| JavaTM 2 Platform Std. Ed. v1.4.2
java.security
Class AccessController
java.lang.Object
 java.security.AccessController
- public final class AccessController
- extends Object
The AccessController class is used for access control operations
and decisions.
More specifically, the AccessController class is used for
three purposes:
- to decide whether an access to a critical system
resource is to be allowed or denied, based on the security policy
currently in effect,
- to mark code as being "privileged", thus affecting subsequent
access determinations, and
- to obtain a "snapshot" of the current calling context so
access-control decisions from a different context can be made with
respect to the saved context.
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...
}
- See Also:
AccessControlContext
| Methods inherited from class java.lang.Object |
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
doPrivileged
public static Object doPrivileged(PrivilegedAction action)
- Performs the specified
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.
- Parameters:
action - the action to be performed.
- Returns:
- the value returned by the action's
run method. - See Also:
doPrivileged(PrivilegedAction,AccessControlContext),
doPrivileged(PrivilegedExceptionAction)
doPrivileged
public static Object doPrivileged(PrivilegedAction action,
AccessControlContext context)
- Performs the specified
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.
- Parameters:
action - the action to be performed.context - an access control context representing the
restriction to be applied to the caller's domain's
privileges before performing the specified action.
- Returns:
- the value returned by the action's
run method. - See Also:
doPrivileged(PrivilegedAction),
doPrivileged(PrivilegedExceptionAction,AccessControlContext)
doPrivileged
public static Object doPrivileged(PrivilegedExceptionAction action)
throws PrivilegedActionException
- Performs the specified
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.
- Parameters:
action - the action to be performed
- Returns:
- the value returned by the action's
run method
- Throws:
PrivilegedActionException - if the specified action's
run method threw a checked exception- See Also:
doPrivileged(PrivilegedAction),
doPrivileged(PrivilegedExceptionAction,AccessControlContext)
doPrivileged
public static Object doPrivileged(PrivilegedExceptionAction action,
AccessControlContext context)
throws PrivilegedActionException
- Performs the specified
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.
- Parameters:
action - the action to be performedcontext - an access control context representing the
restriction to be applied to the caller's domain's
privileges before performing the specified action
- Returns:
- the value returned by the action's
run method
- Throws:
PrivilegedActionException - if the specified action's
run method
threw a checked exception- See Also:
doPrivileged(PrivilegedAction),
doPrivileged(PrivilegedExceptionAction,AccessControlContext)
getContext
public static AccessControlContext getContext()
- This method takes a "snapshot" of the current calling context, which
includes the current Thread's inherited AccessControlContext,
and places it in an AccessControlContext object. This context may then
be checked at a later point, possibly in another thread.
- Returns:
- the AccessControlContext based on the current context.
- See Also:
AccessControlContext
checkPermission
public static void checkPermission(Permission perm)
throws AccessControlException
- Determines whether the access request indicated by the
specified permission should be allowed or denied, based on
the security policy currently in effect.
This method quietly returns if the access request
is permitted, or throws a suitable AccessControlException otherwise.
- Parameters:
perm - the requested permission.
- Throws:
AccessControlException - if the specified permission
is not permitted, based on the current security policy.
Copyright 2003 Sun Microsystems, Inc. All rights reserved
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