initial implementation of nullable support during validation

Author: erosb

core/src/main/java/org/everit/json/schema/ValidatingVisitor.java

@@ -24,6 +24,14 @@ private static boolean isNull(Object obj) {
         this.failureReporter = new CollectingFailureReporter(schema);
     }
 
+    @Override
+    void visit(Schema schema) {
+        if (schema.isNullable() == Boolean.FALSE && isNull(subject)) {
+            failureReporter.failure("value cannot be null", "nullable");
+        }
+        super.visit(schema);
+    }
+
     ValidatingVisitor(Object subject, ValidationFailureReporter failureReporter) {
         this.subject = subject;
         this.failureReporter = failureReporter;

core/src/test/java/org/everit/json/schema/NullableValidationTest.java

@@ -0,0 +1,26 @@
+package org.everit.json.schema;
+
+import org.json.JSONObject;
+import org.junit.Test;
+
+public class NullableValidationTest {
+
+    @Test
+    public void testNullableFalse_JSONNull() {
+        TestSupport.failureOf(StringSchema.builder().requiresString(false).nullable(false))
+                .input(JSONObject.NULL)
+                .expectedKeyword("nullable")
+                .expectedMessageFragment("value cannot be null")
+                .expect();
+    }
+
+    @Test
+    public void testNullableFalse_nullReference() {
+        TestSupport.failureOf(StringSchema.builder().requiresString(false).nullable(false))
+                .input(null)
+                .expectedKeyword("nullable")
+                .expectedMessageFragment("value cannot be null")
+                .expect();
+    }
+
+}

core/src/test/java/org/everit/json/schema/StringSchemaTest.java

@@ -28,7 +28,7 @@
 import static org.junit.Assert.assertTrue;
 
 public class StringSchemaTest {
-    
+
     private static Schema loadWithNullableSupport(JSONObject rawSchemaJson) {
         return SchemaLoader.builder().nullableSupport(true).schemaJson(rawSchemaJson).build().load().build();
     }