Saving Images

This java program can save images in JPG, PNG or BMP format, using the "Save Image" option. The default format is PNG. Choose a filename that ends with ".bmp", ".jpg", or ".png" to pick the format. JPG compression sacrifices image quality for small size. BMP preserves the original image quality with no compression. PNG compresses images somewhat less than JPG but retains the original image quality.

To save the images, the java SecurityManager must grant file-writing permission to this program. You can arrange this with the Sun Policy Tool utility (you will need system administrator priveleges). Below is a brief explanation of how to grant file writing permission, but you may also want to check out http://java.sun.com/security/index.html and read about Policy Permissions and the use of the Policy Tool.

The following scenario describes granting file writing permission to a java program on a Windows system. Suppose we have a java app named "SomeJavaApp" which can save image files. The app is packaged in a jar archive file titled "SomeJavaApp.jar." When we run the java app as an application (see javaapp.html for help running java applications) and try to save an image file, we get a permission-denied exception message. The default behavior of the java Security Manager denies java programs the permission to write files to the local file system in order to prevent hostile apps from doing harm. In order to change this default behavior, we will use the Sun Policy Tool to grant our java app permission to write image files to our system.

First of all, you'll need to locate the policytool.exe file in order to launch it. Many Windows systems will have more than one policytool.exe file, so be sure to find the one that comes with your java runtime software, which should have a location resembling :
C:\Program Files\Java\jre6\bin
Dreamweaver, Adobe Designer and other programs also come with a policytool.exe file which we won't use in this example.

The Policy Tool will enable us to grant this file-writing permission to a CodeBase (a location where java software resides), and to specify a Target Name (which restricts which files may be written by the java software in the CodeBase). For this example, assume the "SomeJavaApp.jar" file is located in the directory "C:\SomeJavaApp\", residing just below the C: drive. Run the Sun policytool program by typing "policytool" on the command line (or by double-clicking the policytool.exe file).



Once the policytool window is up, you will see the Policy File field which shows the user policy file. If the field is blank, you will be notified that the default user policy file was not found. Remember the default location where the file wasn't found, because you'll need to save a new policy file in that location. You do this by using the menu item File->Save As to save a file named ".java.policy" in the default directory (or you can try saving it in the Windows directory if you can't remember the default location).

Now add a policy : press Add Policy and the Policy Entry window will appear.



The CodeBase field holds the URL of the CodeBase to which we grant a permission. Our CodeBase is the jar archive file in our directory. Since the URL is a local file, you would enter it as follows in the CodeBase field :

 file:/C:/SomeJavaApp/SomeJavaApp.jar

The only item provided in the Target Name dropdown box is the special token "<<ALL FILES>>". If you choose this as a target, the java software in the CodeBase will be permitted to write to any file in any location! You can restrict write permission to a single file by typing the path to that file in the target Name field. In our case, we might want to save multiple files, but restrict the writing to a single directory. We can use wildcard syntax (see the Sun documentation) to target all files in a single directory. In the target Name field, you would type in :

C:\SomeJavaApp\*

Press "OK" and "Done" to accept the entries made in the Permission and Policy Entry windows. Then be sure to save the changes to the policy file using File->Save. Hopefully it worked, but if not, make sure you properly saved the user policy file and typed everything in correctly. You can edit or remove the policies in the policy file as needed with the Policy Tool.