Buffer vs Unsafe - Outside JVM

<p>I have a requirement to use a space in the available RAM which the GC has no control on. I read a few articles on the same which gave me the introduction on two approaches. They are specified in the following code.</p><pre><br /><code>package com.directmemory;<br /></code><br /></pre><p>import java.lang.reflect.Field; import java.nio.ByteBuffer;</p><p>import sun.misc.Unsafe;</p><p>public class DirectMemoryTest {</p><pre><br /><code>public static void main(String[] args) {<br /><br /> //Approach 1<br /> ByteBuffer directByteBuffer = ByteBuffer.allocateDirect(8);<br /> directByteBuffer.putDouble(1.0);<br /> directByteBuffer.flip();<br /> System.out.println(directByteBuffer.getDouble());<br /><br /> //Approach 2<br /> Unsafe unsafe = getUnsafe();<br /> long pointer = unsafe.allocateMemory(8);<br /> unsafe.putDouble(pointer, 2.0);<br /> unsafe.putDouble(pointer+8, 3.0);<br /><br /> System.out.println(unsafe.getDouble(pointer));<br /> System.out.println(unsafe.getDouble(pointer+8));<br /> System.out.println(unsafe.getDouble(pointer+16));<br />}<br /><br />public static Unsafe getUnsafe() {<br /> try {<br /> Field f = Unsafe.class.getDeclaredField("theUnsafe");<br /> f.setAccessible(true);<br /> return (Unsafe) f.get(null);<br /> } catch (Exception e) {<br /> e.printStackTrace();<br /> }<br /> return null;<br />}<br /></code><br /></pre><p>}</p><p>I have a couple of questions</p><p>1) Why should I ever pay attention to Approach 1 mentioned in the code because as per my understanding ByteBuffer.allocateDirect() cannot return me a buffer with a storage capacity of greater than 2GB ? So if my requirement is to store say 3 GB of data, I have to create a new buffer and store the data there which would mean that apart from storing data I have additional responsibility of identifying the respective buffer (out of the list of 'n' buffers) which maintains a pointer to direct memory.</p><p>2) Isn't approach 2 a little faster than approach 1 because I don't have to first find the buffer and then the data, I just need an indexing mechanism of for an object's field and use getDouble/getInt methods and pass the absolute address ?</p><p>3) Is the allocation of direct memory (is it right to say off heap memory ?) related to a PID ? If on a machine, I have 2 java processes, allocateMemory calls in PID 1 and PID 2 give me never intersecting memory blocks to use ?</p><p>4) Why is the last sysout statement not resulting in 0.0 ? The idea is that every double uses 8 bytes so I store 1.0 at address returned by allocateMemory say address = 1, the 2.0 at address 1+8 which is 9 and then stop. So shouldn't the default value be 0.0 ?</p><br /><h2>Click here view more answers <a href="%20http://stackoverflow.com/questions/34469568/buffer-vs-unsafe-outside-jvm">http://ift.tt/1OlveY3</a></h2>