java直接内存与堆内存的比较

来源：互联网发布：私有密码算法范畴编辑：程序博客网时间：2024/04/30 03:23

对于nio这个库可以直接操作内存的使用，可以做个对比，看哪种情况下适合使用。

import java.nio.ByteBuffer;public class AccessDirectBuffer {    //直接分配内存    public static void directAccess() {        long startTime = System.currentTimeMillis();        ByteBuffer b = ByteBuffer.allocateDirect(500);        for (int i = 0; i < 100000; i++) {            for (int j = 0; j < 99; j++)                b.putInt(j);            b.flip();            for (int j = 0; j < 99; j++)                b.getInt();            b.clear();        }        long endTime = System.currentTimeMillis();        System.out.println("directAccess:" + (endTime - startTime));    }    //分配堆内存    public static void bufferAccess() {        long startTime = System.currentTimeMillis();        ByteBuffer b = ByteBuffer.allocate(500);        for (int i = 0; i < 100000; i++) {            for (int j = 0; j < 99; j++)                b.putInt(j);            b.flip();            for (int j = 0; j < 99; j++)                b.getInt();            b.clear();        }        long endTime = System.currentTimeMillis();        System.out.println("bufferAccess" + (endTime - startTime));    }    public static void main(String args[]) {        bufferAccess();        directAccess();        bufferAccess();        directAccess();    }}

输出结果：

bufferAccess57
directAccess:28
bufferAccess55
directAccess:10

可以看到直接内存读写有较大优势，但是看看下面的情况：

/** * Alipay.com Inc. * Copyright (c) 2004-2015 All Rights Reserved. */import java.nio.ByteBuffer;/** * * @author yunmo.hl * @version $Id: AllocDirectBuffer.java, v 0.1 2015-12-15/12/14-15:02 yunmo.hl Exp $$ */public class AllocDirectBuffer {    public static void directAllocate() {        long startTime = System.currentTimeMillis();        for (int i = 0; i < 200000; i++) {            ByteBuffer.allocateDirect(1000);        }        long endTime = System.currentTimeMillis();        System.out.println("directAllocate:" + (endTime - startTime));    }    public static void bufferAllocate() {        long startTime = System.currentTimeMillis();        for (int i = 0; i < 200000; i++) {            ByteBuffer.allocate(1000);        }        long endTime = System.currentTimeMillis();        System.out.println("bufferAllocate:" + (endTime - startTime));    }    public static void main(String[] args) {        bufferAllocate();        directAllocate();        bufferAllocate();        directAllocate();    }}

输出结果：

bufferAllocate:38
directAllocate:282
bufferAllocate:71
directAllocate:403

可以看到在内存空间申请上，直接内存毫无优势。

如果内存空间本身需要频繁申请，则不适合使用直接内存。

1 0