Java memory map (
jmap tool to get memory information for a particular Java™ process, or list of processes. The tool shows statistics about classes on the heap, including the number of objects and their aggregate size. The command syntax is as follows:
jmap [<option>] [<vmid>]
<vmid> is the Attach API virtual machine identifier for the Java process. This ID is typically the same as
the operating system process ID, unless you specified the
-Dcom.ibm.tools.attach.id system property when you started the process.
VMID is shown in
jps or other Attach API-based tools. Multiple VMIDs can be specified, separated by a space.
If you do not specify a VMID, the command reads input from
stdin. You can therefore get information for all processes by piping the output of the
jps command to
jps -q | jmap -histo
IDs of dead processes are silently ignored.
On its own,
jmap prints help information. To obtain memory information, a
-histo argument must be supplied, where the available
<options> are as follows:
-histo: Prints statistics about classes on the heap, including the number of objects and their aggregate size
-histo:live: Prints statistics for live objects only
-J: supplies arguments to the Java VM that is running the
jmapcommand. You can use multiple
-Joptions, for example:
jmap -J-Xms2m -J-Xmx10m
The output has the following format:
num object count total size class name ------------------------------------------------- 1 3354 107328 [C 2 717 57360 java.lang.Class 3 2427 38832 java.lang.String 4 50 13200 [J 5 717 11472 java.lang.J9VMInternals$ClassInitializationLock 6 342 8208 java.lang.StringBuilder 7 151 7248 jdk.internal.org.objectweb.asm.Item 8 396 6336 [Ljava.lang.Object;
Restrictions: This tool is not supported and is subject to change or removal in future releases. Although similar in usage and output to the HotSpot tool of the same name, this tool is a different implementation that is specific to OpenJ9.
The following tool limitations apply:
- Displays information only for local processes that are owned by the current user, due to security considerations. You can display information for remote processes by using
ssh user@host jmap <options> <pid>.
- Displaying data from core dumps is not supported; use
- Other options , such as
-F(force a dump of an unresponsive process) can be accomplished using
kill -QUIT <pid>.
The tool uses the Attach API, and has the following limitations:
- Displays information for OpenJ9 Java processes only
- Does not show information for processes whose Attach API is disabled. Note: The Attach API is disabled by default on z/OS.
For more information about the Attach API, including how to enable and secure it, see Java Attach API.