diff --git a/.editorconfig b/.editorconfig
index 81752c6..2ae904c 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -21,6 +21,7 @@ charset = utf-8
 indent_size = 4
 # isort plugin configuration
 known_first_party = jsonresolver
+known_third_party = werkzeug
 multi_line_output = 2
 default_section = THIRDPARTY
 
diff --git a/AUTHORS.rst b/AUTHORS.rst
index 81dcfae..8375d28 100644
--- a/AUTHORS.rst
+++ b/AUTHORS.rst
@@ -12,3 +12,4 @@ Authors
 JSON data resolver with support for plugins.
 
 - Jiri Kuncar <jiri.kuncar@cern.ch>
+- Krzysztof Nowak <k.nowak@cern.ch>
diff --git a/jsonresolver/core.py b/jsonresolver/core.py
index cbdc983..21fa66c 100644
--- a/jsonresolver/core.py
+++ b/jsonresolver/core.py
@@ -33,8 +33,7 @@ def __init__(self, plugins=None, entry_point_group=None):
             )
         if entry_point_group:
             self.pm.load_setuptools_entrypoints(entry_point_group)
-
-        self._build_url_map()
+        self.url_map = None
 
     def _build_url_map(self):
         """Build an URL map from registered plugins."""
@@ -43,6 +42,8 @@ def _build_url_map(self):
 
     def resolve(self, url):
         """Resolve given URL and use regitered loader."""
+        if self.url_map is None:
+            self._build_url_map()
         parts = urlsplit(url)
         loader, args = self.url_map.bind(parts.hostname).match(parts.path)
         return loader(**args)
diff --git a/jsonresolver/hookspec.py b/jsonresolver/hookspec.py
index 4a7601d..5b0f687 100644
--- a/jsonresolver/hookspec.py
+++ b/jsonresolver/hookspec.py
@@ -3,9 +3,9 @@
 # This file is part of jsonresolver
 # Copyright (C) 2015 CERN.
 #
-# jsonresolver is free software; you can redistribute it and/or modify
-# it under the terms of the Revised BSD License; see LICENSE file for
-# more details.
+# jsonresolver is free software; you can redistribute it and/or
+# modify it under the terms of the Revised BSD License; see LICENSE
+# file for more details.
 
 """Define hook specification."""
 
diff --git a/run-tests.sh b/run-tests.sh
index ce114b5..307abb8 100755
--- a/run-tests.sh
+++ b/run-tests.sh
@@ -24,7 +24,7 @@
 
 
 pep257 jsonresolver && \
-# FIXME isort -rc -c -df **/*.py && \
+isort -rc -c -df **/*.py && \
 check-manifest --ignore ".travis-*" && \
 sphinx-build -qnNW docs docs/_build/html && \
 python setup.py test && \
diff --git a/tests/demo/internals.py b/tests/demo/internals.py
index 858bff8..cb4da10 100755
--- a/tests/demo/internals.py
+++ b/tests/demo/internals.py
@@ -1,11 +1,23 @@
-import jsonresolver
-import requests
+# -*- coding: utf-8 -*-
+#
+# This file is part of jsonresolver
+# Copyright (C) 2015 CERN.
+#
+# jsonresolver is free software; you can redistribute it and/or
+# modify it under the terms of the Revised BSD License; see LICENSE
+# file for more details.
+
+"""Test adding JSON resolving rule using ``werkzeug.routing.Rule`` object."""
 
+import requests
 from werkzeug.routing import Rule
 
+import jsonresolver
+
 
 @jsonresolver.hookimpl
 def jsonresolver_loader(url_map):
+    """Load the resolver plugin as a Rule to URL map."""
     def endpoint(recid):
         return requests.get(
             'https://cds.cern.ch/record/{recid}?of=recjson'.format(recid=recid)
diff --git a/tests/demo/raising.py b/tests/demo/raising.py
new file mode 100644
index 0000000..ff90555
--- /dev/null
+++ b/tests/demo/raising.py
@@ -0,0 +1,22 @@
+# -*- coding: utf-8 -*-
+#
+# This file is part of jsonresolver
+# Copyright (C) 2015 CERN.
+#
+# jsonresolver is free software; you can redistribute it and/or
+# modify it under the terms of the Revised BSD License; see LICENSE
+# file for more details.
+
+"""Test for detecting the lazy evaluation of decorated JSON resolver."""
+
+import jsonresolver
+
+
+class EndpointCallDetected(Exception):
+    """Raise this ``exception`` to detect when a plugin is called in test."""
+
+
+@jsonresolver.route('/test', host='http://localhost:4000')
+def raising():
+    """Raise an exception."""
+    raise EndpointCallDetected
diff --git a/tests/demo/raising_hook.py b/tests/demo/raising_hook.py
new file mode 100644
index 0000000..c36d5eb
--- /dev/null
+++ b/tests/demo/raising_hook.py
@@ -0,0 +1,22 @@
+# -*- coding: utf-8 -*-
+#
+# This file is part of jsonresolver
+# Copyright (C) 2015 CERN.
+#
+# jsonresolver is free software; you can redistribute it and/or
+# modify it under the terms of the Revised BSD License; see LICENSE
+# file for more details.
+
+"""Test for detecting the lazy evaluation of hook-registered JSON resolver."""
+
+import jsonresolver
+
+
+class HookRegistrationDetected(Exception):
+    """Raise this ``exception`` to detect when a hook is registered."""
+
+
+@jsonresolver.hookimpl
+def jsonresolver_loader(url_map):
+    """Load the raising plugin as a Rule to URL map."""
+    raise HookRegistrationDetected
diff --git a/tests/demo/schema.py b/tests/demo/schema.py
index b81cc83..fe250b3 100644
--- a/tests/demo/schema.py
+++ b/tests/demo/schema.py
@@ -7,10 +7,13 @@
 # modify it under the terms of the Revised BSD License; see LICENSE
 # file for more details.
 
