这个bug让我困扰了好几个小时。我遇到了NullPointerException的问题。但是这个错误出现的不一定,只有在启动应用程序时偶尔会出现。因此我不确定是什么原因导致的。
非常抱歉我提问时使用了冗长的错误日志,但我没有找到其他询问方式。
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)
这个问题出现的代码如下:
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);
}
}
}
}
}
如果你有疑问,错误日志中的第74行(更具体地说是这个at com.myproject.android.ImageDownloaderThread.queueImage(ImageDownloaderThread.java:74)
),引用了queueImage()
函数中的.obtainMessage(MESSAGE_DOWNLOAD, pos, 0, url)
代码行。
编辑
根据Loop的建议,在调用queueImage()
时,mHandler
为空。那么,在执行任何queueImage()
调用之前,如何保证mHandler
由onLooperPrepared()
初始化?
onLooperPrepared()
在Looper循环之前被调用。确切的措辞是“回调方法,如果需要在Looper循环之前执行一些设置,则可以显式地覆盖它。”(更多细节,请参见链接http://developer.android.com/reference/android/os/HandlerThread.html#onLooperPrepared())。现在,为什么`queueImage()`会在`onLooperPrepared()`之前被调用呢? - GreesomHandler
确实是null
。我更喜欢你第二个建议的解决方案,但我不想跳过一些queueImage()
调用。我该如何让queueImage()
等待onLooperPrepared()
执行完成以便完成其初始化呢? - GreesoqueueImage()
何时何地被调用了吗? - Damian PetlaqueueImage
和onLooperPrepared
通常来自不同的线程,因此由于竞争条件可能会丢失消息。在这种情况下需要适当的同步。 - fdermishin