JavaBean之Builder模式

小灰灰 2022-06-04 05:36 190阅读 0赞

### 静态工厂和构造器 ###

重载构造函数，分别包含多个不同的参数

> 无法很好地扩展到大量的可选参数

### JavaBean方式 ###

调用一个无参构造函数来创建对象，然后通过setter方法来设置必要参数

> 对象构造过程被分割到了几个调用，构造过程中可能导致JavaBean处于不一致的状态，类无法通过检验构造器参数的有效性来保证一致性

### Builder模式 ###

> 不直接生成想要的对象，通过调用构造器（或静态工厂），得到builder对象，客户端在builder对象上调用类似setter方法，来设置每个相关的可选参数，最后，再调用build方法来生成不可变对象

public class Person { 
        @Override
        public String toString() {
            return "Person{" +
                    "name='" + name + '\'' +
                    ", address='" + address + '\'' +
                    ", firstName='" + firstName + '\'' +
                    ", lastName='" + lastName + '\'' +
                    '}';
        }
    
        private String name;
        private String address;
        private String firstName;
        private String lastName;
    
        private Person(String name, String address, String firstName, String lastName) {
            this.address = address;
            this.name = name;
            this.lastName = lastName;
            this.firstName = firstName;
        }
    
        public static Builder builder() {
            return new Builder();
        }
    
        public static class Builder { 
    
            private String name;
            private String address;
            private String firstName;
            private String lastName;
    
            public Builder name(final String name) {
                this.name = name;
                return this;
            }
    
            public Builder address(final String address) {
                this.address = address;
                return this;
            }
    
            public Builder lastName(final String lastName) {
                this.lastName = lastName;
                return this;
            }
    
            public Builder firstName(final String firstName) {
                this.firstName = firstName;
                return this;
            }
    
            public Person build() {
                return new Person(name, address, firstName, lastName);
            }
        }
    }

客户端调用

Person p = new Person.Builder().address("上海").firstName("三").lastName("张").name("欧阳").build();
            System.out.println(p);
            Person person = Person.builder().address("上海").firstName("三").lastName("张").name("欧阳").build();
            System.out.println(person);

如果我们使用IDE，我们可以直接使用Lombok插件的@Builder注解，仅需声明类属性

@Builder
    @ToString
    public class LombokPerson { 
        private String name;
        private String address;
        private String firstName;
        private String lastName;
    }

查看生成的class源码

public class LombokPerson {
        private String name;
        private String address;
        private String firstName;
        private String lastName;
    
        @ConstructorProperties({
       "name", "address", "firstName", "lastName"})
        LombokPerson(String name, String address, String firstName, String lastName) {
            this.name = name;
            this.address = address;
            this.firstName = firstName;
            this.lastName = lastName;
        }
    
        public static LombokPerson.LombokPersonBuilder builder() {
            return new LombokPerson.LombokPersonBuilder();
        }
    
        public String toString() {
            return "LombokPerson(name=" + this.name + ", address=" + this.address + ", firstName=" + this.firstName + ", lastName=" + this.lastName + ")";
        }
    
        public static class LombokPersonBuilder {
            private String name;
            private String address;
            private String firstName;
            private String lastName;
    
            LombokPersonBuilder() {
            }
    
            public LombokPerson.LombokPersonBuilder name(String name) {
                this.name = name;
                return this;
            }
    
            public LombokPerson.LombokPersonBuilder address(String address) {
                this.address = address;
                return this;
            }
    
            public LombokPerson.LombokPersonBuilder firstName(String firstName) {
                this.firstName = firstName;
                return this;
            }
    
            public LombokPerson.LombokPersonBuilder lastName(String lastName) {
                this.lastName = lastName;
                return this;
            }
    
            public LombokPerson build() {
                return new LombokPerson(this.name, this.address, this.firstName, this.lastName);
            }
    
            public String toString() {
                return "LombokPerson.LombokPersonBuilder(name=" + this.name + ", address=" + this.address + ", firstName=" + this.firstName + ", lastName=" + this.lastName + ")";
            }
        }
    }

客户端调用：

LombokPerson lombokPerson = LombokPerson.builder().address("上海").firstName("三").lastName("张").name("欧阳").build();
            System.out.println(lombokPerson);

[https://dzone.com/articles/too-many-parameters-java-1][https_dzone.com_articles_too-many-parameters-java-1]

[https_dzone.com_articles_too-many-parameters-java-1]: https://dzone.com/articles/too-many-parameters-java-1