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package java.security;

import sun.security.util.Debug;

import jdk.internal.reflect.CallerSensitive;

import jdk.internal.reflect.Reflection;

/**

 * <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,

 * <li>to mark code as being "privileged", thus affecting subsequent

 * access determinations, and

 * <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} 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} 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} method.

 * The {@code checkPermission} method determines whether access

 * is granted or denied based on the following algorithm:

 *

 *  <pre> {@code

 * for (int i = m; i > 0; i--) {

 *

 *     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)

 *         if (limited permissions were specified in the call to doPrivileged) {

 *             for (each limited permission) {

 *                 if (the limited permission implies the requested permission)

 *                     return;

 *             }

 *         } else

 *             return;

 *     }

 * }

 *

 * // 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}

 * method stops checking if it reaches a caller that

 * was marked as "privileged" via a {@code doPrivileged}

 * call without a context argument (see below for information about a

 * context argument). If that caller's domain has the

 * specified permission and at least one limiting permission argument (if any)

 * implies the requested permission, no further checking is done and

 * {@code 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. If the caller's domain had the specified permission

 * but it was not implied by any limiting permission arguments given in the call

 * to {@code doPrivileged} then the permission checking continues

 * until there are no more callers or another {@code doPrivileged}

 * call matches the requested permission and returns normally.

 *

 * <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> {@code

 * somemethod() {

 *     ...normal code here...

 *     AccessController.doPrivileged(new PrivilegedAction<Void>() {

 *         public Void 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}.

 * The above example shows creation of an implementation

 * of that interface; a concrete implementation of the

 * {@code run} method is supplied.

 * When the call to {@code doPrivileged} is made, an

 * instance of the PrivilegedAction implementation is passed

 * to it. The {@code doPrivileged} method calls the

 * {@code run} method from the PrivilegedAction

 * implementation after enabling privileges, and returns the

 * {@code run} method's return value as the

 * {@code doPrivileged} return value (which is

 * ignored in this example).

 *

 * <p> If you need to return a value, you can do something like the following:

 *

 *  <pre> {@code

 * somemethod() {

 *     ...normal code here...

 *     String user = AccessController.doPrivileged(

 *         new PrivilegedAction<String>() {

 *         public String run() {

 *             return System.getProperty("user.name");

 *             }

 *         });

 *     ...normal code here...

 * }}</pre>

 *

 * <p>If the action performed in your {@code run} method could

 * throw a "checked" exception (those listed in the {@code throws} clause

 * of a method), then you need to use the

 * {@code PrivilegedExceptionAction} interface instead of the

 * {@code PrivilegedAction} interface:

 *

 *  <pre> {@code

 * somemethod() throws FileNotFoundException {

 *     ...normal code here...

 *     try {

 *         FileInputStream fis = AccessController.doPrivileged(

 *         new PrivilegedExceptionAction<FileInputStream>() {

 *             public FileInputStream 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...

 *  }}</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.

 * You can pass {@code Permission} arguments to further limit the

 * scope of the "privilege" (see below).

 *

 *

 * <p> Note that {@code 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

 * <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} 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} 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. You can also limit the scope

 * of the privileged code by passing additional {@code Permission}

 * parameters.

 *

 *  <pre> {@code

 * somemethod() {

 *     AccessController.doPrivileged(new PrivilegedAction<Object>() {

 *         public Object run() {

 *             // Code goes here. Any permission checks within this

 *             // run method will require that the intersection of the

 *             // caller's protection domain and the snapshot's

 *             // context have the desired permission. If a requested

 *             // permission is not implied by the limiting FilePermission

 *             // argument then checking of the thread continues beyond the

 *             // caller of doPrivileged.

 *         }

 *     }, acc, new FilePermission("/temp/*", read));

 *     ...normal code here...

 * }}</pre>

 * <p> Passing a limiting {@code Permission} argument of an instance of

 * {@code AllPermission} is equivalent to calling the equivalent

 * {@code doPrivileged} method without limiting {@code Permission}

 * arguments. Passing a zero length array of {@code Permission} disables

 * the code privileges so that checking always continues beyond the caller of

 * that {@code doPrivileged} method.

 *

 * @see AccessControlContext

 *

 * @author Li Gong

 * @author Roland Schemers

 * @since 1.2

 */

public final class AccessController {

    /**

     * Don't allow anyone to instantiate an AccessController

     */

    private AccessController() { }

    /**

     * Performs the specified {@code PrivilegedAction} 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} method throws an (unchecked)

     * exception, it will propagate through this method.

     *

     * <p> Note that any DomainCombiner associated with the current

     * AccessControlContext will be ignored while the action is performed.

     *

     * @param <T> the type of the value returned by the PrivilegedAction's

     *                  {@code run} method.

     *

     * @param action the action to be performed.

     *

     * @return the value returned by the action's {@code run} method.

     *

     * @exception NullPointerException if the action is {@code null}

     *

     * @see #doPrivileged(PrivilegedAction,AccessControlContext)

     * @see #doPrivileged(PrivilegedExceptionAction)

     * @see #doPrivilegedWithCombiner(PrivilegedAction)

     * @see java.security.DomainCombiner

     */

    @CallerSensitive

    /**

     * Performs the specified {@code PrivilegedAction} 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} method throws an (unchecked)

     * exception, it will propagate through this method.

