Privileged Block API - JDK 5 Documentation v1.2.2, Java 2 SDK 英文文档

API for Privileged Blocks

Last Modified: 22 September, 1998

Overview

This document provides background information about what "privileged" code is and what it is used for, followed by illustrations of the use of the API, with special attention to issues of:

returning values

type safety

exception handling

reflection

Background: What It Means to Have "Privileged" Code

The policy for a JDK installation specifies what permissions -- which types of system resource accesses -- are allowed for code from specified code sources. A "code source" (of type CodeSource) essentially consists of the code location (URL) and a reference to the certificate(s) containing the public key(s) corresponding to the private key(s) used to sign the code (if it was signed).
The policy is represented by a Policy object. More specifically, it is represented by a Policy subclass providing an implementation of the abstract methods in the Policy class (which is in the java.security package).
The source location for the policy information utilized by the Policy object is up to the Policy implementation. The default Policy implementation obtains its information from policy configuration files. See Default Policy Implementation and Policy File Syntax for information about the default Policy implementation and the syntax that must be used in policy files it reads. For information about using the Policy Tool to create a policy file (without needing to know the required syntax), see the Policy Tool documentation (for Solaris) (for Windows).
A "protection domain" encompasses a CodeSource and the permissions granted to code from that CodeSource, as determined by the security policy currently in effect. Thus, classes signed by the same keys and from the same URL are placed in the same domain, and a class belongs to one and only one protection domain. Classes that have the same permissions but are from different code sources belong to different domains.
Today all code shipped as part of the JDK is considered system code and run inside the unique system domain. System code automatically has all permissions.
Each applet or application runs in its appropriate domain, determined by its code source. In order for an applet (or an application running under a security manager) to be allowed to perform a secured action (such as reading or writing a file), the applet or application must be granted permission for that particular action.
More specifically, whenever a resource access is attempted, all code traversed by the execution thread up to that point must have permission for that resource access, unless some code on the thread has been marked as "privileged". That is, suppose access control checking occurs in a thread of execution that has a chain of multiple callers. (Think of this as multiple method calls that potentially cross the protection domain boundaries.) When the AccessController checkPermission method is invoked by the most recent caller, the basic algorithm for deciding whether to allow or deny the requested access is as follows:

If the code for any caller in the call chain does not have the requested permission, AccessControlException is thrown, unless the following is true - a caller whose code is granted the said permission has been marked as "privileged" (see below) and all parties subsequently called by this caller (directly or indirectly) all have the said permission.

Marking code as "privileged" enables a piece of trusted code to temporarily enable access to more resources than are available directly to the code that called it. This is necessary in some situations. For example, an application may not be allowed direct access to files that contain fonts, but the system utility to display a document must obtain those fonts, on behalf of the user. In order to do this, the system utility becomes privileged while obtaining the fonts.

Using the doPrivileged API

The use of the "privileged" feature is described in the following sections.
No return value, no exception thrown
If you don't need to return a value from within the "privileged" block, your call to doPrivileged can look like 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...
  }
 
The AccessController.doPrivileged method takes an object of type java.security.PrivilegedAction and invokes its run method in privileged mode. The implementation guarantees that privileges will be revoked after the run method is executed, even if execution of doPrivileged is interrupted by an asynchronous exception.
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 using an anonymous inner class; 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).
Note that depending on what "privileged code" actually consisted of, you might have to make some changes due to the way inner classes work. For example, if "privileged code" throws an exception or attempts to access local variables then you will have to make some changes, as shown in subsequent sections of this document.
You can also call doPrivileged without using an anonymous inner class, as in:
    somemethod() {
        ...normal code here...
        MyAction mya = new MyAction();
        // become privileged:
        AccessController.doPrivileged(mya);
       ...normal code here...
    }

    class MyAction implements PrivilegedAction {
        public MyAction() {};
        public Object run() {
            // privileged code goes here, for example:
            System.loadLibrary("awt");
            return null; // nothing to return
        }
    }
Be very careful in your use of the "privileged" construct, and always remember to make the privileged code section as small as possible, that is, try and limit the code within the run method to only the code that needs to be run with privileges, and do more general things outside the run method. Also note that the call to doPrivileged should be made in the code that wants to enable its privileges. Do not be tempted to write a utility class that itself calls doPrivileged as that could lead to security holes. You can write utility classes for PrivilegedAction classes though, as shown in the example above.
Returning values
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...
  }
 
Note that this usage requires a dynamic cast on the value returned by doPrivileged. An alternative is to use a final local variable:
   somemethod() {
        ...normal code here...
        final String user[] = {null};
        AccessController.doPrivileged(
          new PrivilegedAction() {
            public Object run() {
                user[0] = System.getProperty("user.name");
                return null; // still need this
            }
          }
        );
        ...normal code here...
  }
 
Yet another alternative would be to write a non-anonymous class that safely handles types for you:
   somemethod() {
        ...normal code here...
        GetPropertyAction gpa = new GetPropertyAction("user.name");
        AccessController.doPrivileged(gpa);
        String user = gpa.getValue();
        ...normal code here...
   }

   class GetPropertyAction implements PrivilegedAction {
      private String property;
      private String value;
      public GetPropertyAction(String prop) { property = prop;}
      public Object run() { 
               value = System.getProperty(property);
               return value;
      } 
      public String getValue() {return value;}

   }
Note there are now no type-casts involved, although the run method still returns a value, so you could still have a "one-liner" if you wanted to:
   somemethod() {
        ...normal code here...
        String user = (String) AccessController.doPrivileged(
                                   new GetPropertyAction("user.name"));
        ...normal code here...
   }
Accessing local variables
If you are using an anonymous inner class, any local variables you access must be final. For example:
   somemethod() {
        ...normal code here...
        final String lib = "awt";
        AccessController.doPrivileged(new PrivilegedAction() {
            public Object run() {
                // privileged code goes here, for example:
                System.loadLibrary(lib);
                return null; // nothing to return
            }
        });
       ...normal code here...
  }
 
The variable lib must be declared final if you intend to use it within the privileged block. See the "Inner Classes" spec for more information on this topic.
If there are cases where you can't make an existing variable final (because it gets set multiple times), then you can create a new final variable right before invoking doPrivileged, and set that variable equal to the other variable. For example:
   somemethod() {
        ...normal code here...
        String lib;
        ...
        // lib gets set multiple times so we can't make it final
        ...
        // create a final String that we can use inside of the run method
        final String fLib = lib;
        AccessController.doPrivileged(new PrivilegedAction() {
            public Object run() {
                // privileged code goes here, for example:
                System.loadLibrary(fLib);
                return null; // nothing to return
            }
        });
       ...normal code here...
  }
 
Handling Exceptions
If the action performed in your run method could throw a "checked" exception (one that must be 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...
  }
 
If a checked exception is thrown during execution of the run method, it is placed in a PrivilegedActionException "wrapper" exception that is then thrown and should be caught by your code, as illustrated in the above example..
Reflection

One subtlety that must be considered is the interaction of this API with reflection. The doPrivileged() method can be invoked reflectively using java.lang.reflect.Method.invoke(). In this case, the privileges granted in privileged mode are not those of Method.invoke() but of the non-reflective code that invoked it. Otherwise, system privileges could erroneously (or maliciously) be conferred on user code. Note that similar requirements exist when using the existing API via reflection.

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