认识lombok 的@Builder注解对初始化的影响

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

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

先上结论：

• 问题1：如果类中用了@Builder注解，而属性没有任何注解话，那么在你初始化这个类的时候，如果你的属性赋值了默认值，则在你用builder方法初始化该类后，属性的默认值则无效即获取会产生空指针异常
• 问题2：在具体要赋默认值字段上加@Builder.default注解可以解决问题1，但是际运行代码之后，我发现一个无奈的问题，builder模式下默认值生效了，但是使用new(以及正常反序列化)得到的实例默认值不会被设置

问题1描述：

我们来剖析下这中间发生了什么

从上面的例子，可以发现Teacher 的address属性为空，这正是我们很常规初始化操作，获取这个address，接着对它进行操作， 如果此时它是null，则会出现空指针异常；
      比较Student中的address则是我们理想中的正常执行过程，是有一个默认值的对象，同时观察Student中的name 和age两个属性值默认值也出现了如同Teacher中的address现象，默认值消失了；
      从表面来看，Student多了@Builder.Default的注解，这个注解确实就是解决这个问题关键，让你想要赋值的默认值来进行正确的初始化了。知道了这个注解的使用只是做到了知其然，我们要做做知其所以然，所以来看看下他们生成的class有什么区别？ 以下代码反编译删除了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 + ")";
    }
}

看两个类的build方法，Student类在调用builde方法时，会判断this.address$set 这个变量是否为false，如果为false，则为这个address对象进行赋值默认值，这个变量就是由@Builder.Default注解产生的.
      而如果你直接对address方法进行赋值话，则会将这个this.address$set进行赋值，这样调用build方法时，就不会再对address进行赋值了.
      比较Teacher，没有对address属性增加@Builder.Default注解，所以在调用build方法时候，就不会产生判断是否要对address进行默认值的初始化了，所以你获取到的address就是null.

这下你知道你程序为什么会出现空指针异常了，为什么添加@Builder.Default注解就能解决问题了。所以对你用的东西进行深入了解，出现问题才能做到知其然知其所以然

问题2描述：

贴上测试代码，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;
}

为什么会这样呢？心里一万头羊驼跑过...

我们来看反编译后的People代码

//
// 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 + ")";
    }
  }
}

从最开始看起，代码中old字段没有赋初值，并且多了一个static方法$default$old，方法的返回值即为设置的默认值。接着往后看，在静态内部类PeopleBuilder 中的build方法中对old字段进行了判断，如果没有被设置值，那么就将$default$old方法中的默认值赋给People实例。现在终于明白之前的困惑，并且顺带了解了@Builder.Default的实现原理。

既然@Builder.Default没有办法解决问题，那么该怎么办呢？

可以换个思路，实例初始化的时候，boolean字段会被初始化为false，利用这个特性把字段名字改为notOld即可。代码如下

public class People {
  private boolean notOld;
}