How can I let a thread wait successfully in Android?

Question

I start 2 threads to receive messages in different pages (A and B) but cannot let one wait successfully.

Here is the code of the receiveMsg thread.

//BufferedReader brClient = new BufferedReader(new InputStreamReader(socket.getInputStream()));
private class receiveMsg implements Runnable {
    @Override
    public void run() {
        try {
            String data;
            while ((data = brClient.readLine()) != null) {
                // diffent operations to fit page A and B
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

When switching pages (A -> B), I tried to let one thread in A wait to avoid competing for the same message with the other in B.

// page A
@Override
public void onPause() {
    super.onPause();
    System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>>>> onPause");
    try {
        receiveMsgThread.wait();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

only to find a exception

java.lang.RuntimeException: Unable to pause activity {com.example.chat/com.example.chat.B_Container_Activity}: java.lang.IllegalMonitorStateException: object not locked by thread before wait()

and I found a solution in the StackOverflow but it didn't work in my project.

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

"didn't work" means there is no exception but the App is stucked when clicking the button in A and trying to swithing pages (A -> B).

I/System.out: >>>>>>>>>>>>>>>>>>>>>>>>>>>>>> onPause
I/ZrHungImpl: sendRawEvent. wpId = 257
E/HiView.HiEvent: length is 0 or exceed MAX: 1024
I/om.example.cha: Thread[7,tid=21291,WaitingInMainSignalCatcherLoop,Thread*=0x7b849a5c00,peer=0x16644030,"Signal Catcher"]: reacting to signal 3
I/om.example.cha: 
I/om.example.cha: Wrote stack traces to tombstoned
I/ZrHungImpl: sendRawEvent. wpId = 258
E/HiView.HiEvent: length is 0 or exceed MAX: 1024

score 1 · Accepted Answer · answered Nov 19 '22 at 06:18

You don't. You NEVER wait on the main thread. Attempting to do so will cause an unresponsive UI and a system popup telling the user your app is unresponsive. It can even cause a watchdog to trip and your app to be killed with an ANR. Instead, let it run in the background and put up a loading screen if you need to until the result is done.

Also, that's not how you use wait(). Wait in Java has to do with concurrency locking. Every object has a wait function, it has nothing to do with threads. It doesn't wait for a thread to be done. You use join for that. But you should still never call join on the main thread.

How can I let a thread wait successfully in Android?

1 Answers1