@Test
    public void mapToJsonStringKeys() {
        Schema stringIntMap = SchemaBuilder.map(Schema.STRING_SCHEMA, Schema.INT32_SCHEMA).build();
        Map<String, Integer> input = new HashMap<>();
        input.put("key1", 12);
        input.put("key2", 15);
        JsonNode converted = parse(converter.fromConnectData(TOPIC, stringIntMap, input));
        validateEnvelope(converted);
        assertEquals(parse("{ \"type\": \"map\", \"keys\": { \"type\" : \"string\", \"optional\": false }, \"values\": { \"type\" : \"int32\", \"optional\": false }, \"optional\": false }"),
                converted.get(JsonSchema.ENVELOPE_SCHEMA_FIELD_NAME));
        assertEquals(JsonNodeFactory.instance.objectNode().put("key1", 12).put("key2", 15),
                converted.get(JsonSchema.ENVELOPE_PAYLOAD_FIELD_NAME));
    }
When schemas are disabled, fields are mapped to Connect maps.
connect/json/src/test/java/org/apache/kafka/connect/json/JsonConverterTest.java