JVM内存与垃圾回收篇第11章直接内存

yuanshuai

发布于 2022-08-17 14:35:26

4510

发布于 2022-08-17 14:35:26

文章被收录于专栏：一只程序原

第 11 章直接内存

1、直接内存概述

直接内存

不是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。
直接内存是在Java堆外的、直接向系统申请的内存区间。
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。即读写性能高。
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

代码示例

代码

/**
 *  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 2020  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

非直接缓存区（BIO）

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

直接缓冲区（NIO）

NIO 直接操作物理磁盘，省去了中间商赚差价

代码测试

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 2020  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 < 3; i++) {
            String dest = "F:\\test\\异界BD中字_" + i + ".mp4";
            // sum += io(src,dest);//54606
            sum += directBuffer(src, dest);//50244
        }

        System.out.println("总花费的时间为：" + sum);
    }

    private static long directBuffer(String src, String dest) {
        long start = System.currentTimeMillis();

        FileChannel inChannel = null;
        FileChannel outChannel = null;
        try {
            inChannel = new FileInputStream(src).getChannel();
            outChannel = new FileOutputStream(dest).getChannel();

            ByteBuffer byteBuffer = ByteBuffer.allocateDirect(_100Mb);
            while (inChannel.read(byteBuffer) != -1) {
                byteBuffer.flip();//修改为读数据模式
                outChannel.write(byteBuffer);
                byteBuffer.clear();//清空
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (inChannel != null) {
                try {
                    inChannel.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }

            }
            if (outChannel != null) {
                try {
                    outChannel.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }

            }
        }

        long end = System.currentTimeMillis();
        return end - start;

    }

    private static long io(String src, String dest) {
        long start = System.currentTimeMillis();

        FileInputStream fis = null;
        FileOutputStream fos = null;
        try {
            fis = new FileInputStream(src);
            fos = new FileOutputStream(dest);
            byte[] buffer = new byte[_100Mb];
            while (true) {
                int len = fis.read(buffer);
                if (len == -1) {
                    break;
                }
                fos.write(buffer, 0, len);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (fis != null) {
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }

            }
            if (fos != null) {
                try {
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }

            }
        }


        long end = System.currentTimeMillis();

        return end - start;
    }
}

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

代码

/**
 * 本地内存的OOM:  OutOfMemoryError: Direct buffer memory
 *
 * @author shkstart  shkstart@126.com
 * @create 2020  0:09
 */
public class BufferTest2 {
    private static final int BUFFER = 1024 * 1024 * 20;//20MB

    public static void main(String[] args) {
        ArrayList<ByteBuffer> 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.BufferTest2
180
Exception in thread "main" java.lang.OutOfMemoryError: Direct buffer memory
	at java.nio.Bits.reserveMemory(Bits.java:694)
	at java.nio.DirectByteBuffer.<init>(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 2020  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.OutOfMemoryError	at sun.misc.Unsafe.allocateMemory(Native Method)	at com.atguigu.java.MaxDirectMemorySizeTest.main(MaxDirectMemorySizeTest.java:20)Process finished with exit code 1