Fix #1799

Author: cowtowncoder

release-notes/VERSION

@@ -6,6 +6,7 @@ Project: jackson-databind
 2.9.3 (not yet released)
 
 #1604: Nested type arguments doesn't work with polymorphic types
+#1799: Allow creation of custom sub-types of `NullNode`, `BooleanNode`, `MissingNode`
 #1804: `ValueInstantiator.canInstantiate()` ignores `canCreateUsingArrayDelegate()`
  (reported byb henryptung@github)
 

src/main/java/com/fasterxml/jackson/databind/JsonNode.java

@@ -99,18 +99,18 @@ public final boolean isContainerNode() {
     }
 
     @Override
-    public final boolean isMissingNode() {
-        return getNodeType() == JsonNodeType.MISSING;
+    public boolean isMissingNode() {
+        return false;
     }
 
     @Override
-    public final boolean isArray() {
-        return getNodeType() == JsonNodeType.ARRAY;
+    public boolean isArray() {
+        return false;
     }
 
     @Override
-    public final boolean isObject() {
-        return getNodeType() == JsonNodeType.OBJECT;
+    public boolean isObject() {
+        return false;
     }
 
     /**

src/main/java/com/fasterxml/jackson/databind/node/ArrayNode.java

@@ -87,6 +87,11 @@ public JsonNodeType getNodeType() {
         return JsonNodeType.ARRAY;
     }
 
+    @Override
+    public boolean isArray() {
+        return true;
+    }
+
     @Override public JsonToken asToken() { return JsonToken.START_ARRAY; }
 
     @Override

src/main/java/com/fasterxml/jackson/databind/node/BooleanNode.java

@@ -20,8 +20,12 @@ public class BooleanNode
     public final static BooleanNode FALSE = new BooleanNode(false);
 
     private final boolean _value;
-    
-    private BooleanNode(boolean v) { _value = v; }
+
+    /**
+     *<p>
+     * NOTE: visibility raised to `protected` in 2.9.3 to allow custom subtypes.
+     */
+    protected BooleanNode(boolean v) { _value = v; }
 
     public static BooleanNode getTrue() { return TRUE; }
     public static BooleanNode getFalse() { return FALSE; }

src/main/java/com/fasterxml/jackson/databind/node/MissingNode.java

@@ -24,7 +24,16 @@ public final class MissingNode
 {
     private final static MissingNode instance = new MissingNode();
 
-    private MissingNode() { }
+    /**
+     *<p>
+     * NOTE: visibility raised to `protected` in 2.9.3 to allow custom subtypes.
+     */
+    protected MissingNode() { }
+
+    @Override
+    public boolean isMissingNode() {
+        return true;
+    }
 
     // Immutable: no need to copy
     @SuppressWarnings("unchecked")

src/main/java/com/fasterxml/jackson/databind/node/NullNode.java

@@ -17,7 +17,11 @@ public final class NullNode
 
     public final static NullNode instance = new NullNode();
 
-    private NullNode() { }
+    /**
+     *<p>
+     * NOTE: visibility raised to `protected` in 2.9.3 to allow custom subtypes.
+     */
+    protected NullNode() { }
 
     public static NullNode getInstance() { return instance; }
 

src/main/java/com/fasterxml/jackson/databind/node/ObjectNode.java

@@ -78,6 +78,11 @@ public JsonNodeType getNodeType() {
         return JsonNodeType.OBJECT;
     }
 
+    @Override
+    public final boolean isObject() {
+        return true;
+    }
+    
     @Override public JsonToken asToken() { return JsonToken.START_OBJECT; }
 
     @Override