We Keep Coding

How to ensure execution order of threads [duplicate]

This question already has answers here:
How threads are executed in the memory?
(2 answers)
Closed 2 years ago.
This is a simplified version of the problem. Given n number of threads, each printing a constant number all the time. For example, Thread-1 should always print 1, Thread-2 should always print 2 and so on...
How to ensure, the threads are executed in order i.e. the output should be as below:
Thread-1: 1
Thread-2: 2
Thread-3: 3
.
.
.
Thread-n: n
I have a naïve solution to do it through wait()/notify() but I guess there might be a better solution than that. Perhaps, using Semaphore maybe? I don't know.
Update:
Based on the answers received, I think I was not very clear. There are some constraints:
All threads should start at once (assume we don't really have control on that)
Once all the threads start, there should be some sort of communication between the threads to execute in order.

This sequentially execution of thread can be handled beautifully using Thread.join() method. To handle it properly, you may have to create MyRunnable(or, use any name you prefer) which implements Runnable interface. Inside MyRunnable, you can inject a parent Thread, and call parent.join() at top of MyRunnable.run() method. The code is given below:
public class SequentialThreadsTest {
static class MyRunnable implements Runnable {
static int objCount; // to keep count of sequential object
private int objNum;
private Thread parent; // keep track of parent thread
MyRunnable(Thread parent) {
this.parent = parent;
this.objNum = objCount + 1;
objCount += 1;
}
#Override
public void run() {
try {
if(parent != null) {
parent.join();
}
System.out.println("Thread-" + objNum + ": " + objNum);
} catch(InterruptedException e) {
e.printStackTrace();
// do something else
} finally {
// do what you need to do when thread execution is finished
}
}
}
public static void main(String[] args) {
int n = 10;
Thread parentThread = null;
for(int i=0; i<n; i++) {
Thread thread = new Thread(new MyRunnable(parentThread));
thread.start();
parentThread = thread;
}
}
}
And the output is:
Thread-1: 1
Thread-2: 2
Thread-3: 3
Thread-4: 4
Thread-5: 5
Thread-6: 6
Thread-7: 7
Thread-8: 8
Thread-9: 9
Thread-10: 10

You haven't specified many details, but if you only want serializable thread execution you can wait for previous thread to finish and then print. Something like this:
public static void main(String[] args) {
Thread thread = null;
for (int i = 0; i < 10; i++) {
int index = i;
Thread previousThread = thread;
thread = new Thread(() -> {
if (previousThread != null) {
try {
previousThread.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println(index);
});
thread.start();
}
}

Try making a queue - this will do exactly what you want. Simply change the value of n to however many threads you have, and add all the threads sequentially (only once). If ever you want to stop the threads from executing, all you have to do is add end to the queue. Obviously, for a larger project, you will need to modify this code a little bit (I would recommend replacing the main method with a class initializer and pass the LinkedBlockingQueue as a pre-built argument)
import java.util.concurrent.LinkedBlockingQueue;
public class HelloWorld{
private static int n = 2;
private static LinkedBlockingQueue<Thread> queue = new LinkedBlockingQueue<>(n+1);
static Thread a = new Thread(()->{
System.out.print("a");
});
static Thread b = new Thread(()->{
System.out.print("b");
});
static Thread end = new Thread(()->{
break_ = true;
});
public static final int END = 20;//this and the counter are just here so the code doesn't run forever
public static volatile int i = 0;
public static volatile boolean break_ = false;
public static void main(String []args){
queue.add(a);
queue.add(b);
//queue.add(end);
outerloop:
while(true){
Thread toBeRun = queue.poll();
try{
toBeRun.run();
queue.add(toBeRun);
i++;
if(i>=END || break_){//i>=END does not need to be here, it's just to stop it from running forever in this example
break;
}
}catch(NullPointerException e){
break;
}
}
}
}
Note: This uses java 8 lambdas. If you're using an older version of java, you will need to create the threads using the run method.

Java start separate threads but do something after all of them finish [duplicate]

What is a way to simply wait for all threaded process to finish? For example, let's say I have:
public class DoSomethingInAThread implements Runnable{
public static void main(String[] args) {
for (int n=0; n<1000; n++) {
Thread t = new Thread(new DoSomethingInAThread());
t.start();
}
// wait for all threads' run() methods to complete before continuing
}
public void run() {
// do something here
}
}
How do I alter this so the main() method pauses at the comment until all threads' run() methods exit? Thanks!

