将原型bean注入单例bean

Question

我刚接触Spring，正在尝试理解“将一个原型bean注入一个单例bean”的概念。根据我在singleton中的理解，每个Spring IoC容器只有一个实例，不管您检索它多少次。validator.validate(requestId）；，因为仍未实例化private RequestValidator validator。我开发了以下示例，其中在单个bean中给出了原型bean的参考，如下所示：

<bean id="requestProcessor" class="com.injection.testing.RequestProcessor">
        <property name="validator" ref="validator" />
</bean>

<bean id="validator" class="com.injection.testing.RequestValidator" scope="prototype" />

RequestProcessor.java

public class RequestProcessor {
    private RequestValidator validator;

    public RequestProcessor(){
        System.out.println("Constructor:: RequestProcessor instance created!");
    }

    public void handleRequest(String requestId){
        System.out.println("Request ID : "+ requestId);
        validator.validate(requestId);
    }

    public RequestValidator getValidator() {
        return validator;
    }

    public void setValidator(RequestValidator validator) {
        this.validator= validator;
    }
}

requestValidator.java

public class RequestValidator {
    private List<String> errorMessages = new ArrayList<String>();

    public RequestValidator() {
        System.out.println("Constructor:: RequestValidator instance created!");
    }

    // Validates the request and populates error messages
    public void validate(String requestId){
        System.out.println("RequestValidator :"+requestId);
    }

    public List<String> getErrorMessages() {
        return errorMessages;
    }
}

public class MainDemo {

    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");

        RequestProcessor processor = (RequestProcessor) context.getBean("requestProcessor");
        processor.handleRequest("1212");
        System.out.println("------------------------");
        processor.handleRequest("1213");
    }
}

Constructor:: RequestProcessor instance created!
Constructor:: RequestValidator instance created!
Request ID : 1212
RequestValidator :1212
------------------------
Request ID : 1213
RequestValidator :1213

如何解决？我的理解正确吗？

另一种方式：

<!-- Lookup way  -->
    <bean id="requestProcessor" class="com.injection.testing.RequestProcessor" >
        <lookup-method name="getValidator" bean="validator" />
    </bean>

    <bean id="validator" class="com.injection.testing.RequestValidator" scope="prototype" />

如果我调用我的main方法，我会看到下面的output+error：这里codevalidator.validate(requestId)；执行，private RequestValidator validator；没有安装，为什么会出现空指针异常。

我在下面的代码中显示了：

public class MainDemo {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");

        RequestValidator requestValidator = (RequestValidator) context.getBean("validator");

        RequestProcessor processor = (RequestProcessor) context.getBean("requestProcessor");
        processor.handleRequest("1212");
        System.out.println("------------------------");
        processor.handleRequest("1213");
    }
}

现在我看到下面的错误：

Constructor:: RequestProcessor instance created!
Constructor:: RequestValidator instance created!
Request ID : 1212
Exception in thread "main" java.lang.NullPointerException
    at com.injection.testing.RequestProcessor.handleRequest(RequestProcessor.java:12)
    at com.injection.testing.MainDemo.main(MainDemo.java:14)

Answer 1

注入只发生一次，当Spring上下文启动时。如果bean具有prototype作用域，那么Spring将为每次注入创建新的原型bean。但不会在每次调用它的方法时都创建prototype bean。让我们考虑下一个例子：

<bean id="firstRequestProcessor" class="com.injection.testing.RequestProcessor">
        <property name="validator" ref="validator" />
</bean>

<bean id="secondRequestProcessor" class="com.injection.testing.RequestProcessor">
        <property name="validator" ref="validator" />
</bean>


<bean id="validator" class="com.injection.testing.RequestValidator" scope="prototype" />

在这种情况下，RequestProcessorbean都将有自己的RequestValidatorbean实例。

查找方法是一种方法，您应该在每次需要新的原型bean实例时调用它。最好让这个方法abstract，因为无论如何Spring都会自动重写这个方法。例如：

public class abstract RequestProcessor {

    public void handleRequest(String requestId){
        System.out.println("Request ID : "+ requestId);
        RequestValidator validator = createValidator(); //here Spring will create new instance of prototype bean
        validator.validate(requestId);
    }

    protected abstract RequestValidator createValidator();
}

<bean id="requestProcessor" class="com.injection.testing.RequestProcessor" >
    <lookup-method name="createValidator" bean="validator" />
</bean>

<bean id="validator" class="com.injection.testing.RequestValidator" scope="prototype" />