Question:
I have a server application listening for clientapplets to connect. The server spawns a new thread each timea connection to a client is established. I want to keeptrack, i.e. count the number of clients connectedand output it to a textfield in a frame. I amnot sure how Java handles shared data among threads so myquestion is: How does one keep track of the number of threadsor connections? I want the number of threads to be displayed ina single frame; I tried something, but a new window is createdeach time a new thread is spawned.
Answer:
Since all threads share the same address space, the simplest solutionis to create a count monitor in the parent thread. Each time theparent thread creates or starts a child thread, it (or the child) incrementsthe count monitor. Each time it stops a thread, it decrements the countmonitor. The count monitor can be passed as a parameter to eachchild thread so it too can increment and decrement it, but in thiscase, access to count needs to be synchronized.
(Note: Java provides a method Thread.activeCount(), whichreturns thenumber of active threads. Unfortunately, this doesn’t get decrementedwhen threads are stopped.)
*/class CountMonitor { private int count = 0; // = # of active threads public synchronized void updateCount(int amt) { // don’t decrement a zero count if (count <= 0 && amt <= 0) { try { wait(); } catch(InterruptedException e) {}; } count += amt; notify(); // start waiting threads } public int getCount() { return count; }}public class Parent { public static void main(String args[]) { CountMonitor c = new CountMonitor(); int N = 5; c.updateCount(1); // for parent Child child[] = new Child[N]; // start some threads for(int i = 0; i < N; i++) { child[i] = new Child(i, c); child[i].start(); } for(int i = 0; i < N; i++) { System.out.println("Parent: thread count = " + c.getCount()); try { Thread.sleep(10); } catch(InterruptedException e) { System.exit(1); } } System.out.println("Parent dying ... "); c.updateCount(-1); }}class Child extends Thread { private int myID; private CountMonitor myCount ; public Child(int i, CountMonitor c) { myID = i; myCount = c;} public void run() { myCount.updateCount(1); for(int i = 0; i < 3; i++) { System.out.println(" Child-" + myID + ": thread count = " +myCount.getCount()); try { sleep(10); } catch(InterruptedException e) { System.exit(1); } System.out.println(" Child-" + myID + " dying ... "); myCount.updateCount(-1); } }} // Child 
