这个错误使我困惑了好几个小时。我正在获取NullPointerException。问题是此错误不一致。当我启动应用程序时会发生这种情况，但偶尔会发生。所以我不确定是什么原因造成的。
我为错误日志中的冗长问题表示歉意，但找不到其他询问方法。
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，thisat com.myproject.android.ImageDownloaderThread.queueImage(ImageDownloaderThread.java:74)引用了.obtainMessage(MESSAGE_DOWNLOAD, pos, 0, url)以下代码中的代码行）queueImage()
根据建议循环的回答，mHandler是空的时候queueImage()被调用。那么，如何保证在执行任何调用之前mHandler被初始化？onLooperPrepared()queueImage()
对我而言，唯一的原因是该queueImage()方法在未初始化之前onLooperPrepared()就mHandler被调用了。
更新
HandlerThread只是调用方法的Thread实现。run()onLooperPrepared()
@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;
}
因此，何时调用取决于启动此线程。如果启动它，并立即在此线程的引用上调用public方法，则可能会遇到竞争状况，mHandler并且无法按时初始化。
一种解决方案是延迟开始处理图像或播放同步技术。但是，我将使用更简单的方法。
只是要清楚一点，您希望mHandler在HandlerThread创建之后立即对其进行初始化，并且您不想从HandlerThread创建主活动的位置显式地进行初始化。
更新2
只需提出以下解决方案。
queueImage()提供简单明了的数据。您可以检查是否mHandler为null，如果为true，则queueImage()向该队列添加参数。在onLoopPrepared()调用when时，检查该队列中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);
}
}
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