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

Method Summary

Methods

Modifier and Type	Method and Description
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 void	checkPermission(String name) Determines whether the access request indicated by the specified permission should be allowed or denied, based on the security policy currently in effect.
static void	checkPermission(String name, String resource) Check for permission and throw an exception if not allowed.
static void	checkPermission(String name, String resource, String extraValue) Checks for permission and throw an exception if not allowed.
static AccessControlContext	getAccessControlContext() Returns the access control context for the running application.
static void	setAccessControlContext(AccessControlContext accessControlContext) Called to set the access control context for the application running in this isolate.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail

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.

setAccessControlContext

public static void setAccessControlContext(AccessControlContext accessControlContext)

Called to set the access control context for the application running in this isolate.

Parameters:

accessControlContext - access control context for the application running in the isolate

Throws:

SecurityException - if the context is already set

getAccessControlContext

public static AccessControlContext getAccessControlContext()

Returns the access control context for the running application.

Throws:

SecurityException - if context has not been set yet

checkPermission

public static void checkPermission(String name)
                            throws SecurityException

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 SecurityException otherwise. May block to ask the user a question.

If the permission check failed because an InterruptedException was thrown, this method will throw a InterruptedSecurityException.

Parameters:

name - name of the requested permission

Throws:

SecurityException - if the specified permission is not permitted, based on the current security policy

checkPermission

public static void checkPermission(String name,
                   String resource)
                            throws SecurityException

Check for permission and throw an exception if not allowed. May block to ask the user a question.

If the permission check failed because an InterruptedException was thrown, this method will throw a InterruptedSecurityException.

Parameters:

name - name of the requested permission

resource - string to insert into the question, can be null if no %2 in the question

name - name of the requested permission

Throws:

SecurityException - if the specified permission is not permitted, based on the current security policy

checkPermission

public static void checkPermission(String name,
                   String resource,
                   String extraValue)
                            throws SecurityException

Checks for permission and throw an exception if not allowed. May block to ask the user a question.

If the permission check failed because an InterruptedException was thrown, this method will throw a InterruptedSecurityException.

Parameters:

name - ID of the permission to check for, the ID must be from com.sun.midp.security.Permissions

resource - string to insert into the question, can be null if no %2 in the question

extraValue - string to insert into the question, can be null if no %3 in the question

name - name of the requested permission

Throws:

SecurityException - if the specified permission is not permitted, based on the current security policy