本篇文章详细介绍了Java中lombok的@Builder注解及@SuperBuilder注解的解析和使用,希望对大家的学习或工作有一定的参考借鉴价值。废话不多说,直接上干货。
Lombok 的@Builder提供了一种非常有用的机制,无需编写样板代码即可使用构建者模式。 @Builder可以放在类,构造函数或方法上。
先定义示例类 Question,类声明中用 @Builder 注解。
// 定义 Quesstion, 使用@Builder注解
@Builder
public class Question {
private Long id;
private String question;
}
用@Builder 注解的类,Lombok会帮我们做以下几个事情(参考下面的示例代码):
public class Question {
private Long id;
private String question;
Question(Long id, String question) {
this.id = id;
this.question = question;
}
public static QuestionBuilder builder() {
return new QuestionBuilder();
}
public static class QuestionBuilder {
private Long id;
private String question;
QuestionBuilder() {
}
public QuestionBuilder id(Long id) {
this.id = id;
return this;
}
public QuestionBuilder question(String question) {
this.question = question;
return this;
}
public Question build() {
return new Question(this.id, this.question);
}
public String toString() {
return "Question.QuestionBuilder(id=" + this.id + ", question=" + this.question + ")";
}
}
@Builder中使用 @Singular 注释集合。
// 定义 Answer,使用 @Builder注解
@Builder
public class Answer {
private Long id;
private String answer;
}
// 定义 Quesstion, 使用@Builder注解
@Builder
public class Question {
private Long id;
private String question;
@Singular
private List answers;
// @Singular("answer")
// private List answerList;
}
在使用@Singular注释注释一个集合字段(使用@Builder注释类),lombok会将该构建器节点视为一个集合,并生成两个adder方法,而不是setter方法。
除此之外,还生成了clear方法,用于清空集合。
public class Question {
private Long id;
private String question;
List answers;
Question(Long id, String question, List answers) {
this.id = id;
this.question = question;
this.answers = answers;
}
public static QuestionBuilder builder() {
return new QuestionBuilder();
}
public static class QuestionBuilder {
private Long id;
private String question;
private ArrayList answers;
QuestionBuilder() {
}
public QuestionBuilder id(Long id) {
this.id = id;
return this;
}
public QuestionBuilder question(String question) {
this.question = question;
return this;
}
public QuestionBuilder answer(Answer answer) {
if (this.answers == null) {
this.answers = new ArrayList();
}
this.answers.add(answer);
return this;
}
public QuestionBuilder answers(Collection<? extends Answer> answers) {
if (answers == null) {
throw new NullPointerException("answers cannot be null");
} else {
if (this.answers == null) {
this.answers = new ArrayList();
}
this.answers.addAll(answers);
return this;
}
}
public QuestionBuilder clearAnswers() {
if (this.answers != null) {
this.answers.clear();
}
return this;
}
public Question build() {
List answers;
switch (this.answers == null ? 0 : this.answers.size()) {
case 0:
answers = Collections.emptyList();
break;
case 1:
answers = Collections.singletonList(this.answers.get(0));
break;
default:
answers = Collections.unmodifiableList(new ArrayList(this.answers));
}
return new Question(this.id, this.question, answers);
}
public String toString() {
return "Question.QuestionBuilder(id=" + this.id + ", question=" + this.question + ", answers=" + this.answers + ")";
}
}
}
从上面代码块,可以看到 在集合字段增加了@Singular注解后,构建器的 build() 方法会更复杂一些,主要是为了保证以下两点:
如果您的标识符是用普通英语编写的,lombok 会假定任何带有 @Singular 的集合的名称是英语复数,并将尝试自动将该名称单数化。 如果可能,add-one 方法将使用此名称。 例如,如果这里我们定义的集合为answers,那么 add-one 方法将自动称为 answer(Answer answer)。 您还可以在@Singular注解中显式指定标识符的单数形式,如上面代码块中被注释的部分:@Singular("answer") private List answerList;
如果 lombok 无法将您的标识符单数化,或者它有歧义,lombok 将生成错误并强制您明确指定单数名称。
@Builder.Default 的使用
如果在构建会话期间从未设置某个字段/参数,则它始终为 0/null/false。 如果您将 @Builder 放在类上(而不是方法或构造函数),您可以直接在字段上指定默认值,并使用 @Builder.Default 注释该字段:
@Builder
public class Answer {
@Builder.Default
private final String id = UUID.randomUUID().toString();
private String answer;
}
@Builder(toBuilder=true)
如果我们想要创建对象的副本或近似副本,我们可以将属性 toBuilder = true 添加到 @Builder 注释中:
Lombok 会在目标类中新增一个 toBuilder() 方法。当调用 toBuilder() 方法时,它会返回一个新的构建器,该构建器使用调用它的实例的属性进行初始化:
public class Answer {
private final String id;
private String answer;
