lombok-builder

Posted on: 2019-10-08 2019-11-19
Tags: Lombok, Pattern

Using Lombok’s @Builder Annotation

1. Overview

Project Lombok’s @Builder is a useful mechanism for using the Builder pattern without writing boilerplate code. We can apply this annotation to a Class or a method.

In this brief tutorial, we’ll look at the different use cases for @Builder.

2. Maven Dependencies

First, we need to add Project Lombok to our pom.xml:

<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <version>1.16.20.0</version>
</dependency>

Maven Central has the latest version of Project Lombok here.

**3. Using @Builder on a Class**

In the first use case, we’re simply implementing a Class, and we want to use a builder to create instances of our class.

The first and only step is to add the annotation to the class declaration:

@Getter
@Builder
public class Widget {
    private final String name;
    private final int id;
}

Lombok does all of the work for us. We can now build a Widget and test it:

Widget testWidget = Widget.builder()
  .name("foo")
  .id(1)
  .build();

assertThat(testWidget.getName())
  .isEqualTo("foo");
assertThat(testWidget.getId())
  .isEqualTo(1);

If we want to create copies or near-copies of objects, we can add the property toBuilder = true to the @Builder annotation:

@Builder(toBuilder = true)
public class Widget {
//...
}

This tells Lombok to add a toBuilder() method to our Class. When we invoke the toBuilder() method, it returns a builder initialized with the properties of the instance it is called on:

Widget testWidget = Widget.builder()
  .name("foo")
  .id(1)
  .build();

Widget.WidgetBuilder widgetBuilder = testWidget.toBuilder();

Widget newWidget = widgetBuilder.id(2).build();
assertThat(newWidget.getName())
  .isEqualTo("foo");
assertThat(newWidget.getId())
  .isEqualTo(2);

We can see in the test code that the builder class generated by Lombok is named like our class, with “Builder” appended to it — WidgetBuilder in this case. We can then modify the properties we wish and build() a new instance.

**4. Using @Builder on a Method**

Suppose we’re using an object that we want to construct with a builder, but we can’t modify the source or extend the *Class*.

First, let’s create a quick example using Lombok’s @Value annotation:

@Value
final class ImmutableClient {
    private int id;
    private String name;
}

Now we have a final Class with two immutable members, getters for them, and an all-arguments constructor.

We covered how to use @Builder on a Class, but we can use it on methods, too. We’ll use this ability to work around not being able to modify or extend ImmutableClient.

Next, we’ll create a new class with a method for creating ImmutableClients:

class ClientBuilder {

    @Builder(builderMethodName = "builder")
    public static ImmutableClient newClient(int id, String name) {
        return new ImmutableClient(id, name);
    }
}

This annotation creates a method named builder() that returns a Builder for creating ImmutableClients.

Now we can build an ImmutableClient:

ImmutableClient testImmutableClient = ClientBuilder.builder()
  .name("foo")
  .id(1)
  .build();
assertThat(testImmutableClient.getName())
  .isEqualTo("foo");
assertThat(testImmutableClient.getId())
  .isEqualTo(1);

5. Conclusion

In this article, we used Lombok’s @Builder annotation on a method to create a builder for a final Class.

Code samples, as always, can be found over on GitHub.

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