将原型注入单例（java配置+注解）

Question

我被问到关于将原型注入单例的面试问题。我很难回答，现在我正在尝试研究这个。

我已经写了以下代码（pring boot）

豆 1：

@Service
@Scope(value = "prototype")
public class MyValidator {
}

豆 2：

@Service
public class ValidatorHolder {

    @Autowired
    MyValidator myValidator;

    public MyValidator getMyValidator() {
        return myValidator;
    }
}

配置：

@SpringBootApplication
@Configuration
@ComponentScan("com.example.domain")
public class DemoApplication {


    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
        ApplicationContext context = new AnnotationConfigApplicationContext(DemoApplication.class);
        ValidatorHolder validatorHolder1 = (ValidatorHolder) context.getBean("validatorHolder");
        ValidatorHolder validatorHolder2 = (ValidatorHolder) context.getBean("validatorHolder");
        System.out.println("=====================================");
        System.out.println(validatorHolder1.getMyValidator() == validatorHolder2.getMyValidator());
        System.out.println("=====================================");

    }
}

此代码返回 true。

正如我在阅读 article link 时所理解的那样 可以配置返回false。

如何在我的代码中执行？ （无 xml）

附言

我尝试重写文章中的代码：

   <bean id="validatorHolder" class="com.example.domain.ValidatorHolder">
        <property name="myValidator" ref="validator"/>
    </bean>

    <bean id="validator" scope="prototype" class="com.example.domain.MyValidator">
        <!-- This instructs the container to proxy the current bean-->
        <aop:scoped-proxy/>
    </bean>

在 main 方法中，我编写了以下代码：

ApplicationContext xmlContext = new FileSystemXmlApplicationContext("classpath:applicationContext.xml");
ValidatorHolder validatorHolder21 = (ValidatorHolder) xmlContext.getBean("validatorHolder");
ValidatorHolder validatorHolder22 = (ValidatorHolder) xmlContext.getBean("validatorHolder");
System.out.println("=====================================");
System.out.println(validatorHolder21.getMyValidator() == validatorHolder22.getMyValidator());
System.out.println("=====================================");

反正我看到true

附言2

让我们研究 Sean Patrick Floyd 的答案（范围代理，b））

我使用以下主要方法类：

@SpringBootApplication
@ComponentScan("com.example.domain")
public class DemoApplication {


    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);

        ApplicationContext context = new AnnotationConfigApplicationContext(DemoApplication.class);
        ValidatorHolder validatorHolder1 = (ValidatorHolder) context.getBean("validatorHolder");
        ValidatorHolder validatorHolder2 = (ValidatorHolder) context.getBean("validatorHolder");
        System.out.println("=====================================");
        System.out.println(validatorHolder1.getMyValidator() == validatorHolder2.getMyValidator());
        System.out.println("=====================================");         
}

当我运行应用程序时 - 我看到了

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'validatorHolder' defined in file [D:\freelance\demo\target\classes\com\example\domain\ValidatorHolder.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.example.domain.ValidatorHolder]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.example.domain.ValidatorHolder.<init>()
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1099)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1044)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:504)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:476)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:303)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:299)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:755)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:759)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:480)
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:689)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:321)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:969)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:958)
    at com.example.DemoApplication.main(DemoApplication.java:20)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.example.domain.ValidatorHolder]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.example.domain.ValidatorHolder.<init>()
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:85)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1092)
    ... 20 common frames omitted
Caused by: java.lang.NoSuchMethodException: com.example.domain.ValidatorHolder.<init>()
    at java.lang.Class.getConstructor0(Class.java:3074)
    at java.lang.Class.getDeclaredConstructor(Class.java:2170)
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:80)
    ... 21 common frames omitted

附言 3

P.S.2 问题与构造函数上错过的@Autowired 有关

修复此问题后

System.out.println(validatorHolder1.getMyValidator() == validatorHolder2.getMyValidator());