private static String $default$id() {
return UUID.randomUUID().toString();
}
Answer(String id, String answer) {
this.id = id;
this.answer = answer;
}
public static AnswerBuilder builder() {
return new AnswerBuilder();
}
public AnswerBuilder toBuilder() {
return (new AnswerBuilder()).id(this.id).answer(this.answer);
}
public static class AnswerBuilder {
private boolean id$set;
private String id$value;
private String answer;
AnswerBuilder() {
}
public AnswerBuilder id(String id) {
this.id$value = id;
this.id$set = true;
return this;
}
public AnswerBuilder answer(String answer) {
this.answer = answer;
return this;
}
public Answer build() {
String id$value = this.id$value;
if (!this.id$set) {
id$value = Answer.$default$id();
}
return new Answer(id$value, this.answer);
}
public String toString() {
return "Answer.AnswerBuilder(id$value=" + this.id$value + ", answer=" + this.answer + ")";
}
}
}
@Builder 并不支持对父类成员属性的构造,为解决这个问题, @SuperBuilder 应运而生,算是 @Builder 的升级版。@SuperBuilder在 lombok v1.18.2 中作为实验性功能引入。
定义示例类 Event和其子类 QuestionEvent。
@SuperBuilder
public class Event {
String message;
}
@SuperBuilder
public class QuestionEvent extends Event {
}
用@SuperBuilder 注解的类,Lombok会帮我们做以下几个事情(参考下面的示例代码):
public class Event {
String message;
protected Event(EventBuilder<?, ?> b) {
this.message = b.message;
}
public static EventBuilder<?, ?> builder() {
return new EventBuilderImpl();
}
private static final class EventBuilderImpl extends EventBuilder {
private EventBuilderImpl() {
}
protected EventBuilderImpl self() {
return this;
}
public Event build() {
return new Event(this);
}
}
public abstract static class EventBuilder> {
private String message;
public EventBuilder() {
}
protected abstract B self();
public abstract C build();
public B message(String message) {
this.message = message;
return this.self();
}
public String toString() {
return "Event.EventBuilder(message=" + this.message + ")";
}
}
}
public class QuestionEvent extends Event {
protected QuestionEvent(QuestionEventBuilder<?, ?> b) {
super(b);
}
public static QuestionEventBuilder<?, ?> builder() {
return new QuestionEventBuilderImpl();
}
private static final class QuestionEventBuilderImpl extends QuestionEventBuilder {
private QuestionEventBuilderImpl() {
}
protected QuestionEventBuilderImpl self() {
return this;
}
public QuestionEvent build() {
return new QuestionEvent(this);
}
}
public abstract static class QuestionEventBuilder> extends Event.EventBuilder {
public QuestionEventBuilder() {
}
protected abstract B self();
public abstract C build();
public String toString() {
return "QuestionEvent.QuestionEventBuilder(super=" + super.toString() + ")";
}
}
}
原因在于,在Java的抽象语法树设计上,每个类只包含了显式声明的变量而不包括父类的成员变量。Lombok针对@Builder注解的内部实现findAllFields方法是从当前类的抽象语法树出发去找所有的成员变量,所以就只能找到当前类的成员变量,而访问不到父类的成员变量。
@SuperBuilder注解的内部实现,在查找所有成员变量之前,先拿到了继承的父类的抽象语法树。
JCClassDecl td = (JCClassDecl) parent.get();
// 获取继承的父类的抽象语法树
JCTree extendsClause = Javac.getExtendsClause(td);
JCExpression superclassBuilderClass = null;
if (extendsClause instanceof JCTypeApply) {
// Remember the type arguments, because we need them for the extends clause of our abstract builder class.
superclassTypeParams = ((JCTypeApply) extendsClause).getTypeArguments();
// A class name with a generics type, e.g., "Superclass".
extendsClause = ((JCTypeApply) extendsClause).getType();
}
if (extendsClause instanceof JCFieldAccess) {
Name superclassName = ((JCFieldAccess) extendsClause).getIdentifier();
String superclassBuilderClassName = superclassName.toString() + "Builder";
superclassBuilderClass = parent.getTreeMaker().Select((JCFieldAccess) extendsClause, parent.toName(superclassBuilderClassName));
} else if (extendsClause != null) {
String superclassBuilderClassName = extendsClause.toString() + "Builder";
superclassBuilderClass = chainDots(parent, extendsClause.toString(), superclassBuilderClassName);
}
页面更新:2024-03-16
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