JVM进阶 — JDK命令

Lists the instrumented Java Virtual Machines (JVMs) on the target system.

如果Java进程关闭了默认开启的UsePerfData参数-XX:-UsePerfData
),那么 jps/jstat
无法探知
到该Java进程

$ jps
1408 Jps
19 LiveCoverMain

参数 备注
m Displays the arguments passed to the main method
. The output may be null for embedded JVMs.
l Displays the full package name
for the application’s main class or the full path name
to the application’s JAR file.
v Displays the arguments passed to the JVM
.

jstat

Monitors Java Virtual Machine (JVM) statistics.

➜  ~ jstat -options
-class
-compiler
-gc
-gccapacity
-gccause
-gcmetacapacity
-gcnew
-gcnewcapacity
-gcold
-gcoldcapacity
-gcutil
-printcompilation

class

Displays statistics about the behavior of the class loader
.

$ jstat -class 19
Loaded  Bytes  Unloaded  Bytes     Time   
 15375 27526.8       54    74.3      68.95

参数 备注
Loaded Number of classes loaded.
Bytes Number of kBs
loaded.
Unloaded Number of classes unloaded.
Bytes Number of Kbytes
unloaded.
Time Time spent performing class loading and unloading operations.

compiler

Displays statistics about the behavior of the Java HotSpot VM Just-in-Time
compiler.

$ jstat -compiler 19
Compiled Failed Invalid   Time   FailedType FailedMethod
   17421      1       0   122.19          1 org/apache/skywalking/apm/dependencies/net/bytebuddy/pool/TypePool$Default$WithLazyResolution doResolve

参数 备注
Compiled Number of compilation tasks performed.
Failed Number of compilations tasks failed.
Invalid Number of compilation tasks that were invalidated.
Time Time spent performing compilation tasks.
FailedType Compile type of the last
failed compilation.
FailedMethod Class name and method of the last
failed compilation.

printcompilation

Displays Java HotSpot VM compilation method
statistics.

$ jstat -printcompilation 19
Compiled  Size  Type Method
   17423    204    1 io/netty/util/internal/MpscLinkedQueue offer

参数 备注
Compiled Number of compilation tasks performed by the most recently
compiled method.
Size Number of bytes of byte code
of the most recently
compiled method.
Type Compilation type of the most recently
compiled method.
Method Class name and method name identifying the most recently
compiled method.

gc

Displays statistics about the behavior of the garbage collected heap.

jstat -gc 19
 S0C    S1C    S0U    S1U      EC       EU        OC         OU       MC     MU    CCSC   CCSU   YGC     YGCT    FGC    FGCT     GCT   
43648.0 43648.0  0.0   356.5  43776.0  17636.4  1835008.0   407774.3  91668.0 82097.8 12108.0 10130.4  33313  553.212   6      0.128  553.340

参数 备注
S0C Current survivor space 0 capacity (kB).
S1C Current survivor space 1 capacity (kB).
S0U Survivor space 0 utilization (kB).
S1U Survivor space 1 utilization (kB).
EC Current eden space capacity (kB).
EU Eden space utilization (kB).
OC Current old space capacity (kB).
OU Old space utilization (kB).
MC 1. Metaspace capacity (kB).

2. Klass Metaspace以及NoKlass Metaspace两者总共 committed
的内存大小
MU 1. Metacspace utilization (kB).

2. Klass Metaspace以及NoKlass Metaspace两者已经使用了的内存大小
CCSC 1. Compressed class space capacity (kB).

2. Klass Metaspace的已经committed的内存大小
CCSU 1. Compressed class space used (kB).

2. Klass Metaspace的已经被使用的内存大小
YGC Number of young generation garbage collection events.
YGCT Young generation garbage collection time.
FGC Number of full GC events.
FGCT Full garbage collection time.
GCT Total garbage collection time.

