Question

我正在编写一个程序，它返回一个字符串的ArrayList。问题是，当我调用方法时，列表尚未填充，所以我得到一个空列表。我尝试了一个线程，但现在我在调用方法时得到一个null引用。顺便说一下，我必须实现异步任务，否则在尝试使用InetAddress时会出现异常。

private class DeviceManager extends Thread {

    private ArrayList<String> deviceList;
    private String networkIP;

    public DeviceManager(String networkIP) {
        this.networkIP = networkIP;
    }

    public void run() {
        getDeviceList();
    }

    public ArrayList<String> getDeviceList() {
        new AsyncTask<Void, Void, Void>() {

            @Override
            protected Void doInBackground(Void... params) {
                try {
                    deviceList = new ArrayList<String>();
                    InetAddress address;

                    Log.i("NetworkIPgetDeviceList", networkIP);

                    String deviceIP = networkIP;

                    for (int i = 0; i < 255; i++) {
                        address = InetAddress.getByName(deviceIP += "" + i);
                        if (address.isReachable(2000)) {    
                            Log.i("Devicefound", deviceIP);
                            deviceList.add(deviceIP);
                        }
                        deviceIP = networkIP;
                    }

                } catch (UnknownHostException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                return null;
            }
        }.execute();
        return deviceList;
    }

    public ArrayList<String> getList() {
        return this.deviceList;
    }
}

Answer 1

你这样做完全错了。 Thread在后台运行，AsyncTask也是如此，所以基本上你是在后台运行后台任务。启

试试这个：

public class DeviceManager {

    private ArrayList<String> deviceList;
    private String networkIP;
    private ConnectionCompleteListener listener;

    public interface ConnectionCompleteListener {
        void onSuccess();

        void onFailure();
    }

    public void setConnectionCompleteListener(ConnectionCompleteListener listener) {
        this.listener = listener;
    }

    public DeviceManager(String networkIP) {
        this.networkIP = networkIP;
    }

    public void getDeviceList() {
        new AsyncTask<Void, Void, Boolean>() {

            @Override
            protected void onPostExecute(Boolean result) {
                if(result) listener.onSuccess();
                else listener.onFailure();
            }

            @Override
            protected Boolean doInBackground(Void... params) {
                try {
                    deviceList = new ArrayList<String>();
                    InetAddress address;

                    Log.i("NetworkIPgetDeviceList", networkIP);

                    String deviceIP = networkIP;

                    for (int i = 0; i < 255; i++) {
                        address = InetAddress.getByName(deviceIP += "" + i);
                        if (address.isReachable(2000)) {
                            Log.i("Devicefound", deviceIP);
                            deviceList.add(deviceIP);
                        }
                        deviceIP = networkIP;
                    }
                    return true;
                } catch (UnknownHostException e) {
                    e.printStackTrace();
                    return false;
                } catch (IOException e) {
                    e.printStackTrace();
                    return false;
                }
                return null;
            }
        }.execute();
    }

    public ArrayList<String> getList() {
        return this.deviceList;
    }
}

然后在你的另一堂课：

private class classname{
    DeviceManager manager=new DeviceMnager(networkIp);
    manger.setConnectionCompleteListener(new DeviceManager.ConnectionCompleteListener() {
        @Override
        public void onSuccess() {
            // get your list here
            manager.getList();
        }

        @Override
        public void onFailure() {
            // connection failed show error
        }
    });
}

Answer 2

Artur您在代码中所做的是启动一个线程来检索设备列表，然后启动另一个线程（AsyncTask）来实际创建设备列表。所以你有三个线程同时在这里运行（假设你在UIThread中使用DeviceManager类）。 getDeviceList()返回 null 的原因是因为AsyncTasks doInBackground尚未运行以收集您的设备列表，因此可能正在等待其计划的机会。所以得出结论，你只需要一个线程（UIThread除外），它可以是一个线程或AsyncTask（更好，因为它提供了更好的控制），因为生锈的大脑已经在他的回答中使用了。我更喜欢将DeviceManager设置为AsyncTask（只是设备管理器的唯一任务是检索设备列表），如下面的代码所示。

in AsyncTask doInBackground在后台线程中运行（顾名思义），onPostExecute在doInBackground之后的UI线程上运行

class DeviceManager extends AsyncTask<String, Void, List<String>> {

    private ConnectionCompleteListener listener;

    public interface ConnectionCompleteListener {
        void onSuccess(List<String> deviceList);

