Answers to Questions and Exercises: Java Web Start

Questions

1. Question: In a link that is to run a Java Web Start application, which file is specified as the a tag's href attribute?

   Answer: You use the application's JNLP file name as the value of the href attribute. When a user clicks the link to the JNLP file, Java Web Start loads the application specified by that JNLP file.

2. Question: Which MIME type must a Web server recognize in order for it to host Java Web Start applications?

   Answer: You must configure the Web server so that files with the .jnlp extension are set to the application/x-java-jnlp-file MIME type.

3. Question: In an application's JNLP file, which two elements must be specified within the resources element?

   Answer: The resources element must contain:

   • The j2se element, which specifies the Java platform on which to run the application.
   • The jar element, which specifies the JAR file for the application.

4. Question: Which interface provides the ability to an application to control how its own resources are cached?

   1. BasicService
   2. DownloadService
   3. PersistenceService
   4. ExtendedService

   Answer: B. The DownloadService interface provides the ability to an application to control how its own resources are cached.

5. Question: True or False: Java Web Start applications run in a secure sandbox by default.

   Answer: True.

6. Question: True or False: If a Java Web Start application is running in a secure sandbox, JAR files for the application can reside on different servers.

   Answer: False. All JAR files for the application must reside on the same server.

7. Question: For a Java Web Start application to support operations outside of the secure sandbox, what must you do?

   Answer: You must include the security element in your .jnlp file and specify all-permissions to enable your application to work outside of the sandbox.

Exercises

1. Exercise: Write the XML code you would add to a JNLP file in order to request that the application have complete access to the client system.

   Answer:

   <security>
      <all-permissions/>
   </security>
   

2. Exercise: For a Java Web Start application, you have two icons, one.gif and two.gif, in the images directory in a JAR file. Write the application code you would use to access these images.

   Answer:

   // Get current classloader
   ClassLoader cl = this.getClass().getClassLoader();
   // Create icons
   Icon firstIcon  = new ImageIcon(cl.getResource("images/one.gif"));
   Icon secondIcon   = new ImageIcon(cl.getResource("images/two.gif"));