+"""Test plugin for JSON schema resolving using decorator."""
+
 import jsonresolver
 
 
 @jsonresolver.route('/schema/<path:name>', host='http://localhost:4000')
 def schema(name):
+    """Return a fixed JSON ``schema``."""
     assert name == 'authors.json'
     return {'type': 'array'}
diff --git a/tests/demo/simple.py b/tests/demo/simple.py
index 0577cbf..0e583e9 100755
--- a/tests/demo/simple.py
+++ b/tests/demo/simple.py
@@ -7,9 +7,12 @@
 # modify it under the terms of the Revised BSD License; see LICENSE
 # file for more details.
 
+"""Test plugin for JSON resolving using ``jsonresolver.route`` decorator."""
+
 import jsonresolver
 
 
 @jsonresolver.route('/test', host='http://localhost:4000')
 def simple():
+    """Return a fixed JSON."""
     return {'test': 'test'}
diff --git a/tests/test_core.py b/tests/test_core.py
index 6693adf..b6c1511 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -14,6 +14,8 @@
 import importlib
 
 import pytest
+from demo.raising import EndpointCallDetected
+from demo.raising_hook import HookRegistrationDetected
 from mock import patch
 from pkg_resources import EntryPoint
 
@@ -21,7 +23,10 @@
 
 
 class MockEntryPoint(EntryPoint):
+    """EntryPoint mock."""
+
     def load(self):
+        """Load module from the mocked entry point."""
         if self.name == 'importfail':
             raise ImportError()
         else:
@@ -29,8 +34,12 @@ def load(self):
 
 
 def _mock_entry_points(name):
+    """Mock the entry points."""
     data = {
         'espresso': [MockEntryPoint('demo.simple', 'demo.simple')],
+        'raising': [MockEntryPoint('demo.raising', 'demo.raising')],
+        'raising_hook': [MockEntryPoint('demo.raising_hook',
+                                        'demo.raising_hook')],
         'someotherstuff': [],
         'doubletrouble': [MockEntryPoint('demo.simple', 'demo.simple'),
                           MockEntryPoint('demo.simple', 'demo.simple')],
@@ -45,10 +54,44 @@ def _mock_entry_points(name):
 
 @patch('pkg_resources.iter_entry_points', _mock_entry_points)
 def test_entry_point_group():
+    """Test the entry point group loading."""
     resolver = JSONResolver(entry_point_group='espresso')
     assert resolver.resolve('http://localhost:4000/test') == {'test': 'test'}
 
 
 def test_plugins():
+    """Test the plugins loading."""
     resolver = JSONResolver(plugins=['demo.simple'])
     assert resolver.resolve('http://localhost:4000/test') == {'test': 'test'}
+
+
+@patch('pkg_resources.iter_entry_points', _mock_entry_points)
+def test_plugin_lazy_execution():
+    """Test the lazy evaluation of loaded plugin."""
+    # Plugin code should be called (i.e. raise exception) on resolve method
+    resolver = JSONResolver(plugins=['demo.raising'])
+    with pytest.raises(EndpointCallDetected) as exc_info:
+        resolver.resolve('http://localhost:4000/test')
+    assert exc_info.type is EndpointCallDetected
+
+    # Same as above but loaded using entry point
+    resolver = JSONResolver(entry_point_group='raising')
+    with pytest.raises(EndpointCallDetected) as exc_info:
+        resolver.resolve('http://localhost:4000/test')
+    assert exc_info.type is EndpointCallDetected
+
+
+@patch('pkg_resources.iter_entry_points', _mock_entry_points)
+def test_plugin_lazy_execution_hooks():
+    """Test the lazy evaluation of loaded plugin through hooks."""
+    # Plugin code should be called (i.e. raise exception) on resolve method
+    resolver = JSONResolver(plugins=['demo.raising_hook'])
+    with pytest.raises(HookRegistrationDetected) as exc_info:
+        resolver.resolve('http://localhost:4000/test')
+    assert exc_info.type is HookRegistrationDetected
+
+    # Same as above but loaded using entry point
+    resolver = JSONResolver(entry_point_group='raising_hook')
+    with pytest.raises(HookRegistrationDetected) as exc_info:
+        resolver.resolve('http://localhost:4000/test')
+    assert exc_info.type is HookRegistrationDetected
diff --git a/tests/test_jsonref.py b/tests/test_jsonref.py
index ef10192..dde8de8 100644
--- a/tests/test_jsonref.py
+++ b/tests/test_jsonref.py
@@ -11,8 +11,8 @@
 
 from __future__ import absolute_import
 
-from jsonref import JsonRef, JsonRefError
 import pytest
+from jsonref import JsonRef, JsonRefError
 
 from jsonresolver import JSONResolver
 from jsonresolver.contrib.jsonref import json_loader_factory
diff --git a/tests/test_jsonschema.py b/tests/test_jsonschema.py
index 3ff7d39..f1572f6 100644
--- a/tests/test_jsonschema.py
+++ b/tests/test_jsonschema.py
@@ -11,8 +11,8 @@
 
 from __future__ import absolute_import
 
-from jsonschema import RefResolutionError, validate
 import pytest
+from jsonschema import RefResolutionError, validate
 
 from jsonresolver import JSONResolver
 from jsonresolver.contrib.jsonschema import ref_resolver_factory