欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页

Lombok:lombok中使用@Builder

程序员文章站 2022-06-04 18:30:19
...

文章摘自:https://www.baeldung.com/lombok-builder

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:

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.

相关标签: lombok @Builder