Closes owlike#17

genson/src/main/java/com/owlike/genson/GensonBuilder.java

@@ -63,6 +63,7 @@ public class GensonBuilder {
   private Converter<Object> nullConverter;
   private DateFormat dateFormat = SimpleDateFormat.getDateInstance();
   private boolean useDateAsTimestamp;
+  private boolean classMetadataWithStaticType = true;
 
   // for the moment we don't allow to override
   private BeanViewDescriptorProvider beanViewDescriptorProvider;
@@ -605,11 +606,38 @@ public GensonBuilder useByteAsInt(boolean enable) {
     return this;
   }
 
+  /**
+   * If set to true, Genson will throw a JsonBindingException when it encounters a property in the incoming json that does not match
+   * a property in the class.
+   * False by default.
+   * @param enable
+   * @return
+   */
   public GensonBuilder failOnMissingProperty(boolean enable) {
     this.failOnMissingProperty = enable;
     return this;
   }
 
+  /**
+   * If set to false, during serialization class metadata will not be serialized for types that are the same as the static type.
+   * Ex:
+   *
+   * <pre>
+   * class Person {
+   *   public Address address;
+   * }
+   * </pre>
+   *
+   * Here if the concrete instance of address is Address then this type will not be serialized as metadata, but if they differ then
+   * it is serialized. By default this option is true, all types are serialized.
+   * @param enable
+   * @return
+   */
+  public GensonBuilder useClassMetadataWithStaticType(boolean enable) {
+    this.classMetadataWithStaticType = enable;
+    return this;
+  }
+
   /**
    * Creates an instance of Genson. You may use this method as many times you want. It wont
    * change the state of the builder, in sense that the returned instance will have always the
@@ -720,7 +748,7 @@ protected Factory<Converter<?>> createConverterFactory() {
       .withNext(new RuntimeTypeConverter.RuntimeTypeConverterFactory());
 
     chainTail = chainTail
-      .withNext(new ClassMetadataConverter.ClassMetadataConverterFactory());
+      .withNext(new ClassMetadataConverter.ClassMetadataConverterFactory(classMetadataWithStaticType));
 
     if (withBeanViewConverter) chainTail = chainTail
       .withNext(new BeanViewConverter.BeanViewConverterFactory(

genson/src/main/java/com/owlike/genson/convert/ClassMetadataConverter.java

@@ -1,6 +1,5 @@
 package com.owlike.genson.convert;
 
-import java.io.IOException;
 import java.lang.reflect.Type;
 
 import com.owlike.genson.*;
@@ -40,6 +39,12 @@
  */
 public class ClassMetadataConverter<T> extends Wrapper<Converter<T>> implements Converter<T> {
   public static class ClassMetadataConverterFactory extends ChainedFactory {
+    private final boolean classMetadataWithStaticType;
+
+    public ClassMetadataConverterFactory(boolean classMetadataWithStaticType) {
+      this.classMetadataWithStaticType = classMetadataWithStaticType;
+    }
+
     @SuppressWarnings({"unchecked", "rawtypes"})
     @Override
     protected Converter<?> create(Type type, Genson genson, Converter<?> nextConverter) {
@@ -52,21 +57,24 @@ protected Converter<?> create(Type type, Genson genson, Converter<?> nextConvert
       if (genson.isWithClassMetadata()
         && !Wrapper.toAnnotatedElement(nextConverter).isAnnotationPresent(
         HandleClassMetadata.class))
-        return new ClassMetadataConverter(rawClass, nextConverter);
+        return new ClassMetadataConverter(rawClass, nextConverter, classMetadataWithStaticType);
       else
         return nextConverter;
     }
   }
 
+  private final boolean classMetadataWithStaticType;
   private final Class<T> tClass;
 
-  public ClassMetadataConverter(Class<T> tClass, Converter<T> delegate) {
+  public ClassMetadataConverter(Class<T> tClass, Converter<T> delegate, boolean classMetadataWithStaticType) {
     super(delegate);
     this.tClass = tClass;
+    this.classMetadataWithStaticType = classMetadataWithStaticType;
   }
 
   public void serialize(T obj, ObjectWriter writer, Context ctx) throws Exception {
-    if (obj != null) {
+    if (obj != null &&
+      (classMetadataWithStaticType || (!classMetadataWithStaticType && !tClass.equals(obj.getClass())))) {
       writer.beginNextObjectMetadata()
         .writeMetadata("class", ctx.genson.aliasFor(obj.getClass()));
     }

genson/src/test/java/com/owlike/genson/functional/MetadataFeatureTest.java

@@ -36,6 +36,14 @@ public void testDeserializeToUnknownType() {
     assertTrue(bean.value instanceof Bean);
   }
 
+  @Test public void testClassMetadataShouldNotBeSerializedForStaticTypes() {
+    Genson genson = new GensonBuilder().useClassMetadata(true).useClassMetadataWithStaticType(false).create();
+
+    Bean bean = new Bean();
+
+    assertEquals("{\"value\":null}", genson.serialize(bean));
+  }
+
   static class Bean {
     Object value;
   }