jcmd VM.set_flag，哪些标志是可写的？

Question

我试图尝试使用jcmd VM.set_flag选项。但是遇到一个错误，说“只能设置‘可写’标志”。什么是可写标志？

得到我的pid：

XXX@XXX-Air:~/javacode$ jcmd -l
6294 Test
6295 jdk.jcmd/sun.tools.jcmd.JCmd -l

试图更改vm标志：

XXX@XXX-Air:~/javacode$ jcmd 6294 VM.set_flag ConcGCThreads 4
6294:
only 'writeable' flags can be set
XXX@XXX-Air:~/javacode$ jcmd 6294 VM.set_flag MaxNewSize 1G
6294:
only 'writeable' flags can be set

编辑:它适用于可管理的标志，下面是成功的命令。

 XXXX@XXX-Air:~/javacode$ jcmd 11441 VM.flags -all | grep MinHeapFreeRatio
    uintx MinHeapFreeRatio                         = 40                                    {manageable} {default}
XXXX@XXX-Air:~/javacode$ jcmd 11441 VM.set_flag MinHeapFreeRatio 45
11441:
Command executed successfully
XXXX@XXX-Air:~/javacode$ jcmd 11441 VM.flags -all | grep MinHeapFreeRatio
    uintx MinHeapFreeRatio                         = 45    

Answer 1

可写标志被标记为{manageable}。

您可以用jcmd 12345 VM.flags -all列出所有标志。然后，您可以将grep用于可管理的(我的Oracle上的jdk8 )：

$ jcmd 12345 VM.flags -all | 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}

Answer 2

关于VM选项的文章指出:-

标记为可管理的的标志可以通过JDK管理接口(JDK)和JConsole动态写入。

要找出所有这样的标志，可以使用VM.flags

打印VM标志选项及其当前值

将-all作为以下选项：

打印VM支持的所有标志。

使用以下命令:-

jcmd <pid> VM.flags -all