        // if you need to know reason for failure you can add
        // parameter to onFailure
        void onFailure();
    }

    public DeviceManager(ConnectionCompleteListener listener) {
        this.listener = listener;
    }

    @Override
    protected List<String> doInBackground(String... params) {
        List<String> deviceList = new ArrayList<>();
        String networkIP = params[0];
        try {
            InetAddress address;
            Log.i("NetworkIPgetDeviceList", networkIP);

            String deviceIP = networkIP;
            for (int i = 0; i < 255; i++) {
                address = InetAddress.getByName(deviceIP += "" + i);
                if (address.isReachable(2000)) {
                    Log.i("Devicefound", deviceIP);
                    deviceList.add(deviceIP);
                }
                deviceIP = networkIP;
            }
        } catch (IOException e) {
            deviceList = null;
            e.printStackTrace();
        }
        return deviceList;
    }

    @Override
    protected void onPostExecute(List<String> deviceList) {
        if (deviceList == null) {
            this.listener.onFailure();
        } else {
            this.listener.onSuccess(deviceList);
        }
    }
}

所以在你的活动中你可以打电话

new DeviceManager(new DeviceManager.ConnectionCompleteListener
            () {
        @Override
        public void onSuccess(List<String> deviceList) {

        }

        @Override
        public void onFailure() {

        }
    }).execute("YOUR_NETWORK_IP");

Answer 3

您正在获取空数组列表，因为当您使用异步任务获取数组列表和异步任务时，doINBackground方法在不同的线程上运行（意味着不在主线程上）。因此，当您的程序运行时，您的程序不会等待异步任务响应。

你可以这样解决这个问题...... 在异步任务类中使用onPostExecute方法并返回arraylist

@Override
protected void onPostExecute(Void result) {
 //return array list here
getList();
}

希望这会对你有所帮助

Answer 4

首先，您不需要将DeviceManager作为线程，因为您在getDeviceList中运行的任务将在另一个新线程中启动。秒你不应该等待主（UI）线程，所以而不是等待回调是一个更好的机制。

如果你坚持使用相同的代码，试试这个..

public class DeviceManager extends Thread {
private ArrayList<String> deviceList;

private String networkIP;
private boolean dataAvailable;

public DeviceManager(String networkIP) {
    this.networkIP = networkIP;
}

public void run() {
    getDeviceList();
}

public ArrayList<String> getDeviceList() {

    new AsyncTask<Void, Void, Void>() {
        @Override
        protected Void doInBackground(Void... params) {
            try {
                deviceList = new ArrayList<String>();
                InetAddress address;

                 Log.i("NetworkIPgetDeviceList", networkIP);

                String deviceIP = networkIP;

                for (int i = 0; i < 255; i++) {
                    System.out.println("checking " + i);
                    address = InetAddress.getByName(deviceIP += "" + i);
                    if (address.isReachable(2000)) {

                         Log.i("Devicefound", deviceIP);

                        deviceList.add(deviceIP);
                    }
                    deviceIP = networkIP;
                }

            } catch (UnknownHostException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
            dataAvailable = true;
            synchronized (DeviceManager.this) {
                DeviceManager.this.notify();
            }
            return null;
        }

    }.execute();

    return deviceList;
}

synchronized public ArrayList<String> getList() {

    while (!dataAvailable) {
        try {
            wait();
        } catch (InterruptedException e) {
        }

    }
    return this.deviceList;
}



}

如何使程序等待java中的方法

4 个答案: