Writing your spring security expression language annotation - PART 1

Spring security expression language is very useful. It helps to secure your service/web methods with one line of code. It supports @PreAuthorize and @Secured. In the coming three posts, I will talk about how to add custom behaviour to the @PreAuthorize annotation.

Part 1 - Customize "hasPermission()" expression
Part 2 - Add new customize method security expression
Part 3 - Override default behaviour of spring security expression (e.g. hasRole() , permitAll() ...)

In this post, I will discuss how to add custom rule for permission checking in your application. This is somewhat similar to what describe in Sold Craft's post. You can reference it for more details.


Step 1: Add configuration in your spring security xml file.

You should first add the DefaultMethodSecurityExpressionHandler. It will instantiate a default MethodSecurityExpressionRoot which provides you all the default security expression (e.g. isAutghenticated(), isAnonymous() ,etc ) .

Besides, you have to add a permsisionEvaluator for that ExpressionHandler. If you are using spring security ACL, you could use AclPermissionEvaluator. In our case, we would create a BasePermissionEvaluator as our permission evaluator. You will see in step 2 that we would define custom rules in this permission evaluator.

<sec:global-method-security pre-post-annotations="enabled">
  <sec:expression-handler ref="expressionHandler"/>
</sec:global-method-security>

<bean id="expressionHandler" class="org.springframework.security.access.expression.method.DefaultMethodSecurityExpressionHandlerr">
    <property name="permissionEvaluator" ref="permissionEvaluator"/>
</bean>  

<bean id="permissionEvaluator" class="org.borislam.security.BasePermissionEvaluator"/>

Step 2: Create your PermissionEvaluator class
You must define a class that implements the org.springframework.security.access.PermissionEvaluator. You have to override the hasPermission() method and define custom rule in this class.

In my example, the user object contains a HashMap which stored the permissions of the user. I will perform checking the permission String against this Hashmap. This HashMap is populated during login by a filter. This part will not be skipped in this example.

For similicity, I just ignore targetDomainObject parameter in my example. By using the targetDomainObject, you can further define security rules on certian domain object of your application.

public class BasePermissionEvaluator implements PermissionEvaluator{
 
 
 @Override
 public boolean hasPermission(Authentication authentication, Object targetDomainObject, Object permission) {
  boolean hasPermission = false;
  if ( authentication != null &&  permission instanceof String){

   //implement the permission checking of your application here   
   //you can just check if the input permission is within your permission list

   //In my example, the user object contains a HashMap which stored the permission of the user.
   //The HashMap<String, PrivilegeResult> is populated during using login by filter. This will not be shown in this example 

   User user = SecurityUtil.getUserCredential();
   HashMap<String, PrivilegeResult> pMap =user.getPrivilegeMap();
   PrivilegeResult privResult = pMap.get(permission); 
   hasPermission =  privResult.isAllowAccess();

  } else {
   hasPermission =false; 
  }
  return hasPermission;
 }

 @Override
 public boolean hasPermission(Authentication authentication,
   Serializable targetId, String targetType, Object permission) {
    throw new RimtimeException("Id and Class permissions are not supperted by this application");
 }
}

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Step 3: Example usage
You could simply add your the @PreAuthorize("hasPermission()") to secure your method.

@PreAuthorize("hasPermission(#user, 'allowDoSomething')")
 public String doSomething()
 {
  //do something
  System.out.println("Do something");
 }

In the next part of this series of tutorial, I will further discuss how to add your new custom method to the security expression root.