jhat

Synopsis

jhat [options ]heap-dump-file

options

The command-line options. SeeOptions.

heap-dump-file

Java binary heap dump file to be browsed. For a dump file that contains multiple heap dumps, you can specify which dump in the file by appending#<number> to the file name, for example,myfile.hprof#3.

Description

Thejhat command parses a Java heap dump file and starts a web server. Thejhat command lets you to browse heap dumps with your favorite web browser. Thejhat command supports predesigned queries such as show all instances of a known classMyClass, and Object Query Language (OQL). OQL is similar to SQL, except for querying heap dumps. Help on OQL is available from the OQL help page shown by thejhat command. With the default port, OQL help is available at http://localhost:7000/oqlhelp/

There are several ways to generate a Java heap dump:

• Use thejmap -dump option to obtain a heap dump at runtime. Seejmap(1).

• Use thejconsole option to obtain a heap dump throughHotSpotDiagnosticMXBean at runtime. Seejconsole(1) and theHotSpotDiagnosticMXBean interface description at
  http://docs.oracle.com/javase/8/docs/jre/api/management/extension/com/sun/management/HotSpotDiagnosticMXBean.html

• Heap dump is generated when anOutOfMemoryError is thrown by specifying the-XX:+HeapDumpOnOutOfMemoryError Java Virtual Machine (JVM) option.

• Use thehprof command. See the HPROF: A Heap/CPU Profiling Tool at
  http://docs.oracle.com/javase/8/docs/technotes/samples/hprof.html

Options

-stack false|true

Turns off tracking object allocation call stack. If allocation site information is not available in the heap dump, then you have to set this flag tofalse. The default istrue.

-refs false|true

Turns off tracking of references to objects. Default istrue. By default, back pointers, which are objects that point to a specified object such as referrers or incoming references, are calculated for all objects in the heap.

-portport-number

Sets the port for thejhat HTTP server. Default is 7000.

-excludeexclude-file

Specifies a file that lists data members that should be excluded from the reachable objects query. For example, if the file listsjava.lang.String.value, then, then whenever the list of objects that are reachable from a specific objecto are calculated, reference paths that involvejava.lang.String.value field are not considered.

-baselineexclude-file

Specifies a baseline heap dump. Objects in both heap dumps with the same object ID are marked as not being new. Other objects are marked as new. This is useful for comparing two different heap dumps.

-debugint

Sets the debug level for this tool. A level of 0 means no debug output. Set higher values for more verbose modes.

-version

Reports the release number and exits

-h

Displays a help message and exits.

-help

Displays a help message and exits.

-Jflag

Passesflag to the Java Virtual Machine on which thejhat command is running. For example,-J-Xmx512m to use a maximum heap size of 512 MB.

See Also

• jmap(1)

• jconsole(1)

• HPROF: A Heap/CPU Profiling Tool at
  http://docs.oracle.com/javase/8/docs/technotes/samples/hprof.html