You put all threads in an array, start them all, and then have a loop
for(i = 0; i < threads.length; i++)
threads[i].join();
Each join will block until the respective thread has completed. Threads may complete in a different order than you joining them, but that's not a problem: when the loop exits, all threads are completed.

One way would be to make a List of Threads, create and launch each thread, while adding it to the list. Once everything is launched, loop back through the list and call join() on each one. It doesn't matter what order the threads finish executing in, all you need to know is that by the time that second loop finishes executing, every thread will have completed.
A better approach is to use an ExecutorService and its associated methods:
List<Callable> callables = ... // assemble list of Callables here
// Like Runnable but can return a value
ExecutorService execSvc = Executors.newCachedThreadPool();
List<Future<?>> results = execSvc.invokeAll(callables);
// Note: You may not care about the return values, in which case don't
// bother saving them
Using an ExecutorService (and all of the new stuff from Java 5's concurrency utilities) is incredibly flexible, and the above example barely even scratches the surface.

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class DoSomethingInAThread implements Runnable
{
public static void main(String[] args) throws ExecutionException, InterruptedException
{
//limit the number of actual threads
int poolSize = 10;
ExecutorService service = Executors.newFixedThreadPool(poolSize);
List<Future<Runnable>> futures = new ArrayList<Future<Runnable>>();
for (int n = 0; n < 1000; n++)
{
Future f = service.submit(new DoSomethingInAThread());
futures.add(f);
}
// wait for all tasks to complete before continuing
for (Future<Runnable> f : futures)
{
f.get();
}
//shut down the executor service so that this thread can exit
service.shutdownNow();
}
public void run()
{
// do something here
}
}

instead of join(), which is an old API, you can use CountDownLatch. I have modified your code as below to fulfil your requirement.
import java.util.concurrent.*;
class DoSomethingInAThread implements Runnable{
CountDownLatch latch;
public DoSomethingInAThread(CountDownLatch latch){
this.latch = latch;
}
public void run() {
try{
System.out.println("Do some thing");
latch.countDown();
}catch(Exception err){
err.printStackTrace();
}
}
}
public class CountDownLatchDemo {
public static void main(String[] args) {
try{
CountDownLatch latch = new CountDownLatch(1000);
for (int n=0; n<1000; n++) {
Thread t = new Thread(new DoSomethingInAThread(latch));
t.start();
}
latch.await();
System.out.println("In Main thread after completion of 1000 threads");
}catch(Exception err){
err.printStackTrace();
}
}
}
Explanation:
CountDownLatch has been initialized with given count 1000 as per your requirement.
Each worker thread DoSomethingInAThread will decrement the CountDownLatch, which has been passed in constructor.
Main thread CountDownLatchDemo await() till the count has become zero. Once the count has become zero, you will get below line in output.
In Main thread after completion of 1000 threads
More info from oracle documentation page
public void await()
throws InterruptedException
Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted.
Refer to related SE question for other options:
wait until all threads finish their work in java

Avoid the Thread class altogether and instead use the higher abstractions provided in java.util.concurrent
The ExecutorService class provides the method invokeAll that seems to do just what you want.

Consider using java.util.concurrent.CountDownLatch. Examples in javadocs

Depending on your needs, you may also want to check out the classes CountDownLatch and CyclicBarrier in the java.util.concurrent package. They can be useful if you want your threads to wait for each other, or if you want more fine-grained control over the way your threads execute (e.g., waiting in their internal execution for another thread to set some state). You could also use a CountDownLatch to signal all of your threads to start at the same time, instead of starting them one by one as you iterate through your loop. The standard API docs have an example of this, plus using another CountDownLatch to wait for all threads to complete their execution.

As Martin K suggested java.util.concurrent.CountDownLatch seems to be a better solution for this. Just adding an example for the same
public class CountDownLatchDemo
{
public static void main (String[] args)
{
int noOfThreads = 5;
// Declare the count down latch based on the number of threads you need
// to wait on
final CountDownLatch executionCompleted = new CountDownLatch(noOfThreads);
for (int i = 0; i < noOfThreads; i++)
{
new Thread()
{
#Override
public void run ()
{
System.out.println("I am executed by :" + Thread.currentThread().getName());
try
{
// Dummy sleep
Thread.sleep(3000);
// One thread has completed its job
executionCompleted.countDown();
}
catch (InterruptedException e)
{
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}.start();
}
try
{
// Wait till the count down latch opens.In the given case till five
// times countDown method is invoked
executionCompleted.await();
System.out.println("All over");
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}

