-XX:MaxDirectMemorySize
最大堆外内存大小，此参数的含义是当Direct ByteBuffer分配的堆外内存到达指定大小后就触发Full GC。
首先可以在jdk文档中找到：关于MaxDirectMemorySize内存的描述：
Sets the maximum total size (in bytes) of the New I/O (the java.nio package) direct-buffer allocations. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the size is set to 0, meaning that the JVM chooses the size for NIO direct-buffer allocations automatically.
The following examples illustrate how to set the NIO size to 1024 KB in different units:
-XX:MaxDirectMemorySize=1m
-XX:MaxDirectMemorySize=1024k
-XX:MaxDirectMemorySize=1048576
参考
关于JDK8中最大堆外内存大小MaxDirectMemorySize

ByteBuffer(字节缓冲区)分类

heap ByteBuffer -> -XX:Xmx
一种是heap ByteBuffer,该类对象分配在JVM的堆内存里面，直接由Java虚拟机负责垃圾回收，
direct ByteBuffer -> -XX:MaxDirectMemorySize
一种是direct ByteBuffer是通过jni在虚拟机外内存中分配的。通过jmap无法查看该快内存的使用情况。只能通过top来看它的内存使用情况。
JVM堆内存大小可以通过-Xmx来设置
direct ByteBuffer可以通过-XX:MaxDirectMemorySize来设置
没有配置MaxDirectMemorySize的，默认MaxDirectMemorySize的大小即等于-Xmx
用JDK8的一定要配置：-Xms -Xmx -XX:MaxDirectMemorySize，【Xmx + MaxDirectMemorySize】的值不能超过docker的最大内存，不然docker内存占满了会被oomkill掉；
参考
JVM参数之MaxDirectMemorySize

启动VM options

-Xmx200m
-Xms50m
-XX:+PrintCommandLineFlags
--add-opens java.base/jdk.internal.misc=ALL-UNNAMED
--add-opens java.base/jdk.internal.access=ALL-UNNAMED

背景了解

// 提供对缓冲区使用情况信息的访问。
public interface BufferPool {String getName();long getCount();long getTotalCapacity();long getMemoryUsed();
}

// The management interface for a buffer pool
public interface BufferPoolMXBean extends PlatformManagedObject {	
// The name of this buffer pool
String getName();	// An estimate of the number of buffers in this pool
long getCount();	// An estimate of the total capacity of the buffers in this pool in bytes
long getTotalCapacity(); // An estimate of the memory that the JVM is using for this buffer pool in bytes, 
// or -1L if an estimate of the memory usage is not available
long getMemoryUsed(); 

练习代码

public class MaxDirectMemorySize {public static void main(String[] args) {printJVMRuntime();printDirectMemory();System.out.println("分配25M本地字节缓冲区 " + ByteBuffer.allocateDirect(25 * 1024 * 1024));System.out.println("创建10M对象 " + new byte[10 * 1024 * 1024]);printJVMRuntime();printDirectMemory();System.out.println("分配25M本地字节缓冲区 " + ByteBuffer.allocateDirect(25 * 1024 * 1024));System.out.println("创建10M对象 " + new byte[10 * 1024 * 1024]);printJVMRuntime();printDirectMemory();}private static void printJVMRuntime() {System.out.println("java虚拟机从操纵系统那里挖到的最大的内存   maxMemory " + Runtime.getRuntime().maxMemory() / 1024 / 1024 + "M");System.out.println("java虚拟机已经从操作系统那里挖过来的内存   totalMemory : " + Runtime.getRuntime().totalMemory() / 1024 / 1024 + "M");System.out.println("java虚拟机从操纵系统挖过来还没用上的内存   freeMemory : " + Runtime.getRuntime().freeMemory() / 1024 / 1024 + "M");}private static void printDirectMemory() {System.out.println("the maximum allocatable direct buffer memory: " + VM.maxDirectMemory() / 1024 / 1024 + "M");System.out.println("BufferPoolMXBean.Name: " + getDirectBufferPoolMBean().getName());System.out.println("BufferPoolMXBean.Count: " + getDirectBufferPoolMBean().getCount());System.out.println("BufferPoolMXBean.TotalCapacity: " + getDirectBufferPoolMBean().getTotalCapacity() / 1024 / 1024 + "M");System.out.println("BufferPoolMXBean.MemoryUsed: " + getDirectBufferPoolMBean().getMemoryUsed() / 1024 / 1024 + "M");System.out.println("BufferPool.Name: " + getNioBufferPool().getName());System.out.println("BufferPool.Count: " + getNioBufferPool().getCount());System.out.println("BufferPool.TotalCapacity: " + getNioBufferPool().getTotalCapacity() / 1024 / 1024 + "M");System.out.println("BufferPool.MemoryUsed: " + getNioBufferPool().getMemoryUsed() / 1024 / 1024 + "M");}public static BufferPoolMXBean getDirectBufferPoolMBean() {return ManagementFactory.getPlatformMXBeans(BufferPoolMXBean.class).stream().filter(e -> e.getName().equals("direct")).findFirst().orElseThrow();}public static VM.BufferPool getNioBufferPool() {return SharedSecrets.getJavaNioAccess().getDirectBufferPool();}
}

