How to cache bitmaps into native memory

explanation

the sample code shows how to store 2 different bitmaps (small ones, but it's just a demo), recycle the original java ones, and later restore them to java instances and use them.

as you might guess, the layout has 2 imageViews. i didn't include it in the code since it's quite obvious.

do remember to change the code to your own package if you need, otherwise things won't work.

MainActivity.java - how to use:

package com.example.jnibitmapstoragetest;...public class MainActivity extends Activity  {  @Override  protected void onCreate(final Bundle savedInstanceState)    {    super.onCreate(savedInstanceState);    //    Bitmap bitmap=BitmapFactory.decodeResource(getResources(),R.drawable.ic_launcher);    final JniBitmapHolder bitmapHolder=new JniBitmapHolder(bitmap);    bitmap.recycle();    //    Bitmap bitmap2=BitmapFactory.decodeResource(getResources(),android.R.drawable.sym_action_call);    final JniBitmapHolder bitmapHolder2=new JniBitmapHolder(bitmap2);    bitmap2.recycle();    //    setContentView(R.layout.activity_main);      {      bitmap=bitmapHolder.getBitmapAndFree();      final ImageView imageView=(ImageView)findViewById(R.id.imageView1);      imageView.setImageBitmap(bitmap);      }      {      bitmap2=bitmapHolder2.getBitmapAndFree();      final ImageView imageView=(ImageView)findViewById(R.id.imageView2);      imageView.setImageBitmap(bitmap2);      }    }  }

JniBitmapHolder.java - the "bridge" between JNI and JAVA :

package com.example.jnibitmapstoragetest;...public class JniBitmapHolder  {  ByteBuffer _handler =null;  static    {    System.loadLibrary("JniBitmapStorageTest");    }  private native ByteBuffer jniStoreBitmapData(Bitmap bitmap);  private native Bitmap jniGetBitmapFromStoredBitmapData(ByteBuffer handler);  private native void jniFreeBitmapData(ByteBuffer handler);  public JniBitmapHolder()    {}  public JniBitmapHolder(final Bitmap bitmap)    {    storeBitmap(bitmap);    }  public void storeBitmap(final Bitmap bitmap)    {    if(_handler!=null)      freeBitmap();    _handler=jniStoreBitmapData(bitmap);    }  public Bitmap getBitmap()    {    if(_handler==null)      return null;    return jniGetBitmapFromStoredBitmapData(_handler);    }  public Bitmap getBitmapAndFree()    {    final Bitmap bitmap=getBitmap();    freeBitmap();    return bitmap;    }  public void freeBitmap()    {    if(_handler==null)      return;    jniFreeBitmapData(_handler);    _handler=null;    }  @Override  protected void finalize() throws Throwable    {    super.finalize();    if(_handler==null)      return;    Log.w("DEBUG","JNI bitmap wasn't freed nicely.please rememeber to free the bitmap as soon as you can");    freeBitmap();    }  }

Android.mk - the properties file of JNI:

LOCAL_PATH := $(call my-dir)include $(CLEAR_VARS)LOCAL_MODULE    := JniBitmapStorageTestLOCAL_SRC_FILES := JniBitmapStorageTest.cppLOCAL_LDLIBS := -llogLOCAL_LDFLAGS += -ljnigraphicsinclude $(BUILD_SHARED_LIBRARY)APP_OPTIM := debugLOCAL_CFLAGS := -g

JniBitmapStorageTest.cpp - the "magical" stuff goes here :

