diff --git a/scrapy/contrib/downloadermiddleware/httpauth.py b/scrapy/contrib/downloadermiddleware/httpauth.py
index b956838f1..7854e899a 100644
--- a/scrapy/contrib/downloadermiddleware/httpauth.py
+++ b/scrapy/contrib/downloadermiddleware/httpauth.py
@@ -5,23 +5,27 @@ See documentation in docs/topics/downloader-middleware.rst
 """
 
 from w3lib.http import basic_auth_header
-from scrapy.utils.python import WeakKeyCache
+
+from scrapy import signals
 
 
 class HttpAuthMiddleware(object):
     """Set Basic HTTP Authorization header
     (http_user and http_pass spider class attributes)"""
 
-    def __init__(self):
-        self._cache = WeakKeyCache(self._authorization)
+    @classmethod
+    def from_crawler(cls, crawler):
+        o = cls()
+        crawler.signals.connect(o.spider_opened, signal=signals.spider_opened)
+        return o
 
-    def _authorization(self, spider):
+    def spider_opened(self, spider):
         usr = getattr(spider, 'http_user', '')
         pwd = getattr(spider, 'http_pass', '')
         if usr or pwd:
-            return basic_auth_header(usr, pwd)
+            self.auth = basic_auth_header(usr, pwd)
 
     def process_request(self, request, spider):
-        auth = self._cache[spider]
+        auth = getattr(self, 'auth', None)
         if auth and 'Authorization' not in request.headers:
             request.headers['Authorization'] = auth
diff --git a/scrapy/tests/test_downloadermiddleware_httpauth.py b/scrapy/tests/test_downloadermiddleware_httpauth.py
index 1f58d53cb..f43401cb0 100644
--- a/scrapy/tests/test_downloadermiddleware_httpauth.py
+++ b/scrapy/tests/test_downloadermiddleware_httpauth.py
@@ -13,6 +13,7 @@ class HttpAuthMiddlewareTest(unittest.TestCase):
     def setUp(self):
         self.mw = HttpAuthMiddleware()
         self.spider = TestSpider('foo')
+        self.mw.spider_opened(self.spider)
 
     def tearDown(self):
         del self.mw