This magercise demonstrates the creation of a custom security manager. As you cannot replace the security manager within an applet,
this exercise will create an application. The security manager does not permit the application from reading or writing files from disk,
unless the filename ends with the extension .tmp.
When creating your own security manager, you have to subclass SecurityManager. The java.lang.SecurityManager implementation
throws security exceptions when attempting any operation. So, instead of subclassing, overriding everything, and doing something
special for the methods related to file access, you'll create a null security manager that overrides everything and doesn't throw any
security exceptions. This involves compiling the
NullSecurityManager.java file provided.
Next, save the
MySecurityManager.java skeleton. This is an empty subclass of the
NullSecurityManager just created. You will need to fill in the checkRead and checkWrite methods next as they are
the methods used to restrict file access.
Starting with checkWrite, there are two varieties to look into: public void checkWrite(FileDescriptor fd) and
public void checkWrite(String filename). With the FileDescriptor variety, you do not have to do anything. When
creating an object that has a method that returns a FileDescriptor, the object will do the appropriate security checks
before a FileDescriptor is available.
So, this leaves you with overriding public void checkWrite(String filename). If the filename provided in the method
does not end with the file extension .tmp, throw a SecurityException.
There are three varieties of checkRead to worry about: public void checkRead(FileDescriptor fd)
public void checkRead(String filename), and public void checkRead(String filename, Object context). As with
checkWrite, you can ignore the FileDescriptor variety. For the two argument variety, this acts like a proxy where the
context parameter is the results of a previous call to SecurityManager.getSecurityContext. This allows one object to check
access for another object, like in the case of RMI, where the other object could reside with another virtual machine. In this magercise's
particular case, the context is irrelevant, only the filename is important. So, just have the two argument filename variety call the one
argument filename variety.
For the public void checkRead(String filename) variety, have the method throw a SecurityException if the filename
parameter doesn't end with .tmp.
That's all there is to MySecurityManager. Compile the class now.
The
SMTester.java program tests out the new security manager. The program includes tests for reading and
writing the files joe.txt and joe.tmp, before and after a security manager is installed. It is your job to install
the security manager previously defined.
Compile and run the program. The program doesn't read/write to any files. Instead, it uses File.exist and
File.canWrite to see if it has permissions to read or write.
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