JDK8 和 JDK11 查看堆内存大小命令变化

JDK 11 中取消了jmap工具 -heap 参数。该 jmap 命令行工具打印内存相关统计运行中的VM或核心文件。JDK8是正常的。如果直接使用jmap -heap 29982，将会得到如下错误：

[root@VM-0-6-centos ~]# jps -l
26363 jdk.jcmd/sun.tools.jps.Jps
29982 /usr/local/nacos/target/nacos-server.jar
[root@VM-0-6-centos ~]# jmap -heap 29982
Error: -heap option used
Cannot connect to core dump or remote debug server. Use jhsdb jmap instead

需要使用 jhsdb jmap --heap --pid 29982，则可以完整的查看到堆的分配和使用情况。

[root@VM-0-6-centos ~]# jhsdb jmap --heap --pid 29982
Attaching to process ID 29982, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 11.0.5+10-LTS

using thread-local object allocation.
Garbage-First (G1) GC with 4 thread(s)

Heap Configuration:
 MinHeapFreeRatio = 40
 MaxHeapFreeRatio = 70
 MaxHeapSize = 536870912 (512.0MB)
 NewSize = 268435456 (256.0MB)
 MaxNewSize = 268435456 (256.0MB)
 OldSize = 5452592 (5.1999969482421875MB)
 NewRatio = 2
 SurvivorRatio = 8
 MetaspaceSize = 21807104 (20.796875MB)
 CompressedClassSpaceSize = 1073741824 (1024.0MB)
 MaxMetaspaceSize = 17592186044415 MB
 G1HeapRegionSize = 1048576 (1.0MB)

Heap Usage:
G1 Heap:
 regions = 512
 capacity = 536870912 (512.0MB)
 used = 181053824 (172.6663818359375MB)
 free = 355817088 (339.3336181640625MB)
 33.72390270233154% used
G1 Young Generation:
Eden Space:
 regions = 30
 capacity = 281018368 (268.0MB)
 used = 31457280 (30.0MB)
 free = 249561088 (238.0MB)
 11.194029850746269% used
Survivor Space:
 regions = 1
 capacity = 1048576 (1.0MB)
 used = 1048576 (1.0MB)
 free = 0 (0.0MB)
 100.0% used
G1 Old Generation:
 regions = 142
 capacity = 254803968 (243.0MB)
 used = 148547968 (141.6663818359375MB)
 free = 106256000 (101.3336181640625MB)
 58.29892256622942% used