fix #60, Add Nullean value type.

Author: cowtowncoder

release-notes/VERSION

@@ -15,6 +15,8 @@ NOTE: Annotations module will never contain changes in patch versions,
 
 #56: Improve `ObjectIdGenerators.key()` to handle `null` appropriately by returning `null`
 #58: Add new properties for `@JsonIgnoreProperties`, "allowGetters", "allowSetters" 
+#60: Add new value type, `Nullean`, for "nullable booleans", to support proper handling
+  and usage of default values, not just explicit true/false.
 - Add `JsonInclude.Include.NON_ABSENT` value, for excluding "absent" Optional values.
 
 2.5.0 (01-Jan-2015)

src/main/java/com/fasterxml/jackson/annotation/Nullean.java

@@ -0,0 +1,38 @@
+package com.fasterxml.jackson.annotation;
+
+/**
+ * Nullable Boolean value, "nullean". Needed just because Java annotations
+ * can not take 'null' as a value (even as default), so there is no
+ * way to distinguish between explicit `true` and `false`, and lack of
+ * choice (related: annotations are limited to primitives, so
+ * {@link java.lang.Boolean} not allowed as solution).
+ *<p>
+ * Note: although use of `true` and `false` would be more convenient, they
+ * can not be chosen since they are Java keyword and compiler won't allow
+ * the choice. And since enum naming convention suggests all-upper-case,
+ * that is what is done here.
+ *
+ * @since 2.6
+ */
+public enum Nullean
+{
+    /**
+     * Value that indicates that the annotation property is explicitly defined to
+     * be enabled, or true.
+     */
+    TRUE,
+    
+    /**
+     * Value that indicates that the annotation property is explicitly defined to
+     * be disabled, or false.
+     */
+    FALSE,
+
+    /**
+     * Value that indicates that the annotation property does NOT have an explicit
+     * definition of enabled/disabled (or true/false); instead, a higher-level
+     * configuration value is used; or lacking higher-level global setting,
+     * default.
+     */
+    DEFAULT;
+}