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jhat(1)                                 Troubleshooting Tools                                jhat(1)



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.

SEE ALSO
       • jmap(1)

       • jconsole(1)

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



JDK 8                                     21 November 2013                                   jhat(1)