If you make a list of the threads, you can loop through them and .join() against each, and your loop will finish when all the threads have. I haven't tried it though.
http://docs.oracle.com/javase/8/docs/api/java/lang/Thread.html#join()

Create the thread object inside the first for loop.
for (int i = 0; i < threads.length; i++) {
threads[i] = new Thread(new Runnable() {
public void run() {
// some code to run in parallel
}
});
threads[i].start();
}
And then so what everyone here is saying.
for(i = 0; i < threads.length; i++)
threads[i].join();

You can do it with the Object "ThreadGroup" and its parameter activeCount:

As an alternative to CountDownLatch you can also use CyclicBarrier e.g.
public class ThreadWaitEx {
static CyclicBarrier barrier = new CyclicBarrier(100, new Runnable(){
public void run(){
System.out.println("clean up job after all tasks are done.");
}
});
public static void main(String[] args) {
for (int i = 0; i < 100; i++) {
Thread t = new Thread(new MyCallable(barrier));
t.start();
}
}
}
class MyCallable implements Runnable{
private CyclicBarrier b = null;
public MyCallable(CyclicBarrier b){
this.b = b;
}
#Override
public void run(){
try {
//do something
System.out.println(Thread.currentThread().getName()+" is waiting for barrier after completing his job.");
b.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}
}
To use CyclicBarrier in this case barrier.await() should be the last statement i.e. when your thread is done with its job. CyclicBarrier can be used again with its reset() method. To quote javadocs:
A CyclicBarrier supports an optional Runnable command that is run once per barrier point, after the last thread in the party arrives, but before any threads are released. This barrier action is useful for updating shared-state before any of the parties continue.

The join() was not helpful to me. see this sample in Kotlin:
val timeInMillis = System.currentTimeMillis()
ThreadUtils.startNewThread(Runnable {
for (i in 1..5) {
val t = Thread(Runnable {
Thread.sleep(50)
var a = i
kotlin.io.println(Thread.currentThread().name + "|" + "a=$a")
Thread.sleep(200)
for (j in 1..5) {
a *= j
Thread.sleep(100)
kotlin.io.println(Thread.currentThread().name + "|" + "$a*$j=$a")
}
kotlin.io.println(Thread.currentThread().name + "|TaskDurationInMillis = " + (System.currentTimeMillis() - timeInMillis))
})
t.start()
}
})
The result:
Thread-5|a=5
Thread-1|a=1
Thread-3|a=3
Thread-2|a=2
Thread-4|a=4
Thread-2|2*1=2
Thread-3|3*1=3
Thread-1|1*1=1
Thread-5|5*1=5
Thread-4|4*1=4
Thread-1|2*2=2
Thread-5|10*2=10
Thread-3|6*2=6
Thread-4|8*2=8
Thread-2|4*2=4
Thread-3|18*3=18
Thread-1|6*3=6
Thread-5|30*3=30
Thread-2|12*3=12
Thread-4|24*3=24
Thread-4|96*4=96
Thread-2|48*4=48
Thread-5|120*4=120
Thread-1|24*4=24
Thread-3|72*4=72
Thread-5|600*5=600
Thread-4|480*5=480
Thread-3|360*5=360
Thread-1|120*5=120
Thread-2|240*5=240
Thread-1|TaskDurationInMillis = 765
Thread-3|TaskDurationInMillis = 765
Thread-4|TaskDurationInMillis = 765
Thread-5|TaskDurationInMillis = 765
Thread-2|TaskDurationInMillis = 765
Now let me use the join() for threads:
val timeInMillis = System.currentTimeMillis()
ThreadUtils.startNewThread(Runnable {
for (i in 1..5) {
val t = Thread(Runnable {
Thread.sleep(50)
var a = i
kotlin.io.println(Thread.currentThread().name + "|" + "a=$a")
Thread.sleep(200)
for (j in 1..5) {
a *= j
Thread.sleep(100)
kotlin.io.println(Thread.currentThread().name + "|" + "$a*$j=$a")
}
kotlin.io.println(Thread.currentThread().name + "|TaskDurationInMillis = " + (System.currentTimeMillis() - timeInMillis))
})
t.start()
t.join()
}
})
And the result:
Thread-1|a=1
Thread-1|1*1=1
Thread-1|2*2=2
Thread-1|6*3=6
Thread-1|24*4=24
Thread-1|120*5=120
Thread-1|TaskDurationInMillis = 815
Thread-2|a=2
Thread-2|2*1=2
Thread-2|4*2=4
Thread-2|12*3=12
Thread-2|48*4=48
Thread-2|240*5=240
Thread-2|TaskDurationInMillis = 1568
Thread-3|a=3
Thread-3|3*1=3
Thread-3|6*2=6
Thread-3|18*3=18
Thread-3|72*4=72
Thread-3|360*5=360
Thread-3|TaskDurationInMillis = 2323
Thread-4|a=4
Thread-4|4*1=4
Thread-4|8*2=8
Thread-4|24*3=24
Thread-4|96*4=96
Thread-4|480*5=480
Thread-4|TaskDurationInMillis = 3078
Thread-5|a=5
Thread-5|5*1=5
Thread-5|10*2=10
Thread-5|30*3=30
Thread-5|120*4=120
Thread-5|600*5=600
Thread-5|TaskDurationInMillis = 3833
As it's clear when we use the join:
The threads are running sequentially.
The first sample takes 765 Milliseconds while the second sample takes 3833 Milliseconds.
Our solution to prevent blocking other threads was creating an ArrayList:
val threads = ArrayList<Thread>()
Now when we want to start a new thread we most add it to the ArrayList:
addThreadToArray(
ThreadUtils.startNewThread(Runnable {
...
})
)
The addThreadToArray function:
#Synchronized
fun addThreadToArray(th: Thread) {
threads.add(th)
}
The startNewThread funstion:
fun startNewThread(runnable: Runnable) : Thread {
val th = Thread(runnable)
th.isDaemon = false
th.priority = Thread.MAX_PRIORITY
th.start()
return th
}
Check the completion of the threads as below everywhere it's needed:
val notAliveThreads = ArrayList<Thread>()
for (t in threads)
if (!t.isAlive)
notAliveThreads.add(t)
threads.removeAll(notAliveThreads)
if (threads.size == 0){
// The size is 0 -> there is no alive threads.
}

