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.

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

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

For example, suppose there are `Module` MBeans, representing modules within an application. A `DependsOn` 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 `DependsOn` relation type would have two roles, `dependent` and `dependedOn`.

Every role is typed, meaning that an MBean that appears in that role must be an instance of the role's type. In the `DependsOn` example, both roles would be of type `Module`.

Every role has a cardinality, 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 `DependsOn` 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 `dependent` module with exactly one `dependedOn` module.

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.

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 `RelationSupport` 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.

The `DependsOn` example might be coded as follows.

    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

@see Java Platform documentation on JMX technology, in particular the JMX Specification, version 1.4 @since 1.5