@Test
    public void timestampToConnectOptionalWithDefaultValue() {
        Schema schema = Timestamp.builder().optional().defaultValue(new java.util.Date(42)).schema();
        String msg = "{ \"schema\": { \"type\": \"int64\", \"name\": \"org.apache.kafka.connect.data.Timestamp\", \"version\": 1,  \"optional\": true, \"default\": 42 }, \"payload\": null }";
        SchemaAndValue schemaAndValue = converter.toConnectData(TOPIC, msg.getBytes());
        assertEquals(schema, schemaAndValue.schema());
        assertEquals(new java.util.Date(42), schemaAndValue.value());
    }
When schemas are disabled, fields are mapped to Connect maps.
connect/json/src/test/java/org/apache/kafka/connect/json/JsonConverterTest.java