/*
* @(#)AccessController.java 1.55 04/05/05
*
* Copyright 2004 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.security;
import sun.security.util.Debug;
/**
* <p> The AccessController class is used for access control operations
* and decisions.
*
* <p> More specifically, the AccessController class is used for
* three purposes:
*
* <ul>
* <li> to decide whether an access to a critical system
* resource is to be allowed or denied, based on the security policy
* currently in effect,<p>
* <li>to mark code as being "privileged", thus affecting subsequent
* access determinations, and<p>
* <li>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. </ul>
*
* <p> The {@link #checkPermission(Permission) 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, <code>checkPermission</code> will determine
* whether or not to grant "read" access to the file named "testFile" in
* the "/temp" directory.
*
* <pre>
*
* FilePermission perm = new FilePermission("/temp/testFile", "read");
* AccessController.checkPermission(perm);
*
* </pre>
*
* <p> If a requested access is allowed,
* <code>checkPermission</code> 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
* <code>checkPermission</code> method.
* The <code>checkPermission </code>method determines whether access
* is granted or denied based on the following algorithm:
*
* <pre>
* 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);
* </pre>
*
* <p> A caller can be marked as being "privileged"
* (see {@link #doPrivileged(PrivilegedAction) doPrivileged} and below).
* When making access control decisions, the <code>checkPermission</code>
* method stops checking if it reaches a caller that
* was marked as "privileged" via a <code>doPrivileged</code>
* 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
* <code>checkPermission</code>
* returns quietly, indicating that the requested access is allowed.
* If that domain does not have the specified permission, an exception
* is thrown, as usual.
*
* <p> 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:
*
* <pre>
* 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...
* }
* </pre>
*
* <p>
* PrivilegedAction is an interface with a single method, named
* <code>run</code>, that returns an Object.
* The above example shows creation of an implementation
* of that interface; a concrete implementation of the
* <code>run</code> method is supplied.
* When the call to <code>doPrivileged</code> is made, an
* instance of the PrivilegedAction implementation is passed
* to it. The <code>doPrivileged</code> method calls the
* <code>run</code> method from the PrivilegedAction
* implementation after enabling privileges, and returns the
* <code>run</code> method's return value as the
* <code>doPrivileged</code> return value (which is
* ignored in this example).
*
* <p> If you need to return a value, you can do something like the following:
*
* <pre>
* somemethod() {
* ...normal code here...
* String user = (String) AccessController.doPrivileged(
* new PrivilegedAction() {
* public Object run() {
* return System.getProperty("user.name");
* }
* }
* );
* ...normal code here...
* }
* </pre>
*
* <p>If the action performed in your <code>run</code> method could
* throw a "checked" exception (those listed in the <code>throws</code> clause
* of a method), then you need to use the
* <code>PrivilegedExceptionAction</code> interface instead of the
* <code>PrivilegedAction</code> interface:
*
* <pre>
* 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
* // <code>PrivilegedActionException</code>.
* throw (FileNotFoundException) e.getException();
* }
* ...normal code here...
* }
* </pre>
*
* <p> Be *very* careful in your use of the "privileged" construct, and
* always remember to make the privileged code section as small as possible.
*
* <p> Note that <code>checkPermission</code> 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
* <i>different</i> context (for example, from within a worker thread).
* The {@link #getContext() getContext} method and
* AccessControlContext class are provided
* for this situation. The <code>getContext</code> 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:
*
* <pre>
*
* AccessControlContext acc = AccessController.getContext()
*
* </pre>
*
* <p>
* AccessControlContext itself has a <code>checkPermission</code> method
* that makes access decisions based on the context <i>it</i> 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:
*
* <pre>
*
* acc.checkPermission(permission)
*
* </pre>
*
* <p> 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:
*
* <pre>
* 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...
* }
* </pre>
*
* @see AccessControlContext
*
* @version 1.55 04/05/05
* @author Li Gong
* @author Roland Schemers
*/
public final class AccessController {
/**
* Don't allow anyone to instantiate an AccessController
*/
private AccessController() { }
/**
* Performs the specified <code>PrivilegedAction</code> with privileges
* enabled. The action is performed with <i>all</i> of the permissions
* possessed by the caller's protection domain.
* <p>
* If the action's <code>run</code> method throws an (unchecked) exception,
* it will propagate through this method.
*
* @param action the action to be performed.
*
* @return the value returned by the action's <code>run</code> method.
*
* @exception NullPointerException if the action is <code>null</code>
*
* @see #doPrivileged(PrivilegedAction,AccessControlContext)
* @see #doPrivileged(PrivilegedExceptionAction)
*/
public static native <T> T doPrivileged(PrivilegedAction<T> action);
/**
* Performs the specified <code>PrivilegedAction</code> with privileges
* enabled and restricted by the specified
* <code>AccessControlContext</code>.
