AccessController类中的doPrivileged方法

示例中需要在eclipse中创建2个project：ServiceCentre和TestService

java.policy
grant codeBase  " file:/D:/Workspaces/ExchangeConnect_V2_Trunk_Maven_workspace/ServiceCentre/bin/* "   {
    permission java.io.FilePermission  " c:/TestService-1.0.jar " ,  " read " ;
    permission java.lang.RuntimePermission  " createClassLoader " ;
} ;

grant codeBase  " file:/c:/TestService-1.0.jar "   {
    permission java.io.FilePermission  " C:/text.txt " ,  " read " ;
} ;

Project - ServiceCentre

package  test;

import  java.lang.reflect.Method;
import  java.net.URL;
import  java.net.URLClassLoader;

/** */ /**
 *  @author  Donf Yang
  */
public   class  ServiceCentreMain  {

     public   void  loadService()  {
        URL[] urls;
         try   {
            urls  =   new  URL[]  {  new  URL( " file:c:/TestService-1.0.jar " ) } ;
            URLClassLoader ll  =   new  URLClassLoader(urls);
             final  Class a  =  ll.loadClass( " test.TestService " );
            Object o  =  a.newInstance();
            Method m  =  a.getMethod( " doService " ,  null );
            m.invoke(o,  null );

        }   catch  (Exception e)  {
            e.printStackTrace();
        }
    }

     /** */ /**
     *  @param  args
      */
     public   static   void  main(String[] args)  {
        ServiceCentreMain s  =   new  ServiceCentreMain();
        s.loadService();
    }

}

Project - TestService
将TestService打包，放到C盘

package  test;

import  java.io.FilePermission;
import  java.security.AccessController;
import  java.security.Permission;

/** */ /**
 *  @author  Donf Yang
 * 
  */
public   class  TestService  {

     public   void  doService()  {
        
        doFileOperation();

    }

     private   void  doFileOperation()  {
        Permission perm  =   new  FilePermission( " C:/text.txt " ,  " read " );
        AccessController.checkPermission(perm);
        System.out.println( " TestService has permission " );
    }

}

运行这个例子的时候，会出现权限错误，把doService()修改一下，就可以顺利通过

public   void  doService()  {

         //  doFileOperation();

        AccessController.doPrivileged( new  PrivilegedAction()  {
             public  Object run()  {
                doFileOperation();
                 return   null ;
            }
        } );
    }

在这个例子中AccessControlContext的stack顺序为
2.  file:/D:/Workspaces/ExchangeConnect_V2_Trunk_Maven_workspace/ServiceCentre/bin/*
1 . file:/c:/TestService-1.0.jar

2没有权限，1有权限，使用doPrivileged后，不检查2



看一下java.security.AccessController的JavaDoc:
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.


其中提到的no further checking is done的意思是指stack中的checking

加入一个TestService2，文件操作在1，stack为(1,2,3为checking顺序)
3 . file:/D:/Workspaces/ExchangeConnect_V2_Trunk_Maven_workspace/ServiceCentre/bin/*
2 . file:/c:/TestService-1.0.jar
1.  file:/c:/TestService2-1.0.jar
checking顺序为  1->2->3
如果doPrivileged是在2中调用，那么1,2需要具有权限，3不再进行检查
如果doPrivileged是在1中调用，那么1需要具有权限，2,3不再进行检查


总结：
1.  这里容易理解错误的地方是checking顺序，例如一个调用链 MethodA->MethodB->MethodC（这里的3个方法需要在3个不同的ProtectionDomain 中），doPrivileged在MethodB中，很容易理解成检查A,B而不检查C，实际上stack中检查顺序为C->B->A，也就是检查C,B而不检查A

2. ServiceCentre不需要太多权限，而Service就需要使用doPrivileged来避免受到ServiceCentre的权限限制（如果service有足够的权限），Equinox中有很多这样的例子（Equinox扮演Service的角色）。