Applet - Servlet Communication

I have abandoned my earlier quest to make the applet communicate directly with the database, even though users and webpages have said that it's possible. I am now trying to get my applet to pass information (String and boolean format) entered in textfields or indicated by checkboxes, and give this to the servlet, which then stores it appropriately in the database. I've got the applet front开发者_如何学编程 end - the GUI - built, and the servlet - database connection also built. The only problem is the link between the two, applet and servlet. How would one pass String data from an applet to a servlet?

Thanks, Joseph G.

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First up, you have to acknowledge that you can only communicate with the server from where your applet was downloaded from, that includes the port number, unless you want to mess around with permissions, applet signing and all that malarky. This also isn't just an Applet restriction, the same applies to Flash and JavaScript (though in the case of JavaScript there are tricks to get around it).

Using either the "getCodeBase()" or "getDocumentBase()" method on your Applet will get you a URL from which you can get the component parts required to build a new URL that will let you call a servlet.

Thus, your Applet must be being served from the same server that your servlet is hosted on.

e.g. if your Applet is in the following page:

http://www.example.com/myapplet.html

...it means you can make calls to any URL that starts with

http://www.example.com/

...relatively easily.

The following is a crude, untested, example showing how to call a Servlet. This assumes that this snippet of code is being called from within an instance of Applet.

URL codeBase = getCodeBase();
URL servletURL = new URL(codeBase.getProtocol(), codeBase.getHost(), codeBase.getPort(), "/myServlet");

// assumes protocol is http, could be https
HttpURLConnection conn = (HttpURLConnection)servletURL.openConnection();
conn.setDoOutput(true);
conn.setRequestMethod("POST");

PrintWriter out = new PrintWriter(conn.openOutputStream());
out.println("hello world");
out.close();

System.out.println(conn.getResponseCode());

Then in your servlet, you can get the text sent by overriding doPost() and reading the input stream from the request (no exception handling shown and only reads first line of input):

public void doPost(HttpServletRequest req, HttpServletResponse res) {

   BufferedReader reader = req.getReader();
   String line = reader.readLine();
   System.out.println("servlet received text: " + line);

}

Of course, that's just one approach. You could also take your inputs and build up a query string like this (URLEncoding not shown):

String queryString = "inputa=" + view.getInputA() + "&inputb=" + view.getInputB();

and append that to your URL:

URL servletURL = new URL(codeBase.getProtocol(), codeBase.getHost(), codeBase.getPort(), "/myServlet?" + queryString);

However, it seems fairly common to build up some kind of string and stream it to the servlet instead these days.

A recommended format would be JSON as it's semi-structured, while being easy to read and there are plenty of (de)serializers around that should work in your Applet and in your servlet. This means you can have a nice object model for your data which you could share between your Applet and Servlet. Building up a query string of complex inputs can be a mind bender.

Likewise, you could actually use Java serialisation and stream binary to your Servlet which then uses Java serialisation to create the appropriate Java objects. However, if you stick to something like JSON, it'll mean your servlet is more open to re-use since Java serialisation has never been implemented outside of Java (that I am aware of).

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Hm, I guess the applet and the servlet run in two separate Java processes. In that case you'll have to use some remoting technology, e.g. an http call to localhost. In fact, that is what servlets are mainly used and implemented for: Accept and process http requests.