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 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 ObjectNames.

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 RelationType interface, typically a RelationTypeSupport. Alternatively, it can be created implicitly using the Relation Service's createRelationType method.

A relation instance can be created explicitly, as an object implementing the Relation interface, typically a 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 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