如何在 Spring Boot 中创建不可变和单例类？答案

【问题标题】：How to create immutable and singleton class in Spring boot?如何在 Spring Boot 中创建不可变和单例类？
【发布时间】：2021-06-08 11:38:08
【问题描述】：

我有一个需要不可变和单例的人模型类。我需要从我的服务类中自动装配，但出现错误。

那么有什么方法可以在 Spring Boot 中创建一个单例不可变类？我可以亲自设置值（通过自动连接）并在整个应用程序中使用相同的值。

我刚开始学习spring-boot。我在谷歌上搜索并找到了 Lombok 的解决方案，但不知何故它对我不起作用。

我的人物模型：

    @Value
    @Builder
    @AllArgsConstructor(access = AccessLevel.PRIVATE) 
    @Component
    public final class Person {
        private final String firstName;
        private final String lastName;
    }


//My service class
@Component
public class ArgumentReadingService {
    
    @Autowired
    Person person;
    
    public void readArguments() {
        
        person = Person.builder().firstName("Hello").build();
        
        System.out.println("name : "+person.getFirstName());
        
    }

错误：

Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2021-03-10 17:52:29.197 ERROR 8824 --- [           main] o.s.boot.SpringApplication               : Application run failed

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'person' defined in file [C:\Users\User\Desktop\WorkSpace\Orion.Strat1.bidAsk_Stp\target\classes\orion\model\Person.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [orion.model.Person]: No default constructor found; nested exception is java.lang.NoSuchMethodException: orion.model.Person.<init>()
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1316) ~[spring-beans-5.3.4.jar:5.3.4]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1214) ~[spring-beans-5.3.4.jar:5.3.4]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:564) ~[spring-beans-5.3.4.jar:5.3.4]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:524) ~[spring-beans-5.3.4.jar:5.3.4]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335) ~[spring-beans-5.3.4.jar:5.3.4]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.3.4.jar:5.3.4]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) ~[spring-beans-5.3.4.jar:5.3.4]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208) ~[spring-beans-5.3.4.jar:5.3.4]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:944) ~[spring-beans-5.3.4.jar:5.3.4]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:917) ~[spring-context-5.3.4.jar:5.3.4]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:582) ~[spring-context-5.3.4.jar:5.3.4]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:767) [spring-boot-2.4.3.jar:2.4.3]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:759) [spring-boot-2.4.3.jar:2.4.3]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:426) [spring-boot-2.4.3.jar:2.4.3]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:326) [spring-boot-2.4.3.jar:2.4.3]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1311) [spring-boot-2.4.3.jar:2.4.3]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1300) [spring-boot-2.4.3.jar:2.4.3]
    at com.orion.Application.main(Application.java:20) [classes/:na]
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [orion.model.Person]: No default constructor found; nested exception is java.lang.NoSuchMethodException: orion.model.Person.<init>()
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:83) ~[spring-beans-5.3.4.jar:5.3.4]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1308) ~[spring-beans-5.3.4.jar:5.3.4]
    ... 17 common frames omitted
Caused by: java.lang.NoSuchMethodException: orion.model.Person.<init>()
    at java.lang.Class.getConstructor0(Class.java:3082) ~[na:1.8.0_191]
    at java.lang.Class.getDeclaredConstructor(Class.java:2178) ~[na:1.8.0_191]
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:78) ~[spring-beans-5.3.4.jar:5.3.4]
    ... 18 common frames omitted

【问题讨论】：

您希望 Spring 从哪里获取 Person 的名字和姓氏？此外，您已将构造函数指定为私有。 Spring 可能能够通过反射解决这个问题，但它很可能会忽略任何私有构造函数。无论哪种方式都绝对是代码味道，因为如果您愿意，您将无法为其编写单元测试。
模型类应该不是一个单例；它们实际上是每个请求的特定数据。
Builder 模式和 Singleton 并不完全相同。如果您允许您的 Builder 创建无数实例，则将您的构造函数设为私有并不会使其成为单例。

标签： java spring spring-boot autowired lombok

【解决方案1】：

这个配置类和那个注解将始终为您提供那个 Person Bean

@Configuration
public class ServerConfig extends
{

    @Bean
    private Person getPerson(){
        return new Person("name", "lastName");
    }

现在它可以在任何地方进行自动装配。

然而，正如 cmets 所指出的，将模型类用作 Singleton 似乎很奇怪。

【讨论】：