Question

这个错误困扰了我好几个小时。我收到 NullPointerException 。问题是这个错误不一致。它发生在我启动应用程序时，但只是偶尔发生。所以我不确定是什么导致它。

我为错误日志的冗长问题道歉，但我找不到另一种询问方式。

错误日志如下：

FATAL EXCEPTION: main
Process: com.myproject.android, PID: 22175
java.lang.NullPointerException
    at com.myproject.android.ImageDownloaderThread.queueImage(ImageDownloaderThread.java:74)
    at com.myproject.android.NewsItemPagerActivity$NewsItemFragmentStatePagerAdapter.getItem(NewsItemPagerActivity.java:325)
    at android.support.v13.app.FragmentStatePagerAdapter.instantiateItem(FragmentStatePagerAdapter.java:109)
    at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:832)
    at android.support.v4.view.ViewPager.populate(ViewPager.java:982)
    at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
    at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
    at android.view.View.measure(View.java:16497)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
    at android.view.View.measure(View.java:16497)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
    at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:327)
    at android.view.View.measure(View.java:16497)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
    at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2291)
    at android.view.View.measure(View.java:16497)
    at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1912)
    at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1109)
    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1291)
    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:996)
    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5600)
    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
    at android.view.Choreographer.doCallbacks(Choreographer.java:574)
    at android.view.Choreographer.doFrame(Choreographer.java:544)
    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
    at android.os.Handler.handleCallback(Handler.java:733)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:136)
    at android.app.ActivityThread.main(ActivityThread.java:5001)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
    at dalvik.system.NativeStart.main(Native Method)

发生这种情况的代码如下所示：

package com.myproject.android;

import java.io.IOException;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.Message;
import android.util.Log;

/*
 * This class is used to download images in the background thread
 */
public class ImageDownloaderThread<Token> extends HandlerThread {

    private static final String TAG = "ImageDownloader";
    private static final int MESSAGE_DOWNLOAD = 0;

    // This is the handler attached to the looper
    Handler mHandler; 






    // The is used as a reference to the main UI thread's handler
    Handler mResponseHandler;

    // This is a listener object that is used to update the main UI thread with the image that is downloaded
    Listener mListener;

    // This is the interface needed when a listener is created. It forces an implementation of the callback in the main UI thread
    public interface Listener {
        void onImageDownloaded(Bitmap image, int pos);
    }

    // Set the listener
    public void setListener(Listener listener) {
        mListener = listener;
    }





    // Constructor
    public ImageDownloaderThread(Handler responseHandler) {
        super(TAG);
        mResponseHandler = responseHandler; // Set the response handler to the one passed from the main thread
    }


    // This method executes some setup before Looper loops for each message
    @Override
    protected void onLooperPrepared() {

        // Create a message handler to handle the message queue
        mHandler = new MessageHandler(ImageDownloaderThread.this);
    }


    // This method is used to add a message to the message queue, so that it can be handled later
    // ... this method is called by the main UI thread to add the message to the queue of the current thread to be handled later
    public void queueImage(String url, int pos) {

        mHandler
            .obtainMessage(MESSAGE_DOWNLOAD, pos, 0, url)
            .sendToTarget();
    }





    // This method is used to download the image  
    private void handleRequest(String url, int pos) {

        try {

            // first check if the url is empty. if it is, then return
            if (url == null) {
                return;
            }

            // Download the image
            byte[] bitmapBytes = new NewsItemsFetcher().getUrlBytes(url);

            // Generate a bitmap
            final Bitmap bitmap = BitmapFactory.decodeByteArray(bitmapBytes, 0, bitmapBytes.length);

            // Set position as 'final'
            final int position = pos;


            // We are using mResponseHandler.post(Runnable) to send a message to the response handler
            // This message will eventually result in the main thread updating the UI with the image
            mResponseHandler.post(new Runnable() {
                @Override
                public void run() {                 
                    mListener.onImageDownloaded(bitmap, position);

                }
            });

        }

        catch (HttpResponseException httpe) {
            // TODO: Handle http response not OK
            Log.e(TAG, "Error in server response", httpe);
        }

        catch (IOException ioe) {
            // TODO: Handle download error
            Log.e(TAG, "Error downloading image", ioe);
        }

    }


    class MessageHandler extends Handler {

        private final ImageDownloaderThread<Token> mImageDownloader;

        MessageHandler(ImageDownloaderThread<Token> imageDownloader) {
            mImageDownloader = imageDownloader;
        }

        // This method is used to process the message that is waiting in the queue 
        @Override
        public void handleMessage(Message msg) {

            // First, check if the message is to download an image
            if (msg.what == MESSAGE_DOWNLOAD) {

                // Call the handleRequest() function which will eventually download the image
                String url = (String)msg.obj;
                int pos = msg.arg1;


                if (mImageDownloader != null) {
                    mImageDownloader.handleRequest(url, pos);
                }

            }
        }

    }

}

如果您想知道错误日志中的第74行（更具体地说，这是at com.myproject.android.ImageDownloaderThread.queueImage(ImageDownloaderThread.java:74)），请参考.obtainMessage(MESSAGE_DOWNLOAD, pos, 0, url)

中的queueImage()代码行

修改

根据循环的回答中的建议，mHandler在调用queueImage()时 null 。那么，在执行任何mHandler来电之前，如何保证onLooperPrepared() queueImage()初始化？

Answer 1

我唯一的原因是在queueImage()之前调用onLooperPrepared()方法，因此mHandler未初始化。

<强>更新

HandlerThread只是Thread的{{1}}方法，其中run()被调用。

onLooperPrepared()

因此，当它被调用时取决于启动此线程。如果你启动它并立即在这个线程的引用上调用public方法，你可能会遇到竞争条件而@Override public void run() { mTid = Process.myTid(); Looper.prepare(); synchronized (this) { mLooper = Looper.myLooper(); notifyAll(); } Process.setThreadPriority(mPriority); onLooperPrepared();//It's HERE Looper.loop(); mTid = -1; }没有按时初始化。

一种解决方案是延迟开始处理图像或使用同步技术。但是，我会用更简单的方法。

为了清楚起见，您希望在创建mHandler之后初始化mHandler，并且您不希望在主要活动中明确地HandlerThread HandlerThread已创建。

更新2

得出以下解决方案。

queueImage()提供简单明了的数据。您可以检查mHandler是否为空，如果它是{true}将queueImage()的参数添加到该队列。调用onLoopPrepared()时，检查该队列是否有任何内容并处理该数据。

private LinkedBlockingQueue<Pair<String,Integer>> mQueue = new LinkedBlockingQueue<Pair<String,Integer>>();

public void queueImage(String url, int pos) {
    if (mHandler == null) {
        mQueue.put(new Pair<String,Integer>(url, pos));
        return;
    }
    mHandler
        .obtainMessage(MESSAGE_DOWNLOAD, pos, 0, url)
        .sendToTarget();
}

@Override
protected void onLooperPrepared() {

    // Create a message handler to handle the message queue
    mHandler = new MessageHandler(ImageDownloaderThread.this);
    //TODO check the queue here, if there is data take it and process
    //you can call queueImage() once again for each queue item
    Pair<String, Integer> pair = null;
    while((pair = mQueue.poll()) != null) {
        queueImage(pair.first, pair.second);
    }
}

Answer 2

我遇到同样的问题。我在排队消息之前调用wait（）并在onLooperPrepared中调用notifyAll（）调用解决了。这不需要存储待处理消息的其他变量。

HandlerThread中的NullPointerException

修改

2 个答案: