[JVM]了断局: 堆外内存无法 [ -XX:MaxDirectMemorySize ] 限制

水深无声 2023-01-20 09:45 136阅读 0赞

# 一. 前言 #

今天看到一句话 , 有点懵, 所以验证一下.

> 使用sun.misc.Unsafe的allocateMemory方法分配堆外内存。不受`-XX:MaxDirectMemorySize`这个JVM参数的限制

# 二. 结论 #

上面的结论有点不完善:

1.  如果通过**反射的方式拿到Unsafe的实例**,然后用Unsafe的allocateMemory方法分配堆外内存. 确实不受`-XX:MaxDirectMemorySize`这个JVM参数的限制 . 所以限制的内存大小为操作系统的内存.
2.  如果使用Java自带的 `ByteBuffer.allocateDirect(size)` 或者直接 `new DirectByteBuffer(capacity)` , 这样受`-XX:MaxDirectMemorySize` 这个JVM参数的限制. 其实底层都是用的`Unsafe#allocateMemory`,区别是对大小做了限制. 如果超出限制直接OOM.
3.  如果不设置`-XX:MaxDirectMemorySize` 默认的话,是跟堆内存大小保持一致. \[堆内存大小如果不设置的话,默认为操作系统的 1/4, 所以 `DirectMemory`的大小限制JVM的`Runtime.getRuntime().maxMemory()`内存大小 . \], 代码入下:

`sun.misc.VM`

public static void saveAndRemoveProperties(Properties var0) { 
            if (booted) { 
                throw new IllegalStateException("System initialization has completed");
            } else { 
                savedProps.putAll(var0);
                String var1 = (String)var0.remove("sun.nio.MaxDirectMemorySize");
                if (var1 != null) { 
                
                    if (var1.equals("-1")) { 
                    	// 默认值 -1 , 代表是JVM的最大内存 (注意,不是操作系统内存!!!!!!!!!). 
                        directMemory = Runtime.getRuntime().maxMemory();
                    } else { 
                        long var2 = Long.parseLong(var1);
                        if (var2 > -1L) { 
                            directMemory = var2;
                        }
                    }
                }
    
                var1 = (String)var0.remove("sun.nio.PageAlignDirectMemory");
                if ("true".equals(var1)) { 
                    pageAlignDirectMemory = true;
                }
    
                var1 = var0.getProperty("sun.lang.ClassLoader.allowArraySyntax");
                allowArraySyntax = var1 == null ? defaultAllowArraySyntax : Boolean.parseBoolean(var1);
                var0.remove("java.lang.Integer.IntegerCache.high");
                var0.remove("sun.zip.disableMemoryMapping");
                var0.remove("sun.java.launcher.diag");
                var0.remove("sun.cds.enableSharedLookupCache");
            }
        }

# 三. 代码验证: 反射获取Unsafe的allocateMemory方法 #

## 3.1. 测试代码 ##

import com.sun.management.OperatingSystemMXBean;
    import sun.misc.Unsafe;
    
    import java.lang.management.ManagementFactory;
    import java.lang.reflect.Field;
    import java.nio.ByteBuffer;
    import java.util.ArrayList;
    import java.util.List;
    
    public class DirectMemoryOOM_01 { 
        // 内存限制失效
        // -Xms10m -Xmx10m -XX:MaxDirectMemorySize=10m
        public static void main(String[] args) throws Exception { 
    
            Runtime rt = Runtime.getRuntime();
            long vmFree = 0;
            long vmUse = 0;
            long vmTotal = 0;
            long vmMax = 0;
            int byteToMb = 1024 * 1024;
            long size = 1024 * 1024 * 1024;
    
    
            Field f = Unsafe.class.getDeclaredFields()[0];
            f.setAccessible(true);
            Unsafe us = (Unsafe) f.get(null);
    
            List<Long> list = new ArrayList<Long>();
    
    
    
            while (true) { 
    
                System.out.println("************ start ***************************");
                System.out.println("JVM maxDirectMemory : " + sun.misc.VM.maxDirectMemory() / byteToMb);
    
    
                vmTotal = rt.totalMemory() / byteToMb;
                vmFree = rt.freeMemory() / byteToMb;
                vmMax = rt.maxMemory() / byteToMb;
                vmUse = vmTotal - vmFree;
                System.out.println("JVM内存已用的空间为：" + vmUse + " MB");
                System.out.println("JVM内存的空闲空间为：" + vmFree + " MB");
                System.out.println("JVM总内存空间为：" + vmTotal + " MB");
                System.out.println("JVM总内存空间为：" + vmMax + " MB");
    
                System.out.println("======================================");
                // 操作系统级内存情况查询
                OperatingSystemMXBean osmxb = (OperatingSystemMXBean) ManagementFactory.getOperatingSystemMXBean();
                String os = System.getProperty("os.name");
                long physicalFree = osmxb.getFreePhysicalMemorySize() / byteToMb;
                long physicalTotal = osmxb.getTotalPhysicalMemorySize() / byteToMb;
                long physicalUse = physicalTotal - physicalFree;
                System.out.println("操作系统的版本：" + os);
                System.out.println("操作系统物理内存已用的空间为：" + physicalUse + " MB");
                System.out.println("操作系统物理内存的空闲空间为：" + physicalFree + " MB");
                System.out.println("操作系统总物理内存：" + physicalTotal + " MB");
                System.out.println("======================================");
    
                // 获得线程总数
                ThreadGroup parentThread;
                int totalThread = 0;
                for (parentThread = Thread.currentThread().getThreadGroup(); parentThread
                        .getParent() != null; parentThread = parentThread.getParent()) { 
                    totalThread = parentThread.activeCount();
                }
                System.out.println("获得线程总数:" + totalThread);
    
