Applet : Java Glossary

Applet Difficulties

Applets are harder to write than applications. I would recommend that beginners start with applications. With Applets you have the following complications:

1. Your entire logic must fit in the straight jacket of four methods init, start, stop and destroy.
2. Unsigned (ordinary) Applets are restricted in hundreds of not-well-documented ways. They are not even allowed to read files.
3. An Applet must run inside a browser. This adds an extra level of complication and uncertainty as different browsers have different bugs.
4. Unless you spin off a separate Thread, an Applet has no mechanism to do any calculation that takes more than a fraction of a second.

The easiest way to write an Applet is to start with a working one, perhaps one of mine. Write it as a hybrid, and debug it as an application. Once you have it working as an application, test it as an Applet in a browser. If you have an IDE (Integrated Development Environment) like Intellij Idea, you can run it in the IDE as an Applet as an intermediate phase between testing as an application and as an Applet in a browser.

Unsigned Applet Restrictions

• talk to any server but the one you were loaded from. No talking to strangers. If they were loaded from a local disk, they can’t talk to any webserver at all. They can’t load classes, read files, or have socket communications with any webserver but mom, the server htey were loaded from. Applets can send/receive email to/from a different host, but only via the home host.
• I have discovered by experiment that when an Applet runs locally, it is only allowed to read files in the same directory or in a subdirectory of that directory. It can’t read files in the parents or sibling directories, just child/grandchild directories. Oddly, it does not matter where the jar itself is, but where the web page is that has the <applet tag. The files the Applet reads have to be downstream of that page. You will notice multiple snippet subdirectories all over my website. This is because Applets could not get at a central snippet directory. The place where I put the code snippets for a given webpage has to be in a branch off the directory containing the web page. My only other alternative would be to always put the code snippets in the same directory as the web page. I am not sure if the same restriction applies to Applets run on a webserver.
• I have discovered by experiment that when an Applet runs from a website, it is only allowed to read files in the same directory as the web page or in a subdirectory of that directory, (downstream from the webpage). It can’t read files in the parents or sibling directories, just child/grandchild directories. It does not matter where the jar is, just the web page and the files that you try to read. This means unsigned Applets usually end up bundling everything they need to read as resources in the jar.
• I have not experimented with https:. Please let me know if you can use https in an unsigned Applet and if you can mix http: to fetch the Applet jar and http: to fetch the data.
• read or write local files on the client machine. They are not even allowed to look in a directory or test for the existence of a file. You can however read a file (resource) embedded in the jar with Class. getResource or Class.getResourceAsStream. It can also read from the server via a URL. Even when your Applet is loaded from hard disk, it still may not do file I/O even via file: URLs (Uniform Resource Locators).
• Use System.setOut or System. setErr to redirect the console.
• Look at the restricted system properties, or get an enumeration of all the system properties. See Wassup. Don’t use Color. getColor which interrogates the system properties.
• Monitor mouse motion.
• Print in Java version 1.1, though you can in JDK (Java Development Kit) 1,2+. The user has to ok the printing though.
• Send email to a server other than the one it was loaded from.
• Talk to a serial port.
• Talk to a parallel port.
• Change thread priority, even lower it.
• Read or write the clipboard. However, users can manually cut/paste your AWT (Advanced Windowing Toolkit) components without security clearance. The restriction is on programmatic control of the clipboard. Swing components have no cut/paste ability at all.
• Install software.
• Execute any native code. If you use native JNI (Java Native Interface) code, things get really complicated vis a vis security and signed Applets with native code. You are better off to use Java Web Start.
  Java Web Start
  JNI
• Exec external programs.
• Issue an RMI (Remote Method Invocation) call to a remote object running on a different server than the Applet’s.
• Determine the structure of an object (fields, methods, values, etc.) using the Reflection API (Application Programming Interface).
• Use the Preferences API to store or look at configuration information in the registry.
• You can use Properties.load so long as you read the resource with MyClass.class. getResourceAsStream

If your Applet needs to do any of the above things, it must be signed.