Metaspace相关内容可参考: JVM源码分析之Metaspace解密

G1

$ jstat -gc -t 13903
Timestamp        S0C    S1C    S0U    S1U      EC       EU        OC         OU       PC     PU    YGC     YGCT    FGC    FGCT     GCT
       554652.2  0.0   8192.0  0.0   8192.0 5275648.0 4276224.0 3104768.0   755520.9  131072.0 61178.3   8219  169.021   0      0.000  169.021

  1. S0C和S0U始终为0,这是因为使用G1 GC时,JVM不再设置Eden区、Survivor区和Old区的 内存边界
    ,而是将堆划分为若干个 等长
    内存区域
  2. 每个内存区域都可以作为Eden区、Survivor区和Old区,并且可以在不同区域类型之间来回切换
  3. 因此, 逻辑上只有一个Survivor区
    ,当需要迁移Survivor区中的数据(Copying),只需要申请一个或多个内存区域,作为新的Survivor区
  4. 当发生垃圾回收时,JVM可能出现Survivor内存区域内的对象 全被回收
    或者 全被晋升
    的现象
    • 此时,JVM会将这块内存区域回收,并标记为 可分配
    • 结果堆中可能 完全没有Survivor内存区域
      ,S1C和S1U均为0
  5. 554652s = 6.4day

gcnew

Displays statistics of the behavior of the new generation
.

$ jstat -gcnew 19
 S0C    S1C    S0U    S1U   TT MTT  DSS      EC       EU     YGC     YGCT  
43648.0 43648.0  315.7    0.0 15  15 21824.0  43776.0  16609.9  34072  566.323

参数 备注 样例
S0C Current survivor space 0 capacity (kB).
S1C Current survivor space 1 capacity (kB).
S0U Survivor space 0 utilization (kB).
S1U Survivor space 1 utilization (kB).
TT Tenuring threshold
.
15
MTT Maximum tenuring threshold
.
15
DSS Desired survivor size (kB). 1. -XX:TargetSurvivorRatio=50

2. Desired percentage of survivor space used after scavenge.

3. 21824KB == 0.5 * S0C
EC Current eden space capacity (kB).
EU Eden space utilization (kB).
YGC Number of young generation GC events.
YGCT Young generation garbage collection time.

gcold

Displays statistics about the behavior of the old generation
and metaspace
statistics.

$ jstat -gcold 19
   MC       MU      CCSC     CCSU       OC          OU       YGC    FGC    FGCT     GCT   
 92564.0  82773.0  12236.0  10223.6   1835008.0    423476.6  34693     6    0.128  575.369

参数 备注
MC Metaspace capacity (kB).
MU Metaspace utilization (kB).
CCSC Compressed class space capacity (kB).
CCSU Compressed class space used (kB).
OC Current old space capacity (kB).
OU Old space utilization (kB).
YGC Number of young generation GC events.
FGC Number of full GC events.
FGCT Full garbage collection time.
GCT Total garbage collection time.

gccapacity

Displays statistics about the capacities of the generations and their corresponding spaces.

$ jstat -gccapacity 19
 NGCMN    NGCMX     NGC     S0C   S1C       EC      OGCMN      OGCMX       OGC         OC       MCMN     MCMX      MC     CCSMN    CCSMX     CCSC    YGC    FGC
131072.0 131072.0 131072.0 43648.0 43648.0  43776.0  1835008.0  1835008.0  1835008.0  1835008.0      0.0 1128448.0  91668.0      0.0 1048576.0  12108.0  33477     6

参数 备注 样例
NGCMN Minimum new generation capacity (kB).
NGCMX Maximum new generation capacity (kB).
NGC Current new generation capacity (kB).
S0C Current survivor space 0 capacity (kB).
S1C Current survivor space 1 capacity (kB).
EC Current eden space capacity (kB).
OGCMN Minimum old generation capacity (kB).
OGCMX Maximum old generation capacity (kB).
OGC Current old generation capacity (kB).
OC Current old space capacity (kB).
MCMN Minimum metaspace capacity (kB). 0
MCMX Maximum metaspace capacity (kB). 1. Klass Metaspace以及NoKlass Metaspace两者总共的 reserved
的内存大小

2. 默认情况下Klass Metaspace是通过 CompressedClassSpaceSize
这个参数来reserved 1G的内存

3. NoKlass Metaspace默认reserved的内存大小是 2*InitialBootClassLoaderMetaspaceSize


4. 1128448KB == 1.076GB
MC Metaspace capacity (kB). 1. Klass Metaspace以及NoKlass Metaspace两者总共 committed
的内存大小

2. 91668KB == 89.5MB
CCSMN Compressed class space minimum capacity (kB). 0
CCSMX Compressed class space maximum capacity (kB). 1. Klass Metaspace reserved的内存大小

2. 1048576KB == 1GB
CCSC Compressed class space capacity (kB). 1. Klass Metaspace的已committed的内存大小

2. 12108KB == 11.8MB
YGC Number of young generation GC events.
FGC Number of full GC events.

gcnewcapacity

Displays statistics about the sizes of the new generations and its corresponding spaces.

