如何在android中同时运行2个线程

Question

I am new to android programming.Sorry if this type of question have been asked before. I am getting trouble while creating threads.In my code, I have initialized int i=500; and in my first thread t1, i want to increment the value of t1 if(i<5000) and also on my thread t2 want to check the condition where the value of t2 is decremented if(i>0) Please help...Thanks in advance

Answer 1

Here is the plain java Thread implementation for android as you required for this specific increment/decrement problem...

// class lass level declarations
private static int DEF_VALUE = 500;
private static int MIN_VALUE = 0;
private static int MAX_VALUE = 1000;

private AtomicInteger i = new AtomicInteger(DEF_VALUE);
private Thread t1 = null;
private Thread t2 = null;

private void initThreads() {
    Log.i(TAG, "Initializing Threads...");

    t1 = new Thread(new Runnable() {

        @Override
        public void run() {
            Log.i(TAG, "Starting T1.");
            while (i.get() < MAX_VALUE) {
                i.incrementAndGet();
                Log.d(TAG, String.format("Incremented by T1, i = %d", i.get()));
            }
            Log.i(TAG, "Finishing T1.");
        }
    });

    t2 = new Thread(new Runnable() {

        @Override
        public void run() {
            Log.i(TAG, "Starting T1.");
            while (i.get() > MIN_VALUE) {
                i.decrementAndGet();
                Log.d(TAG, String.format("Decremented by T2, i =  %d", i.get()));
            }
            Log.i(TAG, "Finishing T2.");
        }
    });

    t1.start();
    t2.start();
}

Hope this helps...:)

Update: Source updated to use AtomicInteger instead of plain int to avoid concurrent access issues.

Answer 2

You can do this by using AsyncTask: http://developer.android.com/reference/android/os/AsyncTask.html

Do note that in some versions on Android, AsyncTask is executed on a single thread. If you need to execute in paralell this need to be set on the task, from the documentation:

When first introduced, AsyncTasks were executed serially on a single background thread. Starting with DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with HONEYCOMB, tasks are executed on a single thread to avoid common application errors caused by parallel execution. If you truly want parallel execution, you can invoke executeOnExecutor(java.util.concurrent.Executor, Object[]) with THREAD_POOL_EXECUTOR.

In addition, you will run into concurrency issues, so you should take steps to handle them.

It might look like this:

public class DoingTwoThings {

private AsyncTask<Void, Void, Void> taskA;
private AsyncTask<Void, Void, Void> taskB;
private volatile int i;

public DoingTwoThings() {
    createTaskA();
    createTaskB();
    startTasks();
}

private void createTaskA() {
    taskA = new AsyncTask<Void, Void, Void>() {
        @Override
        protected Void doInBackground(Void... voids) {
            doInBackgroundA();
            return null;
        }
    };
}

private void doInBackgroundA() {
    while (i < 5000) {
        i++;
    }
}

private void createTaskB() {
    taskB = new AsyncTask<Void, Void, Void>() {
        @Override
        protected Void doInBackground(Void... voids) {
            doInBackGroundB();
            return null;
        }
    };
}

private void doInBackGroundB() {
    while (i > 0) {
        i--;
    }
}

private void startTasks() {
    // AsyncTasks executed one one thread in Honeycomb+ unless executed in thread pool manually
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
        taskA.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
        taskB.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
    } else {
        taskA.execute();
        taskB.execute();
    }
}}

The code in the overriden "doInBackground()" methods are executed on a different thread. If you need to modify some UI before or after the task is done you can easily override onPreExecute and onPostExecute.