Make it easier to use Gson and the Actuator at the same time

Previously the recommendation when a user wanted to use Gson was to
exclude Jackson from the classpath and add Gson. This worked fine until
the user also want to use the Actuator which requires Jackson. To get
over this hurdle the user could leave Jackson on the classpath and
perform their own HttpMessageConverter configuration and register a
GsonHttpMessageConverter instead of or before any Jackson-based
converter. A little complicated, but it worked.

This commit makes things easier by updating the auto-configuration for
HTTP message converters to prefer Gson when both Gson and Jackson are
on the classpath, i.e. in the presence of both, a
GsonHttpMessageConverter will be auto-configured and a
MappingJackson2HttpMessageConverter won’t be. This allows an
application to easily use Gson while allowing the Actuator to continue
to use Jackson.

Closes gh-2247

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/HttpMessageConvertersAutoConfiguration.java

@@ -25,7 +25,7 @@ import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
-import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingClass;
+import org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration;
 import org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.context.annotation.Bean;
@@ -55,7 +55,7 @@ import com.google.gson.Gson;
  */
 @Configuration
 @ConditionalOnClass(HttpMessageConverter.class)
-@AutoConfigureAfter(JacksonAutoConfiguration.class)
+@AutoConfigureAfter({ GsonAutoConfiguration.class, JacksonAutoConfiguration.class })
 public class HttpMessageConvertersAutoConfiguration {
 
 	@Autowired(required = false)
@@ -70,6 +70,7 @@ public class HttpMessageConvertersAutoConfiguration {
 	@Configuration
 	@ConditionalOnClass(ObjectMapper.class)
 	@ConditionalOnBean(ObjectMapper.class)
+	@ConditionalOnMissingBean(GsonHttpMessageConverter.class)
 	@EnableConfigurationProperties(HttpMapperProperties.class)
 	@SuppressWarnings("deprecation")
 	protected static class MappingJackson2HttpMessageConverterConfiguration {
@@ -119,7 +120,6 @@ public class HttpMessageConvertersAutoConfiguration {
 
 	@Configuration
 	@ConditionalOnClass(Gson.class)
-	@ConditionalOnMissingClass(name = "com.fasterxml.jackson.core.JsonGenerator")
 	@ConditionalOnBean(Gson.class)
 	protected static class GsonHttpMessageConverterConfiguration {
 

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/HttpMessageConvertersAutoConfigurationTests.java

@@ -22,6 +22,8 @@ import java.util.List;
 import org.junit.After;
 import org.junit.Test;
 import org.springframework.beans.DirectFieldAccessor;
+import org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration;
+import org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration;
 import org.springframework.boot.test.EnvironmentTestUtils;
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 import org.springframework.context.annotation.Bean;
@@ -118,17 +120,32 @@ public class HttpMessageConvertersAutoConfigurationTests {
 
 	@Test
 	public void defaultGsonConverter() throws Exception {
-		this.context.register(GsonConfig.class,
+		this.context.register(GsonAutoConfiguration.class,
 				HttpMessageConvertersAutoConfiguration.class);
 		this.context.refresh();
-		// Shouldn't be registered because we have Jackson
-		assertEquals(0, this.context.getBeansOfType(GsonHttpMessageConverter.class)
+		assertConverterBeanExists(GsonHttpMessageConverter.class,
+				"gsonHttpMessageConverter");
+
+		assertConverterBeanRegisteredWithHttpMessageConverters(GsonHttpMessageConverter.class);
+	}
+
+	@Test
+	public void gsonIsPreferredWhenBothGsonAndJacksonAreAvailable() {
+		this.context.register(GsonAutoConfiguration.class,
+				JacksonAutoConfiguration.class,
+				HttpMessageConvertersAutoConfiguration.class);
+		this.context.refresh();
+		assertConverterBeanExists(GsonHttpMessageConverter.class,
+				"gsonHttpMessageConverter");
+		assertConverterBeanRegisteredWithHttpMessageConverters(GsonHttpMessageConverter.class);
+		assertEquals(0,
+				this.context.getBeansOfType(MappingJackson2HttpMessageConverter.class)
 						.size());
 	}
 
 	@Test
 	public void customGsonConverter() throws Exception {
-		this.context.register(GsonConfig.class, GsonConverterConfig.class,
+		this.context.register(GsonAutoConfiguration.class, GsonConverterConfig.class,
 				HttpMessageConvertersAutoConfiguration.class);
 		this.context.refresh();
 		assertConverterBeanExists(GsonHttpMessageConverter.class,
@@ -227,15 +244,6 @@ public class HttpMessageConvertersAutoConfigurationTests {
 		}
 	}
 
-	@Configuration
-	protected static class GsonConfig {
-
-		@Bean
-		public Gson gson() {
-			return new Gson();
-		}
-	}
-
 	@Configuration
 	protected static class GsonConverterConfig {