如何使用 Java 和 Gson 中的构建器模式将选择类字段序列化为 JSON 字符串？

问题描述

我创建了一个带有构建器模式的 User 类，目的是将其序列化为用于 POST 的 JSON 字符串。现在，需求发生了变化，我需要能够修补现有记录，更新记录集中的一个或多个字段，但不是所有字段。下面的示例在其 User 类中有 5 个字段，但想象一下，如果它有 30 甚至 40 个字段，包括 int 类型。

import com.google.gson.*;

class User 
{
    //All final attributes
    private final String firstName;
    private final String lastName;
    private final int age;
    private final String phone;
    private final String address;
 
    private User(UserBuilder builder) {
        this.firstName = builder.firstName;
        this.lastName = builder.lastName;
        this.age = builder.age;
        this.phone = builder.phone;
        this.address = builder.address;
    }
 
    //All getter,and NO setter to provide immutability
    public String getFirstName() {
        return firstName;
    }
    public String getLastName() {
        return lastName;
    }
    public int getAge() {
        return age;
    }
    public String getPhone() {
        return phone;
    }
    public String getAddress() {
        return address;
    }
 
    public static class UserBuilder 
    {
        private String firstName;
        private String lastName;
        private int age;
        private String phone;
        private String address;
 
        public UserBuilder() {
        }

        public UserBuilder(String firstName,String lastName) {
            this.firstName = firstName;
            this.lastName = lastName;
        }

        public UserBuilder firstName(String firstName) {
            this.firstName = firstName;
            return this;
        }
        
        public UserBuilder lastName(String lastName) {
            this.firstName = lastName;
            return this;
        }
        public UserBuilder age(int age) {
            this.age = age;
            return this;
        }
        public UserBuilder phone(String phone) {
            this.phone = phone;
            return this;
        }
        public UserBuilder address(String address) {
            this.address = address;
            return this;
        }
        //Return the finally consrcuted User object
        public User build() {
            User user =  new User(this);
            return user;
        }
    }
}

public class TestUserBuild {

    public static void main(String[] args) {
        // Todo Auto-generated method stub
        User user = new User.UserBuilder().
        //No last name
        //No age
        //No phone
        //no address
        .firstName("Super")
        .build();
                 
        System.out.println(user); // User: Super,null,null
                    
        Gson gson = new Gson();
                    
        System.out.println(gson.toJson(user)); // {"firstName":"Super","age":0}
    }
}

我没有指定年龄，但它在 JSON 字符串中。我认为构建器模式将有助于创建任意数量的 JSON 字符串排列，即更新名字和姓氏，仅更新名字，仅更新年龄，仅更新姓氏和电话号码等...

构建器模式方法不是解决此问题的正确方法吗？如果这是一个可接受的解决方案，我如何利用构建器模式将 User 类序列化为 JSON 字符串，但只有我选择的字段？我可以在 Gson 库中利用某些东西来实现此任务，例如创建自定义类型适配器吗？也许我可以创建一个自定义类型适配器，将所有字段作为输入，检查每个字段是否为 NULL，或者对于整数是否为 0，然后仅使用增量构建 JSON 字符串。

解决方法

您的“年龄”字段值为 0，因为“int”具有默认值。如果您希望年龄字段默认为空，请使用“整数”。

构建器模式的一些优点是不变性（您可以选择允许构建器仅在创建时修改类，从类中删除所有设置器），并且它是更简洁，用于实例化具有多个属性的类。

但是您不需要保持您的构建器模式不可变。如果我正确理解您的需求，您可以在对象创建期间保留构建器模式以实现多功能性，并将设置器保留在类中以便能够轻松更新字段。

import com.google.gson.*;

class User {
//Your attributes don't need to be final
private String firstName;
private String lastName;
private int age;
private String phone;
private String address;

private User(UserBuilder builder) {
    this.firstName = builder.firstName;
    this.lastName = builder.lastName;
    this.age = builder.age;
    this.phone = builder.phone;
    this.address = builder.address;
}

//getters AND setters (omitted for brevity)

//builder class stays as is,omitted for brevity

public class TestUserBuild {

    public static void main(String[] args) {
        User user = new User.UserBuilder()
        .firstName("Super")
        .build();
                 
        //when you need to update
        user.setAge(42);
    }
}

问题是，如果您需要定期更新字段，为什么还需要不变性？您是否应该完全删除不变性约束（通过将 setter 留在类中）？您是否需要能够根据特定的业务规则更新特定的字段组合？如果后者是真的，我会建议远离 anemic model （一个“getter 和 setter 包”）并添加特定于域的方法来更新相关字段。您的 User 类将如下所示：

class User {
    //Your attributes don't need to be final
    private String firstName;
    private String lastName;
    private int age;
    private String phone;
    private String address;

    private User(UserBuilder builder) {
        this.firstName = builder.firstName;
        this.lastName = builder.lastName;
        this.age = builder.age;
        this.phone = builder.phone;
        this.address = builder.address;
    }
    //getters ONLY (omitted for brevity)
    //no setters,only domain-relevant methods which update fields as needed
    public void setIdentity(String firstName,String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }
    public void setCoordinates(String phone,String address) {
        this.phone = phone;
        this.address = address;
    }
}
//builder class stays as is

public class TestUserBuild {

    public static void main(String[] args) {
        User user = new User.UserBuilder()
        .firstName("Super")
        .build();

        //update identity (say,your frontend has an "identity" page with only firstName and lastName on it
        user.setIdentity("Chris","Neve");
        //your frontend page allowing user to update coordinates
        user.setCoordinates("[phone]","7th av,NYC");
    }
}