JVM debugging tool -jmap

jmap

jmap Command is a tool that can output all objects in memory , You can even put VM Medium heap, Binary output into text .

Print out something java process （ Use pid） In memory , all ‘ object ’ The situation of （ Such as ： Produce those objects , And quantity ）.

Command usage ：jmap [option]
(to connect to running process) Connect to a running process

jmap [option] <executable <core>
    (to connect to a core file)      Connect to the core file 

jmap [option] [[email protected]]<remote server IP or hostname>
    (to connect to remote debug server)  Connect to remote debugging service 
    
pid:     Target process PID, Process number , May adopt ps -ef | grep java  see java Process PID;

executable:      produce core dump Of java Executable program ;

core:      The message will be printed core dump file ;

remote-hostname-or-IP:      long-range debug The hostname or ip;

server-id:      only id, Suppose you have multiple remotes on a single host debug service ;

Common commands

Print Java Heap profile , Including the GC Algorithm 、 Heap configuration parameters and heap memory usage in each generation

> jmap -heap 23766
Attaching to process ID 31846, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 24.71-b01
 
using thread-local object allocation.
Parallel GC with 4 thread(s)//GC  The way 
 
Heap Configuration: // Heap memory initialization configuration 
   MinHeapFreeRatio = 0 // Corresponding jvm Launch parameters -XX:MinHeapFreeRatio Set up JVM Heap minimum idle ratio (default 40)
   MaxHeapFreeRatio = 100 // Corresponding jvm Launch parameters  -XX:MaxHeapFreeRatio Set up JVM The maximum idle ratio of the heap (default 70)
   MaxHeapSize      = 2082471936 (1986.0MB) // Corresponding jvm Launch parameters -XX:MaxHeapSize= Set up JVM The maximum size of the heap 
   NewSize          = 1310720 (1.25MB)// Corresponding jvm Launch parameters -XX:NewSize= Set up JVM Heaped ‘ The new generation ’ Default size of 
   MaxNewSize       = 17592186044415 MB// Corresponding jvm Launch parameters -XX:MaxNewSize= Set up JVM Heaped ‘ The new generation ’ Maximum size of 
   OldSize          = 5439488 (5.1875MB)// Corresponding jvm Launch parameters -XX:OldSize=<value>: Set up JVM Heaped ‘ Old generation ’ Size 
   NewRatio         = 2 // Corresponding jvm Launch parameters -XX:NewRatio=:‘ The new generation ’ and ‘ Old generation ’ The size ratio of 
   SurvivorRatio    = 8 // Corresponding jvm Launch parameters -XX:SurvivorRatio= Set up a young generation Eden District and Survivor Area size ratio  
   PermSize         = 21757952 (20.75MB)  // Corresponding jvm Launch parameters -XX:PermSize=<value>: Set up JVM Heaped ‘ Eternal generation ’ Initial size of 
   MaxPermSize      = 85983232 (82.0MB)// Corresponding jvm Launch parameters -XX:MaxPermSize=<value>: Set up JVM Heaped ‘ Eternal generation ’ Maximum size of 
   G1HeapRegionSize = 0 (0.0MB)
 
Heap Usage:// Heap memory usage 
PS Young Generation
Eden Space://Eden Area memory distribution 
   capacity = 33030144 (31.5MB)//Eden Total area capacity 
   used     = 1524040 (1.4534378051757812MB)  //Eden Zone used 
   free     = 31506104 (30.04656219482422MB)  //Eden Area remaining capacity 
   4.614088270399305% used //Eden Area usage ratio 
From Space:  // One of them Survivor Area memory distribution 
   capacity = 5242880 (5.0MB)
   used     = 0 (0.0MB)
   free     = 5242880 (5.0MB)
   0.0% used
To Space:  // the other one Survivor Area memory distribution 
   capacity = 5242880 (5.0MB)
   used     = 0 (0.0MB)
   free     = 5242880 (5.0MB)
   0.0% used
PS Old Generation // Current Old Area memory distribution 
   capacity = 86507520 (82.5MB)
   used     = 0 (0.0MB)
   free     = 86507520 (82.5MB)
   0.0% used
PS Perm Generation// Current  “ Eternal generation ”  Distribution of memory 
   capacity = 22020096 (21.0MB)
   used     = 2496528 (2.3808746337890625MB)
   free     = 19523568 (18.619125366210938MB)
   11.337498256138392% used
 
670 interned Strings occupying 43720 bytes.

Print Java Histogram of objects in heap , Each... Can be obtained from this diagram class The number of objects , Occupied memory size and class full name information , close :live, Only the living objects are counted

> jmap -histo 23766

23766: Unable to open socket file: target process not responding or HotSpot VM not loaded
The -F option can be used when the target process is not responding

> jmap -F -histo 23766
Attaching to process ID 23766, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.191-b12
Iterating over heap. This may take a while...

 num     #instances         #bytes  class name
----------------------------------------------
   1:        245237       26378800  [C
   2:         18355       19262448  [B
   3:         45062       14682856  [I
   4:        245389        5889336  java.util.HashMap$Node
   5:        274529        4392464  java.lang.String
   6:         44989        4087960  [Ljava.util.HashMap$Node;
   7:         68684        3150440  [Ljava.lang.Object;
   8:         18342        1841288  java.lang.Class
   9:         28573        1828672  java.lang.reflect.Field
  10:         42818        1712720  java.util.HashMap
  11:         30488        1556088  [Ljava.lang.String;
  12:         32420        1037440  java.util.LinkedHashMap$Entry

Print object information waiting to be recycled

>  jmap -finalizerinfo 23766

Attaching to process ID 23766, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.191-b12
Number of objects pending for finalization: 0

#  Show the current F-QUEUE There is no waiting in the queue Fializer Threads execute finalizer Object of method .

With hprof Binary format will Java The heap information is output to a file , This document can be used MAT、VisualVM or jhat Wait for tools to check

#  output to a file , Note that the application will be suspended 
> jmap -dump:live,format=b,file=heapDump 23766

Dumping heap to /home/oidd/heapDump ...
Heap dump file created

#  Access... Through a browser 
> jhat -port 5000 heapDump

Reading from heapDump...
Dump file created Thu Sep 17 08:43:21 CST 2020
Snapshot read, resolving...
Resolving 314429 objects...
Chasing references, expect 62 dots..............................................................
Eliminating duplicate references..............................................................
Snapshot resolved.
Started HTTP server on port 5000
Server is ready.


 visit ：http://localhost:5000/