字符串变量最终为null，我不知道为什么

时间：2012-11-25 17:13:13

标签： android

我一直在努力创建一个活动，允许用户使用意图拍摄照片并通过电子邮件发送此图像。我创建文件名并将其存储在一个字符串中，但它最终会将其值​​带走。当它被重置为null时，我最终创建了另一个字符串变量来保存其值，因此我可以确保没有任何东西可以访问更改其值，但第二个字符串也最终为null。我使用字符串的值并将其传递给intent，以便将刚刚拍摄的图像添加为附件。任何人都可以看到这个变量为什么以及如何改变它的价值？

这是我的班级，有评论：

public class ImproveActivity extends Activity implements OnClickListener{
private static final String JPEG_FILE_PREFIX = "Improve_";
private static final String JPEG_FILE_SUFFIX = ".jpg";
private Bitmap mImageBitmap;
private ImageView mImageView;
private File storageDir;
private String mCurrentPhotoPath;
Button share, capture;
private String _path;
private File f;
//This variable holds file name
private String imageFileName;
//I am storing the file name variable here to keep its value
private String temp;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
       setContentView(R.layout.improve_layout);
       mImageView = (ImageView)findViewById(R.id.imgview);
       capture = (Button)findViewById(R.id.capture);
       share = (Button)findViewById(R.id.share);

       capture.setOnClickListener(this);
       share.setOnClickListener(this);

}

private void dispatchTakePictureIntent() {

    if(isIntentAvailable(getApplicationContext(), MediaStore.ACTION_IMAGE_CAPTURE)){

        Intent takePictureIntent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
        ;
        try {
            //f is the image
            f = createImageFile();

            takePictureIntent.putExtra(MediaStore.EXTRA_OUTPUT, Uri.fromFile(f));
            startActivityForResult(takePictureIntent, 1);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            Toast.makeText(getApplicationContext(), "error creating image", Toast.LENGTH_LONG).show();
        }

    }
    else{
        Toast.makeText(getApplicationContext(), "no camera available", Toast.LENGTH_LONG).show();
    }


}

@Override
public void onActivityResult(int requestCode, int resultCode, Intent intent){
    Toast.makeText(getApplicationContext(), "returning", Toast.LENGTH_LONG).show();

    if (requestCode == 1) {

        mImageBitmap = BitmapFactory.decodeFile(mCurrentPhotoPath);
        if (mImageBitmap == null) {
            // bitmap still null
        } else {
            // set bitmap in imageview
            mImageView.setImageBitmap(mImageBitmap);
        }
    }

}

private File createImageFile() throws IOException {
    // Create an image file name
    String timeStamp = 
        new SimpleDateFormat("yyyyMMdd_HHmmss").format(new Date());
   imageFileName = JPEG_FILE_PREFIX + timeStamp + "_";
   //display the current image file name
    Toast.makeText(getApplicationContext(), imageFileName, Toast.LENGTH_LONG).show();

    File file = new File(getAlbumDir().getAbsolutePath());
    file.mkdirs();              
    Toast.makeText(getApplicationContext(), imageFileName, Toast.LENGTH_LONG).show();
    File image = File.createTempFile(
        imageFileName, 
        JPEG_FILE_SUFFIX  
    );


    mCurrentPhotoPath = image.getAbsolutePath();
    temp = imageFileName;
    return image;

}

private File getAlbumDir() {
    return storageDir = new File(
            Environment.getExternalStoragePublicDirectory(
                Environment.DIRECTORY_DCIM
            ), 
            "Camera"
        );
}

public static boolean isIntentAvailable(Context context, String action) {
    final PackageManager packageManager = context.getPackageManager();
    final Intent intent = new Intent(action);
    List<ResolveInfo> list =
            packageManager.queryIntentActivities(intent, PackageManager.MATCH_DEFAULT_ONLY);
    return list.size() > 0;
}


