The jmap command-line utility prints memory-related statistics for a running VM or core file.
The utility can also use the jsadebugd daemon to query a process or core file on a remote machine. Note: The output takes longer to print in this case.
The release of JDK 8 introduced Java Mission Control, Java Flight Recorder, and jcmd utility for diagnosing problems with JVM and Java applications. It is suggested to use the latest utility, jcmd instead of the previous jmap utility for enhanced diagnostics and reduced performance overhead.
If jmap is used with a process or core file without any command-line options, then it prints the list of shared objects loaded (the output is similar to the pmap utility on Oracle Solaris operating system). For more specific information, you can use the options -heap, -histo, or -permstat. These options are described in the subsections that follow.
In addition, the JDK 7 release introduced the -dump:format=b,file=filename option, which causes jmap to dump the Java heap in binary HPROF format to a specified file. This file can then be analyzed with the jhat tool.
If the jmap pid command does not respond because of a hung process, then the -F option can be used (on Oracle Solaris and Linux operating systems only) to force the use of the Serviceability Agent.
For more details on the jmap utility, see the jmap command man page.
The following sections describe jmap command usage and troubleshooting techniques with examples that print memory-related statistics for a running VM or a core file.
The -heap option is used to obtain the following Java heap information:
Information specific to the garbage collection (GC) algorithm, including the name of the GC algorithm (for example, parallel GC) and algorithm-specific details (such as number of threads for parallel GC).
Heap configuration that might have been specified as command-line options or selected by the VM based on the machine configuration.
Heap usage summary: For each generation (area of the heap), the tool prints the total heap capacity, in-use memory, and available free memory. If a generation is organized as a collection of spaces (for example, the new generation), then a space specific memory size summary is included.
Example 2-22 shows output from the jmap -heap command.
The jmap command with the -histo option can be used to obtain a class specific histogram of the heap. Depending on the parameter specified, the jmap -histo command can print out the heap histogram for a running process or a core file.
When the command is executed on a running process, the tool prints the number of objects, memory size in bytes, and fully qualified class name for each class. Internal classes in the Java HotSpot VM are enclosed in angle brackets. The histogram is useful in understanding how the heap is used. To get the size of an object, you must divide the total size by the count of that object type.
Example 2-23 shows output from the jmap -histo command when it is executed on a process with PID number 29620.
When the jmap -histo command is executed on a core file, the tool prints the size, count, and class name for each class. Internal classes in the Java HotSpot VM are prefixed with an asterisk (*).
Example 2-24 shows output of the jmap -histo command when it is executed on a core file.
The permanent generation is the area of the heap that holds all the reflective data of the virtual machine itself, such as class and method objects. This area is also called "method area" in The Java Virtual Machine Specification.
Configuring the size of the permanent generation can be important for applications that dynamically generate and load a very large number of classes (for example, Java Server Pages or web containers). If an application loads too many classes, then it is possible it will terminate with the following error:
Exception in thread thread_name java.lang.OutOfMemoryError: PermGen space
For a description of this and other variants of the OutOfMemoryError exception, see Understand the OutOfMemoryError Exception.
To get further information about the permanent generation, you can use the -permstat option of the jmap command to print statistics for the objects in the permanent generation.
Example 2-25 shows output from the jmap -permstat command executed on a process with PID number 29620.
For each class loader object, the following details are printed:
The address of the class loader object at the snapshot when the utility was run
The number of classes loaded
The approximate number of bytes consumed by metadata for all classes loaded by this class loader
The address of the parent class loader (if any)
A live or dead indication of whether the loader object will be garbage collected in the future
The class name of this class loader
