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Trail: Essential Java Classes
Lesson: Accessing System Resources

Installing Your Security Manager

Once you've completed writing your SecurityManager subclass, you can install it as the current security manager for your Java application. You do this with the setSecurityManager() method from the System class.

Here's a small test application, SecurityManagerTest (in a .java source file), that installs the PasswordSecurityManager class from the previous page as the current security manager. Then to verify that the security manager is in place and operational, the SecurityManagerTest application opens two files--one for reading and one for writing--and copies the contents of the first file into the second.

The main() method begins by installing a new security manager:

try {
    System.setSecurityManager(new PasswordSecurityManager("Booga Booga"));
} catch (SecurityException se) {
    System.out.println("SecurityManager already set!");
The bold line in the previous code snippet creates a new instance of the PasswordSecurityManager class with the password "Booga Booga". This instance is passed to System's setSecurityManager() method, which installs the object as the current security manager for the running application. This security manager will remain in effect for the duration of the execution of this application.

You can set the security manager for your application only once. In other words, your Java application can invoke System.setSecurityManager() only one time during its lifetime. Any subsequent attempt to install a security manager within a Java application will result in a SecurityException.

The rest of the program copies the contents of this file inputtext.txt into an output file named outputtext.txt. This is a simple test to verify that the PasswordSecurityManager has been properly installed.

try {
    DataInputStream fis = new DataInputStream(
                                  new FileInputStream("inputtext.txt"));
    DataOutputStream fos = new DataOutputStream(
                                   new FileOutputStream("outputtext.txt"));
    String inputString;
    while ((inputString = fis.readLine()) != null) {
} catch (IOException ioe) {
    System.err.println("I/O failed for SecurityManagerTest.");
The bold lines in the previous code snippet are restricted file system accesses. These method calls will result in a call to PasswordSecurityManager's checkAccess() method.

Running the Test Program

When you run the SecurityManagerTest application, you are prompted twice for a password: once when the application opens the input file and once when the application opens the output file. If you type in the correct password, the access is granted--the file object--and the application proceeds to the next statement. If you type in an incorrect password, checkXXX() throws a SecurityException, which the test application makes no attempt to catch so the application terminates.

This is an example of the output from the application when you type in the password correctly the first time, but incorrectly the second:

What's the secret password?
Booga Booga
What's the secret password?
Wrong password
java.lang.SecurityException: Not Even!
  at PasswordSecurityManager.checkWrite(
  at SecurityManagerTest.main(
Notice that the error message that the application prints is the error message for the checkWrite(String) method.

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