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

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    Contributors:
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd" >
<concept id="concept_reachability" xml:lang="en-us">
	<title>Reachability</title>
	<prolog>
		<copyright>
			<copyryear year=""></copyryear>
			<copyrholder>
				Copyright (c) 2018 IBM Corporation.
			    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> Objects in a heap dump have references to other objects. These can either be
		via field references (for simple objects), array elements (for object arrays)
		or via various hidden references. For instance every object contains a reference to
		its type, and each class contain a reference to the class loader which loaded
		the class.</p>
		<p>These objects and references form a directed graph. The objects are the nodes,
		the references are the directed links between the nodes. The
		garbage collection roots are the roots of this graph.
		</p>
		<dl>
			<dlentry>
			<dt>Reachable</dt><dd>An object is reachable from another object if there is a
			path following the directed links from the source object to the destination object.
			</dd>
			</dlentry>
			<dlentry>
			<dt>Unreachable object</dt><dd>If there is no path from a garbage collection root
			to an object then it is unreachable. There is then no way a legitimate Java program
			can every get access to this object, so it is safe for the Java virtual machine to
			discard this object from the heap. This process of determining unreachable objects and
			discarding them, thus making room for more objects to be allocated as required, is
			called garbage collection.
			</dd>
			</dlentry>
		</dl>
	</conbody>
	<related-links>
		<link href="gcroots.dita" type="concept">
			<linktext>Garbage collection roots</linktext>
		</link>
	</related-links>
</concept>