multithreading - Interthread communication in Java -

i have 2 java threads need talk each other , i'm not sure i'm going right way.

the basic this: have on thread, "runner" start off long-running process making call command line using the apache exec library. thread, "monitor", periodically monitor , report on state of system.

part of involves returning lines runner process got stdout. currently, runner , monitor run separate threads. monitor has reference runner , can access output collected (so far) calling runner's "get stdout" method.

this seems working fine, whole setup doesn't seem me. partly, i'm worried implementation isn't thread-safe, though i'm not sure how test that. it's been quite while since i've done multithreaded programming, i'm wondering if there's better/cleaner way i'm trying do. code looks this:

public class runner implements callable<string>  {   private bytearrayoutputstream outputstream = new bytearrayoutputstream();   private bytearrayoutputstream errorstream = new bytearrayoutputstream();   public string getstdoutlines()   {     return new string(outputstream.tobytearray());   }   public string getstderrlines()   {     return new string(errorstream.tobytearray());   }    @override   public string call() throws exception   {     /*     use apache exec begin long-running process.     stdout , stderr written 2 local members (outputstream , errorstream).     done apache exec library.     */     pumpstreamhandler streamhandler = new pumpstreamhandler(this.outputstream, this.errorstream);     defaultexecuteresulthandler resulthandler = new defaultexecuteresulthandler();     executor.setstreamhandler(streamhandler);     commandline cli = //details not important here.     executor.execute(cli, resulthandler);     resulthandler.waitfor();     string resultsstring = generatefinalresultstring(outputstream, errorstream); //implementation details of not important here.     return resultsstring; /*some string calculated after process has completed.*/   } }  public class monitor implements runnable {   private runner runner;    public void setrunner(runner r)   {     this.runner = r;   }    @override   public void run()   {     try     {       while (!thread.currentthread().interrupted())       {         //call functions on other thread stdout , stderr         commandstdout = this.runner.getstdoutlines();         commandstderr = this.runner.getstderrlines();         dosendmonitoringmessage(commandstdout, commandstderr);         thread.sleep(some_delay);       }     }     catch(interruptedexception e)     {        system.out.println("monitor shutting down");     }   } }  public class executeallthis {    public void dothestuff()    {       system.out.println("begining...");       runner runner = new runner();       monitor monitor = new monitor();        monitor.setrunner(runner);       executorservice xservice = executors.newfixedthreadpool(2);       xservice.execute(monitor);       future<string> runnerfuture = xservice.submit(runner);       string result = runnerfuture.get(0);       system.out.println("result is: "+result);    } 

edit: i've added detail how i'm calling long-running process writes stdout , stderr strings, since more relevant realized.

your implementation fine.

however, in theory, variables outputstream , errorstream should volatile, otherwise possible monitor thread not latest updates - in theory.

another reason using volatile shared variables, in general case, prevent reading corrupt states. reason not apply here though since string immutable.

another advice, if not sure, use old synchronized. might little slower volatile, not problem; , not in case.