java static variables

Question

i have two java files file1 and file2 as follows in package pak

file1:

package pak;
public  class file1 {
  public  static int x=432;

  public static void main(String[]  args){
    System.out.println("y is "+file2.y);
    while(x==432) {
      System.out.println("x is "+file1.x);
    }

  }
}

file 2:

package pak;
public  class file2 {
  public  static int y=46;
  public static void main(String[]  args){

   System.out.println("x is "+file1.x);
   ++file1.x;
   System.out.println("x is "+file1.x);

  }
}

i will run first file1 and it will be running and when i run file 2 from another shell it should increment x value and it should come out of loop in file 1 can some one pls help me ??????

You need to learn about Interprocess Communication (IPC) - it's not as simple as what you are trying to do. — Nim
– Nim, Commented Mar 14, 2012 at 11:07
When you say that you use 2 shell... it also means you have 2 separate process, right? In that case static variable are not shared between file1 and file2. — Olivier.Roger
– Olivier.Roger, Commented Mar 14, 2012 at 11:08

Péter Török · Accepted Answer · 2012-03-14 11:13:00Z

2

It won't work in such a simple way. Either you

run the code currently within your two main methods from two distinct threads within the same process (i.e. Java app), (and you also need to declare file1.x volatile for this to work), or
run the two distinct processes as described above, but then you need to make them communicate with each other, which is far from trivial. Otherwise the two processes each will have their own copies of file1.x so modifications in one won't be visible for the other.

edited Mar 14, 2012 at 11:13

answered Mar 14, 2012 at 11:07

Péter Török

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2 Comments

Sree Ram Over a year ago

how will i do it for two different processes

Péter Török Over a year ago

As Nim mentioned, this is a topic called Interprocess Communication (IPC). I have no direct experience with it so can't help you more than giving a few links: docs.oracle.com/javase/tutorial/rmi/index.html, publib.boulder.ibm.com/infocenter/iseries/v5r4/…, stackoverflow.com/questions/1416780/…

Peter Lawrey · Accepted Answer · 2012-03-14 11:35:45Z

0

i will run killjava that kills the process in iostat.java and just before killing i need to perform one action

You can do this with a shutdown hook.

public static void main(String... args) throws InterruptedException {
    Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("Final stats");
        }
    }));
    System.out.println("PID " + ManagementFactory.getRuntimeMXBean().getName());
    while (true) {
        System.out.println("stats");
        Thread.sleep(2000);
    }
}

prints

PID 29490@plus-dev-01
stats
stats
stats
stats
stats
Final stats

The last line occurs after I run kill 29490 in another window.

how will i do it for two different processes

This is a complex answer with too many possible solution to mention. (Shared memory, JMS, RMI, signals, Sockets, IPC etc)

Generally, this is done as a high level concept like "stop process" rather than "increment x" even if the result is the same.

The problem is that as its an advanced topic, you would need to have a good understand of interprocess communication to understand the full answer.

The simple solution is; don't do it, find another way to do what you want.

edited Mar 14, 2012 at 11:35

answered Mar 14, 2012 at 11:13

Peter Lawrey

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3 Comments

Sree Ram Over a year ago

is it not possible in any simple way ??

Peter Lawrey Over a year ago

There is a simple way, use one JVM. ;) Its not clear from your example why you would want to use multiple processes.

Sree Ram Over a year ago

actually in my project i will running iostat.java ffrom one shell ...it will be generating logs ...and from another shell i will run killjava that kills the process in iostat.java and just before killing i need to perform one action (i.e record the logs avg ,min,max) in one csv file ....

Azodious · Accepted Answer · 2012-03-14 11:12:20Z

0

You are invoking two different jvm processes. Both the processes will have it's own copy of file1 and file2. Change in static variable within one process will not be seen by other process.

answered Mar 14, 2012 at 11:12

Azodious

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