连续循环应用程序的正确方法是什么

Question

My question isn't specific to Java but that is the language I'm using to achieve what I want.

I'm experimenting with Bluetooth in Java and have written a simple terminal program ie no GUI interface that searches for nearby Bluetooth devices and lists them. My code is as follows:

import javax.bluetooth.*;

public class BluetoothTest implements DiscoveryListener{

    private static boolean isAlive = true;

    public static void main(String[] args) {
        try {
            LocalDevice ld = LocalDevice.getLocalDevice();
            if (LocalDevice.isPowerOn()){
                System.out.println("Power On.");
                System.out.println("Friendly Name: " + ld.getFriendlyName());
                System.out.println("Address: " + ld.getBluetoothAddress());
                DiscoveryAgent da = ld.getDiscoveryAgent();
                da.startInquiry(DiscoveryAgent.GIAC,new BluetoothTest());
                while (isAlive){
                    /* Sleep */
                    try {
                        Thread.sleep(200);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            } else {
                System.out.println("Power Off.");
            }
        } catch (BluetoothStateException e) {
            System.out.println(e.toString());
        }
    }

    public void setAlive(boolean status){
        isAlive = status;
    }

    public void deviceDiscovered(RemoteDevice rd, DeviceClass dc){
        try{
            System.out.println(rd.getFriendlyName(true));
        } catch (java.io.IOException e){
            System.out.println(e.toString());
        }
    }

    public void inquiryCompleted(int discType){
        isAlive = false;
    }

    public void servicesDiscovered(int transID, ServiceRecord[] sr){
    }

    public void serviceSearchCompleted(int transID, int respCode){
    }
}

The startInquiry() of DiscoveryAgent object returns immediately and any devices that are discovered are returned to the DiscoveryListener interface which I have implemented. The problem is unless I include the while() loop the program will terminate before any devices are discovered.

Just how do applications efficiently stay resident? Is it achieved by having a separate 'working' thread and a main thread which spawns the working thread but itself sleeps until the worker had finished?

Answer 1

You may use Object wait / notify mechanism to hold main method until BluetoothTest notifies on a common object lock.

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Just a pseudo code,

Define a static final Object _mutex = new Object();

In main method after call of startInquiry call _mutex.wait(); This will hold the main thread.

In inquiryCompleted call _mutex.notify(); This will release the main thread.

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Please note that this code will work only if startInquiry create a new thread and invokes call back methods. I am not much aware of DiscoverAgent class. So if that's not the case, the above solution might now work.

Answer 2

Java programs will stay running until all threads that are not marked as "daemon" exit. If the Main thread exits and there are no other threads running then the program will quit. If you have spawned another thread (or if DiscoveryAgent is running in another thread) that is not a "daemon" thread, then Java will keep running until the thread exits. Putting the loop in main is a fine way of doing it although as @jatanp mentioned, using wait / notify is cleaner.

Couple of things about your code:

• It is strange to have your Main code also be your DiscoveryListener . I would isolate that functionality in another class:

   public class BluetoothTest { ... private static class OurListener implements DiscoveryListener { } } 

• Because you have 2 threads that are reading from the same variable isAlive , it should be marked as volatile or you need to synchronize around it.