1. 为什么要有链式编程

当我们想方便的将其进行实例化和赋值，往往会调用其构造方法来创建对象。
但一个POJO有很多参数的时候，我们想灵活的为其赋值，往往需要调用其set方法，但在并发条件下，这种做法会将非完全初始化的对象暴漏给外界的其他线程，这样做会产生数据不全的风险。
所以我们采用更加安全的建造者模式，通过链式表达式创建新的对象。

2. 注意

1、@Builder不支持父类对象构建，建议使用@superBuilder进行代替。
2、@Accessors需要将属性chain = true，如此可将操作默认为get&set，令其可以支持链式表达。

3. @Builder

  1. @SuperBuilder
  2. public class Person1 {
  3.     private Long id;
  4.     private String username;
  5.     private String passwd;
  6. }

编译后效果：

  1. package com.testLombok.pojo;
  3. public class Person1 {
  4.     private Long id;
  5.     private String username;
  6.     private String passwd;
  8.     protected Person1(Person1.Person1Builder<?, ?> b) {
  9.         this.id = b.id;
  10.         this.username = b.username;
  11.         this.passwd = b.passwd;
  12.     }
  14.     public static Person1.Person1Builder<?, ?> builder() {
  15.         return new Person1.Person1BuilderImpl();
  16.     }
  18.     private static final class Person1BuilderImpl extends Person1.Person1Builder<Person1, Person1.Person1BuilderImpl> {
  19.         private Person1BuilderImpl() {
  20.         }
  22.         protected Person1.Person1BuilderImpl self() {
  23.             return this;
  24.         }
  26.         public Person1 build() {
  27.             return new Person1(this);
  28.         }
  29.     }
  31.     public abstract static class Person1Builder<C extends Person1, B extends Person1.Person1Builder<C, B>> {
  32.         private Long id;
  33.         private String username;
  34.         private String passwd;
  36.         public Person1Builder() {
  37.         }
  39.         protected abstract B self();
  41.         public abstract C build();
  43.         public B id(Long id) {
  44.             this.id = id;
  45.             return this.self();
  46.         }
  48.         public B username(String username) {
  49.             this.username = username;
  50.             return this.self();
  51.         }
  53.         public B passwd(String passwd) {
  54.             this.passwd = passwd;
  55.             return this.self();
  56.         }
  58.         public String toString() {
  59.             return "Person1.Person1Builder(id=" + this.id + ", username=" + this.username + ", passwd=" + this.passwd + ")";
  60.         }
  61.     }
  62. }

由此可以看出，在使用@Builder时，需要创建一个Build对象，再掉用其对应Build方法生成我们需要得到的对象，这样会占用更多的内存空间，代码也相对变得冗余。

具体使用代码：

  1. EmsUser emsUser = EmsUser.builder().mobile("").email("").build();

4. @Accessors

  1. @Accessors(chain = true)
  2. @Data
  3. public class Person2 {
  4.     private Long id;
  5.     private String username;
  6.     private String passwd;
  7. }

编译后效果：

  1. package com.testLombok.pojo;
  3. public class Person2 {
  4.     private Long id;
  5.     private String username;
  6.     private String passwd;
  8.     public Person2() {
  9.     }
  11.     public Long getId() {
  12.         return this.id;
  13.     }
  15.     public String getUsername() {
  16.         return this.username;
  17.     }
  19.     public String getPasswd() {
  20.         return this.passwd;
  21.     }
  23.     public Person2 setId(Long id) {
  24.         this.id = id;
  25.         return this;
  26.     }
  28.     public Person2 setUsername(String username) {
  29.         this.username = username;
  30.         return this;
  31.     }
  33.     public Person2 setPasswd(String passwd) {
  34.         this.passwd = passwd;
  35.         return this;
  36.     }
  38.     public boolean equals(Object o) {
  39.         if (o == this) {
  40.             return true;
  41.         } else if (!(o instanceof Person2)) {
  42.             return false;
  43.         } else {
  44.             Person2 other = (Person2)o;
  45.             if (!other.canEqual(this)) {
  46.                 return false;
  47.             } else {
  48.                 label47: {
  49.                     Object this$id = this.getId();
  50.                     Object other$id = other.getId();
  51.                     if (this$id == null) {
  52.                         if (other$id == null) {
  53.                             break label47;
  54.                         }
  55.                     } else if (this$id.equals(other$id)) {
  56.                         break label47;
  57.                     }
  59.                     return false;
  60.                 }
  62.                 Object this$username = this.getUsername();
  63.                 Object other$username = other.getUsername();
  64.                 if (this$username == null) {
  65.                     if (other$username != null) {
  66.                         return false;
  67.                     }
  68.                 } else if (!this$username.equals(other$username)) {
  69.                     return false;
  70.                 }
  72.                 Object this$passwd = this.getPasswd();
  73.                 Object other$passwd = other.getPasswd();
  74.                 if (this$passwd == null) {
  75.                     if (other$passwd != null) {
  76.                         return false;
  77.                     }
  78.                 } else if (!this$passwd.equals(other$passwd)) {
  79.                     return false;
  80.                 }
  82.                 return true;
  83.             }
  84.         }
  85.     }
  87.     protected boolean canEqual(Object other) {
  88.         return other instanceof Person2;
  89.     }
  91.     public int hashCode() {
  92.         int PRIME = true;
  93.         int result = 1;
  94.         Object $id = this.getId();
  95.         int result = result * 59 + ($id == null ? 43 : $id.hashCode());
  96.         Object $username = this.getUsername();
  97.         result = result * 59 + ($username == null ? 43 : $username.hashCode());
  98.         Object $passwd = this.getPasswd();
  99.         result = result * 59 + ($passwd == null ? 43 : $passwd.hashCode());
  100.         return result;
  101.     }
  103.     public String toString() {
  104.         return "Person2(id=" + this.getId() + ", username=" + this.getUsername() + ", passwd=" + this.getPasswd() + ")";
  105.     }
  106. }

借助简单的return this实现链式表达，代码实现简单规范，原理清晰透彻。

具体使用代码：

  1. EmsUser emsUser = new EmsUser().setEmail("").setMobile("");

5. 如何选择

由于@Accessors仍为一个不稳定版本的注解，在长期迭代的项目中不推荐使用，建议使用@Builder

6. @Accessors使用中的空指针异常

  1. java.lang.NullPointerException
  2.     at net.sf.cglib.core.ReflectUtils.getMethodInfo(ReflectUtils.java:424)
  3.     at net.sf.cglib.beans.BeanCopier$Generator.generateClass(BeanCopier.java:133)
  4.     at net.sf.cglib.core.DefaultGeneratorStrategy.generate(DefaultGeneratorStrategy.java:25)
  5.     at net.sf.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:216)
  6.     at net.sf.cglib.beans.BeanCopier$Generator.create(BeanCopier.java:90)
  7.     at net.sf.cglib.beans.BeanCopier.create(BeanCopier.java:50)

当借助工具对实体类进行反射浅拷贝时，会校验其set返回值是否为void，此时使用@Accessors便会出现异常

解决办法：

  1. `BeanCopier.create` will throw exception when the `source` or `target` use `@Accessors(chain = true)` in lombok.
  2. But `org.springframework.beans.BeanUtils.copyProperties(source, target);` has no problem.