plugins/org.eclipse.mat.ui.help/concepts/heapdump.dita

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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA 1.1 Concept//EN" "concept.dtd" >
<concept id="concept_heapdump" xml:lang="en-us">
	<title>Heap Dump</title>
	<prolog>
		<copyright>
			<copyryear year=""></copyryear>
			<copyrholder>
				Copyright (c) 2008, 2023 SAP AG, IBM Corporation and others.
			    All rights reserved. This program and the accompanying materials
			    are made available under the terms of the Eclipse Public License 2.0
			    which accompanies this distribution, and is available at
			    https://www.eclipse.org/legal/epl-2.0/
			</copyrholder>
		</copyright>
	</prolog>
	
	<conbody>
		<p>A heap dump is a snapshot of the memory of a Java process at a certain point of time. 
		   There are different formats for persisting this data, and depending on the format, may
		   contain different pieces of information; but, in general, the snapshot contains information
		   about the Java objects and classes in the heap at the moment the snapshot was triggered.
		   In some cases, a full GC is triggered before the heap dump is written so it contains
		   information about the remaining objects.</p>
		<p>The Memory Analyzer is able to work with HPROF binary heap dumps, 
		   IBM system dumps (old versions require preprocessing), and IBM portable heap dumps (PHD) 
		   from a variety of platforms.
		</p>		   

		<p>
			Typical information which can be found in heap dumps (depending on the heap dump type):
		</p>
		
		<ul>
			<li>
				All Objects
				<p>Class, fields, primitive values and references</p>
			</li>
			<li>
				All Classes
				<p>Classloader, name, super class, static fields</p>
				<p>When MAT parses IBM system dumps, the size of classes includes some of the amount
				of native memory in the Java process (but outside of the Java heap) which is related
				to those classes such as native memory for bytecode and JIT compiled code for the class
				methods. In some cases, this may cause the total size reported on the Overview pane to
				exceed the maximum Java heap size. This calculation may be disabled using an option
				in the MAT configuration.</p>
			</li>
			<li>
				Garbage Collection Roots
				<p>Objects defined to be reachable by the JVM</p>
			</li>
			<li>
				Thread Stacks and Local Variables
				<p>The call-stacks of threads at the moment of the snapshot, and per-frame information about local objects</p>
			</li>			
		</ul>
		<p>A heap dump does not contain allocation information so it cannot
			resolve questions like who had created the objects and where they
			have been created.</p>
	</conbody>
	<related-links>
		<link href="../tasks/acquiringheapdump.dita" type="task">
			<linktext>Acquiring Heap Dumps</linktext>
		</link>
	</related-links>
</concept>