如何在 Spring 引導中創建不可變和 singleton class？

Question

我有一個需要不可變的人 model class 和 singleton。我需要從我的服務 class 自動裝配但出現錯誤。

那么有什么辦法可以在spring引導中創建一個singleton不可變的class呢？ 我可以親自設置值（通過自動裝配）並在整個應用程序中使用相同的值。

我剛剛開始學習 spring-boot。我在谷歌上搜索並找到了 Lombok 的解決方案，但不知何故它對我不起作用。

我的人 model：

    @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());
        
    }

錯誤：

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

Answer 1

此配置 class 和該注釋將始終為您提供該 Person Bean

@Configuration
public class ServerConfig extends
{

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

現在它將隨處可用於自動裝配。

然而，正如評論所指出的，使用 model class 作為 Singleton 似乎很奇怪。

Answer 2

為讀者添加更多信息，

看看這個問題更清楚
SpringBoot - @Configuration 創建一個 singleton bean