由java引起的。lang.NullPointerException：尝试调用虚拟方法“int android”。图样位图。空对象引用上的getWidth（）

Question

我有BitmapScalingHelper。爪哇：

public class BitmapScalingHelper
{
    public static Bitmap decodeResource(Resources res, int resId, int dstWidth, int dstHeight)
    {
        Options options = new Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(res, resId, options);
        options.inJustDecodeBounds = false;

        options.inSampleSize = calculateSampleSize(options.outWidth, options.outHeight, dstWidth,
                dstHeight);

        Bitmap unscaledBitmap = BitmapFactory.decodeResource(res, resId, options);

        return unscaledBitmap;
    }

    public static Bitmap decodeFile(String filePath, int dstWidth, int dstHeight)
    {
        Options options = new Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeFile(filePath, options);

        options.inJustDecodeBounds = false;
        options.inSampleSize = calculateSampleSize(options.outWidth, options.outHeight, dstWidth,
                dstHeight);

        Bitmap unscaledBitmap = BitmapFactory.decodeFile(filePath, options);

        return unscaledBitmap;
    }


    public static int calculateSampleSize(int srcWidth, int srcHeight, int dstWidth, int dstHeight)
    {
        final float srcAspect = (float)srcWidth / (float)srcHeight;
        final float dstAspect = (float)dstWidth / (float)dstHeight;

        if (srcAspect > dstAspect)
        {
            return srcWidth / dstWidth;
        }
        else
        {
            return srcHeight / dstHeight;
        }
    }

    public static Bitmap createScaledBitmap(Bitmap unscaledBitmap, int dstWidth, int dstHeight)
    {
        Rect srcRect = calculateSrcRect(unscaledBitmap.getWidth(), unscaledBitmap.getHeight());

        Rect dstRect = calculateDstRect(unscaledBitmap.getWidth(), unscaledBitmap.getHeight(),
                dstWidth, dstHeight);

        Bitmap scaledBitmap = Bitmap.createBitmap(dstRect.width(), dstRect.height(),
                Config.ARGB_8888);

        Canvas canvas = new Canvas(scaledBitmap);
        canvas.drawBitmap(unscaledBitmap, srcRect, dstRect, new Paint(Paint.FILTER_BITMAP_FLAG));

        return scaledBitmap;
    }

    public static Rect calculateSrcRect(int srcWidth, int srcHeight)
    {
        System.out.print("Scr" + srcWidth + " " + srcHeight);
        return new Rect(0, 0, srcWidth, srcHeight);
    }

    public static Rect calculateDstRect(int srcWidth, int srcHeight, int dstWidth, int dstHeight)
    {
        final float srcAspect = (float)srcWidth / (float)srcHeight;
        final float dstAspect = (float)dstWidth / (float)dstHeight;

        if (srcAspect > dstAspect)
        {
            return new Rect(0, 0, dstWidth, (int)(dstWidth / srcAspect));
        }
        else
        {
            return new Rect(0, 0, (int)(dstHeight * srcAspect), dstHeight);
        }
    }
}

这门课有：

createScaledBitmap()

...返回缩放的位图图像。

在另一节课上，我有这样的方法：

public Bitmap readSelectedBitmapFromFile(Context context, String fileName)
    {
        DisplayMetrics metrics = new DisplayMetrics();
        WindowManager windowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
        windowManager.getDefaultDisplay().getMetrics(metrics);

        Bitmap scaledBitmap = getDefaultBitmap(context);

        try {
            File themeParentDir = context.getDir(THEME_DIRECTORY_NAME, Context.MODE_PRIVATE); //Creating an internal dir;
            File themeSubDir = new File(themeParentDir, THEME_SUB_DIRECTORY_NAME + getThemeBasedDirectoryNumber(m_SelectedTheme));
            themeSubDir.mkdir();

            File themeFileWithinDir = new File(themeSubDir, fileName); //Getting a file within the dir.