结果分析

-XX:ConcGCThreads=3 -XX:G1ConcRefinementThreads=13 -XX:GCDrainStackTargetSize=64 -XX:InitialHeapSize=52428800 -XX:MarkStackSize=4194304 -XX:MaxHeapSize=209715200 -XX:MinHeapSize=52428800 -XX:+PrintCommandLineFlags -XX:ReservedCodeCacheSize=251658240 -XX:+SegmentedCodeCache -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseG1GC -XX:-UseLargePagesIndividualAllocation

java虚拟机从操纵系统那里挖到的最大的内存   maxMemory 200M
java虚拟机已经从操作系统那里挖过来的内存   totalMemory : 50M
java虚拟机从操纵系统挖过来还没用上的内存   freeMemory : 45Mthe maximum allocatable direct buffer memory: 200MBufferPoolMXBean.Name: direct
BufferPoolMXBean.Count: 1
BufferPoolMXBean.TotalCapacity: 0M
BufferPoolMXBean.MemoryUsed: 0MBufferPool.Name: direct
BufferPool.Count: 1
BufferPool.TotalCapacity: 0M
BufferPool.MemoryUsed: 0M分配25M本地字节缓冲区 java.nio.DirectByteBuffer[pos=0 lim=26214400 cap=26214400]
创建10M对象 [B@5f4da5c3java虚拟机从操纵系统那里挖到的最大的内存   maxMemory 200M
java虚拟机已经从操作系统那里挖过来的内存   totalMemory : 50M
java虚拟机从操纵系统挖过来还没用上的内存   freeMemory : 33Mthe maximum allocatable direct buffer memory: 200MBufferPoolMXBean.Name: direct
BufferPoolMXBean.Count: 2
BufferPoolMXBean.TotalCapacity: 25M
BufferPoolMXBean.MemoryUsed: 25MBufferPool.Name: direct
BufferPool.Count: 2
BufferPool.TotalCapacity: 25M
BufferPool.MemoryUsed: 25M分配25M本地字节缓冲区 java.nio.DirectByteBuffer[pos=0 lim=26214400 cap=26214400]
创建10M对象 [B@443b7951java虚拟机从操纵系统那里挖到的最大的内存   maxMemory 200M
java虚拟机已经从操作系统那里挖过来的内存   totalMemory : 50M
java虚拟机从操纵系统挖过来还没用上的内存   freeMemory : 22Mthe maximum allocatable direct buffer memory: 200MBufferPoolMXBean.Name: direct
BufferPoolMXBean.Count: 3
BufferPoolMXBean.TotalCapacity: 50M
BufferPoolMXBean.MemoryUsed: 50MBufferPool.Name: direct
BufferPool.Count: 3
BufferPool.TotalCapacity: 50M
BufferPool.MemoryUsed: 50M