Question

We’re developing an applet and need it to be able to read/write files in the user’s temporary files directory (e.g. C:\Documents and Settings\USERNAME\Local Settings\Temp).

The applet is signed, the user clicks the ‘allow’ option on applet startup, and the Java Control Panel has “Allow user to grant permissions to signed content” and “Allow user to grant permissions to content from an untrusted authority” enabled.

However, on startup, we get a SecurityException:

java.lang.SecurityException: Unable to create temporary file
at java.io.File.checkAndCreate(Unknown Source)
at java.io.File.createTempFile(Unknown Source)
at java.io.File.createTempFile(Unknown Source)
at com.jniwrapper.util.AppletHelper.b(SourceFile:104)
at com.jniwrapper.util.AppletHelper.a(SourceFile:79)
at com.jniwrapper.util.AppletHelper.b(SourceFile:50)
at com.jniwrapper.util.AppletHelper.init(SourceFile:122)
at com.x.Y.init(Y.java:31)
at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Exception: java.lang.SecurityException: Unable to create temporary file

If we edit the java.policy file to grant all to everything then the applet works OK but this is clearly insecure. What minimal permissions must we grant to allow the applet to read/write/create files in the user’s temporary files directory?

Answer 1

Using the policy file is kinda ok for testing but you should not be relying on it for your finished code, especially when granting a file permission, it is dangerous.

To interact with files you need to do the following.

1. Sign your jar – tons of tutorials like this, you can just do a self signed one.

2. Add the file creation code to a privileged block here is an example

   File myFile = (File) AccessController.doPrivileged(new PrivilegedAction() {
   public Object run() 
   {
       return new File("C:\\MyFolder\\MyFile");
   }
   
   });