图像不会缓存在内部存储器中，并且在没有外部 SD 卡的设备上会崩溃。我已经尝试了我所知道的，但没有任何效果。这是 ImageCache.java

  package com.minecraftpix.android.bitmapfun.util;

  import com.minecraftpix.BuildConfig;

import android.annotation.TargetApi;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Bitmap.CompressFormat;
import android.graphics.BitmapFactory;
import android.graphics.drawable.BitmapDrawable;
import android.os.Bundle;
import android.os.Environment;
import android.os.StatFs;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.util.LruCache;
import android.util.Log;

import java.io.File;
import java.io.FileDescriptor;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.lang.ref.SoftReference;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.HashSet;
import java.util.Iterator;


public class ImageCache {
private static final String TAG = "ImageCache";

// Default memory cache size in kilobytes
private static final int DEFAULT_MEM_CACHE_SIZE = 1024 * 5; // 5MB

// Default disk cache size in bytes
private static final int DEFAULT_DISK_CACHE_SIZE = 1024 * 1024 * 10; // 10MB

// Compression settings when writing images to disk cache
private static final CompressFormat DEFAULT_COMPRESS_FORMAT = CompressFormat.JPEG;
private static final int DEFAULT_COMPRESS_QUALITY = 70;
private static final int DISK_CACHE_INDEX = 0;

// Constants to easily toggle various caches
private static final boolean DEFAULT_MEM_CACHE_ENABLED = true;
private static final boolean DEFAULT_DISK_CACHE_ENABLED = true;
private static final boolean DEFAULT_INIT_DISK_CACHE_ON_CREATE = false;

private DiskLruCache mDiskLruCache;
private LruCache<String, BitmapDrawable> mMemoryCache;
private ImageCacheParams mCacheParams;
private final Object mDiskCacheLock = new Object();
private boolean mDiskCacheStarting = true;

private HashSet<SoftReference<Bitmap>> mReusableBitmaps;

/**
 * Create a new ImageCache object using the specified parameters. This should not be
 * called directly by other classes, instead use
 * {@link ImageCache#getInstance(FragmentManager, ImageCacheParams)} to fetch an ImageCache
 * instance.
 *
 * @param cacheParams The cache parameters to use to initialize the cache
 */
private ImageCache(ImageCacheParams cacheParams) {
    init(cacheParams);
}

/**
 * Return an {@link ImageCache} instance. A {@link RetainFragment} is used to retain the
 * ImageCache object across configuration changes such as a change in device orientation.
 *
 * @param fragmentManager The fragment manager to use when dealing with the retained fragment.
 * @param cacheParams The cache parameters to use if the ImageCache needs instantiation.
 * @return An existing retained ImageCache object or a new one if one did not exist
 */
public static ImageCache getInstance(
        FragmentManager fragmentManager, ImageCacheParams cacheParams) {

    // Search for, or create an instance of the non-UI RetainFragment
    final RetainFragment mRetainFragment = findOrCreateRetainFragment(fragmentManager);

    // See if we already have an ImageCache stored in RetainFragment
    ImageCache imageCache = (ImageCache) mRetainFragment.getObject();

    // No existing ImageCache, create one and store it in RetainFragment
    if (imageCache == null) {
        imageCache = new ImageCache(cacheParams);
        mRetainFragment.setObject(imageCache);
    }

    return imageCache;
}

/**
 * Initialize the cache, providing all parameters.
 *
 * @param cacheParams The cache parameters to initialize the cache
 */
private void init(ImageCacheParams cacheParams) {
    mCacheParams = cacheParams;

    // Set up memory cache
    if (mCacheParams.memoryCacheEnabled) {
        if (BuildConfig.DEBUG) {
            Log.d(TAG, "Memory cache created (size = " + mCacheParams.memCacheSize + ")");
        }

        // If we're running on Honeycomb or newer, then
        if (Utils.hasHoneycomb()) {
            mReusableBitmaps = new HashSet<SoftReference<Bitmap>>();
        }

        mMemoryCache = new LruCache<String, BitmapDrawable>(mCacheParams.memCacheSize) {

            /**
             * Notify the removed entry that is no longer being cached
             */
            @Override
            protected void entryRemoved(boolean evicted, String key,
                    BitmapDrawable oldValue, BitmapDrawable newValue) {
                if (RecyclingBitmapDrawable.class.isInstance(oldValue)) {
                    // The removed entry is a recycling drawable, so notify it 
                    // that it has been removed from the memory cache
                    ((RecyclingBitmapDrawable) oldValue).setIsCached(false);
                } else {
                    // The removed entry is a standard BitmapDrawable

                    if (Utils.hasHoneycomb()) {
                        // We're running on Honeycomb or later, so add the bitmap
                        // to a SoftRefrence set for possible use with inBitmap later
                        mReusableBitmaps.add(new SoftReference<Bitmap>(oldValue.getBitmap()));
                    }
                }
            }

