General Idea

The general idea of Java Security Manager is to restrict what an application can do. This is important as it provides a separate environment for programs to do whatever they do without affecting the host system or other applications.

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For instance, we only allow application MyApp to read and write files in some directory.

Here are some useful links:

Through the Security Manager, the applications are controlled with security policies ( .policy files).

Consider this source code file.

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package com.turreta.securitymanager;

import java.io.File;

public class SecurityManagerDemo {

    public static void main(String[] args) {
        File file = new File("c:/where-ever-file.txt");
        System.out.println("Does file exist? " + file.exists());
    }
}

package com.turreta.securitymanager;

import java.io.File;

public class SecurityManagerDemo {

public static void main(String[] args) {

File file = new File("c:/where-ever-file.txt");

System.out.println("Does file exist? " + file.exists());

}

Running Java without Security Manager

Run your application from the command line prompt as follows.

java com.turreta.securityManager.SecurityManagerDemo

1	java com.turreta.securityManager.SecurityManagerDemo

Outputs

Does file exist? false

1	Does file exist? false

Running Java with Security Manager

Now run it with -D option.

java -Djava.security.manager com.turreta.securityManager.SecurityManagerDemo

1	java -Djava.security.manager com.turreta.securityManager.SecurityManagerDemo

Outputs

Exception in thread "main" java.security.AccessControlException: access denied ("java.io.FilePermission" "c:\where-ever-file.txt" "read")
	at java.security.AccessControlContext.checkPermission(AccessControlContext.java:472)
	at java.security.AccessController.checkPermission(AccessController.java:884)
	at java.lang.SecurityManager.checkPermission(SecurityManager.java:549)
	at java.lang.SecurityManager.checkRead(SecurityManager.java:888)
	at java.io.File.exists(File.java:814)
	at com.turreta.securitymanager.SecurityManagerDemo.main(SecurityManagerDemo.java:9)