如何解决如何使用 Java 和 Gson 中的构建器模式将选择类字段序列化为 JSON 字符串?
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("+331231231","7th av,NYC");
}
}
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