jmap

Print shared object mappings for a Java process (output like pmap)

$ jmap [java_pid]

$ jmap -heap [filename.jar] [java_pid]

$ jmap -histo [java_pid]

$ jmap -dump:format=b,file=[path/to/file] [java_pid]

$ jmap -dump:live,format=b,file=[path/to/file] [java_pid]

jmap [ options ] <pid>

-dump[:live,format=b,file=<filename>]
    Dump heap to file (format=b for binary hprof; live filters to live objects only)

-finalizerinfo
    Print information on objects awaiting finalization

-heap
    Show summary of heap configuration and usage

-histo[:live]
    Print histogram of heap objects (live shows only live objects)

-clstats | -permstat
    Print classloader statistics (permstat deprecated)

-F
    Force use of ClassAttachedNativeMemoryCounting for heap dump

-h | -help
    Display help message

-J<flag>
    Pass JVM flag directly to the runtime

jmap is a command-line utility included with the JDK for inspecting the memory usage of Java Virtual Machine (JVM) processes. It provides detailed information about the heap, object histograms, finalizable objects, and class loader statistics, making it invaluable for diagnosing memory leaks, out-of-memory errors, and performance issues in Java applications.

By specifying a process ID (pid), jmap connects to the target JVM and generates reports on memory allocation. Common use cases include generating heap dumps for analysis with tools like Eclipse MAT or VisualVM, viewing live object histograms to identify dominant classes consuming memory, or summarizing heap configuration and usage. It operates non-intrusively in most cases but can pause the application briefly during heap dumps.

jmap requires the JVM attach mechanism to be enabled (default since JDK 8) and is best used with the same JDK version as the target process for compatibility. It's particularly useful in production environments where full GC pauses need monitoring or when troubleshooting high memory footprints.

Requires JVM attach listener enabled (-XX:+EnableDynamicAgentLoading); may pause application during operations; use same JDK bitness and version as target JVM; full heap dumps can be large and I/O intensive.

jmap -dump:live,format=b,file=heap.hprof <pid>
jmap -heap <pid>
jmap -histo:live <pid>

Introduced in JDK 1.5 as part of jvmtools; enhanced in JDK 8+ with better attach mechanism and histo:live support; largely superseded by jcmd in JDK 11+ but remains available.

jcmd(1), jstat(1), jstack(1), jhat(1)