#include <jni.h>#include <jni.h>#include <android/log.h>#include <stdio.h>#include <android/bitmap.h>#include <cstring>#include <unistd.h>#define  LOG_TAG    "DEBUG"#define  LOGD(...)  __android_log_print(ANDROID_LOG_DEBUG,LOG_TAG,__VA_ARGS__)#define  LOGE(...)  __android_log_print(ANDROID_LOG_ERROR,LOG_TAG,__VA_ARGS__)extern "C"  {  JNIEXPORT jobject JNICALL Java_com_example_jnibitmapstoragetest_JniBitmapHolder_jniStoreBitmapData(JNIEnv * env, jobject obj, jobject bitmap);  JNIEXPORT jobject JNICALL Java_com_example_jnibitmapstoragetest_JniBitmapHolder_jniGetBitmapFromStoredBitmapData(JNIEnv * env, jobject obj, jobject handle);  JNIEXPORT void JNICALL Java_com_example_jnibitmapstoragetest_JniBitmapHolder_jniFreeBitmapData(JNIEnv * env, jobject obj, jobject handle);  }class JniBitmap  {  public:    uint32_t* _storedBitmapPixels;    AndroidBitmapInfo _bitmapInfo;    JniBitmap()      {      _storedBitmapPixels = NULL;      }  };JNIEXPORT void JNICALL Java_com_example_jnibitmapstoragetest_JniBitmapHolder_jniFreeBitmapData(JNIEnv * env, jobject obj, jobject handle)  {  JniBitmap* jniBitmap = (JniBitmap*) env->GetDirectBufferAddress(handle);  if (jniBitmap->_storedBitmapPixels == NULL)    return;  delete[] jniBitmap->_storedBitmapPixels;  jniBitmap->_storedBitmapPixels = NULL;  delete jniBitmap;  }JNIEXPORT jobject JNICALL Java_com_example_jnibitmapstoragetest_JniBitmapHolder_jniGetBitmapFromStoredBitmapData(JNIEnv * env, jobject obj, jobject handle)  {  JniBitmap* jniBitmap = (JniBitmap*) env->GetDirectBufferAddress(handle);  if (jniBitmap->_storedBitmapPixels == NULL)    {    LOGD("no bitmap data was stored. returning null...");    return NULL;    }  //  //creating a new bitmap to put the pixels into it - using Bitmap Bitmap.createBitmap (int width, int height, Bitmap.Config config) :  //  //LOGD("creating new bitmap...");  jclass bitmapCls = env->FindClass("android/graphics/Bitmap");  jmethodID createBitmapFunction = env->GetStaticMethodID(bitmapCls, "createBitmap", "(IILandroid/graphics/Bitmap$Config;)Landroid/graphics/Bitmap;");  jstring configName = env->NewStringUTF("ARGB_8888");  jclass bitmapConfigClass = env->FindClass("android/graphics/Bitmap$Config");  jmethodID valueOfBitmapConfigFunction = env->GetStaticMethodID(bitmapConfigClass, "valueOf", "(Ljava/lang/String;)Landroid/graphics/Bitmap$Config;");  jobject bitmapConfig = env->CallStaticObjectMethod(bitmapConfigClass, valueOfBitmapConfigFunction, configName);  jobject newBitmap = env->CallStaticObjectMethod(bitmapCls, createBitmapFunction, jniBitmap->_bitmapInfo.height, jniBitmap->_bitmapInfo.width, bitmapConfig);  //  // putting the pixels into the new bitmap:  //  int ret;  void* bitmapPixels;  if ((ret = AndroidBitmap_lockPixels(env, newBitmap, &bitmapPixels)) < 0)    {    LOGE("AndroidBitmap_lockPixels() failed ! error=%d", ret);    return NULL;    }  uint32_t* newBitmapPixels = (uint32_t*) bitmapPixels;  int pixelsCount = jniBitmap->_bitmapInfo.height * jniBitmap->_bitmapInfo.width;  memcpy(newBitmapPixels, jniBitmap->_storedBitmapPixels, sizeof(uint32_t) * pixelsCount);  AndroidBitmap_unlockPixels(env, newBitmap);  //LOGD("returning the new bitmap");  return newBitmap;  }JNIEXPORT jobject JNICALL Java_com_example_jnibitmapstoragetest_JniBitmapHolder_jniStoreBitmapData(JNIEnv * env, jobject obj, jobject bitmap)  {  AndroidBitmapInfo bitmapInfo;  uint32_t* storedBitmapPixels = NULL;  //LOGD("reading bitmap info...");  int ret;  if ((ret = AndroidBitmap_getInfo(env, bitmap, &bitmapInfo)) < 0)    {    LOGE("AndroidBitmap_getInfo() failed ! error=%d", ret);    return NULL;    }  LOGD("width:%d height:%d stride:%d", bitmapInfo.width, bitmapInfo.height, bitmapInfo.stride);  if (bitmapInfo.format != ANDROID_BITMAP_FORMAT_RGBA_8888)    {    LOGE("Bitmap format is not RGBA_8888!");    return NULL;    }  //  //read pixels of bitmap into native memory :  //  //LOGD("reading bitmap pixels...");  void* bitmapPixels;  if ((ret = AndroidBitmap_lockPixels(env, bitmap, &bitmapPixels)) < 0)    {    LOGE("AndroidBitmap_lockPixels() failed ! error=%d", ret);    return NULL;    }  uint32_t* src = (uint32_t*) bitmapPixels;  storedBitmapPixels = new uint32_t[bitmapInfo.height * bitmapInfo.width];  int pixelsCount = bitmapInfo.height * bitmapInfo.width;  memcpy(storedBitmapPixels, src, sizeof(uint32_t) * pixelsCount);  AndroidBitmap_unlockPixels(env, bitmap);  JniBitmap *jniBitmap = new JniBitmap();  jniBitmap->_bitmapInfo = bitmapInfo;  jniBitmap->_storedBitmapPixels = storedBitmapPixels;  return env->NewDirectByteBuffer(jniBitmap, 0);  }

If you just want to cache bitmaps off the heap, a simpler solution is to use parcel memory.

This is the Gist of it (full code bellow). You can use it for Parcelable instances other than Bitmap. Use it like this:

private final CachedParcelable<Bitmap> cache = new CachedParcelable<>(Bitmap.CREATOR);cache.put(bitmap);bitmap = cache.get();cache.close();

public final class CachedParcelable<T extends Parcelable> implements AutoCloseable {    private final Parcelable.Creator<T> creator;    private Parcel cache;    public CachedParcelable(Parcelable.Creator<T> creator) {        this.creator = creator;    }    public synchronized T get() {        if (cache == null) return null;        try {            cache.setDataPosition(0);            return creator.createFromParcel(cache);        } catch (BadParcelableException e) {            //        } catch (RuntimeException e) {            if (creator != Bitmap.CREATOR) throw e;        }        return null;    }    public synchronized void put(T value) {        if (cache != null) cache.recycle();        if (value == null) {            cache = null;            return;        }        try {            cache = Parcel.obtain();            value.writeToParcel(cache, 0);        } catch (RuntimeException e) {            if (creator != Bitmap.CREATOR) throw e;        }    }    @Override    public synchronized void close() {        if (cache != null) {            cache.recycle();            cache = null;        }    }}