jhat - Online Linux Manual PageSection : 1
Updated : 21 November 2013
Source : JDK 8
Note : Troubleshooting Tools

NAMEjhat − Analyzes the Java heap​. This command is experimental and unsupported​.

SYNOPSISjhat [ 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​.

DESCRIPTIONThe 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
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