$ jstat -gcnewcapacity 19
  NGCMN      NGCMX       NGC      S0CMX     S0C     S1CMX     S1C       ECMX        EC      YGC   FGC
  131072.0   131072.0   131072.0  43648.0  43648.0  43648.0  43648.0    43776.0    43776.0 34383     6

参数 备注
NGCMN Minimum new generation capacity (kB).
NGCMX Maximum new generation capacity (kB).
NGC Current new generation capacity (kB).
S0CMX Maximum survivor space 0 capacity (kB).
S0C Current survivor space 0 capacity (kB).
S1CMX Maximum survivor space 1 capacity (kB).
S1C Current survivor space 1 capacity (kB).
ECMX Maximum eden space capacity (kB).
EC Current eden space capacity (kB).
YGC Number of young generation GC events.
FGC Number of full GC events.

gcoldcapacity

Displays statistics about the sizes of the old generation.

$ jstat -gcoldcapacity 19
   OGCMN       OGCMX        OGC         OC       YGC   FGC    FGCT     GCT   
  1835008.0   1835008.0   1835008.0   1835008.0 34478     6    0.128  572.454

参数 备注
OGCMN Minimum old generation capacity (kB).
OGCMX Maximum old generation capacity (kB).
OGC Current old generation capacity (kB).
OC Current old space capacity (kB).
YGC Number of young generation GC events.
FGC Number of full GC events.
FGCT Full garbage collection time.
GCT Total garbage collection time.

gcmetacapacity

Displays statistics about the sizes of the metaspace.

$ jstat -gcmetacapacity 19
   MCMN       MCMX        MC       CCSMN      CCSMX       CCSC     YGC   FGC    FGCT     GCT   
       0.0  1130496.0    92564.0        0.0  1048576.0    12236.0 34593     6    0.128  574.271

参数 备注
MCMN Minimum metaspace capacity (kB).
MCMX Maximum metaspace capacity (kB).
MC Metaspace capacity (kB).
CCSMN Compressed class space minimum capacity (kB).
CCSMX Compressed class space maximum capacity (kB).
YGC Number of young generation GC events.
FGC Number of full GC events.
FGCT Full garbage collection time.
GCT Total garbage collection time.

gcutil

Displays a summary about garbage collection statistics.

$ jstat -gcutil 19
  S0     S1     E      O      M     CCS    YGC     YGCT    FGC    FGCT     GCT   
  0.16   0.00  14.11  23.02  89.37  83.50  34596  574.184     6    0.128  574.312

参数 备注 样例
S0 Survivor space 0 utilization as a percentage of the space’s current capacity.
S1 Survivor space 1 utilization as a percentage of the space’s current capacity.
E Eden space utilization as a percentage of the space’s current capacity.
O Old space utilization as a percentage of the space’s current capacity.
M Metaspace utilization as a percentage of the space’s current capacity. 1. Klass Metaspace以及NoKlass Metaspace两者总共的使用率

2. MC=92564.0, MU=82773.0, CCSC=12236.0, CCSU=10223.6

3. M = MU/MC = 89.4

4. 有时候M达到90%以上,不一定说明metaspace使用了很多,因为内存是慢慢commit的
CCS Compressed class space utilization as a percentage. 1. NoKlass Metaspace的使用率

2. MC=92564.0, MU=82773.0, CCSC=12236.0, CCSU=10223.6

3. CCS = CCSU/CCSC = 83.5
YGC Number of young generation GC events.
YGCT Young generation garbage collection time.
FGC Number of full GC events.
FGCT Full garbage collection time.
GCT Total garbage collection time.

gccause

Displays a summary about garbage collection statistics (same as -gcutil), with the cause of the last
and current
(when

applicable) garbage collection events.

