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 	<h1 class="title topictitle1">Immediate Dominators</h1>


 	<div class="body refbody"><p class="shortdesc">Find out who is keeping alive a set of objects.
 	</p>

 		<div class="section"><h2 class="title sectiontitle">Motivation</h2>

 			<img class="image floatright" src="immediate_dominators_graph.png" alt="Object graph of an hash table structure."/>
 			<p class="p">
 				To find out why a single object is still in the heap is easy: follow
 				the
 				<a class="xref" href="path_to_gc_roots.html" title="Find out who is keeping alive a set of objects.">shortest path</a>
 				to any GC root. But what if you have thousands of objects? Expanding
 				every single path is too time consuming. Immediate dominators is a
 				very effective way to find out who is keeping a set of objects
 				alive.
 			</p>

 			<p class="p">
 				Let's consider the object graph on the left: The blue objects
 				<img class="image" src="../../mimes/icons/obj_blue.png" alt="blue"/>
 				represent
 				<samp class="ph codeph">java.util.HashMap</samp>
 				: the map itself, the backing array with the buckets and finally the
 				map entries referring to keys and values. The yellow objects
 				<img class="image" src="../../mimes/icons/obj_yellow.png" alt="yellow"/>
 				are the values stored in the map, for example strings. The red
 				object
 				<img class="image" src="../../mimes/icons/obj_red.png" alt="red"/>
 				is holding a reference to the map and thereby preventing its garbage
 				collection.
 			</p>

 			<p class="p">
 				In this case, the
 				<a class="xref" href="../../concepts/dominatortree.html">dominator tree</a>
 				is identical to the object graph. Keep in mind that the tree
 				structure can differ from the object graph!
 			</p>

 			<p class="p">
 				The
 				<strong class="ph b">immediate dominators</strong>
 				of the yellow objects are the hash map entries. If all references to
 				the entry objects were gone, all yellow strings were gone too.
 			</p>

 			<p class="p">
 				The
 				<strong class="ph b">skip pattern</strong>
 				tells the query to skip those immediate dominators which match the
 				pattern. In the exemplary graph, it skips all blue hash map objects
 				and spits out the red object. The resulting table says: this one red
 				object keeps alive three yellow strings.
 			</p>

 		</div>

 		<div class="section"><h2 class="title sectiontitle">Arguments</h2>

 			<table cellpadding="4" cellspacing="0" summary="" border="1" class="simpletable"><tr class="sthead">
 					<th valign="bottom" align="left" id="d4207e100" class="stentry">Argument</th>

 					<th valign="bottom" align="left" id="d4207e103" class="stentry">Description</th>

 				</tr>
 <tr class="strow">
 					<td valign="top" headers="d4207e100" class="stentry">objects</td>

 					<td valign="top" headers="d4207e103" class="stentry">An arbitrary set of objects to be analyzed.</td>

 				</tr>
 <tr class="strow">
 					<td valign="top" headers="d4207e100" class="stentry">-skip</td>

 					<td valign="top" headers="d4207e103" class="stentry">
 						<p class="p"> A regular expression specifying which objects to skip while
 							going up the dominator tree. If the dominator of an object
 							matches the pattern, then the dominator of that dominator will be
 							taken, and so on, until an object not matching the skip pattern
 							is reached.</p>

 						<p class="p"> If the object is not dominated by any other object, it is
 							placed in a category ROOT.</p>

 					</td>

 				</tr>
 </table>

 		</div>

 		<div class="section" id="ref_inspections_immediate_dominators__result"><h2 class="title sectiontitle">Result</h2>

 			<p class="p">The sample below shows the immediate dominators of all strings
 				in this particular heap dump.</p>

 			<p class="p">
 				<img class="image" src="immediate_dominators_table.png" alt="Table displaying the immediate dominators."/>
 			</p>

 			<p class="p">
 				Read the selected row as follows:
 				<strong class="ph b">7.669</strong>
 				instances of
 				<strong class="ph b">ConfigurationElement</strong>
 				are responsible for
 				<strong class="ph b">16.130</strong>
 				strings. The configuration elements alone take up
 				<strong class="ph b">368.112</strong>
 				bytes (shallow size) while the strings use
 				<strong class="ph b">387.120</strong>
 				bytes.
 			</p>

