at another file.

Look below:

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package dev.artsman.blog;
/* ommited imports */
@WebServlet(name = "HomeController", urlPatterns = "/home")
public class HomeController extends HttpServlet {

  @Override
  protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    req.setAttribute("secretMessage", "Hello World!");
    req.getRequestDispatcher("/secret-message.jsp").forward(req, resp);
  }

}
  1. Line 8, We simplesly have set an atribute secretMessage on request
  2. Line 9, We forward that request/response to our view namming secret-message.jsp
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/* WEB-INF/script.js */
function showSecretMessage() {
  document.getElementById('secret-message').innerHTML = secretMessage
}
  1. This method only insert at element secret-message the content of one variable secretMessage
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<%@ page pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
  <head>
    <title>JSP Tips</title>
    <script type="text/javascript" src="script.js"></script>
  </head>
  <body>
    <h1>Getting message from Java and using on Javascript.</h1>
    <p>Click on button to see the secret message: <span id="secret-message"></span></p>
    <button type="button" onclick="showSecretMessage()">Show me!</button>
    <script type="text/javascript">
      let secretMessage = "${secretMessage}"
    </script>
  </body>
</html>
  1. Line 6, we simplesly make the import of the script.js
  2. Line 10, it’s defined the place for show de message
  3. Line 13, at this point, we make the bind beetween Java’s variable and Javascript!

That allows us do what we need to do on right place:

  • Java's code only in JSP file
  • Javascript's code in JS file
    • We don’t need use var, limit the scope using let!

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Why doing simple things appear so difficult?
xoff.