                System.out.println("======================================");
    
                long p = us.allocateMemory(size);
                for (int i = 0; i < size; i++) { 
                    us.putByte(p + i, Byte.MAX_VALUE);
                }
                list.add(p);
    
                System.out.println("************ end ***************************");
    
    
                Thread.sleep(1000);
            }
        }
    }

## 3.2.设置JVM参数 ##

-Xms10m -Xmx10m -XX:MaxDirectMemorySize=10m

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3poYW5nbG9uZ180NDQ0_size_16_color_FFFFFF_t_70]

## 3.2.输出结果: 不报错,无法限制内存. ##

************ start ***************************
    JVM maxDirectMemory : 10
    JVM内存已用的空间为：2 MB
    JVM内存的空闲空间为：7 MB
    JVM总内存空间为：9 MB
    JVM总内存空间为：9 MB
    ======================================
    操作系统的版本：Mac OS X
    操作系统物理内存已用的空间为：16368 MB
    操作系统物理内存的空闲空间为：16 MB
    操作系统总物理内存：16384 MB
    ======================================
    获得线程总数:2
    ======================================
    
    无限死循环中.....................
    无限死循环中.....................
    无限死循环中.....................
    无限死循环中.....................
    无限死循环中.....................
    无限死循环中.....................
    无限死循环中.....................

# 四. 代码验证: ByteBuffer.allocateDirect方式获取内存 #

## 4.1. 测试代码 ##

package com.unsafe;
    
    import sun.misc.Unsafe;
    
    import java.lang.management.ManagementFactory;
    import com.sun.management.OperatingSystemMXBean;
    import java.nio.ByteBuffer;
    import java.util.ArrayList;
    import java.util.List;
    
    public class DirectMemoryOOM_02 { 
        // 内存限制失效
        // -Xms10m -Xmx10m -XX:MaxDirectMemorySize=10m
        public static void main(String[] args) throws Exception { 
    
            Runtime rt = Runtime.getRuntime();
            long vmFree = 0;
            long vmUse = 0;
            long vmTotal = 0;
            long vmMax = 0;
            int byteToMb = 1024 * 1024;
    
            int size = 1024 * 1024 * 1024;
    
            List<ByteBuffer> list = new ArrayList<ByteBuffer>();
    
    
            while (true) { 
    
                System.out.println("************ start ***************************");
    
                System.out.println("JVM maxDirectMemory : " + sun.misc.VM.maxDirectMemory() / byteToMb);
    
                vmTotal = rt.totalMemory() / byteToMb;
                vmFree = rt.freeMemory() / byteToMb;
                vmMax = rt.maxMemory() / byteToMb;
                vmUse = vmTotal - vmFree;
                System.out.println("JVM内存已用的空间为：" + vmUse + " MB");
                System.out.println("JVM内存的空闲空间为：" + vmFree + " MB");
                System.out.println("JVM总内存空间为：" + vmTotal + " MB");
                System.out.println("JVM总内存空间为：" + vmMax + " MB");
    
                System.out.println("======================================");
                // 操作系统级内存情况查询
                OperatingSystemMXBean osmxb = (OperatingSystemMXBean) ManagementFactory.getOperatingSystemMXBean();
                String os = System.getProperty("os.name");
                long physicalFree = osmxb.getFreePhysicalMemorySize() / byteToMb;
                long physicalTotal = osmxb.getTotalPhysicalMemorySize() / byteToMb;
                long physicalUse = physicalTotal - physicalFree;
                System.out.println("操作系统的版本：" + os);
                System.out.println("操作系统物理内存已用的空间为：" + physicalUse + " MB");
                System.out.println("操作系统物理内存的空闲空间为：" + physicalFree + " MB");
                System.out.println("操作系统总物理内存：" + physicalTotal + " MB");
                System.out.println("======================================");
    
                // 获得线程总数
                ThreadGroup parentThread;
                int totalThread = 0;
                for (parentThread = Thread.currentThread().getThreadGroup(); parentThread
                        .getParent() != null; parentThread = parentThread.getParent()) { 
                    totalThread = parentThread.activeCount();
                }
                System.out.println("获得线程总数:" + totalThread);
    
                System.out.println("======================================");
    
                // 分配 Direct Memory
                ByteBuffer by =  ByteBuffer.allocateDirect(size);
                list.add(by);
    
                System.out.println("************ end ***************************");
    
    
                Thread.sleep(1000);
            }
        }
    }

## 4.2. 设置JVM参数 ##

-Xms10m -Xmx10m -XX:MaxDirectMemorySize=10m

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3poYW5nbG9uZ180NDQ0_size_16_color_FFFFFF_t_70 1]

## 4.3. 输出结果: OutOfMemoryError: Direct buffer memory ##

************ start ***************************
    JVM maxDirectMemory : 10
    JVM内存已用的空间为：2 MB
    JVM内存的空闲空间为：7 MB
    JVM总内存空间为：9 MB
    JVM总内存空间为：9 MB
    ======================================
    操作系统的版本：Mac OS X
    操作系统物理内存已用的空间为：13986 MB
    操作系统物理内存的空闲空间为：2398 MB
    操作系统总物理内存：16384 MB
    ======================================
    获得线程总数:1
    ======================================
    Exception in thread "main" java.lang.OutOfMemoryError: Direct buffer memory
    	at java.nio.Bits.reserveMemory(Bits.java:695)
    	at java.nio.DirectByteBuffer.<init>(DirectByteBuffer.java:123)
    	at java.nio.ByteBuffer.allocateDirect(ByteBuffer.java:311)
    	at com.unsafe.DirectMemoryOOM_02.main(DirectMemoryOOM_02.java:68)
    Disconnected from the target VM, address: '127.0.0.1:58987', transport: 'socket'

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