     *

     * <p> This method preserves the current AccessControlContext's

     * DomainCombiner (which may be null) while the action is performed.

     *

     * @param <T> the type of the value returned by the PrivilegedAction's

     *                  {@code run} method.

     *

     * @param action the action to be performed.

     *

     * @return the value returned by the action's {@code run} method.

     *

     * @exception NullPointerException if the action is {@code null}

     *

     * @see #doPrivileged(PrivilegedAction)

     * @see java.security.DomainCombiner

     *

     * @since 1.6

     */

    @CallerSensitive

    public static <T> T doPrivilegedWithCombiner(PrivilegedAction<T> action) {

        AccessControlContext acc = getStackAccessControlContext();

        if (acc == null) {

            return AccessController.doPrivileged(action);

        }

        DomainCombiner dc = acc.getAssignedCombiner();

        return AccessController.doPrivileged(action,

                                             preserveCombiner(dc, Reflection.getCallerClass()));

    }

    /**

     * Performs the specified {@code PrivilegedAction} with privileges

     * enabled and restricted by the specified {@code 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 {@code AccessControlContext}.

     * <p>

     * If the action's {@code run} method throws an (unchecked) exception,

     * it will propagate through this method.

     * <p>

     * If a security manager is installed and the specified

     * {@code AccessControlContext} was not created by system code and the

     * caller's {@code ProtectionDomain} has not been granted the

     * {@literal "createAccessControlContext"}

     * {@link java.security.SecurityPermission}, then the action is performed

     * with no permissions.

     *

     * @param <T> the type of the value returned by the PrivilegedAction's

     *                  {@code run} 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}, then no additional restriction is applied.

     *

     * @return the value returned by the action's {@code run} method.

     *

     * @exception NullPointerException if the action is {@code null}

     *

     * @see #doPrivileged(PrivilegedAction)

     * @see #doPrivileged(PrivilegedExceptionAction,AccessControlContext)

     */

    @CallerSensitive

    /**

     * Performs the specified {@code PrivilegedAction} with privileges

     * enabled and restricted by the specified

     * {@code AccessControlContext} and with a privilege scope limited

     * by specified {@code Permission} arguments.

     *

     * 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}.

     * <p>

     * If the action's {@code run} method throws an (unchecked) exception,

     * it will propagate through this method.

     * <p>

     * If a security manager is installed and the specified

     * {@code AccessControlContext} was not created by system code and the

     * caller's {@code ProtectionDomain} has not been granted the

     * {@literal "createAccessControlContext"}

     * {@link java.security.SecurityPermission}, then the action is performed

     * with no permissions.

     *

     * @param <T> the type of the value returned by the PrivilegedAction's

     *                  {@code run} 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},

     *                then no additional restriction is applied.

     * @param perms the {@code Permission} arguments which limit the

     *              scope of the caller's privileges. The number of arguments

     *              is variable.

     *

     * @return the value returned by the action's {@code run} method.

     *

     * @throws NullPointerException if action or perms or any element of

     *         perms is {@code null}

     *

     * @see #doPrivileged(PrivilegedAction)

     * @see #doPrivileged(PrivilegedExceptionAction,AccessControlContext)

     *

     * @since 1.8

     */

    @CallerSensitive

    public static <T> T doPrivileged(PrivilegedAction<T> action,

        AccessControlContext context, Permission... perms) {

        AccessControlContext parent = getContext();

        if (perms == null) {

            throw new NullPointerException("null permissions parameter");

        }

        Class <?> caller = Reflection.getCallerClass();

        return AccessController.doPrivileged(action, createWrapper(null,

            caller, parent, context, perms));

    }

    /**

     * Performs the specified {@code PrivilegedAction} with privileges

     * enabled and restricted by the specified

     * {@code AccessControlContext} and with a privilege scope limited

     * by specified {@code Permission} arguments.

     *

     * 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}.

     * <p>

     * If the action's {@code run} method throws an (unchecked) exception,

     * it will propagate through this method.

     *

     * <p> This method preserves the current AccessControlContext's

     * DomainCombiner (which may be null) while the action is performed.

     * <p>

     * If a security manager is installed and the specified

     * {@code AccessControlContext} was not created by system code and the

     * caller's {@code ProtectionDomain} has not been granted the

     * {@literal "createAccessControlContext"}

     * {@link java.security.SecurityPermission}, then the action is performed

     * with no permissions.

     *

     * @param <T> the type of the value returned by the PrivilegedAction's

     *                  {@code run} 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},

     *                then no additional restriction is applied.

     * @param perms the {@code Permission} arguments which limit the

     *              scope of the caller's privileges. The number of arguments

     *              is variable.

     *

     * @return the value returned by the action's {@code run} method.

     *

     * @throws NullPointerException if action or perms or any element of

     *         perms is {@code null}

     *

     * @see #doPrivileged(PrivilegedAction)

     * @see #doPrivileged(PrivilegedExceptionAction,AccessControlContext)

     * @see java.security.DomainCombiner

     *

     * @since 1.8

     */

    @CallerSensitive

    public static <T> T doPrivilegedWithCombiner(PrivilegedAction<T> action,

        AccessControlContext context, Permission... perms) {

        AccessControlContext parent = getContext();

        DomainCombiner dc = parent.getCombiner();

        if (dc == null && context != null) {

            dc = context.getCombiner();

        }