Computers as an Expressive Medium

1
Computers as an Expressive Medium

• Lab 8 jar signing, debugging
• Mayhew Seavey

2
This weeks episode

• jar signing Not just for pottery celebrities
  anymore.
• Debugging Not just for exterminators anymore.

3
This weeks episode

• jar signing Not just for pottery celebrities
  anymore.
• Debugging Not just for exterminators anymore.
• And MORE crappy puns!

4
Using Web crawling on Courseware

• Processing window extends BApplet. You can view
  Applets on the web, which is why they work
  (usually) seamlessly with the Courseware.
• HOWEVER, browsers have security problems when a
  Java applet tries to access other files at
  different sites.

5
Using Web crawling on Courseware

• For instance, this call works both in Processing
  and on the Courseware (provided image1.jpg is in
  your data directory)
• BImage im loadImage(image1.jpg)
• But this call works only in the Processing
  program, not online
• BImage im loadImage(http//idt.gatech.edu/msea
  vey/image1.jpg)

6
Signing your jar file

• To allow browsers to control the security of your
  applet, you must sign it first.
• Two steps
• Create a keystore file.
• Sign your jar file.
• Before you can do this, you must either
• upload your jar file to your Steel account.
• For the following commands, use a shell prompt.
• install the Java SDK on your machine.
• get to a command line in the directory of your
  jar.

7
Signing your jar file

• Create a keystore file. Run the following
  command
• keytool -genkey -alias -keystore
  
• The -keystore part is optional. It
  allows you to specify where the .keystore file
  will go. If you dont specify, it will place it
  in your default system path. This should be fine
  for most applications.

8
Signing your jar file

• After you run keytool, it will prompt you for
  some things
• keystore password this is used when signing your
  jars.
• Name, Organizational Unit, blah blah not very
  important, just provides viewers information on
  who theyre trusting.
• key password also used when signing your jars.
  Not a problem to make it the same as the keystore
  password.

9
Signing your jar file

• Sign your jar file. Run the following command
• jarsigner -keystore
• is just the file name of the jar.
• just needs to match the alias you used
  for the keystore.
• -keystore is also optional. The
  default will be the same location as keytool.
• You will be prompted for your keystore password.

10
Viewing the jar file

• The jar file is signed! Upload it to the
  courseware like you would normally.
• When anyone loads the applet, a window will come
  up asking if they accept the certificate. Just
  click Yes, and the applet will load.

11
Debugging

• Stack Trace The indecipherable lines of red that
  pop up when an error occurs.
• This shows what methods were running in what
  classes when the program broke, as well as the
  line numbers in their java files.

12
Debugging

• Common errors, and what they mean
• ArrayOutOfBoundsException
• You are trying to access an index in an array
  that doesnt have that index. For example,
  thisArray-1 or thisArray6 in an array with
  length 6 (max index of 5). Check to make sure any
  for loops have the right conditions.

13
Debugging

• Common errors, and what they mean
• NullPointerException
• You are trying to access an object that hasnt
  been initialized, or is set to null. Remember
  that in addition to declaring a variable (telling
  the program this variable exists, has this type,
  and this name), you must initialize the variable
  (give it a value or point it at an object).
• String myString // declares String variable.
• myString new String() // sets variable to
  point to a String object.

14
Debugging Tips

• Split up tasks into smaller methods. Processing
  will tell you which method the error occurred in,
  so the smaller they are, the closer you can
  pinpoint it.
• Processing also gives you a line number. Not as
  useful when it doesnt give you a way to scroll
  to that line, but you can copy and paste your
  code into another editor (NetBeans, even
  Dreamweaver) that does have a go to line
  function.

15
Debugging Tips

• Use println intelligently to try and figure out
  whats wrong. You can use this to keep track of
  what variables are at different points in the
  program, or even just to tell you what methods
  the program is going into. Doing this can help
  give a sense as to the flow of what is happening
  in the program.

16
End of Slides

• Lets do some more hands-on debugging stuff.
• Betcha thought itd be a Farscape picture this
  week, huh?

17
End of Slides

• Lets do some more hands-on debugging stuff.
• Betcha thought itd be a Farscape picture this
  week, huh?