Java Object Mapper: What, Why and How

Posted on by Ibrahim Chowdhury

An object mapper can simplify the process of data serialization and deserialization, as well as data transformation and validation. An object mapper can also handle complex data structures, such as nested objects, arrays, collections, and polymorphic types.

Why use an object mapper?

An object mapper is a tool that can convert data between different formats, such as JSON, XML, CSV, etc. and Java objects. An object mapper can also map data from one Java object to another Java object, such as a DTO (Data Transfer Object) or an entity.

Using an object mapper can have several benefits

  • Reducing boilerplate code: An object mapper can automate the mapping logic, so you don’t have to write repetitive code for converting data between different formats or objects.
  • Improving performance: An object mapper can optimize the mapping process, such as caching, buffering, streaming, etc. to improve the speed and efficiency of data conversion.
  • Enhancing readability and maintainability: An object mapper can make the code more readable and maintainable, as you can focus on the business logic rather than the mapping details.
  • Supporting customization and extensibility: An object mapper can provide various options and features to customize and extend the mapping behaviour, such as annotations, configuration, converters, filters, etc.

How to use an object mapper?

There are many object mapper libraries available for Java, such as Jackson, Gson, ModelMapper, MapStruct, etc. Each library has its own advantages and disadvantages, and you can choose the one that suits your needs and preferences.

we will use Jackson as an example of an object mapper library. Jackson is one of the most popular and widely used object mapper libraries for Java. It supports JSON, XML, YAML, CBOR, and other data formats. It also provides various modules and annotations to customize and extend the mapping functionality.

To use Jackson, you need to add the following dependency to your pom.xml file:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.16.0</version>
</dependency>

Then, you need to create an instance of the ObjectMapper class, which is the main class of Jackson. The ObjectMapper class provides various methods to read and write data between different formats and Java objects.

For example, to convert a JSON string to a Java object, you can use the readValue method:

String json = "{\"name\":\"John\",\"age\":30}";
User user = objectMapper.readValue(json, User.class);

To convert a Java object to a JSON string, you can use the writeValueAsString method:

User user = new User("John", 30);
String json = objectMapper.writeValueAsString(user);

You can also use the updateValue method to update an existing Java object with the data from another Java object.

To customize and extend the mapping behaviour, you can use various annotations, such as @JsonProperty, @JsonIgnore, @JsonFormat, @JsonView, @JsonSubTypes, etc. You can also use the configure method to set some global settings, such as indentation, null handling, date format, etc.

Reference

  • https://www.baeldung.com/jackson-object-mapper-tutorial
  • https://www.tutorialspoint.com/jackson/jackson_objectmapper.htm
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