            /**
             * Measure item size in kilobytes rather than units which is more practical
             * for a bitmap cache
             */
            @Override
            protected int sizeOf(String key, BitmapDrawable value) {
                final int bitmapSize = getBitmapSize(value) / 1024;
                return bitmapSize == 0 ? 1 : bitmapSize;
            }
        };
    }

    // By default the disk cache is not initialized here as it should be initialized
    // on a separate thread due to disk access.
    if (cacheParams.initDiskCacheOnCreate) {
        // Set up disk cache
        initDiskCache();
    }
}

/**
 * Initializes the disk cache.  Note that this includes disk access so this should not be
 * executed on the main/UI thread. By default an ImageCache does not initialize the disk
 * cache when it is created, instead you should call initDiskCache() to initialize it on a
 * background thread.
 */
public void initDiskCache() {
    // Set up disk cache
    synchronized (mDiskCacheLock) {
        if (mDiskLruCache == null || mDiskLruCache.isClosed()) {
            File diskCacheDir = mCacheParams.diskCacheDir;
            if (mCacheParams.diskCacheEnabled && diskCacheDir != null) {
                if (!diskCacheDir.exists()) {
                    diskCacheDir.mkdirs();
                }
                if (getUsableSpace(diskCacheDir) > mCacheParams.diskCacheSize) {
                    try {
                        mDiskLruCache = DiskLruCache.open(
                                diskCacheDir, 1, 1, mCacheParams.diskCacheSize);
                        if (BuildConfig.DEBUG) {
                            Log.d(TAG, "Disk cache initialized");
                        }
                    } catch (final IOException e) {
                        mCacheParams.diskCacheDir = null;
                        Log.e(TAG, "initDiskCache - " + e);
                    }
                }
            }
        }
        mDiskCacheStarting = false;
        mDiskCacheLock.notifyAll();
    }
}

/**
 * Adds a bitmap to both memory and disk cache.
 * @param data Unique identifier for the bitmap to store
 * @param value The bitmap drawable to store
 */
public void addBitmapToCache(String data, BitmapDrawable value) {
    if (data == null || value == null) {
        return;
    }

    // Add to memory cache
    if (mMemoryCache != null) {
        if (RecyclingBitmapDrawable.class.isInstance(value)) {
            // The removed entry is a recycling drawable, so notify it 
            // that it has been added into the memory cache
            ((RecyclingBitmapDrawable) value).setIsCached(true);
        }
        mMemoryCache.put(data, value);
    }

    synchronized (mDiskCacheLock) {
        // Add to disk cache
        if (mDiskLruCache != null) {
            final String key = hashKeyForDisk(data);
            OutputStream out = null;
            try {
                DiskLruCache.Snapshot snapshot = mDiskLruCache.get(key);
                if (snapshot == null) {
                    final DiskLruCache.Editor editor = mDiskLruCache.edit(key);
                    if (editor != null) {
                        out = editor.newOutputStream(DISK_CACHE_INDEX);
                        value.getBitmap().compress(
                                mCacheParams.compressFormat, mCacheParams.compressQuality, out);
                        editor.commit();
                        out.close();
                    }
                } else {
                    snapshot.getInputStream(DISK_CACHE_INDEX).close();
                }
            } catch (final IOException e) {
                Log.e(TAG, "addBitmapToCache - " + e);
            } catch (Exception e) {
                Log.e(TAG, "addBitmapToCache - " + e);
            } finally {
                try {
                    if (out != null) {
                        out.close();
                    }
                } catch (IOException e) {}
            }
        }
    }
}