$ jstat -gccause 19
  S0     S1     E      O      M     CCS    YGC     YGCT    FGC    FGCT     GCT    LGCC                 GCC                 
  0.00   0.83   2.23  22.60  89.67  84.02  33919  563.393     6    0.128  563.521 Allocation Failure   No GC

参数 备注 样例
LGCC Cause of last
garbage collection
Allocation Failure
GCC Cause of current
garbage collection
No GC

jmap

  1. Prints shared object memory maps or heap memory details for a process, core file, or remote debug server.
  2. 由于jmap将访问 堆中的所有对象
    ,为了保证次此过程不被应用线程干扰
    • jmap需要借助 安全点机制
      ,让所有线程停留在不改变堆中数据的状态
  3. 因此, jmap导出的堆快照必定是安全点位置的
    ,可能导致基于该堆快照的分析结果存在 偏差

    • 例如编译生成的机器码中,某些对象的生命周期在两个安全点之间,那么:live选项将无法探知到这些对象
  4. 如果某个线程长时间无法跑到安全点,jmap将一直等待下去
    • 垃圾回收器会主动将jstat所需要的摘要数据保存至 固定位置
    • 因此jstat只需要直接读取即可
  5. jmap/jinfo/jstack/jcmd
    ,均依赖于JVM的 Attach API
    ,因此 只能监控本地Java进程
  6. 如果开启 -XX:+DisableAttachMechanism
    ,那么基于 Attach API
    的命令也将无法执行

clstats

Prints class loader wise statistics of Java heap. For each class loader, its name, how active it is, address, parent class loader, and the number and size of classes it has loaded are printed.

$ jmap -clstats 19
Attaching to process ID 19, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.131-b11
finding class loader instances ..done.
computing per loader stat ..done.
please wait.. computing liveness.liveness analysis may be inaccurate ...
class_loader    classes bytes   parent_loader   alive?  type

<bootstrap>     2508    4508619   null          live    <internal>
0x000000009388bb88      1       1473      null          dead    sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x0000000090bcccb8      1       1472    0x0000000090024a50      dead    sun/reflect/DelegatingClassLoader@0x0000000100009df8
...
total = 634     12693   20378456            N/A         alive=1, dead=633           N/A

finalizerinfo

Prints information about objects that are awaiting finalization.

$ jmap -finalizerinfo 19
Attaching to process ID 19, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.131-b11
Number of objects pending for finalization: 0

heap

Prints a heap summary of the garbage collection used, the head configuration, and generation-wise heap usage. In addition, the number and size of interned Strings are printed.

$ jmap -heap 19
Attaching to process ID 19, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.131-b11

using parallel threads in the new generation.
using thread-local object allocation.
Concurrent Mark-Sweep GC

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 2013265920 (1920.0MB)
   NewSize                  = 134217728 (128.0MB)
   MaxNewSize               = 134217728 (128.0MB)
   OldSize                  = 1879048192 (1792.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 1
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
New Generation (Eden + 1 Survivor Space):
   capacity = 89522176 (85.375MB)
   used     = 32297224 (30.80103302001953MB)
   free     = 57224952 (54.57396697998047MB)
   36.07734467937866% used
Eden Space:
   capacity = 44826624 (42.75MB)
   used     = 31529392 (30.068771362304688MB)
   free     = 13297232 (12.681228637695312MB)
   70.33630728024488% used
From Space:
   capacity = 44695552 (42.625MB)
   used     = 767832 (0.7322616577148438MB)
   free     = 43927720 (41.892738342285156MB)
   1.7179159125274928% used
To Space:
   capacity = 44695552 (42.625MB)
   used     = 0 (0.0MB)
   free     = 44695552 (42.625MB)
   0.0% used
concurrent mark-sweep generation:
   capacity = 1879048192 (1792.0MB)
   used     = 33776136 (32.21143341064453MB)
   free     = 1845272056 (1759.7885665893555MB)
   1.7975130251475744% used

21703 interned Strings occupying 2142592 bytes.

histo[:live]

Prints a histogram of the heap. For each Java class, the number of objects, memory size in bytes, and the fully qualified class names are printed. The JVM internal class names are printed with an asterisk (*) prefix. If the live suboption is specified, then only active objects are counted.

$ jmap -histo:live 19
num     #instances         #bytes  class name
----------------------------------------------
   1:         72102        6440744  [C
   2:          5996        4159456  [B
   3:         19079        1783760  [Ljava.lang.Object;
   4:         71213        1709112  java.lang.String
...
Total        529634       28005336

dump:[live,] format=b, file=filename

Dumps the Java heap in hprof
binary format to filename. The live suboption is optional, but when specified, only the active objects in the heap are dumped.

