jdk/src/share/classes/javax/management/relation/package.html

<html>
<head>
<title>javax.management.relation package</title>
<!--
Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.
DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

This code is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License version 2 only, as
published by the Free Software Foundation.  Oracle designates this
particular file as subject to the "Classpath" exception as provided
by Oracle in the LICENSE file that accompanied this code.

This code is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
version 2 for more details (a copy is included in the LICENSE file that
accompanied this code).

You should have received a copy of the GNU General Public License version
2 along with this work; if not, write to the Free Software Foundation,
Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
or visit www.oracle.com if you need additional information or have any
questions.
-->
</head>
<body bgcolor="white">
      <p>Provides the definition of the Relation Service.  The
	Relation Service is used to record relationships between
	MBeans in an MBean Server.  The Relation Service is itself an
	MBean.  More than one instance of a {@link
	javax.management.relation.RelationService RelationService}
	MBean can be registered in an MBean Server.</p>

      <p>A <em>relation type</em> defines a relationship between MBeans.
	It contains <em>roles</em> that the MBeans play in the
	relationship.  Usually there are at least two roles in a
	relation type.</p>

      <p>A <em>relation</em> is a named instance of a relation type,
	where specific MBeans appear in the roles, represented by
	their {@link javax.management.ObjectName ObjectName}s.</p>

      <p>For example, suppose there are <code>Module</code> MBeans,
	representing modules within an application.  A
	<code>DependsOn</code> relation type could express the
	relationship that some modules depend on others, which could
	be used to determine the order in which the modules are
	started or stopped.  The <code>DependsOn</code> relation type
	would have two roles, <code>dependent</code> and
	<code>dependedOn</code>.</p>

      <p>Every role is <em>typed</em>, meaning that an MBean that
	appears in that role must be an instance of the role's type.
	In the <code>DependsOn</code> example, both roles would be of
	type <code>Module</code>.</p>

      <p>Every role has a <em>cardinality</em>, which provides lower
	and upper bounds on the number of MBeans that can appear in
	that role in a given relation instance.  Usually, the lower
	and upper bounds are both 1, with exactly one MBean appearing
	in the role.  The cardinality only limits the number of MBeans
	in the role per relation instance.  The same MBean can appear
	in the same role in any number of instances of a relation
	type.  In the <code>DependsOn</code> example, a given module
	can depend on many other modules, and be depended on by many
	others, but any given relation instance links exactly one
	<code>dependent</code> module with exactly one
	<code>dependedOn</code> module.</p>

      <p>A relation type can be created explicitly, as an object
	implementing the {@link javax.management.relation.RelationType
	RelationType} interface, typically a {@link
	javax.management.relation.RelationTypeSupport
	RelationTypeSupport}.  Alternatively, it can be created
	implicitly using the Relation Service's {@link
	javax.management.relation.RelationServiceMBean#createRelationType(String,
	RoleInfo[]) createRelationType} method.</p>

      <p>A relation instance can be created explicitly, as an object
	implementing the {@link javax.management.relation.Relation
	Relation} interface, typically a {@link
	javax.management.relation.RelationSupport RelationSupport}.
	(A <code>RelationSupport</code> is itself a valid MBean, so it
	can be registered in the MBean Server, though this is not
	required.)  Alternatively, a relation instance can be created
	implicitly using the Relation Service's {@link
	javax.management.relation.RelationServiceMBean#createRelation(String,
	String, RoleList) createRelation} method.</p>

      <p>The <code>DependsOn</code> example might be coded as follows.</p>

<pre>
import java.util.*;
import javax.management.*;
import javax.management.relation.*;

// ...
MBeanServer mbs = ...;

// Create the Relation Service MBean
ObjectName relSvcName = new ObjectName(":type=RelationService");
RelationService relSvcObject = new RelationService(true);
mbs.registerMBean(relSvcObject, relSvcName);

// Create an MBean proxy for easier access to the Relation Service
RelationServiceMBean relSvc =
    MBeanServerInvocationHandler.newProxyInstance(mbs, relSvcName,
						  RelationServiceMBean.class,
						  false);

// Define the DependsOn relation type
RoleInfo[] dependsOnRoles = {
    new RoleInfo("dependent", Module.class.getName()),
    new RoleInfo("dependedOn", Module.class.getName())
};
relSvc.createRelationType("DependsOn", dependsOnRoles);

// Now define a relation instance "moduleA DependsOn moduleB"

ObjectName moduleA = new ObjectName(":type=Module,name=A");
ObjectName moduleB = new ObjectName(":type=Module,name=B");

Role dependent = new Role("dependent", Collections.singletonList(moduleA));
Role dependedOn = new Role("dependedOn", Collections.singletonList(moduleB));
Role[] roleArray = {dependent, dependedOn};
RoleList roles = new RoleList(Arrays.asList(roleArray));
relSvc.createRelation("A-DependsOn-B", "DependsOn", roles);

// Query the Relation Service to find what modules moduleA depends on
Map&lt;ObjectName,List&lt;String&gt;&gt; dependentAMap =
    relSvc.findAssociatedMBeans(moduleA, "DependsOn", "dependent");
Set&lt;ObjectName&gt; dependentASet = dependentAMap.keySet();
// Set of ObjectName containing moduleB
</pre>

    @see <a href="{@docRoot}/../technotes/guides/jmx/">
      Java Platform documentation on JMX technology</a>,
    in particular the 
    <a href="{@docRoot}/../technotes/guides/jmx/JMX_1_4_specification.pdf">
      JMX Specification, version 1.4</a>

      @since 1.5

</BODY>
</HTML>