java | 直接内存 | 犀牛的博客

Direct Memory 直接内存。

直接内存并不属于 jvm 内存，属于操作系统内存。所以图中没有。

常见于 NIO 操作时，用于数据缓冲区
分配回收成本高，但读写性能强
不受 JVM 内存回收管理

copy 速度的比较

package com.redisc;

import lombok.extern.slf4j.Slf4j;

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

@Slf4j(topic = "c.Run")
public class Run {

    static final String FROM = "";
    static final String TO = "";
    static final int _1MB = 1024 * 1024;

    public static void main(String[] args) throws IOException {
        io(); // 传统 IO
        directionBuffer(); // 直接内存IO
    }

    private static void directionBuffer() {
        long start = System.nanoTime();
        try (FileChannel from = new FileInputStream(FROM).getChannel();
             FileChannel to = new FileOutputStream(TO).getChannel();) {
            ByteBuffer bb = ByteBuffer.allocateDirect(_1MB);
            while (true) {
                int len = from.read(bb);
                if (len == -1) {
                    break;
                }
                bb.flip();
                to.write(bb);
                bb.clear();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        long end = System.nanoTime();
        System.out.println("direct 用时" + (end - start) / 1000000);
    }

    private static void io() {
        long start = System.nanoTime();
        try (FileInputStream from = new FileInputStream(FROM);
             FileOutputStream to = new FileOutputStream(TO);) {
            byte[] buf = new byte[_1MB];
            while (true) {
                int len = from.read(buf);
                if (len == -1) {
                    break;
                }
                to.write(buf);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        long end = System.nanoTime();
        System.out.println("io 用时" + (end - start) / 1000000);
    }

}