相关的JVM参数: -XX:+HeapDumpAfterFullGC
-XX:+HeapDumpOnOutOfMemoryError

$ jmap -dump:live,format=b,file=/tmp/cover.hprof 19
Dumping heap to /tmp/cover.hprof ...
Heap dump file created

jinfo

Generates configuration information

$ jinfo 19
Attaching to process ID 19, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.131-b11
Java System Properties:

java.runtime.name = Java(TM) SE Runtime Environment
java.vm.version = 25.131-b11
...
conf.key = lz_live_cover
...
VM Flags:
Non-default VM flags: -XX:CICompilerCount=2 -XX:+CMSClassUnloadingEnabled
....
Command line:  -Dconf.key=lz_live_cover -Dconf.env=pre

sysprops

Prints Java system properties as name-value pairs.

$ jinfo -sysprops 19
Attaching to process ID 19, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.131-b11
java.runtime.name = Java(TM) SE Runtime Environment
java.vm.version = 25.131-b11
...
java.vm.vendor = Oracle Corporation
conf.key = lz_live_cover
...
java.vm.name = Java HotSpot(TM) 64-Bit Server VM
sun.java.launcher = SUN_STANDARD
...

flags

$ jinfo -flags 19
Attaching to process ID 19, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.131-b11
Non-default VM flags: -XX:CICompilerCount=2 -XX:+CMSClassUnloadingEnabled
...
Command line:  -Dconf.key=lz_live_cover -Dconf.env=pre
...

manageable参数

$ java -XX:+PrintFlagsFinal -version | grep manageable
     intx CMSAbortablePrecleanWaitMillis            = 100                                 {manageable}
     intx CMSTriggerInterval                        = -1                                  {manageable}
     intx CMSWaitDuration                           = 2000                                {manageable}
     bool HeapDumpAfterFullGC                       = false                               {manageable}
     bool HeapDumpBeforeFullGC                      = false                               {manageable}
     bool HeapDumpOnOutOfMemoryError                = false                               {manageable}
    ccstr HeapDumpPath                              =                                     {manageable}
    uintx MaxHeapFreeRatio                          = 100                                 {manageable}
    uintx MinHeapFreeRatio                          = 0                                   {manageable}
     bool PrintClassHistogram                       = false                               {manageable}
     bool PrintClassHistogramAfterFullGC            = false                               {manageable}
     bool PrintClassHistogramBeforeFullGC           = false                               {manageable}
     bool PrintConcurrentLocks                      = false                               {manageable}
     bool PrintGC                                   = false                               {manageable}
     bool PrintGCDateStamps                         = false                               {manageable}
     bool PrintGCDetails                            = false                               {manageable}
     bool PrintGCID                                 = false                               {manageable}
     bool PrintGCTimeStamps                         = false                               {manageable}
java version "1.8.0_131"
Java(TM) SE Runtime Environment (build 1.8.0_131-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.131-b11, mixed mode)

flag

Prints the name and value of the specified command-line flag.

$ jinfo -flag HeapDumpAfterFullGC 19
-XX:-HeapDumpAfterFullGC

flag [+|-]name

Enables or disables the specified Boolean command-line flag.

$ jinfo -flag +HeapDumpAfterFullGC 19

$ jinfo -flag HeapDumpAfterFullGC 19
-XX:+HeapDumpAfterFullGC

flag name=value

Sets the specified command-line flag to the specified value.

$ jinfo -flag CMSWaitDuration=1999 19

$ jinfo -flag CMSWaitDuration 19
-XX:CMSWaitDuration=1999

jstack

Prints Java thread stack traces for a Java process, core file, or remote debug server.

jstack的一个常用应用场景为 检测

$ jstack 19120
2019-01-10 09:53:21
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.181-b13 mixed mode):

...