The problem with:
for(i = 0; i < threads.length; i++)
threads[i].join();
...is, that threads[i + 1] never can join before threads[i].
Except the "latch"ed ones, all solutions have this lack.
No one here (yet) mentioned ExecutorCompletionService, it allows to join threads/tasks according to their completion order:
public class ExecutorCompletionService<V>
extends Object
implements CompletionService<V>
A CompletionService that uses a supplied Executor to execute tasks. This class arranges that submitted tasks are, upon completion, placed on a queue accessible using take. The class is lightweight enough to be suitable for transient use when processing groups of tasks.
Usage Examples.
Suppose you have a set of solvers for a certain problem, each returning a value of some type Result, and would like to run them concurrently, processing the results of each of them that return a non-null value, in some method use(Result r). You could write this as:
void solve(Executor e, Collection<Callable<Result>> solvers) throws InterruptedException, ExecutionException {
CompletionService<Result> cs = new ExecutorCompletionService<>(e);
solvers.forEach(cs::submit);
for (int i = solvers.size(); i > 0; i--) {
Result r = cs.take().get();
if (r != null)
use(r);
}
}
Suppose instead that you would like to use the first non-null result of the set of tasks, ignoring any that encounter exceptions, and cancelling all other tasks when the first one is ready:
void solve(Executor e, Collection<Callable<Result>> solvers) throws InterruptedException {
CompletionService<Result> cs = new ExecutorCompletionService<>(e);
int n = solvers.size();
List<Future<Result>> futures = new ArrayList<>(n);
Result result = null;
try {
solvers.forEach(solver -> futures.add(cs.submit(solver)));
for (int i = n; i > 0; i--) {
try {
Result r = cs.take().get();
if (r != null) {
result = r;
break;
}
} catch (ExecutionException ignore) {}
}
} finally {
futures.forEach(future -> future.cancel(true));
}
if (result != null)
use(result);
}
Since: 1.5 (!)
Assuming use(r) (of Example 1) also asynchronous, we had a big advantage. #

