The policy for a Java runtime (specifying which permissions are available for code from various principals) is represented by a Policy object. Whenever a Policy is initialized or refreshed, Permission objects of appropriate classes are created for all permissions allowed by the Policy.
Many permission class types referenced by the policy configuration are ones that exist locally (i.e., ones that can be found on CLASSPATH). Objects for such permissions can be instantiated during Policy initialization. For example, it is always possible to instantiate a java.io.FilePermission, since the FilePermission class is found on the CLASSPATH.
Other permission classes may not yet exist during Policy initialization. For example, a referenced permission class may be in a JAR file that will later be loaded. For each such class, an UnresolvedPermission is instantiated. Thus, an UnresolvedPermission is essentially a "placeholder" containing information about the permission.
Later, when code calls AccessController.checkPermission on a permission of a type that was previously unresolved, but whose class has since been loaded, previously-unresolved permissions of that type are "resolved". That is, for each such UnresolvedPermission, a new object of the appropriate class type is instantiated, based on the information in the UnresolvedPermission.
To instantiate the new class, UnresolvedPermission assumes the class provides a zero, one, and/or two-argument constructor. The zero-argument constructor would be used to instantiate a permission without a name and without actions. A one-arg constructor is assumed to take a String
name as input, and a two-arg constructor is assumed to take a String
name and String
actions as input. UnresolvedPermission may invoke a constructor with a null
name and/or actions. If an appropriate permission constructor is not available, the UnresolvedPermission is ignored and the relevant permission will not be granted to executing code.
The newly created permission object replaces the UnresolvedPermission, which is removed.
Note that the getName
method for an UnresolvedPermission
returns the type
(class name) for the underlying permission that has not been resolved.
@see java.security.Permission
@see java.security.Permissions
@see java.security.PermissionCollection
@see java.security.Policy
@version 1.31 05/11/17
@author Roland Schemers
|
|
|
|
|
|
|
|
|
|
|
|
|
|