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| <title>javax.management.relation package</title> |
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| <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<ObjectName,List<String>> dependentAMap = |
| relSvc.findAssociatedMBeans(moduleA, "DependsOn", "dependent"); |
| Set<ObjectName> dependentASet = dependentAMap.keySet(); |
| // Set of ObjectName containing moduleB |
| </pre> |
| |
| @see <a href="https://jcp.org/aboutJava/communityprocess/mrel/jsr160/index2.html"> |
| JMX Specification, version 1.4</a> |
| |
| @since 1.5 |
| |
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