Performing a long calculation that returns after a timeout

I want to perform a search using iterative deepening, meaning every time I do it, I go deeper and it takes longer. There is a time limit (2 seconds) to get the best result possible. From what I've researched, the best way to do this is using an ExecutorService, a Future and interrupting it when the time runs out. This is what I have at the moment:
In my main function:
ExecutorService service = Executors.newSingleThreadExecutor();
ab = new AB();
Future<Integer> f = service.submit(ab);
Integer x = 0;
try {
x = f.get(1990, TimeUnit.MILLISECONDS);
}
catch(TimeoutException e) {
System.out.println("cancelling future");
f.cancel(true);
}
catch(Exception e) {
throw new RuntimeException(e);
}
finally {
service.shutdown();
}
System.out.println(x);
And the Callable:
public class AB implements Callable<Integer> {
public AB() {}
public Integer call() throws Exception {
Integer x = 0;
int i = 0;
while (!Thread.interrupted()) {
x = doLongComputation(i);
i++;
}
return x;
}
}
I have two problems:
doLongComputation() isn't being interrupted, the program only checks if Thread.interrupted() is true after it completes the work. Do I need to put checks in doLongComputation() to see if the thread has been interrupted?
Even if I get rid of the doLongComputation(), the main method isn't receiving the value of x. How can I ensure that my program waits for the Callable to "clean up" and return the best x so far?

To answer part 1: Yes, you need to have your long task check the interrupted flag. Interruption requires the cooperation of the task being interrupted.
Also you should use Thread.currentThread().isInterrupted() unless you specifically want to clear the interrupt flag. Code that throws (or rethrows) InterruptedException uses Thread#interrupted as a convenient way to both check the flag and clear it, when you're writing a Runnable or Callable this is usually not what you want.
Now to answer part 2: Cancellation isn't what you want here.
Using cancellation to stop the computation and return an intermediate result doesn't work, once you cancel the future you can't retrieve the return value from the get method. What you could do is make each refinement of the computation its own task, so that you submit one task, get the result, then submit the next using the result as a starting point, saving the latest result as you go.
Here's an example I came up with to demonstrate this, calculating successive approximations of a square root using Newton's method. Each iteration is a separate task which gets submitted (using the previous task's approximation) when the previous task completes:
import java.util.concurrent.*;
import java.math.*;
public class IterativeCalculation {
static class SqrtResult {
public final BigDecimal value;
public final Future<SqrtResult> next;
public SqrtResult(BigDecimal value, Future<SqrtResult> next) {
this.value = value;
this.next = next;
}
}
static class SqrtIteration implements Callable<SqrtResult> {
private final BigDecimal x;
private final BigDecimal guess;
private final ExecutorService xs;
public SqrtIteration(BigDecimal x, BigDecimal guess, ExecutorService xs) {
this.x = x;
this.guess = guess;
this.xs = xs;
}
public SqrtResult call() {
BigDecimal nextGuess = guess.subtract(guess.pow(2).subtract(x).divide(new BigDecimal(2).multiply(guess), RoundingMode.HALF_EVEN));
return new SqrtResult(nextGuess, xs.submit(new SqrtIteration(x, nextGuess, xs)));
}
}
public static void main(String[] args) throws Exception {
long timeLimit = 10000L;
ExecutorService xs = Executors.newSingleThreadExecutor();
try {
long startTime = System.currentTimeMillis();
Future<SqrtResult> f = xs.submit(new SqrtIteration(new BigDecimal("612.00"), new BigDecimal("10.00"), xs));
for (int i = 0; System.currentTimeMillis() - startTime < timeLimit; i++) {
f = f.get().next;
System.out.println("iteration=" + i + ", value=" + f.get().value);
}
f.cancel(true);
} finally {
xs.shutdown();
}
}
}

Is this multi-threading code guaranteed to always return zero?

I am taking a book to do some mock test, I have found this question:
import java.util.concurrent.atomic.AtomicInteger;
class AtomicVariableTest {
private static AtomicInteger counter = new AtomicInteger(0);
static class Decrementer extends Thread {
public void run() {
counter.decrementAndGet(); // #1
}
}
static class Incrementer extends Thread {
public void run() {
counter.incrementAndGet(); // #2
}
}
public static void main(String[] args) {
for (int i = 0; i < 5; i++) {
new Incrementer().start();
new Decrementer().start();
}
System.out.println(counter);
}
}
The answer:
This program will always print 0.
But I think there is no guarantee that the threads will have completed when it prints the counter value.
I mean, most of the time it will return 0, but if you are strict with the theory there is no guarantee of this.
Am I correct?

