Welcome to this lesson on Content Negotiation in Spring Boot! In the previous lessons, we focused on validating request data and handling exceptions, essential skills for building robust RESTful APIs. Today, we'll delve into content negotiation—a vital aspect that allows your API to serve responses in different formats, like JSON and XML, based on the client's request.

Content negotiation is crucial when you want a single endpoint to serve results in various formats. This is particularly useful when different clients use your public endpoint and prefer different response formats. For example, mobile applications may request JSON, while legacy systems might require XML. In this lesson, we'll cover content negotiation for XML and JSON formats in Spring Boot. Other formats like YAML, plain text, and PDF can also be supported by adding the appropriate dependencies and configurations.

Here is an example of a TodoItem object returned in JSON format:

JSON
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1{
2    "id": 1,
3    "title": "Read book",
4    "description": "Read the first chapter of 'Effective Java'"
5}

And here is the same TodoItem object returned in XML format:

HTML, XML
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1<TodoItem>
2    <id>1</id>
3    <title>Read book</title>
4    <description>Read the first chapter of 'Effective Java'</description>
5</TodoItem>

First, let's add the necessary dependencies to our build.gradle script:

Groovy
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1implementation 'com.fasterxml.jackson.dataformat:jackson-dataformat-xml'
2implementation 'jakarta.xml.bind:jakarta.xml.bind-api:3.0.1'

• jackson-dataformat-xml: This dependency includes the necessary components for serializing and deserializing XML data using Jackson.
• jakarta.xml.bind-api: This dependency provides the Jakarta XML Binding API, which is used for converting Java objects to XML and vice versa.

To enable content negotiation, you'll need to adjust your controller methods. Instead of using semantic annotations like @GetMapping, you'll use @RequestMapping to explicitly specify the supported media types.

Here's a code snippet for our TodoItemController:

Java
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1@RequestMapping(value = "/{id}", method = RequestMethod.GET, produces = { MediaType.APPLICATION_JSON_VALUE, MediaType.APPLICATION_XML_VALUE })
2public TodoItem getTodoItemById(@PathVariable int id) {
3    return todoItemRepository.findById(id).orElseThrow(() -> new NoSuchElementException("Todo item with id " + id + " not found"));
4}

Using @RequestMapping, we can specify multiple media types that the endpoint supports.

Spring Boot offers two primary strategies for content negotiation:

• URL Parameter Strategy: GET /todo/123?format=xml to request XML data or GET /todo/123?format=json to request JSON data.
• Accept Header Strategy: GET /todo/123 with Accept: application/xml in the request header to request XML data or GET /todo/123 with Accept: application/json in the request header to request JSON data.

In the URL Parameter Strategy, the response format is determined by a parameter in the URL query string. For example, the URL GET /todos?format=xml would request XML data.

Here's a configuration snippet using this strategy:

Java
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1package com.codesignal;
2
3import org.springframework.context.annotation.Configuration;
4import org.springframework.web.servlet.config.annotation.ContentNegotiationConfigurer;
5import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
6
7@Configuration
8public class ContentNegotiationConfig implements WebMvcConfigurer {
9
10    @Override
11    public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
12        configurer
13            .favorParameter(true) // Enable content negotiation via URL parameter
14            .parameterName("format") // Specify the parameter name to look for in the URL
15            .defaultContentType(MediaType.APPLICATION_JSON) // Set the default content type
16            .mediaType("json", MediaType.APPLICATION_JSON) // Map "json" to MediaType.APPLICATION_JSON
17            .mediaType("xml", MediaType.APPLICATION_XML); // Map "xml" to MediaType.APPLICATION_XML
18    }
19}

Explanation:

• favorParameter(true): This method enables content negotiation via a URL parameter. When set to true, Spring will use the specified query parameter to determine the response format.
• parameterName("format"): This method sets the name of the URL parameter to be used for content negotiation. In this case, the format parameter will be used.
• defaultContentType(MediaType.APPLICATION_JSON): This sets the default content type to JSON in case the URL parameter is not specified.
• mediaType("json", MediaType.APPLICATION_JSON) and mediaType("xml", MediaType.APPLICATION_XML): These methods map the string values json and xml to their respective media types.

In the Accept Header Strategy, the client specifies the desired response format using the HTTP Accept header. For example, setting Accept: application/xml in the request header informs the server to respond with XML data.

Here's the configuration snippet for this strategy:

Java
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1package com.codesignal;
2
3import org.springframework.context.annotation.Configuration;
4import org.springframework.web.servlet.config.annotation.ContentNegotiationConfigurer;
5import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
6
7@Configuration
8public class ContentNegotiationConfig implements WebMvcConfigurer {
9
10    @Override
11    public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
12        configurer
13            .favorParameter(false) // Disable content negotiation via URL parameter
14            .ignoreAcceptHeader(false) // Enable content negotiation via Accept header
15            .defaultContentType(MediaType.APPLICATION_JSON) // Set default content type
16            .mediaType("json", MediaType.APPLICATION_JSON) // Map "json" to MediaType.APPLICATION_JSON
17            .mediaType("xml", MediaType.APPLICATION_XML); // Map "xml" to MediaType.APPLICATION_XML
18    }
19}

Explanation:

• favorParameter(false): This method disables content negotiation via URL parameters. When set to false, the server will not consider URL parameters for determining the response format.
• ignoreAcceptHeader(false): This method allows content negotiation using the Accept header. When set to false, Spring will use the Accept header in the HTTP request to determine the response format.
• defaultContentType(MediaType.APPLICATION_JSON): This sets the default content type to JSON in case the Accept header is not specified.
• mediaType("json", MediaType.APPLICATION_JSON) and mediaType("xml", MediaType.APPLICATION_XML): These methods map the string values json and xml to their respective media types.

Today, we learned about content negotiation in Spring Boot, essential for creating flexible APIs that can serve multiple response formats. We covered adding necessary dependencies, making controller adjustments, and configuring both URL parameter and Accept header strategies. Next, you'll practice implementing these strategies in various scenarios to strengthen your understanding and skills. Keep up the good work!