Now that IE (Internet Explorer) has dropped Java, it is reasonable to ask your clients to upgrade to the latest Java, understanding that some will not. For them, you must develop or at least test on earlier versions. Don’t just assume that just because you did not use any new features that you are home free.

Standalone Java applications are not so limited. Exactly what the limitations are is controlled by the security manager in the browser. If the user installs an alternate security class, the Applet may have more powers. The security restrictions are controlled entirely by the browser. There is nothing to stop you from writing a non-conforming browser that has quite different security restrictions for Applets. However, in practice, you write code to sign your jars and bypass each of the five different security schemes used in browsers.

Programmers bitterly complain about these restrictions. The restrictions protect the end user from malicious web Applets they might encounter on the web. Without such protection, vicious Applets could destroy the user’s hard disk, print reams of paper, phone out on a spare serial port and rack up long distance bills, go sniffing on the LAN (Local Area Network) for the company books… You don’t want to give those powers to psychotic strangers — those same people who stay up late at night writing viruses.

Applet Gotchas

• When you use a browser to host your Applets, the JVM (Java Virtual Machine) is started once, when you run your first Applet. All Applets run in the same JVM. They even share the same Thread. You must be careful in your Applet not to hog the CPU (Central Processing Unit), otherwise you will throttle other Applets. This loading strategy is why the first Applet takes longer to start. In contrast, when you run applications, each get their own logically distinct JVM, but they share DLLs (Dynamic Link Libraries) (in Windows).
• Don’t try to make Applets work on your website with naked class files. Always bundle everything, classes, gifs, configuration scripts etc. up into a jar. This bypasses many classpath headaches, and it downloads faster. Further, the chances are much better your Applet will work first time on the website once you get it working locally out of a jar.
• Applets are a pain in the behind. If you possibly can, use Java Web Start instead.
• Don’t try to use any of the Applet methods in your constructor such as getCodeBase. They depend on a private field called appletStub that does not get initialised until just before your init method is called. You will get NullPointerExceptions if you fail to heed this advice.
• Unsigned Applets are extremely restricted in what they can do. Carefully review the list above. In particular, Applets cannot write to the local hard disk or talk to a server other than the one they were loaded from.
• AppletContext.getApplets will get all the Applets on the page, not just other instances of the current class. Further, sometimes this instance is included, sometimes it is not. Don’t count on it working either way.

• Normally, all applets run in the same JVM instance. This allows instances of an Applet on a page to share information via static variables and calling each other’s instance methodse. As of JRE1.6.0_10 you can no longer count on this. The plug-in may run Applets in separate JVMs (Java Virtual Machines). Applets in different JVMs will not be visible to each other when calling AppletContext.getApplets() even if they are within the same document, which would seem to contradict the Java API:

  Oracle’s Javadoc on getApplets class : available:

  • on the web at Oracle.com
  • in the current JDK 1.7.0_17 or in the old JDK 1.6.0_45 on your local Windows J: drive.

  Apparently it is possible to encourage the plug-in to start applets in the same VM if the <applet> tag has a 'java_arguments' param with an identical set of arguments, regardless of what these options actually specify. However, it is explicitly not guaranteed that this will work. Also, according to Oracle

  There is no way to name a JVM instance used to launch a particular applet and forcesubsequent applets into that JVM instance. Phhht!

Windows and Variable Resolution

Windows has a feature whereby the user can increase the size of the default system font. Let’s say you increase it to 125%. Now, when you use an Applet in Internet Explorer, IE will give you Applet region to paint 25% bigger than you asked for on your <applet tag. Windows/IE is inviting you as an Applet programmer to use a 25% bigger font inside your Applet and to magnify your images by 25% to make them easier for the user to see. If you ignore the extra space, your Applet will appear with small fonts, small icons, and a big wide empty border.

