为什么laravel 4.1给出类不存在命名空类的错误？

I'm having an issue in laravel 4.1. I'm following a tutorial series. I have "Acme/Transformers" folder in "app" directory which has two class "Transformer.php" and "LessonTransformer.php". When i tried to access "LessonTransformer.php" in LessonController class of Controller directory i'm getting the following issue.

ReflectionException
Class Acme\Transformers\LessonTransformer does not exist

I updated my composer.json file like following.

"autoload": {
    "classmap": [
        "app/commands",
        "app/controllers",
        "app/models",
        "app/database/migrations",
        "app/database/seeds",
        "app/tests/TestCase.php",
        "app/Acme"
    ],
    "psr-0":{
        "Acme" : "app/Acme"
    }

and ran

"composer dump-autoload -o"

but it made no change in the output. I have no ideas what's going on. Please help me.

Here's is my LessonsController.php class

use Acme\Transformers\LessonTransformer;

class LessonsController extends \BaseController {

    /**
     * @var Acme\Transformers\LessonTransformer
     */
     protected $lessonTransformer;

     function __construct(LessonTransformer $lessonTransformer){
         $this->lessonTransformer = $lessonTransformer;
     }

     /**
      * Display a listing of the resource.
      *
      * @return Response
      */
     public function index()
     {
         $lessons = Lesson::all();
         return Response::json([
             'lessons' => $this->lessonTransformer->transformCollection($lessons->toArray())
         ], 200);
     }

     /**
      * Display the specified resource.
      *
      * @param  int  $id
      * @return Response
      */
     public function show($id)
     {
          $lesson = Lesson::find($id);

          if(!$lesson){
               return Response::json([
                        'error' => [
                            'mesage' => 'Lessons does not exits'
                        ]
               ], 404);
          }

          return Response::json([
               'lesson' => $this->lessonTransformer->transform($lesson)
          ], 200);
      }

      /**
       * Map over the lesson collection and cast it in an array,
       * so that we can map over it.
       *
       * Transform a collection of lesson
       * 
       * @param  array $lessons
       * @return array
       */
      public function transformCollection(array $lessons){
          return array_map([$this,'transform'], $lessons);
      }

      /**
       * Transform a single object(lesson)
       * Instead of returning everything, we return some selected data.
       * We replaced the keys that exposed the DB structure with some
       * other keys because if we wish to change the keys of data in 
       * future then we can change it without affecting the DB structure.
       * 
       * @param  array $lesson
       * @return array
       */
      public function transform($lesson){
          return [
                'title'     => $lesson['title'],
                'body'      => $lesson['body'],
                'active'    => (boolean)$lesson['some_bool']
          ];
      }
 }

Have you added the

namespace Acme\Transformers;

line at the top of the LessonTransformer.php file? Also check for plurality of the filenames. Often mistakes come from a file named LessonTransformer.php and the class named LessonsFormatter (mind the s).

EDIT

These should be the top lines of your files:

app/src/Acme/Transformers/Transformer.php

<?php
namespace Acme\Transformers;

class Transformer
{

app/src/Acme/Transformers/LessonTransformer.php

<?php
namespace Acme\Transformers;

class LessonTransformer extends Transformer
{

app/controllers/LessonsController.php

<?php

use Acme\Transformers\LessonTransformer;

class LessonsController extends BaseController
{

maybe you forget to update your composer.json: add app/Acme to classmap

"autoload": {
    "classmap": [
        "app/commands",
        "app/controllers",
        "app/models",
        "app/database/migrations",
        "app/database/seeds",
        "app/tests/TestCase.php",
        "app/Acme"
    ]
},

and run composer dump-autoloadon your terminal