返回真

但如果有点替换 MyValidator 代码：

@Service
@Scope(value="prototype", proxyMode=ScopedProxyMode.TARGET_CLASS)
public class MyValidator {
    Object object = new Object();

    public Object getObject() {
        return object;
    }

    public void setObject(Object object) {
        this.object = object;
    }
}

---

 System.out.println(validatorHolder1.getMyValidator() == validatorHolder2.getMyValidator());

是的

 System.out.println(validatorHolder1.getMyValidator().getObject() == validatorHolder2.getMyValidator().getObject());

假

甚至

System.out.println(validatorHolder1.getMyValidator().getObject() == validatorHolder1.getMyValidator().getObject());

假

Answer 1

您误解了方法注入技术。您需要使您的 bean 抽象才能工作：

public class MyValidator {}

public abstract class ValidatorHolder {
    public abstract MyValidator getMyValidator();
}

现在您可以按如下方式在 XML 中定义 bean：

<bean class="com.somepackage.MyValidator" scope="prototype" />
<bean class="com.somepackage.ValidatorHolder">
    <lookup-method name="getMyValidator" bean="myValidator" />
</bean>

在这种情况下，Spring 将创建一个ValidatorHolder 的匿名子类，它会在每个被调用的类型中返回原型 bean（一个新副本）。

使用带注释的服务类，查找方法注入是不可能的，但这是您可以使用@Configuration 类的方法：

@Configuration
public class MyConfiguration{

    @Bean
    @Scope("prototype")
    public MyValidator myValidator(){
        return new MyValidator();
    }

    @Bean
    public ValidatorHolder validatorHolder(){
        return new ValidatorHolder(){
            @Override public MyValidator getMyValidator(){
                return myValidator();
            }
        };
    }
}

在这种情况下，您正在自己创建ValidatorHolder 的子类，您可以清楚地看到发生了什么。

但任何一个版本都只有在你将 bean 和提供者方法抽象化时才有效。

最后，定义 spring bean 有三种不同的方式：

• XML
• 带有组件的注释类（例如@Service、@Component） 扫描
• @Configuration 类与 @Bean 方法。

在您的示例代码中，您混合了这三种样式，这几乎不是一个好主意。选择一种技术并坚持下去。

---

关于作用域代理，这可以在所有三种 bean 注册技术中实现。

a) XML

public class MyValidator {}

public class ValidatorHolder {
    private MyValidator myValidator;
    public void setMyValidator(MyValidator myValidator){
        this.myValidator = myValidator;}
    public MyValidator getMyValidator();
}

<bean class="com.somepackage.MyValidator" scope="prototype" />
<bean class="com.somepackage.ValidatorHolder">
    <aop:scoped-proxy />
</bean>

b) 带注释的服务类

@Service @Scope(value="prototype", proxyMode=ScopedProxyMode.TARGET_CLASS)
public class MyValidator {}

@Service
public class ValidatorHolder {
    @Autowired
    public ValidatorHolder(MyValidator myValidator){
        this.myValidator=myValidator;
    }
    private final MyValidator myValidator;
    public MyValidator getMyValidator(){ return myValidator; };
}

c) @Configuration 类，类似 XML 版本的 Bean 类

@Configuration
public class MyConfiguration{

    @Bean
    @Scope("prototype")
    public MyValidator myValidator(){
        return new MyValidator();
    }

    @Bean
    public ValidatorHolder validatorHolder(){
        return new ValidatorHolder(myValidator());
    }

}

请注意，所有代理解决方案将始终返回相同的对象，即代理。但底层功能将委托给不同的对象。通过将此代码添加到 MyValidator 来尝试一下：

private int counter = 1;
public int counter(){
    return counter ++;
}

现在，与您调用此代码的频率无关：

validatorHolder.getMyValidator().counter();

它总是会返回1。

Answer 2

@Service
public class ValidatorHolder {

@Autowired
ApplicatioContext context;

public MyValidator getMyValidator() {
    return context.getBean(MyValidator.class);
}
}

另请阅读http://shekhargulati.com/tag/method-injection/