lombok @Builder注解使用的例子、反編譯之后的代碼詳解

lombok的@Builder實際是建造者模式的一個變種，所以在創建復雜對象時常使用

這里對lombok的@Builder和@Data組合的使用示例

import lombok.Builder;
import lombok.Data;

@Data
@Builder
public class People {
    private String name;
    private String sex;
    private int age;
}

 

使用了@Bulider和@Data注解后，就可以使用鏈式風格優雅地創建對象

public class TestLombok {

    @Test
    public void testBuilderAnnotation(){
        People luoTianyan = People.builder()
                .sex("female")
                .age(23)
                .name("LuoTianyan")
                .build();


        System.out.println(luoTianyan.toString());
        //People(name=LuoTianyan, sex=female, age=23)

        People people = new People("LuoTianyan","female",23);
        System.out.println(luoTianyan.equals(people));
        //true
    }

}

 

class People加上了@Builder和@Data注解后，多了一個靜態內部類PeopleBuilder，People調用靜態方法builder生成PeopleBuilder對象，PeopleBuilder對象可以使用".屬性名(屬性值)"的方式進行屬性設置，再調用build()方法就生成了People對象，並且如果兩個People對象的屬性如果相同，就會認為這兩個對象相等，即重寫了hashCode和equls方法。

 

可以利用Javap、cfr進行反編譯該字節碼；

這里就直接在Intellij IDEA下，查看反編譯的文件People.class；

可以看到，生成的有：

• Getter和Setter方法；
• 訪問類型是private無參構造方法，訪問類型為default的全部參數的構造方法；
• 重寫hashCode、equals、toString方法，則People可以做為Map的key；
• 訪問類型為public的靜態方法builder，返回的是People.PeopleBuilder對象，非單例；
• 訪問類型為public的靜態內部類PeopleBuilder，該類主要有build方法，返回類型是People；
• 最后還有個canEqual方法，判斷是否與People同類型。

 

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//


public class People {
    private String name;
    private String sex;
    private int age;

    People(String name, String sex, int age) {
        this.name = name;
        this.sex = sex;
        this.age = age;
    }

    public static People.PeopleBuilder builder() {
        return new People.PeopleBuilder();
    }

    private People() {
    }

    public String getName() {
        return this.name;
    }

    public String getSex() {
        return this.sex;
    }

    public int getAge() {
        return this.age;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public boolean equals(Object o) {
        if (o == this) {
            return true;
        } else if (!(o instanceof People)) {
            return false;
        } else {
            People other = (People)o;
            if (!other.canEqual(this)) {
                return false;
            } else {
                label39: {
                    Object this$name = this.getName();
                    Object other$name = other.getName();
                    if (this$name == null) {
                        if (other$name == null) {
                            break label39;
                        }
                    } else if (this$name.equals(other$name)) {
                        break label39;
                    }

                    return false;
                }

                Object this$sex = this.getSex();
                Object other$sex = other.getSex();
                if (this$sex == null) {
                    if (other$sex != null) {
                        return false;
                    }
                } else if (!this$sex.equals(other$sex)) {
                    return false;
                }

                if (this.getAge() != other.getAge()) {
                    return false;
                } else {
                    return true;
                }
            }
        }
    }

    protected boolean canEqual(Object other) {
        return other instanceof People;
    }

    public int hashCode() {
        int PRIME = true;
        int result = 1;
        Object $name = this.getName();
        int result = result * 59 + ($name == null ? 43 : $name.hashCode());
        Object $sex = this.getSex();
        result = result * 59 + ($sex == null ? 43 : $sex.hashCode());
        result = result * 59 + this.getAge();
        return result;
    }

    public String toString() {
        return "People(name=" + this.getName() + ", sex=" + this.getSex() + ", age=" + this.getAge() + ")";
    }

    public static class PeopleBuilder {
        private String name;
        private String sex;
        private int age;

        PeopleBuilder() {
        }

        public People.PeopleBuilder name(String name) {
            this.name = name;
            return this;
        }

        public People.PeopleBuilder sex(String sex) {
            this.sex = sex;
            return this;
        }

        public People.PeopleBuilder age(int age) {
            this.age = age;
            return this;
        }

        public People build() {
            return new People(this.name, this.sex, this.age);
        }

        public String toString() {
            return "People.PeopleBuilder(name=" + this.name + ", sex=" + this.sex + ", age=" + this.age + ")";
        }
    }
}

 

自從Java 6起，Javac就支持“JSR 269 Pluggable Annotation Processing API”規范，只要程序實現了該API，就能在javac運行的時候得到調用。

Lombok就是一個實現了"JSR 269 API"的程序。在使用javac的過程中，它產生作用的具體流程如下：

1. Javac對源代碼進行分析，生成一棵抽象語法樹(AST)

2. Javac編譯過程中調用實現了JSR 269的Lombok程序

3. 此時Lombok就對第一步驟得到的AST進行處理，找到Lombok注解所在類對應的語法樹(AST)，然后修改該語法樹(AST)，增加Lombok注解定義的相應樹節點

4. Javac使用修改后的抽象語法樹(AST)生成字節碼文件