Problem with System.setProperty(key,  value)

Posted on 2000-04-03
Medium Priority
Last Modified: 2006-11-17
Instead of "System.getEnv()", which is deprecated since java 1.2,  we use the method "System.getProperty(Key ) for the reading of environment- variables.
With the method "System.setProperty(key, value)" we set a selfdefined environment-variable.

When we start another Programm which read the selfdefined environment-variable with the method "System.getProperty(Key )" we get returnvalue "null".

Which Lifetime has a Property set by the "System.setProperty(key, value)" method?

Question by:ziab
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Expert Comment

ID: 2679502
The System class has a static object named props where all the properties are kept. On initialization, System has a native method initProperties which it calls to fill in the properties in props. The setProperty method adds a property to the props object.
All this means that the properties will be shared between applications that run on the same instance of a JVM. Programs running on different instaces will not share properties (because the setProperty method does nothing to change any external objects in any way).
I am not 100% sure about this, as I never worked with properties, just looked at the docs and source code for System so if other experts can correct/confirm my answer, please do :-)

Expert Comment

ID: 2679578
If you wanna make the new defined key available to an other process (second JVM), try to save your properties in the first process and load them in the second one.

Have a look at java.utils.Properties class and something like

Properties myProps = System.getProperties();

In second program do something like
Properties myProps = Properties.load("myPropertyFileName);

Expert Comment

ID: 2679652
I think the only way to change env variables for an "upcomming" program is to change the env array you use in "java.lang.Runtime.exec".
As far as I can remember env. variables are static they are parameters when you start the program up. And can only be changed within a batch/command script.
Pls. correct me if I'm wrong!

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Author Comment

ID: 2679842
Thanks for the comments,

but we are seaching for a possibility to set, save and hold a selfdefined env-var in the env only with Java.


Accepted Solution

sankars98 earned 300 total points
ID: 2680487

 You can use the following class to get & set the environment variable to the OS level..
 This is windows specific..

 import java.io.*;
 import java.util.*;
 public class EnvironmentVariables extends Hashtable{
   public EnvironmentVariables() {
     String[] cmd = {"cmd", "/c", "SET"};
     try {
         Process proc = Runtime.getRuntime().exec(cmd);
         BufferedReader inp = new BufferedReader(new InputStreamReader(proc.getInputStream()));
         String LINE = inp.readLine();
         while (LINE != null) {
             int eqPos = LINE.indexOf("=");
             if (eqPos >= 0) {
               String name = LINE.substring(0, eqPos);
               String value = LINE.substring(eqPos + 1);
               super.put(name.toUpperCase(), value);
             LINE = inp.readLine();
       } catch (IOException e) {
         System.out.println("Weird I/O error");
   public synchronized String get(Object name) {
     String theName = name.toString().toUpperCase();
     return (String)super.get(theName);
   public synchronized Object put(Object name, Object value) {
       String theName = name.toString().toUpperCase();
       String theValue = value.toString();
       Object result = super.get(theName);
       // this is windows specific
       String[] cmd = {"cmd", "/c", "SET " + theName + "=" + theValue};
       try {
         Process proc = Runtime.getRuntime().exec(cmd);
         BufferedReader inp = new BufferedReader(new InputStreamReader(proc.getInputStream()));
         String LINE = inp.readLine();
         while (LINE != null) {}
           super.put(name.toString().toUpperCase(), value.toString());
           return result;
       } catch (IOException e) {
         System.out.println("Weird I/O error");
         return null;

 Usage :

 EnvironmentVariables eVariables = new EnvironmentVariables();
 String value = eVariables.get("MYENVIRONMENTVARIABLE");

Sankar S.

Author Comment

ID: 2689872
Thanks Sankar S.

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