@Override
public void onClick(View v) {
    switch(v.getId()){
    case R.id.capture :
        dispatchTakePictureIntent();
        //displaying the value of the string imageFileName, it is what it should be, the file name
        Toast.makeText(getApplicationContext(), imageFileName, Toast.LENGTH_LONG).show();

        //displaying the file name in temp at this point it is still the file name I need
        Toast.makeText(getApplicationContext(), "temp is " +temp, Toast.LENGTH_LONG).show();
        break;
        //Once the image is saved, clicking share to open chooser
    case R.id.share:

        //displaying the temp string the file name is gone it is now null ????
        Toast.makeText(getApplicationContext(), "temp is " +temp, Toast.LENGTH_LONG).show();

        Intent picMessageIntent = new Intent(android.content.Intent.ACTION_SEND);  
        picMessageIntent.setType("image/jpg");  
        File downloadedPic =  new File(  
            Environment.getExternalStoragePublicDirectory(  
            Environment.DIRECTORY_DCIM),  
            "Camera/"+ temp+"jpg");  //I want to use the file name string to send the image via email
        picMessageIntent.putExtra(Intent.EXTRA_STREAM, Uri.fromFile(downloadedPic));  
        startActivity(Intent.createChooser(picMessageIntent, "Send your picture using:"));  
        break;
    }
}

如果有人能说清楚这是怎么回事，我将不胜感激。

这是logcat，我打开活动，拍照，保存，然后返回到活动视图，我按分享，吐司显示选择器打开时的temp值。

11-25 17:56:34.573: D/dalvikvm(12037): GC_EXTERNAL_ALLOC freed 1176 objects / 87312 bytes in 53ms
11-25 17:56:39.313: W/IInputConnectionWrapper(12037): showStatusIcon on inactive InputConnection
11-25 17:56:49.803: W/System.err(12037): java.lang.NullPointerException: Argument must not be null
11-25 17:56:49.808: W/System.err(12037):    at java.io.FileInputStream.<init>(FileInputStream.java:78)
11-25 17:56:49.808: W/System.err(12037):    at java.io.FileInputStream.<init>(FileInputStream.java:134)
11-25 17:56:49.808: W/System.err(12037):    at android.graphics.BitmapFactory.decodeFile(BitmapFactory.java:349)
11-25 17:56:49.808: W/System.err(12037):    at android.graphics.BitmapFactory.decodeFile(BitmapFactory.java:399)
11-25 17:56:49.808: W/System.err(12037):    at com.seweb.app.ImproveActivity.onActivityResult(ImproveActivity.java:94)
11-25 17:56:49.808: W/System.err(12037):    at android.app.Activity.dispatchActivityResult(Activity.java:3890)
11-25 17:56:49.808: W/System.err(12037):    at android.app.ActivityThread.deliverResults(ActivityThread.java:3511)
11-25 17:56:49.808: W/System.err(12037):    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3115)
11-25 17:56:49.808: W/System.err(12037):    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3143)
11-25 17:56:49.813: W/System.err(12037):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2684)
11-25 17:56:49.813: W/System.err(12037):    at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3815)
11-25 17:56:49.813: W/System.err(12037):    at android.app.ActivityThread.access$2400(ActivityThread.java:125)
11-25 17:56:49.813: W/System.err(12037):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2037)
11-25 17:56:49.818: W/System.err(12037):    at android.os.Handler.dispatchMessage(Handler.java:99)
11-25 17:56:49.818: W/System.err(12037):    at android.os.Looper.loop(Looper.java:123)
11-25 17:56:49.818: W/System.err(12037):    at android.app.ActivityThread.main(ActivityThread.java:4627)
11-25 17:56:49.818: W/System.err(12037):    at java.lang.reflect.Method.invokeNative(Native Method)
11-25 17:56:49.818: W/System.err(12037):    at java.lang.reflect.Method.invoke(Method.java:521)
11-25 17:56:49.823: W/System.err(12037):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
11-25 17:56:49.823: W/System.err(12037):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
11-25 17:56:49.823: W/System.err(12037):    at dalvik.system.NativeStart.main(Native Method)
11-25 17:56:50.273: D/dalvikvm(12037): GC_EXTERNAL_ALLOC freed 1920 objects / 108152 bytes in 136ms