/**
 * Get from memory cache.
 *
 * @param data Unique identifier for which item to get
 * @return The bitmap drawable if found in cache, null otherwise
 */
public BitmapDrawable getBitmapFromMemCache(String data) {
    BitmapDrawable memValue = null;

    if (mMemoryCache != null) {
        memValue = mMemoryCache.get(data);
    }

    if (BuildConfig.DEBUG && memValue != null) {
        Log.d(TAG, "Memory cache hit");
    }

    return memValue;
}

/**
 * Get from disk cache.
 *
 * @param data Unique identifier for which item to get
 * @return The bitmap if found in cache, null otherwise
 */
public Bitmap getBitmapFromDiskCache(String data) {
    final String key = hashKeyForDisk(data);
    Bitmap bitmap = null;

    synchronized (mDiskCacheLock) {
        while (mDiskCacheStarting) {
            try {
                mDiskCacheLock.wait();
            } catch (InterruptedException e) {}
        }
        if (mDiskLruCache != null) {
            InputStream inputStream = null;
            try {
                final DiskLruCache.Snapshot snapshot = mDiskLruCache.get(key);
                if (snapshot != null) {
                    if (BuildConfig.DEBUG) {
                        Log.d(TAG, "Disk cache hit");
                    }
                    inputStream = snapshot.getInputStream(DISK_CACHE_INDEX);
                    if (inputStream != null) {
                        FileDescriptor fd = ((FileInputStream) inputStream).getFD();

                        // Decode bitmap, but we don't want to sample so give
                        // MAX_VALUE as the target dimensions
                        bitmap = ImageResizer.decodeSampledBitmapFromDescriptor(
                                fd, Integer.MAX_VALUE, Integer.MAX_VALUE, this);
                    }
                }
            } catch (final IOException e) {
                Log.e(TAG, "getBitmapFromDiskCache - " + e);
            } finally {
                try {
                    if (inputStream != null) {
                        inputStream.close();
                    }
                } catch (IOException e) {}
            }
        }
        return bitmap;
    }
}

/**
 * @param options - BitmapFactory.Options with out* options populated
 * @return Bitmap that case be used for inBitmap
 */
protected Bitmap getBitmapFromReusableSet(BitmapFactory.Options options) {
    Bitmap bitmap = null;

    if (mReusableBitmaps != null && !mReusableBitmaps.isEmpty()) {
        final Iterator<SoftReference<Bitmap>> iterator = mReusableBitmaps.iterator();
        Bitmap item;

        while (iterator.hasNext()) {
            item = iterator.next().get();

            if (null != item && item.isMutable()) {
                // Check to see it the item can be used for inBitmap
                if (canUseForInBitmap(item, options)) {
                    bitmap = item;

                    // Remove from reusable set so it can't be used again
                    iterator.remove();
                    break;
                }
            } else {
                // Remove from the set if the reference has been cleared.
                iterator.remove();
            }
        }
    }

    return bitmap;
}

/**
 * Clears both the memory and disk cache associated with this ImageCache object. Note that
 * this includes disk access so this should not be executed on the main/UI thread.
 */
public void clearCache() {
    if (mMemoryCache != null) {
        mMemoryCache.evictAll();
        if (BuildConfig.DEBUG) {
            Log.d(TAG, "Memory cache cleared");
        }
    }

    synchronized (mDiskCacheLock) {
        mDiskCacheStarting = true;
        if (mDiskLruCache != null && !mDiskLruCache.isClosed()) {
            try {
                mDiskLruCache.delete();
                if (BuildConfig.DEBUG) {
                    Log.d(TAG, "Disk cache cleared");
                }
            } catch (IOException e) {
                Log.e(TAG, "clearCache - " + e);
            }
            mDiskLruCache = null;
            initDiskCache();
        }
    }
}

/**
 * Flushes the disk cache associated with this ImageCache object. Note that this includes
 * disk access so this should not be executed on the main/UI thread.
 */
public void flush() {
    synchronized (mDiskCacheLock) {
        if (mDiskLruCache != null) {
            try {
                mDiskLruCache.flush();
                if (BuildConfig.DEBUG) {
                    Log.d(TAG, "Disk cache flushed");
                }
            } catch (IOException e) {
                Log.e(TAG, "flush - " + e);
            }
        }
    }
}

/**
 * Closes the disk cache associated with this ImageCache object. Note that this includes
 * disk access so this should not be executed on the main/UI thread.
 */
public void close() {
    synchronized (mDiskCacheLock) {
        if (mDiskLruCache != null) {
            try {
                if (!mDiskLruCache.isClosed()) {
                    mDiskLruCache.close();
                    mDiskLruCache = null;
                    if (BuildConfig.DEBUG) {
                        Log.d(TAG, "Disk cache closed");
                    }
                }
            } catch (IOException e) {
                Log.e(TAG, "close - " + e);
            }
        }
    }
}

