Support null values returned for mbean attributes.

patch by Jürgen Hötzel for JMX-7

src/main/clojure/clojure/java/jmx.clj

@@ -146,7 +146,9 @@
    (into {} (zipmap (keys m) (map objects->data (vals m)))))
 
   Object
-  (objects->data [obj] obj))
+  (objects->data [obj] obj)
+  nil
+  (objects->data [_] nil))
 
 (def ^{:private true} guess-attribute-map
      {"java.lang.Integer" "int"

src/test/clojure/clojure/java/test_jmx.clj

@@ -105,14 +105,6 @@
            (jmx/invoke-signature "java.lang:type=Threading" :getThreadInfo ["long"] 1)
            (jmx/invoke-signature "java.lang:type=Threading" :getThreadInfo ["long" "int"] 1 (new java.lang.Integer 1))))
 
-(deftest test-objects->data
-  (testing "it works recursively on maps"
-           (let [some-map {:foo (jmx/raw-read "java.lang:type=Memory" :HeapMemoryUsage)}]
-             (is (map? (:foo (jmx/objects->data some-map))))))
-  (testing "it leaves everything else untouched"
-           (is (= "foo" (jmx/objects->data "foo")))))
-
-
 (deftest test-objects->data
   (let [objects (jmx/raw-read "java.lang:type=Memory" :HeapMemoryUsage)
         prox (jmx/objects->data objects)]
@@ -122,7 +114,14 @@
         props (jmx/objects->data raw-props)]
     (are [k] (contains? props k)
          :java.class.path
-         :path.separator)))
+         :path.separator))
+  (testing "it works recursively on maps"
+           (let [some-map {:foo (jmx/raw-read "java.lang:type=Memory" :HeapMemoryUsage)}]
+             (is (map? (:foo (jmx/objects->data some-map))))))
+  (testing "it handles null references"
+    (is (nil? (jmx/objects->data nil))))
+  (testing "it leaves everything else untouched"
+    (is (= "foo" (jmx/objects->data "foo")))))
 
 (deftest test-creating-attribute-infos
   (let [infos (@#'jmx/map->attribute-infos [[:a 1] [:b 2]])