Why Lombok's @Builder removes default field value when creating object with default constructor?

Question

Consider the following code:

@RunWith(JUnit4.class)
public class TestClass {

    @Test
    public void builderTest() throws Exception {
        List<Object> list = new LombokBuilderTest().list;
        assertNotNull(list);
    }
}

@NoArgsConstructor
@AllArgsConstructor
@Builder
class LombokBuilderTest {
    @Builder.Default
    List<Object> list = new ArrayList<>();
}

The test fails even though there is default value for list property. If you comment @Builder annotation it works as expected. Why does Lombok work this way? I expect to have empty ArrayList assigned to list property when using default constructor.

Answer 1

This is a known issue (1, 2). When you annotate a class with @Builder it's "expected" that you'd use the builder to instantiate the class. The problem comes when you use the @NoArgsConstructor which bypasses the building process.

The generated code for your example looks something like this:

class LombokBuilderTest {

    public static class LombokBuilderTestBuilder {
        private List<Object> list;
        private boolean list$set;

        LombokBuilderTestBuilder() {}

        public LombokBuilderTestBuilder list(final List<Object> list) {
            this.list = list;
            list$set = true;
            return this;
        }

        public LombokBuilderTest build() {
            return new LombokBuilderTest((list$set ? list : LombokBuilderTest.$default$list()));
        }

        @Override
        public String toString() {
            return (("LombokBuilderTest.LombokBuilderTestBuilder(list=" + this.list) + ")");
        }
    }

    List<Object> list;

    private static List<Object> $default$list() {
        return new ArrayList<>();
    }

    public static LombokBuilderTestBuilder builder() {
        return new LombokBuilderTestBuilder();
    }

    public LombokBuilderTest() {}

    public LombokBuilderTest(final List<Object> list) {
        this.list = list;
    }

    public static void main(String[] args) {
        List<Object> list = new LombokBuilderTest().list;
        System.out.println(list);
    }
}

As you can see, your initialized value was moved from the field declaration to the $default$list method and using the no-agrs constructor is the only way to "corrupt" the instantiation.

As explained in the 2nd link, the reason is that if the assignment expression (new ArrayList<>() in your case) is costly, then using the builder would cause the expression to be executed twice which is a performance issue. The workaround would be to write your own no-args constructor and do the field initialization there.