HTML (Hypertext Markup Language) to Invoke an Applet

Here is tip to find the optimal size parameters. Run the Applet as an application. Drag the frame to the optimum size. Use Paint Shop Pro to capture a screen snapshot of the frame, excluding the menu bar. Then look in the lower right corner of the PSP window (or use Mioplanet Pixel Ruler) to discover the size for the <applet height and width tags. Then measure including the menu bar to discover the size for the Frame. setSize( width, height ) when running as an application. It will need about 24 pixels extra height. This trick saves a lot of guesswork and experimentation to home in on the optimal values.

When debugging Applets, remember to click Shift-Reload in your browser, not just plain Reload to attempt re-running with your new version. Shift-Reload supposedly flushes the cache of class files. This usually does not work. You have to exit the browser and restart. It is a good idea to put something unique in every incarnation of your code so you can tell if you are running the old or new code. I do it by flipping a background colour or setting a micro version number.

You can go crazy debugging Applets because the browser sometimes caches old copies of jars, classes, html… It is best to start each test with at least a fresh loading of your browser, preferably with all its caches deleted using a bat file before each test. I often start different a different browser for each test to get a clean start, e.g. Opera, Firefox, SeaMonkey, Netscape, IE.

It is best to debug as an application, then at the last minute convert to an Applet.

Applet Tags

<applet code="MyApplet.class" width="330" height="240">
</applet>

Make sure you get your <s s ‘s ’s s and >s in the right places.

A more typical Applet invocation might look like this:

<applet
archive="../mypackage.jar"
code="com.mindprod.mypackage.MyClass.class"
width="565" height="46"
alt="Java needed to display this Applet.">
<param name="flavour" value="strawberry">
Java needed to display this Applet.
</applet>

<applet, from a CSS point of view, is just another tag. In your CSS style sheet, you can apply properties to it, adjust the margin spacing, put borders around it… In your HTML markup you can give it a class or id. Unfortunately Mozilla, Netscape and Firefox all have a common bug and don’t display a border around Applets even when you ask them to. Opera and IE work properly.

<OBJECT> and <EMBED>

Starting with JDK 1.4.2, you can arranged that jars be cached at the client site using the EMBED tag cache_archive parameter.

That was the polite version, but here is what I really think of <OBJECT> and <OBJECT> tags. Anti-Java, rather corrupt people launched a successful attempt to derail Java by somehow convincing some weak minded folk at the W3C to deprecate the <applet tags. Browser makers have all sensibly resisted this foul play and all continue to support <applet. The replacement is an utterly preposterous and verbose scheme using <OBJECT> and <OBJECT> tags that works in different ways in different browsers. The scheme is beyond ridiculous. It is so baroque, you can’t even hand code it. You have to use an automated tool to convert <applet tags into pages and pages of gobbledegook. Don’t use them! If you ignore my advice, you might as well add to the flakiness by using the Java Deployment Toolkit to generate the tags with JavaScript.

Tell the W3C (World Wide Web Consortium) and Microsoft to shove their <OBJECT> and <EMBED> and lunatic classids to a foul-smelling place. Please, do not use these tags!

You can use a simple  Jump to Java Button to handle a missing JRE.

Applet ⇒ application: Hybrid Switch Hitters

• Simulate the getParameter method to get at the Applet parameters.
• Adjust the coordinate system so part of the Applet is not hidden under the Frame menu bar. Layout managers deal with this automatically, but if you do absolute positioning, you have to compensate manually..
• It does not simulate getAppletContext. Code defensively to deal with a null return.

There is equivalent code for JApplet.

Switch Hitter: application ⇒ Applet

Transparency

Loading A Web Page

this.getAppletContext().showDocument( url, window );

InterApplet Communication

1. Applet Applet. getAppletContext(). getApplet( receiver )
   that accesses another Applet uniquely identified via a name you assign in the HTML <applet name= tag.
2. Applet Enumeration Applet.getAppletContext().getApplets()
   that gets you a list of all the Applets, of any class, not just yours, running on the page (including yourself).

The browser may decide to kill (forget) Applets not on the current page, and then again it may not. Presumably, if you held onto a reference to offscreen Applets in your onscreen Applet, or in a common doubly linked object, those offscreen objects would have to stay alive.

