A DomainCombiner provides a means to dynamically
update the ProtectionDomains associated with the current
AccessControlContext.
A DomainCombiner is passed as a parameter to the
appropriate constructor for AccessControlContext.
The newly constructed context is then passed to the
AccessController.doPrivileged(..., context) method
to bind the provided context (and associated DomainCombiner)
with the current execution Thread. Subsequent calls to
AccessController.getContext or
AccessController.checkPermission
cause the DomainCombiner.combine to get invoked.
The combine method takes two arguments. The first argument represents
an array of ProtectionDomains from the current execution Thread,
since the most recent call to AccessController.doPrivileged.
If no call to doPrivileged was made, then the first argument will contain
all the ProtectionDomains from the current execution Thread.
The second argument represents an array of inherited ProtectionDomains,
which may be null. ProtectionDomains may be inherited
from a parent Thread, or from a privileged context. If no call to
doPrivileged was made, then the second argument will contain the
ProtectionDomains inherited from the parent Thread. If one or more calls
to doPrivileged were made, and the most recent call was to
doPrivileged(action, context), then the second argument will contain the
ProtectionDomains from the privileged context. If the most recent call
was to doPrivileged(action), then there is no privileged context,
and the second argument will be null.
The combine method investigates the two input arrays
of ProtectionDomains and returns a single array containing the updated
ProtectionDomains. In the simplest case, the combine
method merges the two stacks into one. In more complex cases,
the combine method returns a modified
stack of ProtectionDomains. The modification may have added new
ProtectionDomains, removed certain ProtectionDomains, or simply
updated existing ProtectionDomains. Re-ordering and other optimizations
to the ProtectionDomains are also permitted. Typically the
combine method bases its updates on the information
encapsulated in the DomainCombiner.
After the AccessController.getContext method
receives the combined stack of ProtectionDomains back from
the DomainCombiner, it returns a new
AccessControlContext that has both the combined ProtectionDomains
as well as the DomainCombiner. |
