認識lombok 的@Builder注解對初始化的影響

參考：https://www.jianshu.com/p/4e4cef2e82e1

參考：https://blog.csdn.net/qq_28411869/article/details/84104893

先上結論：

• 問題1：如果類中用了@Builder注解，而屬性沒有任何注解話，那么在你初始化這個類的時候，如果你的屬性賦值了默認值，則在你用builder方法初始化該類后，屬性的默認值則無效即獲取會產生空指針異常
• 問題2：在具體要賦默認值字段上加@Builder.default注解可以解決問題1，但是際運行代碼之后，我發現一個無奈的問題，builder模式下默認值生效了，但是使用new(以及正常反序列化)得到的實例默認值不會被設置

問題1描述：

我們來剖析下這中間發生了什么

從上面的例子，可以發現Teacher 的address屬性為空，這正是我們很常規初始化操作，獲取這個address，接着對它進行操作， 如果此時它是null，則會出現空指針異常；
      比較Student中的address則是我們理想中的正常執行過程，是有一個默認值的對象，同時觀察Student中的name 和age兩個屬性值默認值也出現了如同Teacher中的address現象，默認值消失了；
      從表面來看，Student多了@Builder.Default的注解，這個注解確實就是解決這個問題關鍵，讓你想要賦值的默認值來進行正確的初始化了。知道了這個注解的使用只是做到了知其然，我們要做做知其所以然，所以來看看下他們生成的class有什么區別？ 以下代碼反編譯刪除了equal和hashcode方法

Teacher.class

public class Teacher {
private String name;
private List<String> address = new ArrayList();

Teacher(String name, List<String> address) {
    this.name = name;
    this.address = address;
}

public static Teacher.TeacherBuilder builder() {
    return new Teacher.TeacherBuilder();
}

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

public List<String> getAddress() {
    return this.address;
}

public void setName(String name) {
    this.name = name;
}
public void setAddress(List<String> address) {
    this.address = address;
}
protected boolean canEqual(Object other) {
    return other instanceof Teacher;
}

public String toString() {
    return "Teacher(name=" + this.getName() + ", address=" + this.getAddress() + ")";
}

public static class TeacherBuilder {
    private String name;
    private List<String> address;

    TeacherBuilder() {
    }

    public Teacher.TeacherBuilder name(String name) {
        this.name = name;
        return this;
    }

    public Teacher.TeacherBuilder address(List<String> address) {
        this.address = address;
        return this;
    }

    public Teacher build() {
        return new Teacher(this.name, this.address);
    }

    public String toString() {
        return "Teacher.TeacherBuilder(name=" + this.name + ", address=" + this.address + ")";
    }
}

Student.class

public class Student {
private String name = "c";
private int age = 25;
private long num;
private List<String> address;

private static List<String> $default$address() {
    return new ArrayList();
}

Student(String name, int age, long num, List<String> address) {
    this.name = name;
    this.age = age;
    this.num = num;
    this.address = address;
}

public static Student.StudentBuilder builder() {
    return new Student.StudentBuilder();
}

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

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

public long getNum() {
    return this.num;
}

public List<String> getAddress() {
    return this.address;
}

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

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

public void setNum(long num) {
    this.num = num;
}

public void setAddress(List<String> address) {
    this.address = address;
}
protected boolean canEqual(Object other) {
    return other instanceof Student;
}

  public String toString() {
    return "Student(name=" + this.getName() + ", age=" + this.getAge() + ", num=" + this.getNum() + ", address=" + this.getAddress() + ")";
}

public static class StudentBuilder {
    private String name;
    private int age;
    private long num;
    private boolean address$set;
    private List<String> address;

    StudentBuilder() {
    }

    public Student.StudentBuilder name(String name) {
        this.name = name;
        return this;
    }

    public Student.StudentBuilder age(int age) {
        this.age = age;
        return this;
    }

    public Student.StudentBuilder num(long num) {
        this.num = num;
        return this;
    }

    public Student.StudentBuilder address(List<String> address) {
        this.address = address;
        this.address$set = true;
        return this;
    }

    public Student build() {
        List address = this.address;
        if(!this.address$set) {
            address = Student.$default$address();
        }

        return new Student(this.name, this.age, this.num, address);
    }

    public String toString() {
        return "Student.StudentBuilder(name=" + this.name + ", age=" + this.age + ", num=" + this.num + ", address=" + this.address + ")";
    }
}

看兩個類的build方法，Student類在調用builde方法時，會判斷this.address$set 這個變量是否為false，如果為false，則為這個address對象進行賦值默認值，這個變量就是由@Builder.Default注解產生的.
      而如果你直接對address方法進行賦值話，則會將這個this.address$set進行賦值，這樣調用build方法時，就不會再對address進行賦值了.
      比較Teacher，沒有對address屬性增加@Builder.Default注解，所以在調用build方法時候，就不會產生判斷是否要對address進行默認值的初始化了，所以你獲取到的address就是null.

這下你知道你程序為什么會出現空指針異常了，為什么添加@Builder.Default注解就能解決問題了。所以對你用的東西進行深入了解，出現問題才能做到知其然知其所以然

問題2描述：

貼上測試代碼，清晰些。

import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.NoArgsConstructor;
import lombok.ToString;
 
public class testLombok {
  public static void main(String[] args) {
    People p1 = new People();
    System.out.println(p1); //People(old=false)
    People p2 = People.builder().build(); //People(old=true)
    System.out.println(p2);
  }
}
 
 
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
class People {
  @Builder.Default
  private boolean old = true;
}

為什么會這樣呢？心里一萬頭羊駝跑過...

我們來看反編譯后的People代碼

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
 
import java.beans.ConstructorProperties;
 
class People {
  private boolean old;
 
  private static boolean $default$old() {
    return true;
  }
 
  public static People.PeopleBuilder builder() {
    return new People.PeopleBuilder();
  }
 
  public String toString() {
    return "People(old=" + this.old + ")";
  }
 
  public People() {
  }
 
  @ConstructorProperties({"old"})
  public People(boolean old) {
    this.old = old;
  }
 
  public static class PeopleBuilder {
    private boolean old$set;
    private boolean old;
 
    PeopleBuilder() {
    }
 
    public People.PeopleBuilder old(boolean old) {
      this.old = old;
      this.old$set = true;
      return this;
    }
 
    public People build() {
      return new People(this.old$set ? this.old : People.$default$old());
    }
 
    public String toString() {
      return "People.PeopleBuilder(old=" + this.old + ")";
    }
  }
}

從最開始看起，代碼中old字段沒有賦初值，並且多了一個static方法$default$old，方法的返回值即為設置的默認值。接着往后看，在靜態內部類PeopleBuilder 中的build方法中對old字段進行了判斷，如果沒有被設置值，那么就將$default$old方法中的默認值賦給People實例。現在終於明白之前的困惑，並且順帶了解了@Builder.Default的實現原理。

既然@Builder.Default沒有辦法解決問題，那么該怎么辦呢？

可以換個思路，實例初始化的時候，boolean字段會被初始化為false，利用這個特性把字段名字改為notOld即可。代碼如下

public class People {
  private boolean notOld;
}