第 11 章 直接内存

1、直接内存概述

直接内存不属于虚拟机运行时数据区的一部分，也不是《java虚拟机规范》中定义的内存区域。它是java堆外的、直接向系统申请的内存区间。直接内存来源于nio，通过存在堆中的directbytebuffer操作native内存。通常，访问直接内存的速度会优于java堆。因此，出于性能考虑，读写频繁的场合可能会考虑使用直接内存。java的nio库允许java程序使用直接内存，用于数据缓冲区。

/** 
 * IO                  NIO (New IO / Non-Blocking IO) 
 * byte[] / char[]     Buffer 
 * Stream              Channel 
 * 
 * 查看直接内存的占用与释放 
 * @author shkstart  shkstart@126.com 
 * @create 2025  0:22 
 */
public class BufferTest {
    private static final int BUFFER = 1024 * 1024 * 1024;//1GB
    public static void main(String[] args){
        //直接分配本地内存空间
        ByteBuffer byteBuffer = ByteBuffer.allocateDirect(BUFFER);
        System.out.println("直接内存分配完毕，请求指示！");
        Scanner scanner = new Scanner(System.in);
        scanner.next();
        System.out.println("直接内存开始释放！");
        byteBuffer = null;
        System.gc();
        scanner.next();
    }
}

呐，直接占用了 1G 的本地内存

释放后，Java程序的内存占用明显减少

2、BIO 与 NIO

原来采用BIO的架构，在读写本地文件时，我们需要从用户态切换成内核态

NIO 直接操作物理磁盘，省去了中间商赚差价

测试代码：分别使用 BIO 和 NIO 复制大文件，看看用时差别

/** 
 * @author shkstart  shkstart@126.com 
 * @create 2025  0:04 
 */
public class BufferTest1 {
    private static final String TO = "F:\\test\\异界BD中字.mp4";
    private static final int _100Mb = 1024 * 1024 * 100;
    public static void main(String[] args) {
        long sum = 0;
        String src = "F:\\test\\异界BD中字.mp4";
        for (int i = 0; i 

深入 ByteBuffer 源码

ByteBuffer.allocateDirect() 方法

public static ByteBuffer allocateDirect(int capacity) {
    return new DirectByteBuffer(capacity);
}

DirectByteBuffer 类的构造器用到了 Unsafe 类分配本地内存

DirectByteBuffer(int cap) {                   // package-private
    super(-1, 0, cap, cap);
    boolean pa = VM.isDirectMemoryPageAligned();
    int ps = Bits.pageSize();
    long size = Math.max(1L, (long)cap + (pa ? ps : 0));
    Bits.reserveMemory(size, cap);
    long base = 0;
    try {
        base = unsafe.allocateMemory(size);
    } catch (OutOfMemoryError x) {
        Bits.unreserveMemory(size, cap);
        throw x;
    }
    unsafe.setMemory(base, size, (byte) 0);
    if (pa && (base % ps != 0)) {
        // Round up to page boundary
        address = base + ps - (base & (ps - 1));
    } else {
        address = base;
    }
    cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
    att = null;
}

3、直接内存与 OOM

直接内存也可能导致OutofMemoryError异常。由于直接内存在Java堆外，因此它的大小不会直接受限于-Xmx指定的最大堆大小，但是系统内存是有限的，Java堆和直接内存的总和依然受限于操作系统能给出的最大内存。直接内存的缺点为：分配回收成本较高，不受JVM内存回收管理。直接内存大小可以通过MaxDirectMemorySize设置。如果不指定，默认与堆的最大值-Xmx参数值一致。

代码示例 1

/** 
 * 本地内存的OOM:  OutOfMemoryError: Direct buffer memory 
 * 
 * @author shkstart  shkstart@126.com 
 * @create 2025  0:09 
 */
public class BufferTest2 {
    private static final int BUFFER = 1024 * 1024 * 20;//20MB
    public static void main(String[] args) {
        ArrayList list = new ArrayList();
        int count = 0;
        try {
            while(true){
                ByteBuffer byteBuffer = ByteBuffer.allocateDirect(BUFFER);
                list.add(byteBuffer);
                count++;
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        } finally {
            System.out.println(count);
        }
    }
}

本地内存持续增长，直至程序抛出异常：java.lang.OutOfMemoryError: Direct buffer memory

异常信息

"C:\Program Files\Java\jdk1.8.0_144\bin\java" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\lib\idea_rt.jar=1296:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_144\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\rt.jar;C:\Users\Heygo\Desktop\JVMDemo\out\production\chapter11" com.atguigu.java.BufferTest2180Exception in thread "main" java.lang.OutOfMemoryError: Direct buffer memoryat java.nio.Bits.reserveMemory(Bits.java:694)at java.nio.DirectByteBuffer.(DirectByteBuffer.java:123)at java.nio.ByteBuffer.allocateDirect(ByteBuffer.java:311)at com.atguigu.java.BufferTest2.main(BufferTest2.java:21)Process finished with exit code 1

代码示例 2

代码：直接通过 Unsafe 类申请本地内存

/** 
 * -Xmx20m -XX:MaxDirectMemorySize=10m 
 * @author shkstart  shkstart@126.com 
 * @create 2025  0:36 
 */
public class MaxDirectMemorySizeTest {
    private static final long _1MB = 1024 * 1024;
    public static void main(String[] args) throws IllegalAccessException {
        Field unsafeField = Unsafe.class.getDeclaredFields()[0];
        unsafeField.setAccessible(true);
        Unsafe unsafe = (Unsafe)unsafeField.get(null);
        while(true){
            unsafe.allocateMemory(_1MB);
        }
    }
}

JVM 参数

-Xmx20m -XX:MaxDirectMemorySize=10m

抛出 OOM 异常

"C:\Program Files\Java\jdk1.8.0_144\bin\java" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\lib\idea_rt.jar=1362:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_144\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\rt.jar;C:\Users\Heygo\Desktop\JVMDemo\out\production\chapter11" com.atguigu.java.MaxDirectMemorySizeTestException in thread "main" java.lang.OutOfMemoryErrorat sun.misc.Unsafe.allocateMemory(Native Method)at com.atguigu.java.MaxDirectMemorySizeTest.main(MaxDirectMemorySizeTest.java:20)Process finished with exit code 1