"t3" #12 prio=5 os_prio=31 tid=0x00007faa948b4800 nid=0xf07 waiting for monitor entry [0x00007000043c8000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at me.zhongmingmao.advanced.command.SyncThread.run(ThreadDeadlock.java:39)
	- waiting to lock <0x0000000795700530> (a java.lang.Object)
	- locked <0x0000000795700550> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:748)

"t2" #11 prio=5 os_prio=31 tid=0x00007faa948b4000 nid=0x3c03 waiting for monitor entry [0x0000700005581000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at me.zhongmingmao.advanced.command.SyncThread.run(ThreadDeadlock.java:39)
	- waiting to lock <0x0000000795700550> (a java.lang.Object)
	- locked <0x0000000795700540> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:748)

"t1" #10 prio=5 os_prio=31 tid=0x00007faa948b3000 nid=0x4203 waiting for monitor entry [0x000070000547e000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at me.zhongmingmao.advanced.command.SyncThread.run(ThreadDeadlock.java:39)
	- waiting to lock <0x0000000795700540> (a java.lang.Object)
	- locked <0x0000000795700530> (a java.lang.Object)
	at java.lang.Thread.run(Thread.java:748)

...

Found one Java-level deadlock:
=============================
"t3":
  waiting to lock monitor 0x00007faa94811b58 (object 0x0000000795700530, a java.lang.Object),
  which is held by "t1"
"t1":
  waiting to lock monitor 0x00007faa9480da08 (object 0x0000000795700540, a java.lang.Object),
  which is held by "t2"
"t2":
  waiting to lock monitor 0x00007faa94811aa8 (object 0x0000000795700550, a java.lang.Object),
  which is held by "t3"

...

Found 1 deadlock.

jcmd

Sends diagnostic command requests to a running Java Virtual Machine (JVM).

jcmd可以替代上面除了jstat之外的所有命令,并没有保留jstat的输出格式

查看进程

$ jcmd -l
19 fm.lizhi.live.cover.LiveCoverMain lz-live-cover-pre-deployment-5f45b6955d-f4xjj
3241 sun.tools.jcmd.JCmd -l

查看性能统计

$ jcmd 19 PerfCounter.print
java.ci.totalTime=445938928367
java.cls.loadedClasses=20317
...
java.threads.daemon=121
java.threads.live=124
java.threads.livePeak=125
java.threads.started=2458
...

可执行的操作

$ jcmd 19 help
19:
The following commands are available:
JFR.stop
JFR.start
JFR.dump
JFR.check
VM.native_memory
VM.check_commercial_features
VM.unlock_commercial_features
ManagementAgent.stop
ManagementAgent.start_local
ManagementAgent.start
GC.rotate_log
Thread.print
GC.class_stats
GC.class_histogram
GC.heap_dump
GC.run_finalization
GC.run
VM.uptime
VM.flags
VM.system_properties
VM.command_line
VM.version
help

VM.uptime

$ jcmd 19 VM.uptime
19:
87089.850 s

GC.run

$ jcmd 19 GC.run
19:
Command executed successfully

Thread.print

$ jcmd 3721 Thread.print
Found one Java-level deadlock:
=============================
"t3":
  waiting to lock monitor 0x00007fd53105ff58 (object 0x0000000795700530, a java.lang.Object),
  which is held by "t1"
"t1":
  waiting to lock monitor 0x00007fd53105be08 (object 0x0000000795700540, a java.lang.Object),
  which is held by "t2"
"t2":
  waiting to lock monitor 0x00007fd53105fea8 (object 0x0000000795700550, a java.lang.Object),
  which is held by "t3"

JFR

$ jcmd 3721 JFR.check
3721:
Java Flight Recorder not enabled.
Use VM.unlock_commercial_features to enable.

$ jcmd 3721 VM.unlock_commercial_features
3721:
Commercial Features now unlocked.

$ jcmd 3721 JFR.check
3721:
No available recordings.
Use JFR.start to start a recording.

$ jcmd 3721 JFR.start name=abc,duration=120s
3721:
Started recording 1. No limit (duration/maxsize/maxage) in use.
Use JFR.dump name=abc,duration=120s filename=FILEPATH to copy recording data to file.

$ jcmd 3721 JFR.check
3721:
Recording: recording=1 name="abc,duration=120s" (running)

$ cmd 3721 JFR.dump name=abc,duration=120s filename=abc.jfr
3721:
Dumped recording "abc,duration=120s", 391.4 kB written to:
/Users/zhongmingmao/Documents/source_code/github/jvm_demo/abc.jfr

$ jcmd 3721 JFR.stop name=abc,duration=120s
3721:
Stopped recording "abc,duration=120s".

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http://zhongmingmao.me/2019/01/08/jvm-advanced-command/

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