Question

在Spring应用程序中，我在构造函数上使用了@Autowired(required=false)。这意味着如果在xml文件中没有自动装配的bean，则不应抛出NoSuchBeanDefinitionException，因为提到了required=false。但我收到了UnsatisfiedDependencyException，NoSuchBeanDefinitionException例外情况。

---- TextEditor

public class TextEditor {

    private SpellChecker x;
    private String name;    

    @Autowired(required=false)
    public TextEditor(SpellChecker x) {
        System.out.println("Inside TextEditor constructor." );
        this.x = x;
    }


    public SpellChecker getY() {
        return x;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void spellCheck() {
        x.checkSpelling();
    }
}

---- SpellChecker

public class SpellChecker {
    public SpellChecker() {
        System.out.println("Inside SpellChecker constructor.");
    }

    public void checkSpelling() {
        System.out.println("Inside checkSpelling.");
    }
}

---- Beans.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:aks="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context"
    aks:schemaLocation="http://www.springframework.org/schema/beans 
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd 
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <context:annotation-config />

    <bean id="textEditor" class="com.tutorialspoint.TextEditor">
        <!-- <property name="x" ref="a" /> -->
        <property name="name" value="Generic Text Editor" />
    </bean>

    <!-- <bean id="a" class="com.tutorialspoint.SpellChecker" /> -->

</beans>

---- MainApp
public class MainApp {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext(
                "Beans.xml");//Beans.xml, Beans1.xml
        TextEditor te = (TextEditor) context.getBean("textEditor");
        //te.spellCheck();
        System.out.println(te.getY());
    }
}

---控制台（实际结果）

Apr 24, 2014 4:30:00 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@15eb0a9: startup date [Thu Apr 24 16:30:00 IST 2014]; root of context hierarchy
Apr 24, 2014 4:30:00 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [Beans.xml]
Apr 24, 2014 4:30:00 PM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@2d9c06: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,textEditor]; root of factory hierarchy
Apr 24, 2014 4:30:00 PM org.springframework.beans.factory.support.DefaultSingletonBeanRegistry destroySingletons
INFO: Destroying singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@2d9c06: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,textEditor]; root of factory hierarchy
Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'textEditor' defined in class path resource [Beans.xml]: Unsatisfied dependency expressed through constructor argument with index 0 of type [com.tutorialspoint.SpellChecker]: : No matching bean of type [com.tutorialspoint.SpellChecker] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {}; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No matching bean of type [com.tutorialspoint.SpellChecker] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {}
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:730)
    at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:196)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1002)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:906)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:484)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:455)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:293)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:290)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:192)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:585)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:895)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:425)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
    at com.tutorialspoint.MainApp.main(MainApp.java:8)
Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No matching bean of type [com.tutorialspoint.SpellChecker] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {}
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:924)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:793)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:707)
    at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:795)
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:723)
    ... 15 more

---预期结果程序应该在没有任何Exception的情况下运行，因为构造函数提到了@Autowire(required=false)。即使找不到bean，也不应该提到异常，因为提到了{required=false。

Answer 1

发生此异常是因为 required = false并不意味着它将注入null 。当应用于构造函数时，Spring将尝试确定哪个构造函数最适合创建实例。在这种情况下，您只有一个构造函数需要SpellChecker但没有该类型的对象。

在Spring doc（http://docs.spring.io/spring/docs/4.0.x/javadoc-api/org/springframework/beans/factory/annotation/Autowired.html）中引用：

任何给定bean类只有一个构造函数（最大值）可以带有这个注释，指示构造函数在用作Spring bean时要自动装配。这样的构造函数不必公开。

无论如何你可以添加一个默认的构造函数（它可以是私有的），这样当Spring无法执行它的@Autowired魔法时它会使用那个。在TextEditor类中，您可以添加：

@Deprecated
private TextEditor() {
    // You could leave x = null or create a default value for that field 
    // if you have one (eg. x = new DefaultSpellChecker();)
}

请注意@Deprecated用于避免编译器警告您有一个没有人使用的私有构造函数。

Answer 2

我有类似的问题。实际上，您可以使用

创建多个构造函数

@Autowired(required = false)

但请注意，您不能在默认（不带参数）构造函数中添加此注释，因为@Autowired注释至少需要一个参数

在给出NoSuchBeanDefinitionException的构造函数上@Autowired（required = false）

2 个答案: