I wanted use ByteBuffer.allocateDirect to keep data on native memory but it looks like it keeps data on heap.

I used this code to check that DirectByteBuffer keeps data on heap.

import android.util.Log
import androidx.appcompat.app.AppCompatActivity
import java.nio.ByteBuffer

class MainActivity : AppCompatActivity() {

    val tenMb = 10 * 1024 * 1024
    val byteBuffer = ByteBuffer.allocateDirect(tenMb)

    init {
        if (byteBuffer.hasArray()) {
            Log.i(TAG, "Byte buffer has heap array ${byteBuffer.array().size / 1024f / 1024f} MB")
        } else {
            Log.i(TAG, "Byte buffer doesn't have heap array")
        }
    }

    companion object {
        private const val TAG = "BYTE_BUFFER_TEST"
    }
}

I got heap dump and saw that byte array with size 10 MB is kept on heap.

Image of heap dump

Also, I tried to allocate 300 MB to get OOM because a standard heap for Android app is 256 MB and I got it.

FATAL EXCEPTION: main
Process: com.android.myapplication, PID: 14976
java.lang.OutOfMemoryError: Failed to allocate a 314572819 byte allocation with 25165824 free bytes and 253MB until OOM, target footprint 27560064, growth limit 268435456
    at dalvik.system.VMRuntime.newNonMovableArray(Native Method)
    at java.nio.DirectByteBuffer$MemoryRef.<init>(DirectByteBuffer.java:71)
    at java.nio.ByteBuffer.allocateDirect(ByteBuffer.java:265)
    at net.idrnd.test.myapplication.MainActivity.<init>(MainActivity.kt:10)
    at java.lang.Class.newInstance(Native Method)
    at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:95)
    at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:44)
    at android.app.Instrumentation.newActivity(Instrumentation.java:1441)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3839)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4126)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:108)
    at android.app.servertransaction.TransactionExecutor.executeNonLifecycleItem(TransactionExecutor.java:195)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:157)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:90)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2544)
    at android.os.Handler.dispatchMessage(Handler.java:107)
    at android.os.Looper.loopOnce(Looper.java:232)
    at android.os.Looper.loop(Looper.java:317)
    at android.app.ActivityThread.main(ActivityThread.java:8501)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:552)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:878)

I have two questions:

1. Why DirectByteBuffer keeps data on heap? I was sure that we use for this goal HeapByteBuffer and use DirectByteBuffer to keep data on native memory.
2. How to keep data on native memory if DirectByteBuffer doesn’t work properly and I don’t want to use NDK?