In web application development (which is the usual scenario where Java and JavaScript interact):
there are 2 places where you can make the server and client side languages interact with each other:
At rendering time, this is, when generating the HTML output to the user.
At runtime, this is, when the HTML has been generated and it is at client side (e.g. browser client).
For the first case, let's say we have a Servlet that adds an attribute to the request and forward to a new (JSP) view. This is an example of the servlet:
@WebServlet("/hello")
public class HelloServlet extends HttpServlet {
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String name = request.getParameter("name");
// Prepare messages.
request.setAttribute("name", name);
request.getRequestDispatcher("/WEB-INF/hello.jsp").forward(request, response);
}
}
Then in your view (hello.jsp file inside WEB-INF folder), you have a code like this:
<!DOCTYPE html>
<html lang="en">
<head>
<title>Servlet Hello World</title>
<script>
function changeName(var newName) {
document.getElementById('divName').innerHtml = 'Hello ' + newName;
}
window.onload = function () {
changeName('${name}');
}
</script>
</head>
<body>
<div id="divName">
</div>
<form id="name" action="hello">
Change name to: <input type="text" name="name" />
<br />
<input type="text" value="Change the name" />
</form>
</body>
</html>
By using this approach, we can communicate Java generates variables (NEVER SCRIPTLETS) directly to JavaScript when rendering the view. So we can access to Java page, request, session and context attributes only when creating the HTML that is going to the client.
Note that if you want to re-execute this Java code, you should fire a new request to the server in order to execute the Java code. This is where ajax come handy.
Ajax lets you communicate with the server side asynchronously, the server will prepare a response for your request, and then in client side you will define how to use it. For this purpose, it is better to use a common format for the communication. The most preferred format nowadays is JSON. Ajax interactions to servlet is widely covered here: How to use Servlets and Ajax? (no need to reinvent the wheel in this post).
In standalone or mobile applications:
Java will run on the client machine. Note that here Java can execute JavaScript code through a JavaScript engine like Rhino or nashorn. This is useful when you have lot of functionalities already written in javascript (like an external library) and do not want to migrate all the code to Java. You can just use ScriptEngineManager and StringEngine classes to execute the code in JavaScript in your Java application. A real-world example of using JavaScript in a Java application is Pentaho Kettle, written in Java, that allows applying transformations to your code through JavaScript scripts.
More info on Nashorn:
