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## java.security
Class AccessController

    java.lang.Object
      |
      +--java.security.AccessController

public final class AccessController
extends Object

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 from this
                     // point forward require both the current context and
                     // the snapshot's context to have the desired permission.
                  }
             }, acc);
             ...normal code here...
       }

See Also:
  `AccessControlContext`

Method Summary
`static void` | `checkPermission(Permission perm)` Determines whether the access request indicated by the specified permission should be allowed or denied, based on the security policy currently in effect.
`static Object` | `doPrivileged(PrivilegedAction action)` Performs the specified `PrivilegedAction` with privileges enabled.
`static Object` | `doPrivileged(PrivilegedAction action, AccessControlContext context)` Performs the specified `PrivilegedAction` with privileges enabled and restricted by the specified `AccessControlContext`.
`static Object` | `doPrivileged(PrivilegedExceptionAction action)` Performs the specified `PrivilegedExceptionAction` with privileges enabled.
`static Object` | `doPrivileged(PrivilegedExceptionAction action, AccessControlContext context)` Performs the specified `PrivilegedExceptionAction` with privileges enabled and restricted by the specified `AccessControlContext`.
`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.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Method Detail

### 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:
    `PrivilgedActionException` - the specified action's `run` method threw a checked exception.
  See Also:
    `doPrivileged(PrivilegedExceptionAction,AccessControlContext)`, `doPrivileged(PrivilegedAction)`

### 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 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.
  Throws:
    `PrivilegedActionException` - 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.

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