 			<p class="p">
 				The
 				<strong class="ph b">retained size</strong>
 				of each object set is not calculated right away for performance
 				reasons. Usually, the number of objects and shallow sizes should
 				provide an indication what to analyze further. Of course, one can
 				calculate the retained sizes using context menu.
 			</p>

 			<p class="p">
 				The
 				<strong class="ph b">ROOT</strong>
 				element contains all those objects, which are
 				<strong class="ph b">not</strong>
 				dominated by another object. Those are typically instances, which
 				are kept alive through multiple paths which end in different
 				<a class="xref" href="../../concepts/gcroots.html">GC Roots</a>
 				. Semantically, the ROOT element is the virtual root node of the
 				<a class="xref" href="../../concepts/dominatortree.html">dominator tree</a>
 				.
 			</p>

 			<p class="p">
 				<img class="image" src="immediate_dominators_context.png" alt="Context menu available in the immediate dominator table."/>
 			</p>

 			<p class="p">
 				As shown above, the
 				<strong class="ph b">context menu</strong>
 				gives access to both sets of objects: The dominators (e.g. the
 				configuration elements) and the dominated objects (e.g. the
 				strings).
 			</p>

 			<p class="p">In this example, the retained set of the 7.669 configuration
 				elements would - among other objects - contain the 16.130 strings.
 			</p>

 		</div>

 	</div>

 	<div class="related-links"><div class="relinfo relconcepts"><strong>Related concepts</strong><br/>
 <div><a class="link" href="../../concepts/dominatortree.html">Dominator Tree</a></div>
 <div><a class="link" href="../../concepts/gcroots.html">Garbage Collection Roots</a></div>
 </div>
 </div>

 </body>
 </html>
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	<!DOCTYPE html
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	<html xml:lang="en-us" lang="en-us">
	<head>
	<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
	<meta name="DC.Type" content="reference"/>
	<meta name="DC.Title" content="Immediate Dominators"/>
	<meta name="abstract" content="Find out who is keeping alive a set of objects."/>
	<meta name="description" content="Find out who is keeping alive a set of objects."/>
	<meta name="DC.Relation" scheme="URI" content="../../concepts/dominatortree.html"/>
	<meta name="DC.Relation" scheme="URI" content="../../concepts/gcroots.html"/>
	<meta name="copyright" content="Copyright (c) 2008, 2010 SAP AG and others. All rights reserved. This program and the accompanying materials are made available under the terms of the Eclipse Public License v1.0 which accompanies this distribution, and is available at http://www.eclipse.org/legal/epl-v10.html " type="primary"/>
	<meta name="DC.Rights.Owner" content="Copyright (c) 2008, 2010 SAP AG and others. All rights reserved. This program and the accompanying materials are made available under the terms of the Eclipse Public License v1.0 which accompanies this distribution, and is available at http://www.eclipse.org/legal/epl-v10.html " type="primary"/>
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	<link rel="stylesheet" type="text/css" href="../../styles/commonltr.css"/>
	<title>Immediate Dominators</title>
	</head>
	<body id="ref_inspections_immediate_dominators">


	<h1 class="title topictitle1">Immediate Dominators</h1>




	<div class="body refbody"><p class="shortdesc">Find out who is keeping alive a set of objects.
	</p>

	<div class="section"><h2 class="title sectiontitle">Motivation</h2>

	<img class="image floatright" src="immediate_dominators_graph.png" alt="Object graph of an hash table structure."/>
	<p class="p">
	To find out why a single object is still in the heap is easy: follow
	the
	<a class="xref" href="path_to_gc_roots.html" title="Find out who is keeping alive a set of objects.">shortest path</a>
	to any GC root. But what if you have thousands of objects? Expanding
	every single path is too time consuming. Immediate dominators is a
	very effective way to find out who is keeping a set of objects
	alive.
	</p>

	<p class="p">
	Let's consider the object graph on the left: The blue objects
	<img class="image" src="../../mimes/icons/obj_blue.png" alt="blue"/>
	represent
	<samp class="ph codeph">java.util.HashMap</samp>
	: the map itself, the backing array with the buckets and finally the
	map entries referring to keys and values. The yellow objects
	<img class="image" src="../../mimes/icons/obj_yellow.png" alt="yellow"/>
	are the values stored in the map, for example strings. The red
	object
	<img class="image" src="../../mimes/icons/obj_red.png" alt="red"/>
	is holding a reference to the map and thereby preventing its garbage
	collection.
	</p>

