§6 Object Teams API

(a) Interface to the role registry

Each team instance internally has a registry of known role objects indexed by their base object. Programmers may make use of this registry using the following reflective methods defined in org.objectteams.Team:

boolean hasRole ( Object aBase ) ;

This method checks whether a role for the passed base object already exists in the target team.

boolean hasRole ( Object aBase, Class expectedRole ) ;

This method checks whether a instance of type expectedRole as a role for the passed base object aBase already exists in the target team. The role may also be of any subtype of the specified role type.

Object getRole ( Object aBase ) ;

If the passed base object aBase already has a role in the target team, this role is returned. Otherwise null is returned.

<T> T getRole ( Object aBase, Class<T> expectedRole ) ;

If the passed base object aBase already has a role in the target team that is assignable to the type represented by expectedRole, this role is returned. Otherwise null is returned.

Object[] getAllRoles () ;

Retrieves all existing (registered) roles in the target team.
This method uses internal structures of weak references. For that reason it may return role instances which were about to be reclaimed by the garbage collector. If performance permits, it is thus advisable to always call System.gc() prior to calling getAllRoles() in order to achieve deterministic results (see also §2.1.(f)).

<T> T[] getAllRoles ( Class<T> expectedRole ) ;

Retrieves all existing (registered) roles in the target team that are assignable to the type represented by expectedRole. Class expectedRole must be a bound role otherwise an IllegalArgumentException is thrown.
See the note about garbage collection above.

void unregisterRole ( Object aRole ) ;

This method unregisters the passed role object from the target team. Thus the corresponding base looses this role. After calling this method the role should no longer be used.

void unregisterRole ( Object aRole, Class roleClass ) ;

This method unregisters the passed role object from the target team. Thus the corresponding base looses this role. After calling this method the role should no longer be used. The only difference to the previous method is improved speed because no search for the corresponding registry has to be performed.

It is desirable and possible to use these methods within guards (see §5.4). These methods allow to write the specification of guards in a more concise and more expressive way. Determined by the signature, the first four methods can only be used in a base-level guard (§5.4.2) because they require a reference to a base object.

Example code (Guards and Reflection):

public team class SpecialConditions {

  public void participate(Account as BonusAccount ba) {}

  public class BonusAccount playedBy Account

    base when(SpecialConditions.this.hasRole(base, BonusAccount.class))

  {

    callin void creditBonus(int amount) {

      base.creditBonus(amount + bonus);

    }

    void creditBonus(int amount) <- replace void credit(int i)

      base when (i > 1000);

  }

}

(b) Behavioral reflection

The following reflective methods defined in org.objectteams.Team can be used to inspect the dynamic behavior of a team:

boolean isExecutingCallin () ;

This method is used to inspect whether a control flow has already been intercepted by at least one callin binding of the current team. It can be used to avoid undesirable re-entrance to a team.

boolean isActive () ;

This method checks whether the team instance is active for the current thread.

boolean isActive ( Thread aThread ) ;

This method checks whether the team instance is active for the thread aThread.

(c) Class literals for roles

The Java syntax for so-called class literals, MyClass.class (see JLS §15.8.2) can be used for role types with slightly changed semantics: Role types are virtual types (§1.3.1) that are bound dynamically (§1.3.1.(e)). This applies to role class literals, too. From this follows the constraint that a role class literal can only be used within the non-static context of a team, ie., for evaluating a role class literal an enclosing team instance must be in scope.
Unlike regular type checking for role types, the class literal itself does not have a dependent type. Thus type checking of calls to methods like hasRole(Object, Class) cannot detect, whether the Class instance has actually been obtained from the correct team instance. Any attempt to pass a class that is not known as a bound role within the given team results in an IllegalArgumentException at run-time.

(a) Interfaces for role encapsulation

A set of pre-defined types exist that do not extend java.lang.Object and have no features except the operators == and !=.

These predefined types are

org.objectteams.IConfined

regular interface

org.objectteams.Team.IConfined

role interface

org.objectteams.Team.Confined

role class

These types provide no new functionality but inheriting from these types influences the semantics with respect to encapsulation. The purpose and usage of these types is described in §7.

(b) Interface for explicit lowering

The following role interface exists for the purpose of allowing explicit lowering:

org.objectteams.Team.ILowerable

role interface

This interface was introduced in detail in §2.2.(d).

(c) Team activation methods

Every team can be activated and deactivated by predefined methods of the class org.objectteams.Team.

activate() and activate(Thread th)

Methods for activation of a team

deactivate() and deactivate(Thread th)

Methods for deactivation of a team

The usage of these Methods is described in §5.2.(b).

(d) Exceptions

The following Exceptions can be thrown during the execution of an ObjectTeam/Java program:

ResultNotProvidedException

Thrown if a replace callin without a base call does not provide the necessary (primitive type) base result (see §4.3.(e)).

LiftingFailedException

Thrown if an actual ambiguity occurs during lifting (see §2.3.4.(c)).

WrongRoleException

Thrown during lifting if the base object has, with respect to the same team instance, previously been lifted to a role type that is not conform to the currently requested type (see §2.3.4.(d) and §2.4.3).

DuplicateRoleException

Thrown during explicit role creation, if a new role is created for a base object, which already has a role of the required type in the given team (see §2.4.1.(c)).

RoleCastException

Thrown during cast of an externalized role, if the casted expression is anchored to a different team instance than the cast type (see §1.2.4.(b)).