尝试显示消息框时，NetBeans IDE 6.9.1中的Glassfish服务器停止工作

Question

I'm working with JSF web applications. Since Java doesn't support a direct way to display a message box in web applications, I decided to try something new and thought that the methods which are used to display a message box in conventional desktop applications should be tried out. For the sake of simplicity, I have tried the following method in Java Servlet.

JOptionPane.showMessageDialog(null, "A message from Java Servlet.");

without expecting that it would work but it indeed worked and a message appeared on the web browser! I felt that I had found something new but also found that the Glassfish server in my NetBeans 6.9.1 had stopped working in two or three attempts. It turned off with no warning , no error and no exception at all!

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The screen shot can be seen below.

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Now, if the use of this method in Java web is illegal, it should not be allowed to use this method at all in Java web and some errors or exceptions should be raised when an attempt is made to do so. It should be a compile-time error and if it is so then, how did the message appear through Java Servlet on the web browser? and also, Why did the Glassfish server stop working?

Answer 1

I have no idea why Glassfish stopped, I guess that it's been caused by dangling Swing/AWT threads in the background. It has always been an extremely bad idea to manually spawn (unmanaged) threads in a lifelong running Java EE webapplication. Once the request finishes, you loses the control. You'd have to collect references to the threads in the session or application scope, preferably in a pool which get properly cleaned up on session or application destroy/shutdown.

But that message is actually not displayed by the webbowser, but by the webserver. This means that your intention will only work when both the webserver and webbrowser runs at physically the same machine. This does obviously not happen when you publish your website into a production environment on the world wide web which get visited by clients using physically different machines.

All the webbrowser retrieves and executes is just plain HTML/CSS/JavaScript. In JSF, you need JavaScript's alert() , confirm() , etc or just some absolutely positioned <div> in an overlay if you want a more fine grained control over the markup and look'n'feel and/or executing managed bean actions. The latter is available in flavor of a single JSF component by a lot of 3rd party component libraries, such as PrimeFaces with <p:dialog> and <p:confirmDialog> and RichFaces with <rich:popupPanel> .

If you really need to run Swing/AWT in a web application for some reason, then you should rather be creating an applet instead and ultimately embed it in your web page by HTML <applet> or <object> tag. This will be downloaded from the server to the client and executed in the client's environment.