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jhat

Section: Troubleshooting Tools (1)
Updated: 21 November 2013
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NAME

jhat - Analyzes the Java heap. This command is experimental and unsupported.

SYNOPSIS


jhat [ options ] heap-dump-file

options: The command-line options. See Options.
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

The jhat command parses a Java heap dump file and starts a web server. The jhat command lets you to browse heap dumps with your favorite web browser. The jhat command supports predesigned queries such as show all instances of a known class MyClass, 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 the jhat 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 the jmap -dump option to obtain a heap dump at runtime. See jmap(1).
*: Use the jconsole option to obtain a heap dump through HotSpotDiagnosticMXBean at runtime. See jconsole(1) and the HotSpotDiagnosticMXBean interface description at http://docs.oracle.com/javase/8/docs/jre/api/management/extension/com/sun/management/HotSpotDiagnosticMXBean.html
*: Heap dump is generated when an OutOfMemoryError is thrown by specifying the -XX:+HeapDumpOnOutOfMemoryError Java Virtual Machine (JVM) option.
*: Use the hprof 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 to false. The default is true.
-refs false|true: Turns off tracking of references to objects. Default is true. 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.
-port port-number: Sets the port for the jhat HTTP server. Default is 7000.
-exclude exclude-file: Specifies a file that lists data members that should be excluded from the reachable objects query. For example, if the file lists java.lang.String.value, then, then whenever the list of objects that are reachable from a specific object o are calculated, reference paths that involve java.lang.String.value field are not considered.
-baseline exclude-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.
-debug int: 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: Dsiplays a help message and exits.
-help: Displays a help message and exits.
-Jflag: Passes flag to the Java Virtual Machine on which the jhat command is running. For example, -J-Xmx512m to use a maximum heap size of 512 MB.