Question

我已经编写了一段代码，假设使用url从互联网上下载图像并将其显示在ImageView中，问题是我得到一个空指针异常，我无法找到问题

以下是活动

中的代码段

public void onClick(View v) {
    switch (v.getId()) {
    case R.id.btnShow:
        DownloadImageTask download=new DownloadImageTask(this);
        String imageUrl = ((EditText) findViewById(R.id.editUrl)).getText().toString(); 
        download.execute(imageUrl); 

        break;

    }
}

这里是AsyncTask代码

public class DownloadImageTask extends AsyncTask<String, Integer, Bitmap> {
    private Activity mActivity;  
    private ProgressDialog mDialog;


    public DownloadImageTask(Activity mActivity) {
        super();
        this.mActivity = mActivity;
    }

    protected void onPreExecute() {
        ImageView imageView = (ImageView) mActivity.findViewById(R.id.imageView1);
        imageView.setImageBitmap(null);
        mDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
        mDialog.setCancelable(true);
        mDialog.setMessage("Loading...");
        mDialog.setProgress(0);       
    }

    protected Bitmap doInBackground(String... urls) {
        Log.d("doInBackground", "starting download of image");        
        Bitmap image = downloadImage(urls[0]);
        return image;
    } 
    protected void onProgressUpdate(Integer... progress) {   
        mDialog.show();
        mDialog.setProgress(progress[0]);        
    }

    protected void onPostExecute(Bitmap result) {
        if (result != null) {            
            ImageView imageView = (ImageView) mActivity.findViewById(R.id.imageView1);
            imageView.setImageBitmap(result);
        }
        else {

        }

        mDialog.dismiss();
    }

    private Bitmap downloadImage(String urlString) {
        URL url;        
        try {
            url = new URL(urlString);
            HttpURLConnection httpCon = (HttpURLConnection) url.openConnection();          

            InputStream is = httpCon.getInputStream();          
            int fileLength = httpCon.getContentLength();
            mDialog.setMax(fileLength);
            ByteArrayOutputStream buffer = new ByteArrayOutputStream();
            int nRead, totalBytesRead = 0;          
            byte[] data = new byte[2048];

            while ((nRead = is.read(data, 0, data.length)) != -1) {
                buffer.write(data, 0, nRead);
                totalBytesRead += nRead;                
                publishProgress(totalBytesRead);               
            }          

            byte[] image = buffer.toByteArray();             
            Bitmap bitmap = BitmapFactory.decodeByteArray(image, 0, image.length);             
            return bitmap;
        } catch (Exception e) {            
            e.printStackTrace();
        }     
        return null;
    }
}

这就是logCat错误

12-17 16:15:33.938: E/AndroidRuntime(29324): FATAL EXCEPTION: main
12-17 16:15:33.938: E/AndroidRuntime(29324): Process: com.example.movieandroidproject, PID: 29324
12-17 16:15:33.938: E/AndroidRuntime(29324): java.lang.NullPointerException
12-17 16:15:33.938: E/AndroidRuntime(29324):    at com.example.asyncTask.DownloadImageTask.onPreExecute(DownloadImageTask.java:30)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:587)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at android.os.AsyncTask.execute(AsyncTask.java:535)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at com.example.movieandroidproject.ChooseMovieAdd.onClick(ChooseMovieAdd.java:89)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at android.view.View.performClick(View.java:4461)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at android.view.View$PerformClick.run(View.java:18525)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at android.os.Handler.handleCallback(Handler.java:733)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at android.os.Handler.dispatchMessage(Handler.java:95)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at android.os.Looper.loop(Looper.java:136)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at android.app.ActivityThread.main(ActivityThread.java:5118)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at java.lang.reflect.Method.invokeNative(Native Method)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at java.lang.reflect.Method.invoke(Method.java:515)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:794)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:610)
12-17 16:15:33.938: E/AndroidRuntime(29324):    at dalvik.system.NativeStart.main(Native Method)

Answer 1

首先，您应该关闭输入流。

buffer.close();
is.close();

sencodly你应该检查是否有任何抛出和捕获的异常，因为你在函数的末尾返回null。

try {
        url = new URL(urlString);
        HttpURLConnection httpCon = (HttpURLConnection) url.openConnection();          

        InputStream is = httpCon.getInputStream();          
        int fileLength = httpCon.getContentLength();
        mDialog.setMax(fileLength);
        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
        int nRead, totalBytesRead = 0;          
        byte[] data = new byte[2048];

        while ((nRead = is.read(data, 0, data.length)) != -1) {
            buffer.write(data, 0, nRead);
            totalBytesRead += nRead;                
            publishProgress(totalBytesRead);               
        }          

        byte[] image = buffer.toByteArray();             
        Bitmap bitmap = BitmapFactory.decodeByteArray(image, 0, image.length);             
        return bitmap;
    } catch (Exception e) { 
        // here           
        e.printStackTrace();
    }

Answer 2

调用

时，您的mDialog字段未初始化

    mDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);

Answer 3

public DownloadImageTask(Activity mActivity) {
    super();
    this.mActivity = mActivity;
    this.mDialog = new ProgressDialog(this);
}

但这是一个糟糕的解决方案。至于我，最好在活动中创建showProgress和hideProgress方法，并在你的任务中的onPreExecute和onPostExecute中调用它们。

Answer 4

我认为问题不在AsyncTask中，而是问题在进度条中。你还没有初始化它：

mProgress = (ProgressBar) findViewById(R.id.progress_bar);   // This line is missing...

Answer 5

您可以简化用于下载图像的方法

public Bitmap downloadImage(String src){
        try {
            URL url = new URL(src);
            HttpURLConnection connection = (HttpURLConnection)url.openConnection();
            connection.setDoInput(true);
            connection.connect();
            InputStream inputStream = connection.getInputStream();
            Bitmap myBitmap = BitmapFactory.decodeStream(inputStream);
            return  myBitmap;
        }catch (Exception e){
            e.printStackTrace();
            return null;
        }
    }

然后你就像使用它一样：

protected Bitmap doInBackground(String... urls) {
        Log.d("doInBackground", "starting download of image");        
        Bitmap image = downloadImage(urls[0]);
        return image;
    } 
    protected void onProgressUpdate(Integer... progress) {   
        mDialog.show();
        mDialog.setProgress(progress[0]);        
    }

    protected void onPostExecute(Bitmap result) {
        if (result != null) {            
            ImageView imageView = (ImageView) mActivity.findViewById(R.id.imageView1);
            imageView.setImageBitmap(result);
        }
        else {

        }

        mDialog.dismiss();
    }

使用AsyncTask下载图像（url）

5 个答案: