spel2js

About

SpEL2JS is a plugin that will parse Spring Expression Language within a defined context in JavaScript. This is useful in single-page applications where duplication of authorization expressions for UI purposes can lead to inconsistencies. This library implements a JavaScript version of the parser based on the documentation in the link above. I did my best to followed the docs as closely as possible, but if you come accross an expression that behaves differently than you would expect then please open an issue.

Getting Started

Install SpEL2JS:

$ npm i -S spel2js 
# or
$ bower i -S spel2js

Or download the zip

Include the dependency using a module loader or script tag.

Usage

SpEL2JS exports a singleton with two members:

import spel2js from 'spel2js';

console.log(spel2js);
/*
{
  StandardContext,
  SpelExpressionEvaluator
}
*/

StandardContext

The StandardContext is a factory that creates a evaluation context for an expression. NOTE: This is not the same as the Java EvaluationContext class, though it serves a similar purpose.

let spelContext = spel2js.StandardContext.create(authentication, principal);

The create() method takes two arguments: authentication and principal

authentication is an instance of Spring's Authentication class from Spring Security.

principal is any object representing the user (this is just used for reference, and can be any value or structure)

SpelExpressionEvaluator

The heavy lifting is done using the SpelExpressionEvaluator which exposes two functions: compile() and eval()

compile() pre-compiles a SpEL expression, and returns an object with an eval() method that takes a context and optional locals:

import { StandardContext, SpelExpressionEvaluator } from 'spel2js';

const expression = '#toDoList.owner == authentication.details.name';
const spelContext = StandardContext.create(authentication, principal);
const locals = {
  toDoList: {
    owner: 'Darth Vader'  
  }
};

const compiledExpression = SpelExpressionEvaluator.compile(expression);

compiledExpression.eval(spelContext, locals); // true

eval() is just a shortcut for immediately evaluating an expression instead of pre-compiling:

import { StandardContext, SpelExpressionEvaluator } from 'spel2js';

const expression = '#toDoList.owner == authentication.details.name';
const spelContext = StandardContext.create(authentication, principal);
const locals = {
  toDoList: {
    owner: 'Darth Vader'  
  }
};

SpelExpressionEvaluator.eval(expression, spelContext, locals); // true

Recommended Usage

Create a single context that contains information about the current user and reuse it for all evaluations. This way, you only have to supply an expression and locals when evaluating.

Always pre-compile your expressions! Compilation takes much longer than evaluation; doing it up-front saves CPU when evaluating later.

Example

Say you are creating a shared to-do list, and you want to allow only the owner of the list to make changes, but anyone can view:

//ListController.java

@Controller
@RequestMapping('/todolists')
public class ListController {

  public static final String ADD_LIST_ITEM_PERMISSION = "#toDoList.owner == authentication.details.name";  
  ...
  
  @PreAuthorize(ADD_LIST_ITEM_PERMISSION)
  @RequestMapping(value="/{toDolistId}/items", method=RequestMethod.POST)
  public ResponseEntity<ListItem> addListItem(@MagicAnnotation ToDoList toDoList, @RequestBody ListItem newListItem) {
    //add the item to the list
    return new ResponseEntity<ListItem>(newListItem, HttpStatus.CREATED);
  }

  ...
}

//spel-service.js

import { StandardContext, SpelExpressionEvaluator } from 'spel2js';

// wraps spel2js in a stateful service that simplifies evaluation
angular.module('ToDo').factory('SpelService', function () {
    
  return {
    context: null,
    
    // assume this is called on page load
    setContext(authentication, principal) {
      this.context = StandardContext.create(authentication, principal);
    },
    
    getContext() { return this.context; },
    
    compile(expression) {
      const compiledExpression = SpelExpressionEvaluator.compile(expression); 
      return {
        eval(locals) { 
          return compiledExpression.eval(this.getContext(), locals);
        }
      };
    },
    
    eval(expression, locals) {
      return SpelExpressionEvaluator.eval(expression, this.getContext(), locals);
    }
  };
  
});


//list-controller.js

angular.module('ToDo').controller('ListController', ['$http', '$scope', 'SpelService', function ($http, $scope, SpelService) {
  
  // retrieve all permissions and pre-compile them
  $http.get('/api/permissions').success(function (permissions) {
    angular.forEach(permissions, function (spelExpression, key) {
      $scope.permissions[key] = SpelService.compile(spelExpression);
    });
  });
  
  // $scope will be used as locals
  $scope.list = {
    name: 'My List',
    owner: 'Ben March',
    items: [
      {
        text: 'List item number 1!'
      }
    ]
  }
  
  // EXPAMPLE 1: authorize a request before making it
  $scope.addListItem = function (list, newListItem) {
    if ($scope.permissions.ADD_LIST_ITEM_PERMISSION.eval($scope)) {
      $http.post('/todolists/' + list.id + '/items', item).success(function () {...});  
    }
  }
}]);

<!--list-controller.html-->

<div ng-controller="ListController">
  ...
  <li ng-repeat="listItem in list.items">
    <p>{{listItem.text}}</p>
  </li>
  <li class="list-actions">
    <input type="text" ng-model="newListItem.text" />
    
    <!-- EXAMPLE 2: Hide the button if the user does not have permission -->
    <button ng-click="addListItem(list, newListItem)" ng-if="permissions.ADD_LIST_ITEM_PERMISSION.eval(this)">Add</button>
  </li>
  ...
</div>

Now the UI can always stay in sync with the server-side authorities.

Features

This is now in a stable state and will be released as 0.2.0. The following features are tested and working:

• Primitive Literals
• Property references
• Compound expressions
• Comparisons
• Method references
• Local variable reference ("#someVar")
• Math
• Ternary operators
• Safe navigation
• Assignment
• Complex literals
• Projection/selection
• Increment/Decrement
• Logical operators (and, or, not)
• hasRole() (if you use spel2js.StandardContext)

The following are not implemented yet because I'm not sure of the best approach:

• Qualified identifiers/Type references/Bean References
• hasPermission() for custom permission evaluators

If someone wants to implement a REST-compliant way in Spring to expose the permissions (and maybe the custom PermissionEvaluators) that would be awesome.

Building Locally

$ npm i
$ npm run build
$ npm test

Credits

Credit is given to all of the original authors of the Java SpEL implementation at the time of this library's creation:

• Andy Clement
• Juergen Hoeller
• Giovanni Dall'Oglio Risso
• Sam Brannen
• Mark Fisher
• Oliver Becker
• Clark Duplichien
• Phillip Webb
• Stephane Nicoll
• Ivo Smid

This repository was scaffolded with generator-microjs.

License

Since this was ported from the Spring Framework, this library is under version 2.0 of the Apache License.