/**
 * A holder class that contains cache parameters.
 */
public static class ImageCacheParams {
    public int memCacheSize = DEFAULT_MEM_CACHE_SIZE;
    public int diskCacheSize = DEFAULT_DISK_CACHE_SIZE;
    public File diskCacheDir;
    public CompressFormat compressFormat = DEFAULT_COMPRESS_FORMAT;
    public int compressQuality = DEFAULT_COMPRESS_QUALITY;
    public boolean memoryCacheEnabled = DEFAULT_MEM_CACHE_ENABLED;
    public boolean diskCacheEnabled = DEFAULT_DISK_CACHE_ENABLED;
    public boolean initDiskCacheOnCreate = DEFAULT_INIT_DISK_CACHE_ON_CREATE;

    /**
     * Create a set of image cache parameters that can be provided to
     * {@link ImageCache#getInstance(FragmentManager, ImageCacheParams)} or
     * {@link ImageWorker#addImageCache(FragmentManager, ImageCacheParams)}.
     * @param context A context to use.
     * @param diskCacheDirectoryName A unique subdirectory name that will be appended to the
     *                               application cache directory. Usually "cache" or "images"
     *                               is sufficient.
     */
    public ImageCacheParams(Context context, String diskCacheDirectoryName) {
        diskCacheDir = getDiskCacheDir(context, diskCacheDirectoryName);
    }

    /**
     * Sets the memory cache size based on a percentage of the max available VM memory.
     * Eg. setting percent to 0.2 would set the memory cache to one fifth of the available
     * memory. Throws {@link IllegalArgumentException} if percent is < 0.05 or > .8.
     * memCacheSize is stored in kilobytes instead of bytes as this will eventually be passed
     * to construct a LruCache which takes an int in its constructor.
     *
     * This value should be chosen carefully based on a number of factors
     * Refer to the corresponding Android Training class for more discussion:
     * http://developer.android.com/training/displaying-bitmaps/
     *
     * @param percent Percent of available app memory to use to size memory cache
     */
    public void setMemCacheSizePercent(float percent) {
        if (percent < 0.05f || percent > 0.8f) {
            throw new IllegalArgumentException("setMemCacheSizePercent - percent must be "
                    + "between 0.05 and 0.8 (inclusive)");
        }
        memCacheSize = Math.round(percent * Runtime.getRuntime().maxMemory() / 1024);
    }
}

/**
 * @param candidate - Bitmap to check
 * @param targetOptions - Options that have the out* value populated
 * @return true if <code>candidate</code> can be used for inBitmap re-use with
 *      <code>targetOptions</code>
 */
private static boolean canUseForInBitmap(
        Bitmap candidate, BitmapFactory.Options targetOptions) {
    int width = targetOptions.outWidth / targetOptions.inSampleSize;
    int height = targetOptions.outHeight / targetOptions.inSampleSize;

    return candidate.getWidth() == width && candidate.getHeight() == height;
}

/**
 * Get a usable cache directory (external if available, internal otherwise).
 *
 * @param context The context to use
 * @param uniqueName A unique directory name to append to the cache dir
 * @return The cache dir
 */
public static File getDiskCacheDir(Context context, String uniqueName) {
    // Check if media is mounted or storage is built-in, if so, try and use external cache dir
    // otherwise use internal cache dir
    final String cachePath =
            Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState()) ||
                    !isExternalStorageRemovable() ? getExternalCacheDir(context).getPath() :
                            context.getCacheDir().getPath();

    return new File(cachePath + File.separator + uniqueName);
}

/**
 * A hashing method that changes a string (like a URL) into a hash suitable for using as a
 * disk filename.
 */
public static String hashKeyForDisk(String key) {
    String cacheKey;
    try {
        final MessageDigest mDigest = MessageDigest.getInstance("MD5");
        mDigest.update(key.getBytes());
        cacheKey = bytesToHexString(mDigest.digest());
    } catch (NoSuchAlgorithmException e) {
        cacheKey = String.valueOf(key.hashCode());
    }
    return cacheKey;
}

private static String bytesToHexString(byte[] bytes) {
    // http://stackoverflow.com/questions/332079
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < bytes.length; i++) {
        String hex = Integer.toHexString(0xFF & bytes[i]);
        if (hex.length() == 1) {
            sb.append('0');
        }
        sb.append(hex);
    }
    return sb.toString();
}

/**
 * Get the size in bytes of a bitmap in a BitmapDrawable.
 * @param value
 * @return size in bytes
 */
@TargetApi(12)
public static int getBitmapSize(BitmapDrawable value) {
    Bitmap bitmap = value.getBitmap();

    if (Utils.hasHoneycombMR1()) {
        return bitmap.getByteCount();
    }
    // Pre HC-MR1
    return bitmap.getRowBytes() * bitmap.getHeight();
}