	<p class="p">
	In this case, the
	<a class="xref" href="../../concepts/dominatortree.html">dominator tree</a>
	is identical to the object graph. Keep in mind that the tree
	structure can differ from the object graph!
	</p>

	<p class="p">
	The
	<strong class="ph b">immediate dominators</strong>
	of the yellow objects are the hash map entries. If all references to
	the entry objects were gone, all yellow strings were gone too.
	</p>

	<p class="p">
	The
	<strong class="ph b">skip pattern</strong>
	tells the query to skip those immediate dominators which match the
	pattern. In the exemplary graph, it skips all blue hash map objects
	and spits out the red object. The resulting table says: this one red
	object keeps alive three yellow strings.
	</p>

	</div>

	<div class="section"><h2 class="title sectiontitle">Arguments</h2>

	<table cellpadding="4" cellspacing="0" summary="" border="1" class="simpletable"><tr class="sthead">
	<th valign="bottom" align="left" id="d4207e100" class="stentry">Argument</th>

	<th valign="bottom" align="left" id="d4207e103" class="stentry">Description</th>

	</tr>
	<tr class="strow">
	<td valign="top" headers="d4207e100" class="stentry">objects</td>

	<td valign="top" headers="d4207e103" class="stentry">An arbitrary set of objects to be analyzed.</td>

	</tr>
	<tr class="strow">
	<td valign="top" headers="d4207e100" class="stentry">-skip</td>

	<td valign="top" headers="d4207e103" class="stentry">
	<p class="p"> A regular expression specifying which objects to skip while
	going up the dominator tree. If the dominator of an object
	matches the pattern, then the dominator of that dominator will be
	taken, and so on, until an object not matching the skip pattern
	is reached.</p>

	<p class="p"> If the object is not dominated by any other object, it is
	placed in a category ROOT.</p>

	</td>

	</tr>
	</table>

	</div>

	<div class="section" id="ref_inspections_immediate_dominators__result"><h2 class="title sectiontitle">Result</h2>

	<p class="p">The sample below shows the immediate dominators of all strings
	in this particular heap dump.</p>

	<p class="p">
	<img class="image" src="immediate_dominators_table.png" alt="Table displaying the immediate dominators."/>
	</p>

	<p class="p">
	Read the selected row as follows:
	<strong class="ph b">7.669</strong>
	instances of
	<strong class="ph b">ConfigurationElement</strong>
	are responsible for
	<strong class="ph b">16.130</strong>
	strings. The configuration elements alone take up
	<strong class="ph b">368.112</strong>
	bytes (shallow size) while the strings use
	<strong class="ph b">387.120</strong>
	bytes.
	</p>

	<p class="p">
	The
	<strong class="ph b">retained size</strong>
	of each object set is not calculated right away for performance
	reasons. Usually, the number of objects and shallow sizes should
	provide an indication what to analyze further. Of course, one can
	calculate the retained sizes using context menu.
	</p>

	<p class="p">
	The
	<strong class="ph b">ROOT</strong>
	element contains all those objects, which are
	<strong class="ph b">not</strong>
	dominated by another object. Those are typically instances, which
	are kept alive through multiple paths which end in different
	<a class="xref" href="../../concepts/gcroots.html">GC Roots</a>
	. Semantically, the ROOT element is the virtual root node of the
	<a class="xref" href="../../concepts/dominatortree.html">dominator tree</a>
	.
	</p>

	<p class="p">
	<img class="image" src="immediate_dominators_context.png" alt="Context menu available in the immediate dominator table."/>
	</p>

	<p class="p">
	As shown above, the
	<strong class="ph b">context menu</strong>
	gives access to both sets of objects: The dominators (e.g. the
	configuration elements) and the dominated objects (e.g. the
	strings).
	</p>

	<p class="p">In this example, the retained set of the 7.669 configuration
	elements would - among other objects - contain the 16.130 strings.
	</p>

	</div>

	</div>

	<div class="related-links"><div class="relinfo relconcepts"><strong>Related concepts</strong><br/>
	<div><a class="link" href="../../concepts/dominatortree.html">Dominator Tree</a></div>
	<div><a class="link" href="../../concepts/gcroots.html">Garbage Collection Roots</a></div>
	</div>
	</div>

	</body>
	</html>