            if(themeFileWithinDir.exists())
            {
                // Part 1: Decode image
                Bitmap unscaledBitmap = BitmapScalingHelper.decodeFile(themeFileWithinDir.getPath(), metrics.widthPixels, metrics.heightPixels);

                // Part 2: Scale image
                scaledBitmap = BitmapScalingHelper.createScaledBitmap(unscaledBitmap, metrics.widthPixels, metrics.heightPixels);
                unscaledBitmap.recycle();
            }

            m_SelectedBitmap = scaledBitmap;

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

        return scaledBitmap;
    }

这段代码在许多设备上运行良好。但它在一些设备上崩溃了。谁能帮帮我吗？

我得到的日志如下：

Fatal Exception: java.lang.RuntimeException: Unable to start activity ComponentInfo: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.graphics.Bitmap.getWidth()' on a null object reference
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3254)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3350)
       at android.app.ActivityThread.access$1100(ActivityThread.java:222)
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1795)
       at android.os.Handler.dispatchMessage(Handler.java:102)
       at android.os.Looper.loop(Looper.java:158)
       at android.app.ActivityThread.main(ActivityThread.java:7229)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
Caused by java.lang.NullPointerException: Attempt to invoke virtual method 'int android.graphics.Bitmap.getWidth()' on a null object reference
       at in.plackal.lovecyclesfree.util.BitmapScalingHelper.createScaledBitmap(SourceFile:62)
       at in.plackal.lovecyclesfree.general.ThemeManager.readSelectedBitmapFromFile(SourceFile:202)
       at in.plackal.lovecyclesfree.activity.SplashActivity.onCreate(SourceFile:70)
       at android.app.Activity.performCreate(Activity.java:6876)
       at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1135)
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3207)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3350)
       at android.app.ActivityThread.access$1100(ActivityThread.java:222)
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1795)
       at android.os.Handler.dispatchMessage(Handler.java:102)
       at android.os.Looper.loop(Looper.java:158)
       at android.app.ActivityThread.main(ActivityThread.java:7229)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)

如果是权限问题，它不应该在Android-M版本下崩溃，但在一些Android-M之前的设备上也会崩溃。

Answer 1

简单的回答：遵循以下步骤...

1. 长按“已安装的应用程序”图标

现在它工作正常了

在此处输入图像描述

在此处输入图像描述

Answer 2

对于Android 10，将此行添加到清单：

android:requestLegacyExternalStorage="true"

请注意，您还必须添加所需的参数：

读外部存储器，写外部存储器

Answer 3

您所面临的问题是，您正在尝试createScaledBitmap函数中的uncaledBitmap上的getWidth（）。显然，您的uncaledBitmap有时是null；并且调用getWidth（）会导致空指针异常。

根本原因是无论出于何种原因，decodeResource都会返回空值。

原因可能包括-

1. 无阅读权限

我建议您修改代码，在解码后的位图上包含一个空检查，记录它，然后在您看到错误发生的特定设备上进行调试。

也可能是您正在重新使用的选项变量在第二次调用解码资源时被不同地解释。您可以尝试在那里传递一个空值。

修改后的代码应该如下-

public class BitmapScalingHelper
{
    public static Bitmap decodeResource(Resources res, int resId, int dstWidth, int dstHeight)
    {
        Options options = new Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(res, resId, options);
        options.inJustDecodeBounds = false;

        options.inSampleSize = calculateSampleSize(options.outWidth, options.outHeight, dstWidth,
                dstHeight);

        options = new Options(); 
        //May use null here as well. The funciton may interpret the pre-used options variable in ways hard to tell.
        Bitmap unscaledBitmap = BitmapFactory.decodeResource(res, resId, options);

        if(unscaledBitmap == null)
        {
            Log.e("ERR","Failed to decode resource - " + resId + " " + res.toString());
            return null;
        }

        return unscaledBitmap;
    }
}