异步任务无法正常工作

Question

The following is my code:

public class Imagechanger extends AsyncTask<Object,Void,Void >{

    @Override
    protected Void doInBackground(Object... params) {
        if(!isCancelled()){
            final ImageView v = (ImageView)params[0];
            final Context c = (Context)params[1];
                while(true){
                    try {
                        this.wait(4*1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    if(c.getResources().getDrawable(R.drawable.banner1).equals(v.getBackground()))
                    v.setImageDrawable(c.getResources().getDrawable(R.drawable.banner2));
                    else
                        v.setImageDrawable(c.getResources().getDrawable(R.drawable.banner1));
                }
        }
        return null;
    }

}

the above code is throwing the following exception.

04-26 12:24:06.563: ERROR/AndroidRuntime(6332): java.lang.RuntimeException: An error occured while executing doInBackground()
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at android.os.AsyncTask$3.done(AsyncTask.java:200)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:273)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at java.util.concurrent.FutureTask.setException(FutureTask.java:124)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:307)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at java.util.concurrent.FutureTask.run(FutureTask.java:137)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1068)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:561)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at java.lang.Thread.run(Thread.java:1096)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332): Caused by: java.lang.IllegalMonitorStateException: object not locked by thread before wait()
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at java.lang.Object.wait(Native Method)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at java.lang.Object.wait(Object.java:326)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at com.helios.NauticDates.SimpleCalendarViewActivity$Imagechanger.doInBackground(SimpleCalendarViewActivity.java:525)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at com.helios.NauticDates.SimpleCalendarViewActivity$Imagechanger.doInBackground(SimpleCalendarViewActivity.java:1)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at android.os.AsyncTask$2.call(AsyncTask.java:185)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
04-26 12:24:06.563: ERROR/AndroidRuntime(6332):     ... 4 more

What is going wrong? I am not able to understand. Line 525 is the line with the wait instruction.

thank you in advance.

Answer 1

Here you are trying to call the wait() method on an object , here you need to hold the synchronized lock on that object

try { synchronized (this) {
  this.wait();}
} catch (InterruptedException e) {
e.printStackTrace();
}