1) How can I download a file (for instance, a Microsoft Word document) from a server using a servlet and an applet?
Try telling the applet to call getAppletContext().showDocument("http://foo.com/whatever/blah.doc"). That makes the browser responsible for fetching and saving the document (including putting up the "Save As" dialog).
You may also want to check the web server configuration to make sure the approprate MIME-type configuration is set. If your servlet is the one fetching the document (or creating it on the fly), then it must set the MIME-type using response.setContentType("application/ms-word") or equivalent.
2) How do I get an applet to load a new page into the browser?
In short, get the applet's context and tell it to show a different document: getAppletContext().showDocument(new URL("http://foo.com/whatever/blah.doc")). You can also provide a target frame string as in getAppletContext().showDocument(new URL("http://foo.com/whatever/blah.doc"), "target"). There are four special targets strings. Any other string will act as a named top-level window, if the frame doesn't exist.
"_self" Show in same frame as applet
"_parent" Show in the applet's parent frame
"_top" Show in the top-level frame of the applet's window
"_blank" Show in a new, unnamed top-level window
3) What is the syntax of the
Support for OBJECT is limited.
4) How do I create an applet that accepts an unlimited number of parameters?
While there is some upper bounds to the number of parameters the browser will let you specify, if you would like to let a user specify an undefined number of values, you can let them specify a parameter with a number at the end and keep counting until no more parameters following that pattern are there, as in:
To do this, you just increment a counter for the getParameter() calls:
import java.applet.*;
public class Params extends Applet {
public void init() {
String value;
int i = 0;
while ((value = getParameter("Param" + i)) != null) {
System.out.println ("Param" + i + ": " + value);
i++;
}
}
}
5) How can I connect from an applet to a database on the server?
There are two ways of connecting to a database on the server side.
1. The hard way. Untrusted applets cannot touch the hard disk of a computer. Thus, your applet cannot use native or other local files (such as JDBC database drivers) on your hard drive. The first alternative solution is to create a digitally signed applet which may use locally installed JDBC drivers, able to connect directly to the database on the server side.
2. The easy way. Untrusted applets may only open a network connection to the server from which they were downloaded. Thus, you must place a database listener (either the database itself, or a middleware server) on the server node from which the applet was downloaded. The applet would open a socket connection to the middleware server, located on the same computer node as the webserver from which the applet was downloaded. The middleware server is used as a mediator, connecting to and extract data from the database.
6) How do I display a message in the browser's status bar?
To show a message in the status bar, you would call the showStatus() method of the AppletContext:
getAppletContext().showStatus(message);
Keep in mind that the browser can overwrite the message at any time or the browser might not have a visible status bar. This basically means you should not use the status bar for messages that might be missed.
7) What are the main differences between applets and applications?
Applets are created to be embedded within a browser. As such, they have methods that are automatically called by the browser at certain times: init() when first loaded, start() when page loaded/entered, stop() when page left, destroy() when applet unloaded, and paint() when the screen needs to be displayed. In addition, applets have severe security restrictions as they can be loaded from untrusted sources over the Internet. Applications on the other hand need to be installed locally, start execution with the main() method, and have no inherent security restrictions. Beyond this, applications control their own operations.
8) How can I get the real local host IP address in an applet?
Applet security restrictions do not let you get this in an untrusted applet via InetAddress.getLocalHost().
However, you can get this address by creating a Socket connection back to the web server from which you came and asking the Socket for the local address:
URL url = getDocumentBase();
String host = url.getHost();
Socket socket = new Socket(host, 80);
InetAddress addr = socket.getLocalAddress();
String hostAddr = addr.getHostAddress();
System.out.println("Addr: " + hostAddr);
9) How do I load a serialized applet?
Instead of specifying the CODE attribute of an
This allows you to preinitialize your applet to some saved state, instead of having to reinitialize your applet each time it is loaded.
Support for this in browsers is severly limited. Don't rely on it.
10) From one applet, how do I communicate with another applet loaded from the same server?
It really doesn't matter that they are loaded from the same server, as long as they are running in the same JVM in the browser, you can call methods between them.
If you include a name="appletA" value in the applet tag of the applet you want to call methods on, you get a reference to it by using the AppletContext:
MyApplet appl = (MyApplet) getAppletContext().getApplet("appletA");
if (null != appl) {
appl.register("Hello there!", this);
}
Checking that you get a reference to the class you expect is left as an exercize to the reader :-)
1) In an applet, is the start() or paint() method executed first?
The initial order in which the lifecycle methods of applets are executed when loaded is:
init()
start()
paint()
And when done:
stop()
destroy()
This also holds when you call repaint() in the init() method. However, the init() method will not be called if you load a serialized applet.
2) What are the main differences between JavaBeans and applets?
Applets are Java programs which are meant to be run by an applet-runner (which is typically inside a web browser). By definition, applets are applets because they inherit from java.applet.Applet. Also, applets are pretty constrained in what they are/aren't allowed to do.
Traditional JavaBeans are Java software components. A bean may be a single, simple, standalone class or it may be a complicated behemoth but, as long as it follows the rules and conventions of JavaBeans, then it's a JavaBean.
3) How do I use an applet that resides on a remote machine?
You would specify the location of the class files in the CODEBASE attribute of the
4) When you communicate with a servlet from an Applet or an application, how can you ensure that the session information is preserved? That is, how do you manage cookies in applet-servlet communication?
For sessions, your best bet is to have the servlet rewrite the URLs to include the session information. With cookies read I am writing an application that retrieves a URL, but that URL requires a cookie. Is there a way to hold that cookie within my application and send it back to the URL when requested? Essentially, you can send the cookie back easily with HTTP. For instance, urlConnection.setRequestProperty("cookie",_cookie); Where the _cookie is coming from a previous call: String _cookie = urlConnection.getHeaderField("set-cookie");
5) How can I change my application into applet?
Create a subclass of java.applet.Applet
Take the code in the main() method of the application and move it to the init() method of the applet
Create an HTML file to load the applet
If necessary, move any requirements that untrusted applets cannot do into an appropriate server-side task
If necessary, modify all getting of support files
Test
6) In detail, how do the applet life cycle methods init(), start(), paint(), stop(), and destroy() work?
The life cycle methods of an applet are init(), start(), stop(), and destroy(). While frequently grouped with the methods, the paint() method is not really part of the applet's life cycle. The method is just called by the browser whenever a part of the screen has become invalidated or in response to a programmer-generated repaint request. The update() method falls into the same category as paint().
As far as the other methods go:
init() - called once when applet is loaded
start() - called every time the page the applet is located on is loaded, after init() or after leaving the page and returning
stop() - called every time the page the applet is located on is left
destroy() - called when applet is unloaded
If loading a serialized version of an applet from a .ser file (OBJECT attribute in
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