Spring Boot REST – Consume and Produce JSON

Learn to create spring boot REST service which accept request payload and produce response body in JSON format i.e. application/json media-type.

1. JSON Support in Spring boot

Spring Boot provides integration with three JSON mapping libraries.

1. Gson
2. Jackson
3. JSON-B

Jackson is the preferred and default library in Spring boot.

2. Jackson

2.1. Auto Configuration

Spring boot, by default, includes Jackson 2 dependency and is part of spring-boot-starter-json. Using JacksonAutoConfiguration class, spring boot automatically configures Jackson with following behavior:

• an ObjectMapper in case none is already configured.
• a Jackson2ObjectMapperBuilder in case none is already configured.
• auto-registration for all Module beans with all ObjectMapper beans (including the defaulted ones).

2.2. JSON Response

Any Spring @RestController in a Spring Boot application should render JSON response by default as long as Jackson2 is on the classpath.

In given example, EmployeeList will be serialized by Jackson2 and serves a JSON representation to client.

@GetMapping
public EmployeeList getAllEmployees() 
{
    EmployeeList list = service.getAllEmployees();

    return list;
}

Similarly, for create or update operation, client can send the JSON payload in request body. In given example, Employee object will be populated with JSON request.

@PostMapping
public Employee createOrUpdateEmployee(@Valid Employee employee)
                                                throws RecordNotFoundException {
    Employee updated = service.createOrUpdateEmployee(employee);
    return new ResponseEntity<Employee>(updated, new HttpHeaders(), HttpStatus.OK);
}

2.3. Customize Jackson ObjectMapper

Spring boot auto-configures MappingJackson2HttpMessageConverter as one of default converters to handle request and response body conversion.

We can customize the default conversion behavior using either property files or custom bean definitions.

2.3.1. Property Configuration

spring.jackson.date-format= # For instance, `yyyy-MM-dd HH:mm:ss`.
spring.jackson.default-property-inclusion= # Controls the inclusion of properties during serialization. 
spring.jackson.deserialization.*= # Jackson on/off features for deserialization.
spring.jackson.generator.*= # Jackson on/off features for generators.
spring.jackson.joda-date-time-format= # Joda date time format string.
spring.jackson.locale= # Locale used for formatting.
spring.jackson.mapper.*= # Jackson general purpose on/off features.
spring.jackson.parser.*= # Jackson on/off features for parsers.
spring.jackson.property-naming-strategy= # PropertyNamingStrategy.
spring.jackson.serialization.*= # Jackson on/off features for serialization.
spring.jackson.time-zone= #  Time zone
spring.jackson.visibility.*= # To limit which methods (and fields) are auto-detected.

2.3.2. Bean Configurations

Use anyone of below bean configuration to override to JSON behavior in spring boot.

@Configuration
public class WebConfig 
{
    @Bean
    public Jackson2ObjectMapperBuilderCustomizer customJson()
    {
        return builder -> {

            // human readable
            builder.indentOutput(true);

            // exclude null values
            builder.serializationInclusion(JsonInclude.Include.NON_NULL);

            // all lowercase with under score between words
            builder.propertyNamingStrategy(PropertyNamingStrategy.SNAKE_CASE);
        };
    }
}

@Configuration
public class WebConfig 
{
    @Bean
    @Primary
    public ObjectMapper customJson(){
        return new Jackson2ObjectMapperBuilder()
           .indentOutput(true)
           .serializationInclusion(JsonInclude.Include.NON_NULL)
           .propertyNamingStrategy(PropertyNamingStrategy.SNAKE_CASE)
           .build();
}
}

@Configuration
public class WebConfig 
{
    @Bean
    public Jackson2ObjectMapperBuilder customJson() {
        return new Jackson2ObjectMapperBuilder()
            .indentOutput(true)
            .serializationInclusion(JsonInclude.Include.NON_NULL)
            .propertyNamingStrategy(PropertyNamingStrategy.SNAKE_CASE);
    }
}

@Configuration
public class WebConfig 
{
    @Bean
    public MappingJackson2HttpMessageConverter customJson(){
    return new MappingJackson2HttpMessageConverter(
        new Jackson2ObjectMapperBuilder()
            .indentOutput(true)
            .serializationInclusion(JsonInclude.Include.NON_NULL)
            .propertyNamingStrategy(PropertyNamingStrategy.SNAKE_CASE)
            .build()
    );
} 
}

3. Gson

3.1. Dependency

Include gson in the spring boot application by adding the appropriate Gson dependency.

<dependencies>
    <dependency>
      <groupId>com.google.code.gson</groupId>
      <artifactId>gson</artifactId>
      <version>2.8.5</version>
    </dependency>
</dependencies>

3.2. Auto Configuration

Spring boot detects presence of Gson.class and uses GsonAutoConfiguration for configuring the Gson instance.

To Make Gson preferred json mapper, use this property in application.properties file.

spring.http.converters.preferred-json-mapper=gson

Read More : Gson with Spring boot

2.3. Customization

spring.gson.date-format= # Format to use when serializing Date objects.
spring.gson.disable-html-escaping= # Whether to disable the escaping of HTML characters such as '<', '>', etc.
spring.gson.disable-inner-class-serialization= # Whether to exclude inner classes during serialization.
spring.gson.enable-complex-map-key-serialization= # Whether to enable serialization of complex map keys (i.e. non-primitives).
spring.gson.exclude-fields-without-expose-annotation= # Whether to exclude all fields from consideration for serialization or deserialization that do not have the "Expose" annotation.
spring.gson.field-naming-policy= # Naming policy that should be applied to an object's field during serialization and deserialization.
spring.gson.generate-non-executable-json= # Whether to generate non executable JSON by prefixing the output with some special text.
spring.gson.lenient= # Whether to be lenient about parsing JSON that doesn't conform to RFC 4627.
spring.gson.long-serialization-policy= # Serialization policy for Long and long types.
spring.gson.pretty-printing= # Whether to output serialized JSON that fits in a page for pretty printing.
spring.gson.serialize-nulls= # Whether to serialize null fields.

3. JSON-B

JSON Binding (JSON-B) is the new Java EE specification (JSR 367) for converting JSON messages to Java Objects and back. Using JSON-B, we have a standard way of handling this conversion in spring boot applications.

3.1. Dependency

Add the required Maven dependencies.

<dependency>
    <groupId>javax.json.bind</groupId>
    <artifactId>javax.json.bind-api</artifactId>
    <version>1.0</version>
</dependency>
<dependency>
    <groupId>org.eclipse</groupId>
    <artifactId>yasson</artifactId>
    <version>1.0</version>
</dependency>
<dependency>
    <groupId>org.glassfish</groupId>
    <artifactId>javax.json</artifactId>
    <version>1.1</version>
</dependency>

3.2. Auto Configuration

Set the prereffed JSON mapper to JSON-B.

spring.http.converters.preferred-json-mapper=jsonb

With that in place, now all request and response conversions in spring boot with happen using JSON-B.

Drop me your questions related to producing and consuming JSON in spring boot rest apis.

Happy Learning !!