Lære Native Memory: Beyond the JVM

What Is Native Memory?

Native memory refers to the memory managed directly by the operating system, outside the control of the Java Virtual Machine (JVM). This includes memory allocated for system libraries, native code, and resources used by Java applications through native interfaces like JNI (Java Native Interface).

How Native Memory Differs from Java Heap and Stack

The Java heap is where Java objects and arrays live, managed automatically by the JVM's garbage collector. The Java stack holds method call frames and local variables, managed per thread and cleared as methods finish. In contrast, native memory is not managed by the JVM. It is allocated and freed manually by native code or by the JVM itself for internal needs, such as direct buffers or thread stacks.

Why Native Memory Matters

Native memory usage can impact your application's stability and performance. If native memory is exhausted, your program may crash with an "OutOfMemoryError," even if the Java heap has free space. Understanding native memory helps you troubleshoot memory leaks and optimize resource usage beyond what the JVM manages automatically.

Practical Example: Using Direct ByteBuffer for High-Performance I/O

When you need to handle large files or network data efficiently, you can use DirectByteBuffer in Java. Unlike regular byte buffers, direct buffers allocate memory outside the Java heap, using native memory. This can speed up I/O operations, as data can be transferred directly to and from native OS buffers without extra copying.

Here’s how you might use a direct buffer to read a large file:

Main.java


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package com.example;

import java.io.FileInputStream;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

public class DirectBufferExample {
    public static void main(String[] args) throws Exception {
        String filePath = "large-data.bin";
        try (FileInputStream fis = new FileInputStream(filePath);
             FileChannel channel = fis.getChannel()) {

            // Allocate a 16 MB direct buffer in native memory
            ByteBuffer buffer = ByteBuffer.allocateDirect(16 * 1024 * 1024);
            int bytesRead;
            long totalBytes = 0;
            while ((bytesRead = channel.read(buffer)) != -1) {
                totalBytes += bytesRead;
                buffer.clear(); // Prepare buffer for next read
            }
            System.out.println("Total bytes read: " + totalBytes);
        }
    }
}

Key points:

Allocating with ByteBuffer.allocateDirect uses native memory, not JVM heap memory;
This approach reduces garbage collection pressure and can improve performance for large or frequent I/O;
Always release file handles and channels to avoid native memory leaks.

You might use this technique in high-performance servers, media processing tools, or any application where you want to minimize JVM heap usage during I/O.

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