Make fields and properties annotated with @nullable optional.

build.gradle

@@ -100,6 +100,7 @@ dependencies {
 
     testImplementation 'junit:junit:4.13.2'
     testImplementation 'org.assertj:assertj-core:3.6.2'
+    testImplementation 'com.google.code.findbugs:jsr305:3.0.2'
     testAnnotationProcessor project
 }
 // required in Gradle 8, since the above line 'testAnnotationProcessor project'

src/main/java/org/jilt/internal/AbstractBuilderGenerator.java

@@ -14,6 +14,7 @@
 import org.jilt.utils.Utils;
 
 import javax.annotation.processing.Filer;
+import javax.lang.model.element.AnnotationMirror;
 import javax.lang.model.element.ExecutableElement;
 import javax.lang.model.element.Modifier;
 import javax.lang.model.element.Name;
@@ -144,12 +145,29 @@ private TypeName returnTypeForSetterFor(VariableElement attribute) {
     private Set<VariableElement> initOptionalAttributes() {
         Set<VariableElement> ret = new HashSet<VariableElement>();
         for (VariableElement attribute : attributes) {
-            if (attribute.getAnnotation(Opt.class) != null)
+            if (this.determineIfAttributeIsOptional(attribute)) {
                 ret.add(attribute);
+            }
         }
         return ret;
     }
 
+    private boolean determineIfAttributeIsOptional(VariableElement attribute) {
+        if (attribute.getAnnotation(Opt.class) != null) {
+            return true;
+        }
+        for (AnnotationMirror annotation : attribute.getAnnotationMirrors()) {
+            if (annotationIsCalledNullable(annotation)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    private static boolean annotationIsCalledNullable(AnnotationMirror annotation) {
+        return "Nullable".equals(annotation.getAnnotationType().asElement().getSimpleName().toString());
+    }
+
     private String initBuilderClassPackage() {
         String annotationBuilderPackageName = builderAnnotation.packageName();
         if (annotationBuilderPackageName.isEmpty()) {

src/test/java/org/jilt/test/FullNameTest.java

@@ -0,0 +1,22 @@
+package org.jilt.test;
+
+import org.jilt.test.data.nullable.FullName;
+import org.junit.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.jilt.test.data.nullable.FullNameBuilder.fullName;
+
+public class FullNameTest {
+    @Test
+    public void nullable_attribute() throws Exception {
+        FullName value = fullName()
+                .firstName("First")
+                .lastName(null)
+                // middleName is implicitly optional, because of @Nullable
+                .build();
+
+        assertThat(value.firstName).isEqualTo("First");
+        assertThat(value.middleName).isNull();
+        assertThat(value.lastName).isNull();
+    }
+}

src/test/java/org/jilt/test/data/nullable/FullName.java

@@ -0,0 +1,17 @@
+package org.jilt.test.data.nullable;
+
+import org.jilt.Builder;
+import org.jilt.BuilderStyle;
+
+import javax.annotation.Nullable;
+
+public final class FullName {
+    public final String firstName, middleName, lastName;
+
+    @Builder(style = BuilderStyle.TYPE_SAFE)
+    public FullName(String firstName, @Nullable String middleName, String lastName) {
+        this.firstName = firstName;
+        this.middleName = middleName;
+        this.lastName = lastName;
+    }
+}