Applet Classpath

1. If you have any classes not in a package, these non-uniquely named classes could conflict with ones on the end-user’s local hard disk. To avoid this problem, put any classes ever used by Applets in some globally uniquely named package, e.g. com.mindprod.business
2. You, as developer, may forget to include a necessary class in your jar, usually from some peripheral package. It will work fine on your machine, but will fail on the customer’s machine. On your machine, the JVM finds the missing class via the classpath. On the end user’s machine that search fails.
3. You as developer may test an Applet jar, yet actually may be using some fresher classes on your local hard disk. You inadvertently ship stale code.

Applet vs Application

• You can get the browser to render HTML for you.
• You can direct the browser to fetch and display material off the web.
• People run your Applet code as a side effect of viewing a web page. They are much more willing to do that than download and install your code.
• The user automatically gets the latest version of the software.
• There is nothing to install.

• To do anything interesting, you must use signed Applets. There are many different signing schemes to support. Signing is complicated.
• Applets must co-exist with a browser. There is less RAM (Random Access Memory) available to work.
• Applets must co-exist with a variety of browsers and their variety of bugs and lack of support. IE supports only Java 1.14 with no RMI, Swing, Collections etc. It is almost impossible to write code that dodges all the bugs in all the browsers.
• Applets must be loaded over the Internet afresh each time. This can be time consuming for fat ones. Modern browsers now cache Applets, so this is no longer a disadvantage.
• Web Start is designed to distribute and update applications, not Applets.
• To start an Applet, you must first start a browser.
• You can’t use native code compilers on Applets, only applications. This means your speed will be limited to the JVM supported by the browser.
• You may not be able to force the user to update the browser to something recent.

Applet vs Servlet

• With Applets, the user can find out about keying errors right away, not with cryptic messages that appear only after you hit submit and wait several seconds.
• With Applets, data entry can be much more sophisticated, with keystroke validation, code lookup, pattern validation, bounds checking, mandatory field validation, prompting, help etc.
• With Applets, data communications can go on in the background and can be cached or procrastinated giving the illusion of instant response.
• With Applets, you don’t have to fill in a giant form only to discover the server won’t accept it.
• With Applets, you can run your applications on a cheap vanilla HTML server. You don’t need a complex, expensive to maintain, Servlet womb.
• With Applets, you don’t have to resort to dog’s breakfast kludges like Ajax to provide even tiny bits of client side intelligence.
• With Applets, you have have sufficient local intelligence to send much terser binary compressed messages back and forth with the server. You can also exchange serialised Object trees.

• With Applets, a recent Java must be properly installed at the client. This is more difficult to arrange than you might imagine especially with the FUD (Fear Uncertainty and Doubt) from Sun’s competitors spreading misinformation about the danger of using Java when actually it is the safest language going.
• With Applets, you can’t use low cost web designers to help create your applications. With Servlets and a framework markup language the designers don’t have to write Java code, just HTML, peppered with a bit of gibberish.
• With Applets that connect with a database, you need a mixture of client size and server side code. This requires two sets of skills, and keeping server and client size code in sync.
• Applets that do almost anything interesting have to be digitally signed. This is an annoyance to users and frightens them off.

Applet vs WebStart

• Once Java is loaded it will render immediately. It will render inline on your webpage. I use Applets to display prices on this website in the user’s local currency. There my be dozens of tiny applets on a single page. Java Web Start apps take an inordinately long time to launch. One reason is they always launch a private copy of java.exe for each app.
• Applets just run. You don’t need to persuade the user to click something, or give permission. They are embedded in ordinary HTML pages.

• The app does not run inside the browser. The browser cannot interfere. If you test with one browser, your app will work with them all. Applets can be a pain, working in one browser but not another.
• The app does not get downloaded each time, only when it has changed.
• The user can run your app even when disconnected from the Internet.

Splash Logo

Applet Sound

Runtime Parameters

Learning More