fix #21 Use of "id" field does not work for field reference

src/main/java/com/networknt/schema/BaseJsonValidator.java

@@ -20,12 +20,15 @@
 import org.apache.commons.lang.StringUtils;
 import org.slf4j.Logger;
 
+import java.net.MalformedURLException;
+import java.net.URL;
 import java.util.Set;
 
 public abstract class BaseJsonValidator implements JsonValidator {
     private String schemaPath;
     private JsonNode schemaNode;
     private JsonSchema parentSchema;
+    private JsonSchema subSchema;
     private ValidatorTypeCode validatorType;
     private String errorCode;
 
@@ -35,6 +38,7 @@ public BaseJsonValidator(String schemaPath, JsonNode schemaNode, JsonSchema pare
         this.schemaNode = schemaNode;
         this.parentSchema = parentSchema;
         this.validatorType = validatorType;
+        this.subSchema = obainSubSchemaNode(schemaNode);
     }
 
     protected String getSchemaPath() {
@@ -48,6 +52,28 @@ protected JsonNode getSchemaNode() {
     protected JsonSchema getParentSchema() {
         return parentSchema;
     }
+
+    protected JsonSchema getSubSchema() {
+        return subSchema;
+    }
+
+    protected boolean hasSubSchema() {
+        return subSchema != null;
+    }
+
+    protected JsonSchema obainSubSchemaNode(JsonNode schemaNode){
+        JsonNode node = schemaNode.get("id");
+        if(node == null) return null;
+    	if(node.equals(schemaNode.get("$schema"))) return null;
+
+        try {
+            JsonSchemaFactory factory = new JsonSchemaFactory();
+            URL url = new URL(node.textValue());
+            return factory.getSchema(url);
+        } catch (MalformedURLException e) {
+            return null;
+        }
+    }
 
     public Set<ValidationMessage> validate(JsonNode node) {
         return validate(node, node, AT_ROOT);

src/main/java/com/networknt/schema/JsonSchema.java

@@ -78,7 +78,10 @@ public JsonNode getRefSchemaNode(String ref) {
                 } else {
                     node = node.get(key);
                 }
-                if (node == null) {
+                if (node == null && schema.hasSubSchema()){
+                    node = schema.getSubSchema().getRefSchemaNode(ref);
+                }
+                if (node == null){
                     break;
                 }
             }