INT-4045: Fix NPE in ObjectToJsonTransformer

JIRA: https://jira.spring.io/browse/INT-4045

Handle the situation when the first element of a container type (or map)
is null; set the generic type of the container to `Object`.

Also, clarify how the compatibility between the transformer and Spring AMQP
message converter is achieved.

spring-integration-core/src/main/java/org/springframework/integration/json/JsonToObjectTransformer.java

@@ -19,17 +19,23 @@ package org.springframework.integration.json;
 import org.springframework.beans.factory.BeanClassLoaderAware;
 import org.springframework.integration.mapping.support.JsonHeaders;
 import org.springframework.integration.support.AbstractIntegrationMessageBuilder;
-import org.springframework.integration.support.json.JsonObjectMapperProvider;
 import org.springframework.integration.support.json.JsonObjectMapper;
+import org.springframework.integration.support.json.JsonObjectMapperProvider;
 import org.springframework.integration.transformer.AbstractTransformer;
 import org.springframework.messaging.Message;
 
 /**
- * Transformer implementation that converts a JSON string payload into an instance of the provided target Class.
- * By default this transformer uses {@linkplain org.springframework.integration.support.json.JsonObjectMapperProvider} factory
- * to get an instance of Jackson 1 or Jackson 2 JSON-processor {@linkplain JsonObjectMapper} implementation
- * depending on the jackson-databind or jackson-mapper-asl libs on the classpath.
- * Any other {@linkplain JsonObjectMapper} implementation can be provided.
+ * Transformer implementation that converts a JSON string payload into an instance of the
+ * provided target Class. By default this transformer uses
+ * {@linkplain org.springframework.integration.support.json.JsonObjectMapperProvider}
+ * factory to get an instance of Jackson 1 or Jackson 2 JSON-processor
+ * {@linkplain JsonObjectMapper} implementation depending on the jackson-databind or
+ * jackson-mapper-asl libs on the classpath. Any other {@linkplain JsonObjectMapper}
+ * implementation can be provided.
+ * <p>Since version 3.0, you can omit the target class and the target type can be
+ * determined by the {@link JsonHeaders} type entries - including the contents of a
+ * one-level container or map type.
+ * <p>The type headers can be classes or fully-qualified class names.
  *
  * @author Mark Fisher
  * @author Artem Bilan

spring-integration-core/src/main/java/org/springframework/integration/json/ObjectToJsonTransformer.java

@@ -27,11 +27,25 @@ import org.springframework.util.LinkedCaseInsensitiveMap;
 import org.springframework.util.StringUtils;
 
 /**
- * Transformer implementation that converts a payload instance into a JSON string representation.
- * By default this transformer uses {@linkplain org.springframework.integration.support.json.JsonObjectMapperProvider} factory
- * to get an instance of a Jackson or Jackson 2 JSON-processor {@linkplain JsonObjectMapper} implementation
- * depending on the jackson-databind or jackson-mapper-asl libs on the classpath.
- * Any other {@linkplain JsonObjectMapper} implementation can be provided.
+ * Transformer implementation that converts a payload instance into a JSON string
+ * representation. By default this transformer uses
+ * {@linkplain org.springframework.integration.support.json.JsonObjectMapperProvider}
+ * factory to get an instance of a Jackson or Jackson 2 JSON-processor
+ * {@linkplain JsonObjectMapper} implementation depending on the jackson-databind or
+ * jackson-mapper-asl libs on the classpath. Any other {@linkplain JsonObjectMapper}
+ * implementation can be provided.
+ * <p>Since version 3.0, adds headers to represent the object types that were mapped
+ * from (including one level of container and Map content types). These headers
+ * are compatible with the Spring AMQP Json type mapper such that messages mapped
+ * or converted by either technology are compatible. One difference, however, is the
+ * Spring AMQP converter, when converting to JSON, sets the header types to the class
+ * name. This transformer sets the header types to the class itself.
+ * <p>The compatibility is achieved because, when mapping the Spring Integration
+ * message in the outbound endpoint (via the {@code DefaultAmqpHeaderMapper}), the
+ * classes are converted to String at that time.
+ * <p>Note: the first element of container/map types are used to determine the
+ * container/map content types. If the first element is null, the type is set to
+ * {@link Object}.
  *
  * @author Mark Fisher
  * @author James Carr

spring-integration-core/src/main/java/org/springframework/integration/support/json/Jackson2JsonObjectMapper.java

@@ -107,7 +107,7 @@ public class Jackson2JsonObjectMapper extends AbstractJacksonJsonObjectMapper<Js
 
 	@Override
 	public <T> T fromJson(JsonParser parser, Type valueType) throws Exception {
-		return this.objectMapper.readValue(parser, this.constructType(valueType));
+		return this.objectMapper.readValue(parser, constructType(valueType));
 	}
 
