The AccessController class is used for access control operations and decisions.
More specifically, the AccessController class is used for three purposes:
The checkPermission
method
determines whether the access request indicated by a specified
permission should be granted or denied. A sample call appears
below. In this example, checkPermission will determine
whether or not to grant "read" access to the file named "testFile" in
the "/temp" directory.
FilePermission perm = new FilePermission("/temp/testFile", "read");
AccessController.checkPermission(perm);
If a requested access is allowed,
checkPermission returns quietly. If denied, an
AccessControlException is
thrown. AccessControlException can also be thrown if the requested
permission is of an incorrect type or contains an invalid value.
Such information is given whenever possible.
Suppose the current thread traversed m callers, in the order of caller 1
to caller 2 to caller m. Then caller m invoked the
checkPermission method.
The checkPermission method determines whether access
is granted or denied based on the following algorithm:
i = m;
while (i > 0) {
if (caller i's domain does not have the permission)
throw AccessControlException
else if (caller i is marked as privileged) {
if (a context was specified in the call to doPrivileged)
context.checkPermission(permission)
return;
}
i = i - 1;
};
// Next, check the context inherited when
// the thread was created. Whenever a new thread is created, the
// AccessControlContext at that time is
// stored and associated with the new thread, as the "inherited"
// context.
inheritedContext.checkPermission(permission);
A caller can be marked as being "privileged"
(see doPrivileged
and below).
When making access control decisions, the checkPermission
method stops checking if it reaches a caller that
was marked as "privileged" via a doPrivileged
call without a context argument (see below for information about a
context argument). If that caller's domain has the
specified permission, no further checking is done and
checkPermission
returns quietly, indicating that the requested access is allowed.
If that domain does not have the specified permission, an exception
is thrown, as usual.
The normal use of the "privileged" feature is as follows. If you don't need to return a value from within the "privileged" block, do the following:
somemethod() {
...normal code here...
AccessController.doPrivileged(new PrivilegedAction() {
public Object run() {
// privileged code goes here, for example:
System.loadLibrary("awt");
return null; // nothing to return
}
});
...normal code here...
}
PrivilegedAction is an interface with a single method, named
run, that returns an Object.
The above example shows creation of an implementation
of that interface; a concrete implementation of the
run method is supplied.
When the call to doPrivileged is made, an
instance of the PrivilegedAction implementation is passed
to it. The doPrivileged method calls the
run method from the PrivilegedAction
implementation after enabling privileges, and returns the
run method's return value as the
doPrivileged return value (which is
ignored in this example).
If you need to return a value, you can do something like the following:
somemethod() {
...normal code here...
String user = (String) AccessController.doPrivileged(
new PrivilegedAction() {
public Object run() {
return System.getProperty("user.name");
}
}
);
...normal code here...
}
If the action performed in your run method could
throw a "checked" exception (those listed in the throws clause
of a method), then you need to use the
PrivilegedExceptionAction interface instead of the
PrivilegedAction interface:
somemethod() throws FileNotFoundException {
...normal code here...
try {
FileInputStream fis = (FileInputStream) AccessController.doPrivileged(
new PrivilegedExceptionAction() {
public Object run() throws FileNotFoundException {
return new FileInputStream("someFile");
}
}
);
} catch (PrivilegedActionException e) {
// e.getException() should be an instance of FileNotFoundException,
// as only "checked" exceptions will be "wrapped" in a
// PrivilegedActionException.
throw (FileNotFoundException) e.getException();
}
...normal code here...
}
Be *very* careful in your use of the "privileged" construct, and always remember to make the privileged code section as small as possible.
Note that checkPermission always performs security checks
within the context of the currently executing thread.
Sometimes a security check that should be made within a given context
will actually need to be done from within a
different context (for example, from within a worker thread).
The getContext
method and
AccessControlContext class are provided
for this situation. The getContext method takes a "snapshot"
of the current calling context, and places
it in an AccessControlContext object, which it returns. A sample call is
the following:
AccessControlContext acc = AccessController.getContext()
AccessControlContext itself has a checkPermission method
that makes access decisions based on the context it encapsulates,
rather than that of the current execution thread.
Code within a different context can thus call that method on the
previously-saved AccessControlContext object. A sample call is the
following:
acc.checkPermission(permission)
There are also times where you don't know a priori which permissions to check the context against. In these cases you can use the doPrivileged method that takes a context:
somemethod() {
AccessController.doPrivileged(new PrivilegedAction() {
public Object run() {
// Code goes here. Any permission checks within this
// run method will require that the intersection of the
// callers protection domain and the snapshot's
// context have the desired permission.
}
}, acc);
...normal code here...
}
PrivilegedAction with privileges
enabled. The action is performed with all of the permissions
possessed by the caller's protection domain.
If the action's run method throws an (unchecked) exception,
it will propagate through this method.
PrivilegedAction with privileges
enabled and restricted by the specified
AccessControlContext.
The action is performed with the intersection of the permissions
possessed by the caller's protection domain, and those possessed
by the domains represented by the specified
AccessControlContext.
If the action's run method throws an (unchecked) exception,
it will propagate through this method.
PrivilegedExceptionAction with
privileges enabled. The action is performed with all of the
permissions possessed by the caller's protection domain.
If the action's run method throws an unchecked
exception, it will propagate through this method.
PrivilegedExceptionAction with
privileges enabled and restricted by the specified
AccessControlContext. The action is performed with the
intersection of the the permissions possessed by the caller's
protection domain, and those possessed by the domains represented by the
specified AccessControlContext.
If the action's run method throws an unchecked
exception, it will propagate through this method.
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.)
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.
toString method returns a string that
"textually represents" this object. The result should
be a concise but informative representation that is easy for a
person to read.
It is recommended that all subclasses override this method.
The toString method for class Object
returns a string consisting of the name of the class of which the
object is an instance, the at-sign character `@', and
the unsigned hexadecimal representation of the hash code of the
object. In other words, this method returns a string equal to the
value of:
getClass().getName() + '@' + Integer.toHexString(hashCode())
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.