Environment Variables In Java

Environment Variables  are variables that are added in any Operating system that can be accessed from anywhere in OS without using the complete(Real) location.

In a java Application System.getenv(); is used to find out the system environment variables

Example:-

import java.util.Map;

public class EnvMap {
    public static void main (String[] args) {
        Map<String, String> env = System.getenv();
        for (String envName : env.keySet()) {
            System.out.format("%s=%s%n",
                              envName,
                              env.get(envName));
        }
    }
}

The Security Manager

The security manager is an object that is used to check whether the operation we want to perform is not violating security policy of the operation system.
Sometime we want to perform any operation and got a exception

Exception in thread "AWT-EventQueue-1" java.security.AccessControlException: access denied (java.io.FilePermission characteroutput.txt write)
        at java.security.AccessControlContext.checkPermission(AccessControlContext.java:323)
        at java.security.AccessController.checkPermission(AccessController.java:546)
        at java.lang.SecurityManager.checkPermission(SecurityManager.java:532)
        at java.lang.SecurityManager.checkWrite(SecurityManager.java:962)
        at java.io.FileOutputStream.<init>(FileOutputStream.java:169)
        at java.io.FileOutputStream.<init>(FileOutputStream.java:70)
        at java.io.FileWriter.<init>(FileWriter.java:46)

...

To Prevent from this exception we can use SecurityManager class to check for the permission before performing operation.

The security manager is an object of type SecurityManager; to obtain a reference to this object, invoke System.getSecurityManager.

SecurityManager appsm = System.getSecurityManager();

If there is no security manager, this method returns null.

The SecurityManager class defines many other methods used to verify other kinds of operations. For example, SecurityManager.checkAccess verifies thread accesses, and SecurityManager.checkPropertyAccess verifies access to the specified property. Each operation or group of operations has its owncheckXXX() method.
In addition, the set of checkXXX() methods represents the set of operations that are already subject to the protection of the security manager. Typically, an application does not have to directly invoke any checkXXX() methods.

System Properties in java

There are various system properties that we need to use during our programs to perform some action or access some file.
There are some commonly used properties.

Key	Meaning
"file.separator"	Character that separates components of a file path. This is "/" on UNIX and "\" on Windows.
"java.class.path"	Path used to find directories and JAR archives containing class files. Elements of the class path are separated by a platform-specific character specified in the path.separator property.
"java.home"	Installation directory for Java Runtime Environment (JRE)
"java.vendor"	JRE vendor name
"java.vendor.url"	JRE vendor URL
"java.version"	JRE version number
"line.separator"	Sequence used by operating system to separate lines in text files
"os.arch"	Operating system architecture
"os.name"	Operating system name
"os.version"	Operating system version
"path.separator"	Path separator character used in java.class.path
"user.dir"	User working directory
"user.home"	User home directory
"user.name"	User account name

These are following keys that can be used to find out the property.
"System"  class has a properties Object associated with it that is used to find out those properties.

Example:-