Document HttpMessageConverters detection changes

This commit updates the reference documentation to better reflect
changes applied in gh-48310. As of Spring Boot 4.0.1,
`HttpMessageConverter` beans are all considered as "custom" converters
and will be set ahead of the converters list in the auto-configuration
support.

For more specific changes, developers should contribute
`ClientHttpMessageConvertersCustomizer` or
`ServerHttpMessageConvertersCustomizer` beans to better reflect their
intent: should the converter be configured for client, server or both?
How should this converter be used?

Previously, Spring Boot was relying on heuristics but they would cause
issues for server-only converters, or for specific configuration that
were not meant to replace the default converter for a given media type.

Closes gh-48574

Author: bclozel

documentation/spring-boot-docs/src/docs/antora/modules/reference/pages/web/servlet.adoc

@@ -98,7 +98,10 @@ Any javadoc:org.springframework.http.converter.HttpMessageConverter[] bean that
 You can also override default converters in the same way.
 
 If you need to add or customize converters, you can declare one or more javadoc:org.springframework.boot.http.converter.autoconfigure.ClientHttpMessageConvertersCustomizer[] or
-javadoc:org.springframework.boot.http.converter.autoconfigure.ServerHttpMessageConvertersCustomizer[] as beans, as shown in the following listing:
+javadoc:org.springframework.boot.http.converter.autoconfigure.ServerHttpMessageConvertersCustomizer[] as beans. There, you can choose whether converter instances should be added
+before default ones (`addCustomConverter`) or if they should override a specific default converter (like `withJsonConverter`).
+
+See the following listing for an example:
 
 include-code::MyHttpMessageConvertersConfiguration[]
 

documentation/spring-boot-docs/src/main/java/org/springframework/boot/docs/web/servlet/springmvc/messageconverters/MyHttpMessageConvertersConfiguration.java

@@ -16,9 +16,17 @@
 
 package org.springframework.boot.docs.web.servlet.springmvc.messageconverters;
 
+import java.text.SimpleDateFormat;
+
+import tools.jackson.databind.json.JsonMapper;
+
 import org.springframework.boot.http.converter.autoconfigure.ClientHttpMessageConvertersCustomizer;
+import org.springframework.boot.http.converter.autoconfigure.ServerHttpMessageConvertersCustomizer;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.http.converter.HttpMessageConverters.ClientBuilder;
+import org.springframework.http.converter.HttpMessageConverters.ServerBuilder;
+import org.springframework.http.converter.json.JacksonJsonHttpMessageConverter;
 
 @Configuration(proxyBeanMethods = false)
 public class MyHttpMessageConvertersConfiguration {
@@ -29,4 +37,32 @@ public ClientHttpMessageConvertersCustomizer myClientConvertersCustomizer() {
 			.addCustomConverter(new AnotherHttpMessageConverter());
 	}
 
+	@Bean
+	public JacksonConverterCustomizer jacksonConverterCustomizer() {
+		JsonMapper jsonMapper = JsonMapper.builder().defaultDateFormat(new SimpleDateFormat("yyyy-MM")).build();
+		return new JacksonConverterCustomizer(jsonMapper);
+	}
+
+	// contribute a custom JSON converter to both client and server
+	static class JacksonConverterCustomizer
+			implements ClientHttpMessageConvertersCustomizer, ServerHttpMessageConvertersCustomizer {
+
+		private final JsonMapper jsonMapper;
+
+		JacksonConverterCustomizer(JsonMapper jsonMapper) {
+			this.jsonMapper = jsonMapper;
+		}
+
+		@Override
+		public void customize(ClientBuilder builder) {
+			builder.withJsonConverter(new JacksonJsonHttpMessageConverter(this.jsonMapper));
+		}
+
+		@Override
+		public void customize(ServerBuilder builder) {
+			builder.withJsonConverter(new JacksonJsonHttpMessageConverter(this.jsonMapper));
+		}
+
+	}
+
 }

documentation/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/web/servlet/springmvc/messageconverters/MyHttpMessageConvertersConfiguration.kt

@@ -17,9 +17,13 @@
 package org.springframework.boot.docs.web.servlet.springmvc.messageconverters
 
 import org.springframework.boot.http.converter.autoconfigure.ClientHttpMessageConvertersCustomizer
+import org.springframework.boot.http.converter.autoconfigure.ServerHttpMessageConvertersCustomizer
 import org.springframework.context.annotation.Bean
 import org.springframework.context.annotation.Configuration
 import org.springframework.http.converter.HttpMessageConverters
+import org.springframework.http.converter.json.JacksonJsonHttpMessageConverter
+import tools.jackson.databind.json.JsonMapper
+import java.text.SimpleDateFormat
 
 @Configuration(proxyBeanMethods = false)
 class MyHttpMessageConvertersConfiguration {
@@ -33,5 +37,26 @@ class MyHttpMessageConvertersConfiguration {
 		}
 	}
 
+	@Bean
+	fun jacksonConverterCustomizer(): JacksonConverterCustomizer {
+		val jsonMapper = JsonMapper.builder()
+			.defaultDateFormat(SimpleDateFormat("yyyy-MM"))
+			.build()
+		return JacksonConverterCustomizer(jsonMapper)
+	}
+
+	// contribute a custom JSON converter to both client and server
+	class JacksonConverterCustomizer(private val jsonMapper: JsonMapper) :
+		ClientHttpMessageConvertersCustomizer, ServerHttpMessageConvertersCustomizer {
+
+		override fun customize(builder: HttpMessageConverters.ClientBuilder) {
+			builder.withJsonConverter(JacksonJsonHttpMessageConverter(this.jsonMapper))
+		}
+
+		override fun customize(builder: HttpMessageConverters.ServerBuilder) {
+			builder.withJsonConverter(JacksonJsonHttpMessageConverter(this.jsonMapper))
+		}
+	}
+
 }