http://lovestblog.cn/blog/2014/06/18/jvm-attach/

Attach是什么

jstack $PID

"Attach Listener" daemon prio=5 tid=0x00007fb0c6800800 nid=0x440b waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" daemon prio=5 tid=0x00007fb0c4815800 nid=0x4d03 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
....

”Attach Listener”和“Signal Dispatcher”，这两个线程是我们这次要讲的Attach机制的关键，先偷偷告诉各位，其实Attach Listener这个线程在jvm起来的时候可能并没有的，后面会细说。

Attach能做些什么

总结起来说，比如内存dump，线程dump，类信息统计(比如加载的类及大小以及实例个数等)，动态加载agent(使用过btrace的应该不陌生)，动态设置vm flag(/nfs/shareset/home/blog/content/zh/notes/java/attach)，打印vm flag，获取系统属性等，这些对应的源码(AttachListener.cpp)如下

static AttachOperationFunctionInfo funcs[] = {
  { "agentProperties",  get_agent_properties },
  { "datadump",         data_dump },
  { "dumpheap",         dump_heap },
  { "load",             JvmtiExport::load_agent_library },
  { "properties",       get_system_properties },
  { "threaddump",       thread_dump },
  { "inspectheap",      heap_inspection },
  { "setflag",          set_flag },
  { "printflag",        print_flag },
  { "jcmd",             jcmd },
  { NULL,               NULL }
};

Attach在jvm里如何实现的

前面也提到了，jvm在启动过程中可能并没有启动Attach Listener这个线程，可以通过jvm参数来启动.

Container aware

https://github.com/apangin/jattach

Java 10 update to support container

https://jarekprzygodzki.dev/post/generating-jvm-memory-dumps-from-jre/

Prior to Java 10 jmap, jstack and jcmd could not attach from a process on the host machine to a JVM running inside a Docker container because of how the attach mechanism interacts with pid and mount namespaces. Java 10 fixes this by the JVM inside the container finding its PID in the root namespace and using this to watch for a JVM attachment.