* 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
* <code>AccessControlContext</code>.
* <p>
* If the action's <code>run</code> method throws an (unchecked) exception,
* it will propagate through this method.
*
* @param action the action to be performed.
* @param context an <i>access control context</i>
* representing the restriction to be applied to the
* caller's domain's privileges before performing
* the specified action. If the context is
* <code>null</code>,
* then no additional restriction is applied.
*
* @return the value returned by the action's <code>run</code> method.
*
* @exception NullPointerException if the action is <code>null</code>
*
* @see #doPrivileged(PrivilegedAction)
* @see #doPrivileged(PrivilegedExceptionAction,AccessControlContext)
*/
public static native <T> T doPrivileged(PrivilegedAction<T> action,
AccessControlContext context);
/**
* Performs the specified <code>PrivilegedExceptionAction</code> with
* privileges enabled. The action is performed with <i>all</i> of the
* permissions possessed by the caller's protection domain.
* <p>
* If the action's <code>run</code> method throws an <i>unchecked</i>
* exception, it will propagate through this method.
*
* @param action the action to be performed
*
* @return the value returned by the action's <code>run</code> method
*
* @exception PrivilegedActionException if the specified action's
* <code>run</code> method threw a <i>checked</i> exception
* @exception NullPointerException if the action is <code>null</code>
*
* @see #doPrivileged(PrivilegedAction)
* @see #doPrivileged(PrivilegedExceptionAction,AccessControlContext)
*/
public static native <T> T
doPrivileged(PrivilegedExceptionAction<T> action)
throws PrivilegedActionException;
/**
* Performs the specified <code>PrivilegedExceptionAction</code> with
* privileges enabled and restricted by the specified
* <code>AccessControlContext</code>. 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 <code>AccessControlContext</code>.
* <p>
* If the action's <code>run</code> method throws an <i>unchecked</i>
* exception, it will propagate through this method.
*
* @param action the action to be performed
* @param context an <i>access control context</i>
* representing the restriction to be applied to the
* caller's domain's privileges before performing
* the specified action. If the context is
* <code>null</code>,
* then no additional restriction is applied.
*
* @return the value returned by the action's <code>run</code> method
*
* @exception PrivilegedActionException if the specified action's
* <code>run</code> method
* threw a <i>checked</i> exception
* @exception NullPointerException if the action is <code>null</code>
*
* @see #doPrivileged(PrivilegedAction)
* @see #doPrivileged(PrivilegedExceptionAction,AccessControlContext)
*/
public static native <T> T
doPrivileged(PrivilegedExceptionAction<T> action,
AccessControlContext context)
throws PrivilegedActionException;
/**
* Returns the AccessControl context. i.e., it gets
* the protection domains of all the callers on the stack,
* starting at the first class with a non-null
* ProtectionDomain.
*
* @return the access control context based on the current stack or
* null if there was only privileged system code.
*/
private static native AccessControlContext getStackAccessControlContext();
/**
* Returns the "inherited" AccessControl context. This is the context
* that existed when the thread was created. Package private so
* AccessControlContext can use it.
*/
static native AccessControlContext getInheritedAccessControlContext();
/**
* 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.
*
* @see AccessControlContext
*
* @return the AccessControlContext based on the current context.
*/
public static AccessControlContext getContext()
{
AccessControlContext acc = getStackAccessControlContext();
if (acc == null) {
// all we had was privileged system code. We don't want
// to return null though, so we construct a real ACC.
return new AccessControlContext(null, true);
} else {
return acc.optimize();
}
}
/**
* 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.
*
* @param perm the requested permission.
*
* @exception AccessControlException if the specified permission
* is not permitted, based on the current security policy.
* @exception NullPointerException if the specified permission
* is <code>null</code> and is checked based on the
* security policy currently in effect.
*/
public static void checkPermission(Permission perm)
throws AccessControlException
{
//System.err.println("checkPermission "+perm);
//Thread.currentThread().dumpStack();
AccessControlContext stack = getStackAccessControlContext();
// if context is null, we had privileged system code on the stack.
if (stack == null) {
Debug debug = AccessControlContext.getDebug();
if (debug != null) {
if (Debug.isOn("stack"))
Thread.currentThread().dumpStack();
if (Debug.isOn("domain")) {
debug.println("domain (context is null)");
}
debug.println("access allowed "+perm);
}
return;
}
AccessControlContext acc = stack.optimize();
acc.checkPermission(perm);
}
}
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