/**
 * Check if external storage is built-in or removable.
 *
 * @return True if external storage is removable (like an SD card), false
 *         otherwise.
 */
@TargetApi(9)
public static boolean isExternalStorageRemovable() {
    if (Utils.hasGingerbread()) {
        return Environment.isExternalStorageRemovable();
    }
    return true;
}

/**
 * Get the external app cache directory.
 *
 * @param context The context to use
 * @return The external cache dir
 */
@TargetApi(8)
public static File getExternalCacheDir(Context context) {
    if (Utils.hasFroyo()) {
        return context.getExternalCacheDir();
    }

    // Before Froyo we need to construct the external cache dir ourselves
    final String cacheDir = "/Android/data/" + context.getPackageName() + "/cache/";
    return new File(Environment.getExternalStorageDirectory().getPath() + cacheDir);
}

/**
 * Check how much usable space is available at a given path.
 *
 * @param path The path to check
 * @return The space available in bytes
 */
@TargetApi(9)
public static long getUsableSpace(File path) {
    if (Utils.hasGingerbread()) {
        return path.getUsableSpace();
    }
    final StatFs stats = new StatFs(path.getPath());
    return (long) stats.getBlockSize() * (long) stats.getAvailableBlocks();
}

/**
 * Locate an existing instance of this Fragment or if not found, create and
 * add it using FragmentManager.
 *
 * @param fm The FragmentManager manager to use.
 * @return The existing instance of the Fragment or the new instance if just
 *         created.
 */
private static RetainFragment findOrCreateRetainFragment(FragmentManager fm) {
    // Check to see if we have retained the worker fragment.
    RetainFragment mRetainFragment = (RetainFragment) fm.findFragmentByTag(TAG);

    // If not retained (or first time running), we need to create and add it.
    if (mRetainFragment == null) {
        mRetainFragment = new RetainFragment();
        fm.beginTransaction().add(mRetainFragment, TAG).commitAllowingStateLoss();
    }

    return mRetainFragment;
}

/**
 * A simple non-UI Fragment that stores a single Object and is retained over configuration
 * changes. It will be used to retain the ImageCache object.
 */
public static class RetainFragment extends Fragment {
    private Object mObject;

    /**
     * Empty constructor as per the Fragment documentation
     */
    public RetainFragment() {}

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Make sure this Fragment is retained over a configuration change
        setRetainInstance(true);
    }

    /**
     * Store a single object in this Fragment.
     *
     * @param object The object to store
     */
    public void setObject(Object object) {
        mObject = object;
    }

    /**
     * Get the stored object.
     *
     * @return The stored object
     */
    public Object getObject() {
        return mObject;
    }
}

}

这是来自 Google Developer Console 的堆栈跟踪

    java.lang.RuntimeException: Unable to start activity     ComponentInfo{com.minecraftpix/com.minecraftpix.pics.ImageGridActivity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
at android.app.ActivityThread.access$2300(ActivityThread.java:125)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:123)
at android.app.ActivityThread.main(ActivityThread.java:4627)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:521)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.minecraftpix.android.bitmapfun.util.ImageCache.getDiskCacheDir(ImageCache.java:514)
at com.minecraftpix.android.bitmapfun.util.ImageCache$ImageCacheParams.<init>(ImageCache.java:463)
at com.minecraftpix.pics.ImageGridFragment.onCreate(ImageGridFragment.java:80)
at android.support.v4.app.Fragment.performCreate(Fragment.java:1437)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:877)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1088)
at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1444)
at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:551)
at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1131)
at android.app.Activity.performStart(Activity.java:3781)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2636)
... 11 more

最佳答案

这个:

Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState()) ||
                                 !isExternalStorageRemovable() ?
    getExternalCacheDir(context).getPath() : context.getCacheDir().getPath();

是可疑的，!isExternalStorageRemovable() 检查覆盖了 MEDIA_MOUNTED 检查。仅仅因为外部存储不可移动，并不意味着它不能卸载。例如，在旧版本的 android 上，默认行为是将外部存储作为 USB 大容量存储安装在主机上。这将卸载它并使其对您的应用程序不可用，无论存储介质是否可以物理移除。

在外部存储不可移动但未安装的情况下，getExternalCacheDir() 将返回 null。

这是假设 context 也是非空的。

关于java - 无需外部 SD 卡缓存位图，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/15869920/