Add method to SpecificationVersion to determine from schema node

README.md

@@ -377,7 +377,7 @@ The specification allows for the `$schema` keyword not to be specified, in which
 The following example creates a `SchemaRegistry` that does not specify a default dialect and will throw a `MissingSchemaKeywordException` if the schema does not specify a dialect using the `$schema` keyword.
 
 ```java
-SchemaRegistry registry = SchemaRegistry.builder().dialectRegistry(new BasicDialectRegistry(Dialects.getDraft202012())).build();
+SchemaRegistry registry = SchemaRegistry.withDefaultDialectId(null);
 ```
 
 ### Results and output formats

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

@@ -282,6 +282,26 @@ public static SchemaRegistry withDefaultDialect(SpecificationVersion specificati
      * Creates a new schema registry with a default schema dialect. The schema
      * dialect will only be used if the input does not specify a $schema.
      * <p>
+     * If the dialectId is null then the $schema is mandatory.
+     * <p>
+     * This uses a dialect registry that contains all the supported standard
+     * specification dialects, Draft 4, Draft 6, Draft 7, Draft 2019-09 and Draft
+     * 2020-12.
+     * 
+     * @param dialectId  the default dialect id used when the schema does not
+     *                   specify the $schema keyword
+     * @return the factory
+     */
+    public static SchemaRegistry withDefaultDialectId(String dialectId) {
+        return withDefaultDialectId(dialectId, null);
+    }
+
+    /**
+     * Creates a new schema registry with a default schema dialect. The schema
+     * dialect will only be used if the input does not specify a $schema.
+     * <p>
+     * If the dialectId is null then the $schema is mandatory.
+     * <p>
      * This uses a dialect registry that contains all the supported standard
      * specification dialects, Draft 4, Draft 6, Draft 7, Draft 2019-09 and Draft
      * 2020-12.

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

@@ -19,6 +19,8 @@
 
 import com.networknt.schema.dialect.DialectId;
 
+import tools.jackson.databind.JsonNode;
+
 /**
  * The version of the JSON Schema specification that defines the standard
  * dialects.
@@ -90,4 +92,20 @@ public static Optional<SpecificationVersion> fromDialectId(String dialectId) {
         }
         return Optional.empty();
     }
+
+    /**
+     * Gets the specification version that matches the dialect id indicated by
+     * $schema keyword. The dialect id is an IRI that identifies the meta schema
+     * used to validate the dialect.
+     *
+     * @param schemaNode the schema
+     * @return the specification version if it matches the dialect id
+     */
+    public static Optional<SpecificationVersion> fromSchemaNode(JsonNode schemaNode) {
+        JsonNode schema = schemaNode.get("$schema");
+        if (schema != null && schema.isString()) {
+            return fromDialectId(schema.stringValue());
+        }
+        return Optional.empty();
+    }
 }

src/test/java/com/networknt/schema/SchemaRegistryTest.java

@@ -19,7 +19,9 @@
 
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.atomic.AtomicBoolean;
 
@@ -108,6 +110,14 @@ void noDefaultDialectButSchemaSpecified() {
         });
     }
 
+    @Test
+    void noDefaultDialectWithDialectId() {
+        SchemaRegistry registry = SchemaRegistry.withDefaultDialectId(null);
+        assertThrows(MissingSchemaKeywordException.class, () -> {
+            registry.getSchema("{\"type\":\"object\"}");
+        });
+    }
+
     @Test
     void noDefaultDialectButSchemaSpecifiedButNotInRegistry() {
         SchemaRegistry registry = SchemaRegistry.builder()
@@ -116,4 +126,37 @@ void noDefaultDialectButSchemaSpecifiedButNotInRegistry() {
             registry.getSchema("{\"$schema\":\"https://json-schema.org/draft/2020-12/schema\",\"type\":\"object\"}");
         });
     }
+
+    @Test
+    void noDialectReferredByParentShouldDefaultToDefaultDialect() {
+        String schema = "{\r\n"
+                + "  \"type\": \"object\",\r\n"
+                + "  \"properties\": {\r\n"
+                + "    \"key\": {\r\n"
+                + "      \"type\": \"string\",\r\n"
+                + "      \"description\": \"The unique identifier or name (key) for the pair.\"\r\n"
+                + "    },\r\n"
+                + "    \"value\": {\r\n"
+                + "      \"type\": \"string\",\r\n"
+                + "      \"description\": \"The associated data (value) for the key.\"\r\n"
+                + "    }\r\n"
+                + "  },\r\n"
+                + "  \"required\": [\r\n"
+                + "    \"key\",\r\n"
+                + "    \"value\"\r\n"
+                + "  ],\r\n"
+                + "  \"additionalProperties\": false\r\n"
+                + "}";
+        Map<String, String> schemas = new HashMap<>();
+        schemas.put("https://example.org/schema", schema);
+        SchemaRegistry registry = SchemaRegistry.withDefaultDialect(Dialects.getDraft4(), builder -> builder.schemas(schemas));
+        Schema result = registry.getSchema("{\"$schema\":\"https://json-schema.org/draft/2020-12/schema\",\"type\":\"object\",\"$ref\":\"https://example.org/schema\"}");
+        String input = "{\r\n"
+                + "  \"key\": \"user_id\",\r\n"
+                + "  \"value\": \"123456\"\r\n"
+                + "}";
+        result.validate(input, InputFormat.JSON);
+        Schema nested = registry.getSchema(SchemaLocation.of("https://example.org/schema"));
+        assertEquals(Dialects.getDraft4(), nested.getSchemaContext().getDialect());
+    }
 }

src/test/java/com/networknt/schema/SpecificationVersionTest.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2025 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.networknt.schema;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+import tools.jackson.databind.JsonNode;
+import tools.jackson.databind.json.JsonMapper;
+
+class SpecificationVersionTest {
+    @Test
+    void fromSchemaNode() {
+        String schema = "{\"$schema\":\"https://json-schema.org/draft/2020-12/schema\",\"type\":\"object\",\"$ref\":\"https://example.org/schema\"}";
+        JsonNode schemaNode = JsonMapper.shared().readTree(schema);
+        assertEquals(SpecificationVersionDetector.detectOptionalVersion(schemaNode, false),
+                SpecificationVersion.fromSchemaNode(schemaNode));
+    }
+
+    @Test
+    void fromSchemaNodeMissing() {
+        String schema = "{\"type\":\"object\",\"$ref\":\"https://example.org/schema\"}";
+        JsonNode schemaNode = JsonMapper.shared().readTree(schema);
+        assertEquals(SpecificationVersionDetector.detectOptionalVersion(schemaNode, false),
+                SpecificationVersion.fromSchemaNode(schemaNode));
+    }
+}