With a PermissionCollection, you can:
add method.
implies method.
elements method.
When it is desirable to group together a number of Permission objects
of the same type, the newPermissionCollection method on that
particular type of Permission object should first be called. The default
behavior (from the Permission class) is to simply return null.
Subclasses of class Permission override the method if they need to store
their permissions in a particular PermissionCollection object in order
to provide the correct semantics when the
PermissionCollection.implies method is called.
If a non-null value is returned, that PermissionCollection must be used.
If null is returned, then the caller of newPermissionCollection
is free to store permissions of the
given type in any PermissionCollection they choose
(one that uses a Hashtable, one that uses a Vector, etc).
The PermissionCollection returned by the
Permission.newPermissionCollection
method is a homogeneous collection, which stores only Permission objects
for a given Permission type. A PermissionCollection may also be
heterogeneous. For example, Permissions is a PermissionCollection
subclass that represents a collection of PermissionCollections.
That is, its members are each a homogeneous PermissionCollection.
For example, a Permissions object might have a FilePermissionCollection
for all the FilePermission objects, a SocketPermissionCollection for all the
SocketPermission objects, and so on. Its add method adds a
permission to the appropriate collection.
Whenever a permission is added to a heterogeneous PermissionCollection
such as Permissions, and the PermissionCollection doesn't yet contain a
PermissionCollection of the specified permission's type, the
PermissionCollection should call
the newPermissionCollection method on the permission's class
to see if it requires a special PermissionCollection. If
newPermissionCollection
returns null, the PermissionCollection
is free to store the permission in any type of PermissionCollection it
desires (one using a Hashtable, one using a Vector, etc.). For example,
the Permissions object uses a default PermissionCollection implementation
that stores the permission objects in a Hashtable.
Subclass implementations of PermissionCollection should assume
that they may be called simultaneously from multiple threads,
and therefore should be synchronized properly. Furthermore,
Enumerations returned via the elements method are
not fail-fast. Modifications to a collection should not be
performed while enumerating over that collection.
The equals method implements an equivalence relation
on non-null object references:
x, x.equals(x) should return
true.
x and y, x.equals(y)
should return true if and only if
y.equals(x) returns true.
x, y, and z, if
x.equals(y) returns true and
y.equals(z) returns true, then
x.equals(z) should return true.
x and y, multiple invocations of
x.equals(y) consistently return true
or consistently return false, provided no
information used in equals comparisons on the
objects is modified.
x,
x.equals(null) should return false.
The equals method for class Object implements
the most discriminating possible equivalence relation on objects;
that is, for any non-null reference values x and
y, this method returns true if and only
if x and y refer to the same object
(x == y has the value true).
Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.
java.util.Hashtable.
The general contract of hashCode is:
hashCode method on each of
the two objects must produce the same integer result.
As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)
add.
By default, the object is not readonly. It can be set to
readonly by a call to setReadOnly.
wait methods.
The awakened thread will not be able to proceed until the current thread relinquishes the lock on this object. The awakened thread will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened thread enjoys no reliable privilege or disadvantage in being the next thread to lock this object.
This method should only be called by a thread that is the owner of this object's monitor. A thread becomes the owner of the object's monitor in one of three ways:
synchronized statement
that synchronizes on the object.
Class, by executing a
synchronized static method of that class.
Only one thread at a time can own an object's monitor.
wait methods.
The awakened threads will not be able to proceed until the current thread relinquishes the lock on this object. The awakened threads will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened threads enjoy no reliable privilege or disadvantage in being the next thread to lock this object.
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.
add.super.toString() ( // enumerate all the Permission // objects and call toString() on them, // one per line.. )
super.toString is a call to the toString
method of this
object's superclass, which is Object. The result is
this PermissionCollection's type name followed by this object's
hashcode, thus enabling clients to differentiate different
PermissionCollections object, even if they contain the same permissions.
The current thread must own this object's monitor. The thread
releases ownership of this monitor and waits until another thread
notifies threads waiting on this object's monitor to wake up
either through a call to the notify method or the
notifyAll method. The thread then waits until it can
re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:
synchronized (obj) {
while (<condition does not hold>)
obj.wait();
... // Perform action appropriate to condition
}
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.The current thread must own this object's monitor.
This method causes the current thread (call it T) to place itself in the wait set for this object and then to relinquish any and all synchronization claims on this object. Thread T becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:
A thread can also wake up without being notified, interrupted, or timing out, a so-called spurious wakeup. While this will rarely occur in practice, applications must guard against it by testing for the condition that should have caused the thread to be awakened, and continuing to wait if the condition is not satisfied. In other words, waits should always occur in loops, like this one:
synchronized (obj) {
while (<condition does not hold>)
obj.wait(timeout);
... // Perform action appropriate to condition
}
(For more information on this topic, see Section 3.2.3 in Doug Lea's
"Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
2000), or Item 50 in Joshua Bloch's "Effective Java Programming
Language Guide" (Addison-Wesley, 2001).
If the current thread is interrupted by another thread while it is waiting, then an InterruptedException is thrown. This exception is not thrown until the lock status of this object has been restored as described above.
Note that the wait method, as it places the current thread into the wait set for this object, unlocks only this object; any other objects on which the current thread may be synchronized remain locked while the thread waits.
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.
This method is similar to the wait method of one
argument, but it allows finer control over the amount of time to
wait for a notification before giving up. The amount of real time,
measured in nanoseconds, is given by:
1000000*timeout+nanos
In all other respects, this method does the same thing as the method of one argument. In particular, wait(0, 0) means the same thing as wait(0).
The current thread must own this object's monitor. The thread releases ownership of this monitor and waits until either of the following two conditions has occurred:
notify method
or the notifyAll method.
timeout
milliseconds plus nanos nanoseconds arguments, has
elapsed.
The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:
synchronized (obj) {
while (<condition does not hold>)
obj.wait(timeout, nanos);
... // Perform action appropriate to condition
}
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.