SchemaLoader now handles nullable schema attribute

 * added `nullable` attribute to the `Schema` class as a Boolean, where
    - Boolean.TRUE means explicit nullability
    - Boolean.FALSE means explicit non-nullability
    - null should probably mean implicit non-nullability
 * adding `nullableSupport` as a feature flag to `SchemaLoader` class
 * the "nullable" flag from the schema JSON is mapped to `Schema#nullable`
	if and only if the `nullableSupport` flag is on.
	Tests added verifying it.

Author: erosb

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

@@ -31,6 +31,8 @@ public abstract static class Builder<S extends Schema> {
 
         private Object defaultValue;
 
+        private Boolean nullable = null;
+
         public Builder<S> title(String title) {
             this.title = title;
             return this;
@@ -56,10 +58,15 @@ public Builder<S> defaultValue(Object defaultValue) {
             return this;
         }
 
+        public Builder<S> nullable(Boolean nullable) {
+            this.nullable = nullable;
+            return this;
+        }
+
         public abstract S build();
 
-    }
 
+    }
     private final String title;
 
     private final String description;
@@ -70,6 +77,8 @@ public Builder<S> defaultValue(Object defaultValue) {
 
     private final Object defaultValue;
 
+    private final Boolean nullable;
+
     /**
      * Constructor.
      *
@@ -82,6 +91,7 @@ protected Schema(Builder<?> builder) {
         this.id = builder.id;
         this.schemaLocation = builder.schemaLocation;
         this.defaultValue = builder.defaultValue;
+        this.nullable = builder.nullable;
     }
 
     /**
@@ -189,6 +199,9 @@ public boolean hasDefaultValue() {
         return this.defaultValue != null;
     }
 
+    public Boolean isNullable() {
+        return nullable;
+    }
     /**
      * Describes the instance as a JSONObject to {@code writer}.
      * <p>

core/src/main/java/org/everit/json/schema/loader/LoaderConfig.java

@@ -25,12 +25,20 @@ static LoaderConfig defaultV4Config() {
 
     final boolean useDefaults;
 
+    final boolean nullableSupport;
+
     LoaderConfig(SchemaClient httpClient, Map<String, FormatValidator> formatValidators,
             SpecificationVersion specVersion, boolean useDefaults) {
+        this(httpClient, formatValidators, specVersion, useDefaults, false);
+    }
+
+    LoaderConfig(SchemaClient httpClient, Map<String, FormatValidator> formatValidators,
+            SpecificationVersion specVersion, boolean useDefaults, boolean nullableSupport) {
         this.httpClient = requireNonNull(httpClient, "httpClient cannot be null");
         this.formatValidators = requireNonNull(formatValidators, "formatValidators cannot be null");
         this.specVersion = requireNonNull(specVersion, "specVersion cannot be null");
         this.useDefaults = useDefaults;
+        this.nullableSupport = nullableSupport;
     }
 
 }

core/src/main/java/org/everit/json/schema/loader/SchemaLoader.java

@@ -71,6 +71,8 @@ public static class SchemaLoaderBuilder {
 
         boolean useDefaults = false;
 
+        private boolean nullableSupport = false;
+
         /**
          * Registers a format validator with the name returned by {@link FormatValidator#formatName()}.
          *
@@ -188,6 +190,10 @@ public SchemaLoaderBuilder useDefaults(boolean useDefaults) {
             return this;
         }
 
+        public SchemaLoaderBuilder nullableSupport(boolean nullableSupport) {
+            this.nullableSupport = nullableSupport;
+            return this;
+        }
     }
 
     private static final List<String> NUMBER_SCHEMA_PROPS = asList("minimum", "maximum",
@@ -266,7 +272,11 @@ public SchemaLoader(SchemaLoaderBuilder builder) {
                 specVersion = SpecificationVersion.getByMetaSchemaUrl((String) schemaValue);
             }
         }
-        this.config = new LoaderConfig(builder.httpClient, builder.formatValidators, specVersion, builder.useDefaults);
+        this.config = new LoaderConfig(builder.httpClient,
+                builder.formatValidators,
+                specVersion,
+                builder.useDefaults,
+                builder.nullableSupport);
         this.ls = new LoadingState(config,
                 builder.pointerSchemas,
                 builder.rootSchemaJson == null ? builder.schemaJson : builder.rootSchemaJson,
@@ -362,6 +372,9 @@ private Schema.Builder loadSchemaObject(JsonObject o) {
         ls.schemaJson().maybe(config.specVersion.idKeyword()).map(JsonValue::requireString).ifPresent(builder::id);
         ls.schemaJson().maybe("title").map(JsonValue::requireString).ifPresent(builder::title);
         ls.schemaJson().maybe("description").map(JsonValue::requireString).ifPresent(builder::description);
+        if (config.nullableSupport) {
+            ls.schemaJson().maybe("nullable").map(JsonValue::requireBoolean).ifPresent(builder::nullable);
+        }
         if (config.useDefaults) {
             ls.schemaJson().maybe("default").map(JsonValue::deepToOrgJson).ifPresent(builder::defaultValue);
         }

core/src/test/java/org/everit/json/schema/loader/SchemaLoaderTest.java

@@ -9,6 +9,7 @@
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
@@ -353,7 +354,7 @@ public void stringSchemaWithFormat() {
     public void tupleSchema() {
         ArraySchema actual = (ArraySchema) SchemaLoader.load(get("tupleSchema"));
         assertFalse(actual.permitsAdditionalItems());
-        Assert.assertNull(actual.getAllItemSchema());
+        assertNull(actual.getAllItemSchema());
         assertEquals(2, actual.getItemSchemas().size());
         assertEquals(BooleanSchema.INSTANCE, actual.getItemSchemas().get(0));
         assertEquals(NullSchema.INSTANCE, actual.getItemSchemas().get(1));
@@ -579,4 +580,28 @@ public void applyDefaultNullTest() {
 
         assertEquals(JSONObject.NULL, obj.get("nullDefault"));
     }
+
+    @Test
+    public void nullableBooleansAre_Loaded_withNullableSupport() {
+        SchemaLoader loader = SchemaLoader.builder()
+                .nullableSupport(true)
+                .schemaJson(get("nullableSupport"))
+                .build();
+        ObjectSchema actual = (ObjectSchema) loader.load().build();
+        Schema nullableSchema = actual.getPropertySchemas().get("isNullable");
+        Schema nonNulableSchema = actual.getPropertySchemas().get("nonNullable");
+
+        assertTrue(nullableSchema.isNullable());
+        assertFalse(nonNulableSchema.isNullable());
+    }
+
+    @Test
+    public void nullableBooleansAre_NotLoaded_withoutNullableSupport() {
+        ObjectSchema actual = (ObjectSchema) SchemaLoader.load(get("nullableSupport"));
+        Schema nullableSchema = actual.getPropertySchemas().get("isNullable");
+        Schema nonNulableSchema = actual.getPropertySchemas().get("nonNullable");
+
+        assertNull(nullableSchema.isNullable());
+        assertNull(nonNulableSchema.isNullable());
+    }
 }

core/src/test/resources/org/everit/jsonvalidator/testschemas.json

@@ -587,5 +587,17 @@
         "default": null
       }
     }
+  },
+  "nullableSupport": {
+    "properties": {
+      "nonNullable": {
+        "type": "number",
+        "nullable": false
+      },
+      "isNullable": {
+        "type": "string",
+        "nullable": true
+      }
+    }
   }
 }