《Java 虚拟机原理与性能调优》3.1 运行时数据区：堆、栈、元空间与代码缓存

java -XX:NativeMemoryTracking=summary \
     -Xmx1g -Xss1m \
     -XX:MaxMetaspaceSize=256m \
     -XX:MaxDirectMemorySize=256m \
     -cp out lab.MemoryShape

jcmd <pid> VM.native_memory summary

Native Memory Tracking:

(Omitting categories weighting less than 1KB)

Total: reserved=2058552KB, committed=763192KB
       malloc: 154808KB #18111, peak=154489KB #18113
       mmap:   reserved=1903744KB, committed=608384KB

-                 Java Heap (reserved=1056768KB, committed=526336KB)
                            (mmap: reserved=1056768KB, committed=526336KB, at peak)

-                     Class (reserved=213128KB, committed=264KB)
                            (classes #845)
                            (  instance classes #708, array classes #137)
                            (  Metadata:   )
                            (    reserved=65536KB, committed=448KB)
                            (    used=396KB)
                            (  Class space:)
                            (    reserved=212992KB, committed=128KB)
                            (    used=30KB)

-                    Thread (reserved=249604KB, committed=17284KB)
                            (threads #226)
                            (stack: reserved=248832KB, committed=16512KB, peak=16512KB)

-                      Code (reserved=249735KB, committed=7719KB)
                            (malloc=86KB tag=Code #1880) (at peak)
                            (mmap: reserved=249648KB, committed=7632KB, at peak)

-                        GC (reserved=72612KB, committed=62436KB)

-                     Other (reserved=131072KB, committed=131072KB)
                            (malloc=131072KB tag=Other #4) (at peak)

jcmd <pid> VM.info

Java VM: OpenJDK 64-Bit Server VM Temurin-25.0.5+8 (25.0.5+8, mixed mode, sharing, tiered, compressed oops, compressed class ptrs, g1 gc, bsd-aarch64)
Host: "MacBookPro18,1" arm64, 10 cores, 32G, Darwin 25.3.0, macOS 26.3 (25D125)
Heap address: 0x0000000300800000, size: 1024 MB, Compressed Oops mode: Zero based, Oop shift amount: 3

jcmd <pid> GC.heap_info

garbage-first heap   total reserved 1048576K, committed 526336K, used 259101K [0x0000000300800000, 0x0000000340800000)
 region size 1024K, 76 young (77824K), 24 survivors (24576K)

jcmd <pid> VM.metaspace

Metaspace        used 426K, committed 576K, reserved 278528K
 class space     used 30K, committed 128K, reserved 212992K

Total Usage - 9 loaders, 845 classes (796 shared):
  Non-Class:   11 chunks,      4.02 MB capacity,  407.00 KB ( 10%) committed,   396.27 KB ( 10%) used
      Class:    9 chunks,    265.00 KB capacity,   73.00 KB ( 28%) committed,    30.45 KB ( 11%) used
       Both:   20 chunks,      4.28 MB capacity,  480.00 KB ( 11%) committed,   426.73 KB ( 10%) used

jcmd <pid> Compiler.codecache

CodeHeap 'non-profiled nmethods': size=120032Kb used=32Kb max_used=32Kb free=119999Kb
CodeHeap 'profiled nmethods': size=120032Kb used=122Kb max_used=122Kb free=119909Kb
CodeHeap 'non-nmethods': size=5712Kb used=1190Kb max_used=1205Kb free=4521Kb
CodeCache: size=245776Kb, used=1344Kb, max_used=1359Kb, free=244429Kb
 total_blobs=535, nmethods=157, adapters=287, full_count=0

package com.plume.library.app;

import java.lang.management.GarbageCollectorMXBean;
import java.lang.management.ManagementFactory;
import java.lang.management.MemoryMXBean;
import java.lang.management.MemoryPoolMXBean;
import java.lang.management.MemoryUsage;
import java.lang.management.RuntimeMXBean;
import java.lang.management.ThreadMXBean;

record RuntimeSnapshot(long heapUsed, long heapCommitted, long heapMax,
                       long nonHeapUsed, long metaspaceUsed,
                       long gcCount, long gcTimeMillis,
                       int threadCount, int peakThreadCount,
                       long uptimeMillis) {}

public final class RuntimeProbe {

    public RuntimeSnapshot snapshot() {
        MemoryMXBean memory = ManagementFactory.getMemoryMXBean();
        MemoryUsage heap = memory.getHeapMemoryUsage();
        MemoryUsage nonHeap = memory.getNonHeapMemoryUsage();

        long metaspaceUsed = 0;
        for (MemoryPoolMXBean pool : ManagementFactory.getMemoryPoolMXBeans()) {
            if (pool.getName().contains("Metaspace")) {
                metaspaceUsed += pool.getUsage().getUsed();
            }
        }

        long gcCount = 0;
        long gcTime = 0;
        for (GarbageCollectorMXBean gc : ManagementFactory.getGarbageCollectorMXBeans()) {
            gcCount += gc.getCollectionCount();
            gcTime += gc.getCollectionTime();
        }

        ThreadMXBean threads = ManagementFactory.getThreadMXBean();
        RuntimeMXBean runtime = ManagementFactory.getRuntimeMXBean();

        return new RuntimeSnapshot(
                heap.getUsed(), heap.getCommitted(), heap.getMax(),
                nonHeap.getUsed(), metaspaceUsed,
                gcCount, gcTime,
                threads.getThreadCount(), threads.getPeakThreadCount(),
                runtime.getUptime());
    }
}

Native memory tracking is not enabled

java.lang.OutOfMemoryError: Cannot reserve 33554432 bytes of direct buffer memory (allocated: 134217728, limit: 134217728)
	at java.base/java.nio.Bits.reserveMemory(Bits.java:178)
	at java.base/java.nio.DirectByteBuffer.<init>(DirectByteBuffer.java:108)
	at java.base/java.nio.ByteBuffer.allocateDirect(ByteBuffer.java:367)