 	@Override

spring-integration-core/src/main/java/org/springframework/integration/support/json/JsonObjectMapperAdapter.java

@@ -66,11 +66,14 @@ public abstract class JsonObjectMapperAdapter<N, P> implements JsonObjectMapper<
 	public void populateJavaTypes(Map<String, Object> map, Object object) {
 		map.put(JsonHeaders.TYPE_ID, object.getClass());
 		if (object instanceof Collection && !((Collection<?>) object).isEmpty()) {
-			map.put(JsonHeaders.CONTENT_TYPE_ID, ((Collection<?>) object).iterator().next().getClass());
+			Object firstElement = ((Collection<?>) object).iterator().next();
+			map.put(JsonHeaders.CONTENT_TYPE_ID, firstElement != null ? firstElement.getClass() : Object.class);
 		}
 		if (object instanceof Map && !((Map<?, ?>) object).isEmpty()) {
-			map.put(JsonHeaders.CONTENT_TYPE_ID, ((Map<?, ?>) object).values().iterator().next().getClass());
-			map.put(JsonHeaders.KEY_TYPE_ID, ((Map<?, ?>) object).keySet().iterator().next().getClass());
+			Object firstValue = ((Map<?, ?>) object).values().iterator().next();
+			map.put(JsonHeaders.CONTENT_TYPE_ID, firstValue != null ? firstValue.getClass() : Object.class);
+			Object firstKey = ((Map<?, ?>) object).keySet().iterator().next();
+			map.put(JsonHeaders.KEY_TYPE_ID, firstKey != null ? firstKey.getClass() : Object.class);
 		}
 	}
 

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

@@ -16,12 +16,16 @@
 
 package org.springframework.integration.json;
 
+import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
 
+import java.util.Collections;
+import java.util.List;
 import java.util.Map;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -32,6 +36,7 @@ import org.springframework.context.expression.MapAccessor;
 import org.springframework.expression.Expression;
 import org.springframework.expression.spel.standard.SpelExpressionParser;
 import org.springframework.expression.spel.support.StandardEvaluationContext;
+import org.springframework.integration.mapping.support.JsonHeaders;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.integration.support.json.BoonJsonObjectMapper;
 import org.springframework.integration.support.json.Jackson2JsonObjectMapper;
@@ -150,6 +155,34 @@ public class ObjectToJsonTransformerTests {
 		assertTrue(addressResult.contains("street:\"Main Street\""));
 	}
 
+	@Test
+	public void collectionOrMap() {
+		ObjectToJsonTransformer transformer = new ObjectToJsonTransformer();
+		List<String> list = Collections.singletonList("foo");
+		Message<?> out = transformer.transform(new GenericMessage<>(list));
+		assertThat(out.getHeaders().get(JsonHeaders.TYPE_ID).toString(), containsString("SingletonList"));
+		assertThat(out.getHeaders().get(JsonHeaders.CONTENT_TYPE_ID), equalTo(String.class));
+		Map<String, Long> map = Collections.singletonMap("foo", 1L);
+		out = transformer.transform(new GenericMessage<>(map));
+		assertThat(out.getHeaders().get(JsonHeaders.TYPE_ID).toString(), containsString("SingletonMap"));
+		assertThat(out.getHeaders().get(JsonHeaders.CONTENT_TYPE_ID), equalTo(Long.class));
+		assertThat(out.getHeaders().get(JsonHeaders.KEY_TYPE_ID), equalTo(String.class));
+	}
+
+	@Test
+	public void collectionOrMapWithNullFirstElement() {
+		ObjectToJsonTransformer transformer = new ObjectToJsonTransformer();
+		List<String> list = Collections.singletonList(null);
+		Message<?> out = transformer.transform(new GenericMessage<>(list));
+		assertThat(out.getHeaders().get(JsonHeaders.TYPE_ID).toString(), containsString("SingletonList"));
+		assertThat(out.getHeaders().get(JsonHeaders.CONTENT_TYPE_ID), equalTo(Object.class));
+		Map<String, String> map = Collections.singletonMap("foo", null);
+		out = transformer.transform(new GenericMessage<>(map));
+		assertThat(out.getHeaders().get(JsonHeaders.TYPE_ID).toString(), containsString("SingletonMap"));
+		assertThat(out.getHeaders().get(JsonHeaders.CONTENT_TYPE_ID), equalTo(Object.class));
+		assertThat(out.getHeaders().get(JsonHeaders.KEY_TYPE_ID), equalTo(String.class));
+	}
+
 	@Test
 	public void testBoonJsonObjectMapper() throws Exception {
 		ObjectToJsonTransformer transformer = new ObjectToJsonTransformer(new BoonJsonObjectMapper());