Android memory optimized disk cache

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Preface ：

stay Last article Memory caching is introduced in , The advantage of memory cache is very fast . But it has shortcomings ：

• Small space , The memory cache cannot be very large ;
• Memory may be cleared when it is tight ;
• It will disappear when the application exits , Can't do offline .

Based on the above shortcomings, sometimes we need a second cache , That is disk cache . Everyone should have used news client, Many have offline functions , The realization of this function is disk cache .

DiskLruCache：

stay Android The disk cache used in is mostly based on DiskLruCache Realized , How to use it in detail ？

• Create a disk cache object ：

public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize);

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open() Method accepts four parameters . The first parameter is the cache file address of the data , The second parameter is the version of the current application , The third parameter is the same key How many cache files can be matched . It's usually a biography 1, The fourth parameter is the maximum number of bytes of data that can be cached ,10M？

• Get cache path ：

// Creates a unique subdirectory of the designated app cache directory. Tries to use external
// but if not mounted, falls back on internal storage.
// Create disk cache file . The preferred sdcard, hypothesis sdcard Not mounted or not sdcard Get the default cache Folder 
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);
}

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• Get software version ：

public int getAppVersion(Context context) {
		try {
			PackageInfo packageInfo = context.getPackageManager().getPackageInfo(context.getPackageName(), 0);
			return packageInfo.versionCode;
		} catch (NameNotFoundException e) {
			e.printStackTrace();
		}
		return 1;
	}

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• Complete code, such as the following ：

DiskLruCache mDiskLruCache = null;
try {
	File cacheDir = getDiskCacheDir(context, "thumbnails");
	if (!cacheDir.exists()) {
		cacheDir.mkdirs();
	}
	mDiskLruCache = DiskLruCache.open(cacheDir, getAppVersion(context), 1, 10 * 1024 * 1024);
} catch (IOException e) {
	e.printStackTrace();
}

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• How to use the disk cache created above in detail, such as the following ：

// Join the cache 
public void addBitmapToCache(String key, Bitmap bitmap) {
    // Add to memory cache as before, Put the cache into the memory cache 
    if (getBitmapFromMemCache(key) == null) {
        mMemoryCache.put(key, bitmap);
    }

    // Also add to disk cache, Put the cache into the disk cache 
    synchronized (mDiskCacheLock) {
        if (mDiskLruCache != null && mDiskLruCache.get(key) == null) {
            mDiskLruCache.put(key, bitmap);
        }
    }
}
// Access to the cache 
public Bitmap getBitmapFromDiskCache(String key) {
    synchronized (mDiskCacheLock) {
        // Wait while disk cache is started from background thread
        while (mDiskCacheStarting) {
            try {
                mDiskCacheLock.wait();
            } catch (InterruptedException e) {}
        }
        if (mDiskLruCache != null) {
            return mDiskLruCache.get(key);
        }
    }
    return null;
}

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summary ：

The above is the creation and usage of disk cache . In practice Memory cache And disk cache are used together . Generally, first read data from the memory cache , Assume no more reads from the disk cache .

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