4 个答案:

答案 0 :(得分：4)

您应该在permanently save，数据库或通用文件中SharedPreferences使用字符串。

当您离开活动以使用内置相机应用拍摄照片时，您的应用会进入后台。如果需要额外的内存，后台的任何应用程序都有被操作系统杀死的风险。如果发生这种情况，所有变量都会重置为默认的空值...

<强>加成
来自您的LogCat：

java.lang.NullPointerException: Argument must not be null
    ...
    at com.seweb.app.ImproveActivity.onActivityResult(ImproveActivity.java:94)

我猜第94行是：

mImageBitmap = BitmapFactory.decodeFile(mCurrentPhotoPath);

如果应用程序在后台销毁并重新启动，这是有意义的。当应用完全重启时，只会调用 onCreate()，而不是createImageFile()或任何其他方法。

将mCurrentPhotoPath保存在SharedPreferences imageFileName中，并在onActivityResult()为空时在mCurrentPhotoPath中恢复这些值。

使用SharedPreferences
将这些方法添加到您的活动：

private void savePreferences(){
    // We need an Editor object to make preference changes.
    SharedPreferences.Editor editor = getPreferences(MODE_PRIVATE).edit();
    editor.putString("imageFileName", imageFileName);
    editor.putString("mCurrentPhotoPath", mCurrentPhotoPath);

    // Commit the edits!
    editor.commit();
}

private void restorePreferences() {
    SharedPreferences settings = getPreferences(MODE_PRIVATE);
    imageFileName = settings.getString("imageFileName", "");
    mCurrentPhotoPath = settings.getString("mCurrentPhotoPath", "");
}

我忽略了temp，因为您在调试时说它只是imageFileName的副本。
现在我们需要在适当的时候调用这些方法。首先让我们在离开活动之前保存两个字符串来拍照：

try {
    savePreferences();
    ...
    startActivityForResult(takePictureIntent, 1);
}

然后检查是否需要在onActivityResult()中重新加载这些值：

if (requestCode == 1) {
    if(mCurrentPhotoPath == null)
        restorePreferences();

    mImageBitmap = BitmapFactory.decodeFile(mCurrentPhotoPath);
    ...

答案 1 :(得分：0)

尝试初始化变量：

String temp = "";

答案 2 :(得分：0)

是初始化变量阻止获取NullPointerException，但问题是，为什么此时它为null。你确定，如果你得到一个空字符串就拍照吗？请显示堆栈跟踪，也许你的CreateImageFile-Method出了问题，这个方法有些人会引发异常吗？如果是，则无法初始化此String。

答案 3 :(得分：0)

我建议不要将其保存在shareprefrences中，而是将它保存在savedInstanceState中。

你可以打电话到商店：

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    savedInstanceState.putAll(getIntent().getExtras());
    savedInstanceState.putString("currentPhotoPath", mCurrentPhotoPath);
    super.onSaveInstanceState(savedInstanceState);
}

并在onCreate调用恢复：

        if (savedInstanceState != null) {
mCurrentPhotoPath = savedInstanceState.getString("currentPhotoPath", mCurrentPhotoPath);
    }

所以这就是：

  1. mCurrentPhotoPath是事先创建的文件的路径
  2. 用户进入相机，在销毁活动之前保存mCurrentPhotoPath。
  3. 用户返回Activity，onActivityResult之前调用onCreate。 4.在onCreate中，Activity恢复mCurrentPhotoPath
  4. onActivityResult被称为

    5. 您还可以查看这些页面以获得更好的表达方式：

    https://developer.android.com/training/basics/activity-lifecycle/recreating.html

    https://developer.android.com/training/camera/photobasics.html

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