Support generics in the JsonToObjectTransformer (#2819)

* Support generics in the `JsonToObjectTransformer`

* Deprecate `JsonObjectMapperAdapter` if favor of `default` methods in
the `JsonObjectMapper`
* Introduce `JsonObjectMapper.fromJson(Object, ResolvableType)` to
support generics during deserialization
* Add `JsonHeaders.RESOLVABLE_TYPE` header handling for the
`ResolvableType` management
* Add `ResolvableType` argument into the `JsonToObjectTransformer`
* Change the `JsonToObjectTransformer` logic to consult request message
headers first
* Add `ResolvableType`-based factory method into the `Transformers`
* Document the change

* * Use `0` for conversion to array for headers to remove
in the `JsonToObjectTransformer`

spring-integration-core/src/test/java/org/springframework/integration/json/JsonToObjectTransformerParserTests.java

@@ -23,11 +23,11 @@ import org.junit.runner.RunWith;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Qualifier;
+import org.springframework.core.ResolvableType;
 import org.springframework.integration.channel.QueueChannel;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.integration.support.json.Jackson2JsonObjectMapper;
 import org.springframework.integration.support.json.JsonObjectMapper;
-import org.springframework.integration.support.json.JsonObjectMapperAdapter;
 import org.springframework.integration.test.util.TestUtils;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
@@ -104,12 +104,18 @@ public class JsonToObjectTransformerParserTests {
 		assertThat(result.getJson()).isEqualTo(jsonString);
 	}
 
-	@SuppressWarnings("rawtypes")
-	static class CustomJsonObjectMapper extends JsonObjectMapperAdapter {
+	static class CustomJsonObjectMapper implements JsonObjectMapper<Object, Object> {
 
 		@Override
-		public Object fromJson(Object json, Class valueType) {
-			return new TestJsonContainer((String) json);
+		@SuppressWarnings("unchecked")
+		public <T> T fromJson(Object json, Class<T> valueType) {
+			return (T) new TestJsonContainer((String) json);
+		}
+
+		@Override
+		@SuppressWarnings("unchecked")
+		public <T> T fromJson(Object json, ResolvableType valueType) {
+			return (T) new TestJsonContainer((String) json);
 		}
 
 	}

spring-integration-core/src/test/java/org/springframework/integration/json/JsonToObjectTransformerTests.java

@@ -18,8 +18,12 @@ package org.springframework.integration.json;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
+import java.util.List;
+
 import org.junit.Test;
 
+import org.springframework.core.ParameterizedTypeReference;
+import org.springframework.core.ResolvableType;
 import org.springframework.integration.support.json.Jackson2JsonObjectMapper;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.support.GenericMessage;
@@ -31,24 +35,36 @@ import com.fasterxml.jackson.databind.ObjectMapper;
  * @author Mark Fisher
  * @author Artem Bilan
  * @author Gary Russell
+ *
  * @since 2.0
  */
 public class JsonToObjectTransformerTests {
 
 	@Test
 	public void objectPayload() {
-		JsonToObjectTransformer transformer = new JsonToObjectTransformer(TestPerson.class);
+		JsonToObjectTransformer transformer =
+				new JsonToObjectTransformer(
+						ResolvableType.forType(new ParameterizedTypeReference<List<TestPerson>>() { }));
 		// Since DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES is disabled by default
 		// (see Jackson2JsonObjectMapper)
 		// the extra "foo" property is ignored.
-		String jsonString = "{\"firstName\":\"John\",\"lastName\":\"Doe\",\"age\":42," +
-				"\"address\":{\"number\":123,\"street\":\"Main Street\"}, \"foo\":\"bar\"}";
+		String jsonString = "[{\"firstName\":\"John\",\"lastName\":\"Doe\",\"age\":42," +
+				"\"address\":{\"number\":123,\"street\":\"Main Street\"}, \"foo\":\"bar\"}]";
 		Message<?> message = transformer.transform(new GenericMessage<>(jsonString));
-		TestPerson person = (TestPerson) message.getPayload();
-		assertThat(person.getFirstName()).isEqualTo("John");
-		assertThat(person.getLastName()).isEqualTo("Doe");
-		assertThat(person.getAge()).isEqualTo(42);
-		assertThat(person.getAddress().toString()).isEqualTo("123 Main Street");
+		assertThat(message)
+				.extracting(Message::getPayload)
+				.isInstanceOf(List.class)
+				.asList()
+				.hasSize(1)
+				.element(0)
+				.isInstanceOf(TestPerson.class)
+				.satisfies((actual) -> {
+					TestPerson bean = (TestPerson) actual;
+					assertThat(bean).extracting(TestPerson::getFirstName).isEqualTo("John");
+					assertThat(bean).extracting(TestPerson::getLastName).isEqualTo("Doe");
+					assertThat(bean).extracting(TestPerson::getAge).isEqualTo(42);
+					assertThat(bean).extracting(TestPerson::getAddress).asString().isEqualTo("123 Main Street");
+				});
 	}
 
 	@Test

spring-integration-core/src/test/java/org/springframework/integration/json/ObjectToJsonTransformerParserTests.java

@@ -33,7 +33,7 @@ import org.springframework.integration.channel.QueueChannel;
 import org.springframework.integration.support.DefaultMessageBuilderFactory;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.integration.support.json.Jackson2JsonObjectMapper;
-import org.springframework.integration.support.json.JsonObjectMapperAdapter;
+import org.springframework.integration.support.json.JsonObjectMapper;
 import org.springframework.integration.test.util.TestUtils;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
@@ -174,7 +174,7 @@ public class ObjectToJsonTransformerParserTests {
 		assertThat(expression.getValue(evaluationContext, payload, Boolean.class)).isTrue();
 	}
 
-	static class CustomJsonObjectMapper extends JsonObjectMapperAdapter<Object, Object> {
+	static class CustomJsonObjectMapper implements JsonObjectMapper<Object, Object> {
 
 		@Override
 		public String toJson(Object value) {

spring-integration-core/src/test/java/org/springframework/integration/json/ObjectToJsonTransformerTests.java

@@ -183,11 +183,11 @@ public class ObjectToJsonTransformerTests {
 		List<String> list = Collections.singletonList(null);
 		Message<?> out = transformer.transform(new GenericMessage<>(list));
 		assertThat(out.getHeaders().get(JsonHeaders.TYPE_ID).toString()).contains("SingletonList");
-		assertThat(out.getHeaders().get(JsonHeaders.CONTENT_TYPE_ID)).isEqualTo(Object.class);
+		assertThat(out.getHeaders()).doesNotContainKey(JsonHeaders.CONTENT_TYPE_ID);
 		Map<String, String> map = Collections.singletonMap("foo", null);
 		out = transformer.transform(new GenericMessage<>(map));
 		assertThat(out.getHeaders().get(JsonHeaders.TYPE_ID).toString()).contains("SingletonMap");
-		assertThat(out.getHeaders().get(JsonHeaders.CONTENT_TYPE_ID)).isEqualTo(Object.class);
+		assertThat(out.getHeaders()).doesNotContainKey(JsonHeaders.CONTENT_TYPE_ID);
 		assertThat(out.getHeaders().get(JsonHeaders.KEY_TYPE_ID)).isEqualTo(String.class);
 	}