将位图压缩为JPEG时为什么会出现空指针异常？

问题描述

我正在尝试将我的位图图像存储到Items目录中的jpeg文件中。但是，当尝试使用compress ()将位图压缩为JPEG时，会收到空指针异常。我尝试调试，但发现尝试创建的Items目录失败，因此未创建任何目录。此外，位图也不为空。

我似乎找不到关于为什么未创建目录的问题，并且不知道这是压缩位图时是否得到空指针异常的原因。我尝试过请求存储权限，但这不能解决问题。

更新 ：我现在已经能够创建Items目录，但是在压缩位图时仍然会出现空指针异常。

试图将位图保存到Items目录的代码：

public void saveBitmap () {
    boolean dirCreated;
    askForPermission(Manifest.permission.WRITE_EXTERNAL_STORAGE,STORAGE_PERMISSION_CODE);

    File filePath = Environment.getExternalStorageDirectory ();
        File dir = new File (filePath.getAbsolutePath() + "/Items");

        if (!dir.exists ()) {
            try {
                dirCreated = dir.mkdir();

                if (!dirCreated) {
                    Log.d ("notCreated","the directory has failed");
                }

            } catch (SecurityException e) {
                e.printStackTrace();
            }

        } else {
            Log.d ("dirExists","the directory already exists");//I receive this in the Log cat
        }

        file = new File (dir,name + ".jpg");
        Log.d ("filey","file has been created");//I receive this in the log cat

        try {
            outputStream = new FileOutputStream(file);
        } catch (FileNotFoundException e) {
            e.printStackTrace ();
        }

        if (bitmap != null) {
            Log.d ("bitty","the bimap is not null");//I receive this in the log cat
        }

        bitmap.compress (Bitmap.CompressFormat.JPEG,outputStream);

}

将我的资源转换为位图的代码：

public void convertToBitmap () {
    bitmap = BitmapFactory.decodeResource(context.getResources(),resourceId);

}

要求存储权限的代码：

public void askForPermission (String permission,int requestCode) {
    if (isStoragePermissionGranted()) {
        ActivityCompat.requestPermissions((Activity) context,new String [] {permission},requestCode);
    } else {
        Toast.makeText (context,"Permission already granted",Toast.LENGTH_SHORT)
                .show();

    }
}

public void onRequestPermissionsResult (int requestCode,String [] permissions,int [] grantResults) {
    super.onRequestPermissionsResult (requestCode,permissions,grantResults);

    if (requestCode == STORAGE_PERMISSION_CODE) {
        if (grantResults.length > 0 && grantResults [0] == PackageManager.PERMISSION_GRANTED) {
            Toast.makeText(context,"Storage Permission Granted",Toast.LENGTH_SHORT);
        } else {
            Toast.makeText(context,"Storage Permission Denied",Toast.LENGTH_SHORT);
        }
    }


}

public boolean isStoragePermissionGranted () {

    if (ContextCompat.checkSelfPermission(context,Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
        Toast.makeText(context,"permission granted",Toast.LENGTH_SHORT)
                .show();
        return true;
    }

    return false;
}

清单文件：

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

我收到的错误：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.myapps.myapplication/com.myapps.myapplication.grocery_item}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3374)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3513)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2109)
    at android.os.Handler.dispatchMessage(Handler.java:107)
    at android.os.Looper.loop(Looper.java:214)
    at android.app.ActivityThread.main(ActivityThread.java:7682)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:516)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
 Caused by: java.lang.NullPointerException
    at android.graphics.Bitmap.compress(Bitmap.java:1407)
    at com.myapps.myapplication.BitmapFiles.saveBitmap(BitmapFiles.java:86) //THIS IS THE LINE OF CODE CAUSING THE EXCEPTION
    at com.myapps.myapplication.BitmapFiles.<init>(BitmapFiles.java:40)
    at com.myapps.myapplication.MyDatabaseHelper.createBitmapFiles(MyDatabaseHelper.java:40)
    at com.myapps.myapplication.MyDatabaseHelper.upgradeDatabase(MyDatabaseHelper.java:35)
    at com.myapps.myapplication.MyDatabaseHelper.onUpgrade(MyDatabaseHelper.java:63)
    at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:417)
    at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:341)
    at com.myapps.myapplication.grocery_item.accessDataBase(grocery_item.java:35)
    at com.myapps.myapplication.grocery_item.onCreate(grocery_item.java:20)
    at android.app.Activity.performCreate(Activity.java:7815)
    at android.app.Activity.performCreate(Activity.java:7804)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1318)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3349)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3513) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2109) 
    at android.os.Handler.dispatchMessage(Handler.java:107) 
    at android.os.Looper.loop(Looper.java:214) 
    at android.app.ActivityThread.main(ActivityThread.java:7682) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:516) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950) 

解决方法

检查以确保outputStream不为空，如果输出流为空，请检查compress() will throw a NullPointerException()。

,

正如我在Null Pointer Exception when trying to compress Bitmap中看到的那样，您需要执行以下操作：

app.UseMvc(routes =>
{
    routes.MapRoute(
        name: "default",template: "{controller=Home}/{action=Index}/{id?}");


    routes.MapRoute(
        name: "default",template: "{id?}",new { controller = "Home",action = "Index" });
});