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android - HandlerThread 中的 NullPointerException

转载 作者:塔克拉玛干 更新时间:2023-11-02 22:20:53 29 4
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这个错误让我困惑了几个小时。我收到了 NullPointerException。问题是这个错误不一致。它在我启动应用程序时发生,但只是偶尔发生。所以我不确定是什么原因造成的。

对于错误日志中的冗长问题,我深表歉意,但我找不到另一种提问方式。

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,这个 at com.myproject.android.ImageDownloaderThread.queueImage(ImageDownloaderThread.java:74),引用了 .obtainMessage(MESSAGE_DOWNLOAD, pos, 0, url) queueImage()

中的代码行

编辑

根据Loop的回答中的建议,调用queueImage()mHandlernull .那么,我如何保证 mHandler 在执行任何 queueImage() 调用之前由 onLooperPrepared() 初始化?

最佳答案

对我来说唯一的原因是 queueImage() 方法在 onLooperPrepared() 之前被调用,所以 mHandler 没有初始化。

更新

HandlerThread 只是一个 Thread 实现了 run() 方法,其中调用了 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;
}

所以什么时候调用取决于启动这个线程。如果您启动它并立即对该线程的引用调用公共(public)方法,您可能会遇到竞争条件并且 mHandler 未按时初始化。

一种解决方案是延迟开始处理图像或使用同步技术。但是,我会使用更简单的方法。

明确一点,您希望在创建 HandlerThread 之后立即初始化您的 mHandler,并且您不想从 HandlerThread 已创建。

更新 2

只需提出以下解决方案即可。

queueImage() 提供简单轻量的数据。您可以检查 mHandler 是否为 null,如果为 null,则将 queueImage() 的参数添加到该队列。当 onLoopPrepared() 被调用时,检查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);
}
}

关于android - HandlerThread 中的 NullPointerException,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/25459186/

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