There is guaranteed. And there is not guaranteed. There is no middle ground.
In this case there is no guarantee that the result is always zero. This is because the threads are not joined - and might never even have actually ran before the print!
For example, among other permutations, the sequence this code could have executed is:
counter.decrementAndGet(); // #1
System.out.println(counter); // Main thread
counter.incrementAndGet(); // #2
// (and so on, at some arbitrary point after the print)
Avoiding such undesired interleaving/execution is handled in Java (as per the JLS) under happens-before relationships.
If the threads were joined (to the thread with the print) then a happens-before would have been established - in this case that would mean that the threads started and finished running - and the result would be guarantee to be zero.
public static void main(String[] args) {
final List<Thread> threads = new ArrayList<>();
for (int i = 0; i < 5; i++) {
final new Incrementer i = new Incrementer();
threads.add(i);
i.start();
final new Decrementer d = new Decrementer();
threads.add(d);
d.start();
}
for (final Thread t : threads) { t.join(); }
System.out.println(counter);
}
See one of the many duplicates: Wait until child threads completed : Java
And this is why you use the ExecutorService or ExecutorCompletionService and never deal with thread management manually because it is extremely error prone otherwise.

Create a simple queue with Java threads

I'm trying to create a simple queue with Java Thread that would allow a loop, say a for loop with 10 iterations, to iterate n (< 10) threads at a time and wait until those threads are finished before continuing to iterate.
Here's a better way to illustrate my problem:
for (int i = 1; i <= 10; i++) {
new Thread ( do_some_work() );
if ( no_available_threads ) {
wait_until_available_threads();
}
}
do_some_work() {
// do something that takes a long time
}
Basically what I want to do is a copy of this: Thread and Queue
How can I achieve this the most painless way?

I would use the Java 5 Executors instead of rolling your own. Something like the following:
ExecutorService service = Executors.newFixedThreadPool(10);
// now submit our jobs
service.submit(new Runnable() {
public void run() {
do_some_work();
}
});
// you can submit any number of jobs and the 10 threads will work on them
// in order
...
// when no more to submit, call shutdown, submitted jobs will continue to run
service.shutdown();
// now wait for the jobs to finish
service.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);

Use the Executors, as recommended by the others. However, if you want the fun of doing it yourself, try something like this. (Take care. I wrote it in Notepad and there's some Exceptions you'll need to catch even if I got everything else right. Notepad's poor at catching coding errors.) This is more a concept than an actual solution to anything, but the idea could be generally useful.
private ConcurrentLinkedQueue<MyThread> tQueue =
new ConcurrentLinkedQueue<MyThread>();
class MyThread extends Thread {
public Runnable doSomething;
public void run() {
// Do the real work.
doSomething();
// Clean up and make MyThread available again.
tQueue.add( mythread );
// Might be able to avoid this synch with clever code.
// (Don't synch if you know no one's waiting.)
// (But do that later. Much later.)
synchronized (tQueue) {
// Tell them the queue is no longer empty.
tQueue.notifyAll();
}
}
}
Elsewhere:
// Put ten MyThreads in tQueue.
for (int i = 0; i < 10; i++) tQueue.add( new MyThread() );
// Main Loop. Runs ten threads endlessly.
for (;;) {
MyThread t = tQueue.poll();
if (t == null) {
// Queue empty. Sleep till someone tells us it's not.
do {
// There's a try-catch combo missing here.
synchonized( tQueue ) { tQueue.wait() };
t = tQueue.poll();
} while (t == null) break; // Watch for fake alert!
}
t.doSomething = do_some_work;
t.start();
}
Also, note the clever use of ConcurrentLinkedQueue. You could use something else like ArrayList or LinkedList, but you'd need to synchronize them.

see java.util.concurrent and especially Executors and ExecutorService

Crate Logger.class :
public class Logger extends Thread {
List<String> queue = new ArrayList<String>();
private final int MAX_QUEUE_SIZE = 20;
private final int MAX_THREAD_COUNT = 10;
#Override
public void start() {
super.start();
Runnable task = new Runnable() {
#Override
public void run() {
while (true) {
String message = pullMessage();
Log.d(Thread.currentThread().getName(), message);
// Do another processing
}
}
};
// Create a Group of Threads for processing
for (int i = 0; i < MAX_THREAD_COUNT; i++) {
new Thread(task).start();
}
}
// Pulls a message from the queue
// Only returns when a new message is retrieves
// from the queue.
private synchronized String pullMessage() {
while (queue.isEmpty()) {
try {
wait();
} catch (InterruptedException e) {
}
}
return queue.remove(0);
}
// Push a new message to the tail of the queue if
// the queue has available positions
public synchronized void pushMessage(String logMsg) {
if (queue.size() < MAX_QUEUE_SIZE) {
queue.add(logMsg);
notifyAll();
}
}
}
Then insert bellow code in your main class :
Logger logger =new Logger();
logger.start();
for ( int i=0; i< 10 ; i++) {
logger.pushMessage(" DATE : "+"Log Message #"+i);
}