为什么运行这个简单的Spring配置需要一个no-args构造函数？

Question

我正试图在Mkyong关于Spring的教程。我创建了两个简单的类，Customer和Person，看起来像这样：

客户：

public class Customer {

    private Person person;

    public Customer() {
    }

    //Getters and setters for person here

    public String toString() {
        return "Customer [person=" + person +"]";
    }
}

人：

public class Person {

    private String name;
    private String address;
    private int age;

    public Person() {

    }

    public Person(String name, String address, int age) {
        this.name = name;
        this.address = address;
        this.age = age;
    }

    //All getters and setters

    public String toString() {
        return "Person [address=" + address + ", age=" + age + ", name=" + name + "]";
    }
}

然后我创建了一个Bean配置文件，使用内部bean（这就是我参加教程的原因），看起来像这样：

// Standard bean declarations

    <bean id="CustomerBean" class="com.andrew.SpringInnerBeans.Customer">
        <property name="person">
            <bean class="com.andrew.SpringInnerBeans.Person">
                <property name="name" value="Andrew" />
                <property name="address" value="Address" />
                <property name="age" value="27" />
            </bean>
        </property>
    </bean>
</beans>

最后，我创建了一个MainApp来运行此代码：

public static void main (String[] args) {

    ApplicationContext context = new ClassPathXmlApplicationContext(new String[] {
            "NewBean.xml"});

    Customer cust = (Customer) context.getBean("CustomerBean");
    System.out.println(cust);

}

现在，根据我的理解，这就是正在发生的事情：

加载xml文件，ID为CustomerBean的bean存储在引用cust（它是Customer对象）中。 CustomerBean接受一个参数＆＃34; person＆＃34;，并且创建Person的三个参数的详细信息作为customerBean的内部bean提供。

在MainApp中，调用引用cust的toString方法并显示正确的输出。

由于我在我的示例中专门提供了引用，为什么运行此代码需要无参数构造函数？如果我删除空Customer或空Person构造函数，代码将失败。为什么它在运行时失败，当我设置的bean不需要无参数构造函数时？

编辑：

代码传递示例：

Customer [person=Person [address=4 Ascot House, age=27, name=Andrew]]

代码失败的示例：

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'CustomerBean' defined in class path resource [NewBean.xml]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.andrew.SpringInnerBeans.Customer]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.andrew.SpringInnerBeans.Customer.<init>()
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1095)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1040)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:505)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:476)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:302)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:229)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:298)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:193)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:725)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:757)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:480)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:93)
    at com.andrew.SpringInnerBeans.MainApp.main(MainApp.java:10)
Caused by: org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.andrew.SpringInnerBeans.Customer]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.andrew.SpringInnerBeans.Customer.<init>()
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:85)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1088)
    ... 13 more
Caused by: java.lang.NoSuchMethodException: com.andrew.SpringInnerBeans.Customer.<init>()
    at java.lang.Class.getConstructor0(Unknown Source)
    at java.lang.Class.getDeclaredConstructor(Unknown Source)
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:80)
    ... 14 more

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'CustomerBean' defined in class path resource [NewBean.xml]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.andrew.SpringInnerBeans.Customer]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.andrew.SpringInnerBeans.Customer.<init>()
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1095)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1040)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:505)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:476)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:302)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:229)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:298)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:193)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:725)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:757)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:480)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:93)
    at com.andrew.SpringInnerBeans.MainApp.main(MainApp.java:10)
Caused by: org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.andrew.SpringInnerBeans.Customer]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.andrew.SpringInnerBeans.Customer.<init>()
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:85)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1088)
    ... 13 more
Caused by: java.lang.NoSuchMethodException: com.andrew.SpringInnerBeans.Customer.<init>()
    at java.lang.Class.getConstructor0(Unknown Source)
    at java.lang.Class.getDeclaredConstructor(Unknown Source)
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:80)
    ... 14 more

Answer 1

因为它使用字段注入而不是您的Person（String name，String address，int age）构造函数。请尝试类似：

 ... 
<property name="person">
        <bean class="com.andrew.SpringInnerBeans.Person">
            <constructor-arg index="0" value="Andrew"/>
            <constructor-arg index="1" value="Address"/>
            <constructor-arg index="2" value="27"/>
       </bean>
 </property>
... 

代替。这是Person类的解决方案，对于客户 - 我不知道; ）

Answer 2

再看一下你从Spring容器的角度提出的要求。

• 找到了一个类Customer的bean，只有属性：ok使用no-arg构造函数创建它为new Customer()并暂时保留它。
• 发现了一个只有Person类的bean，只有属性：ok创建它为new Person()，仍然使用no-arg构造函数，给它一个任意名称并暂时保留它
• 使用setter填充Person bean及其属性：ok it ready
• 使用setter和Customer bean作为参数填充Person bean的唯一属性：ok it ready
• 很好，应用程序上下文现已完全填充

因此，您从应用程序上下文中获取bean并且在您自己的代码中没有new，但是Spring已经为您构建了它们，并且确实使用了无参数构造函数。

你可以改为举例：

public class Customer {

    private Person person;

    public Customer(Person person) {
        this.person = person;
    }
    ...
}

你的XML必须是：

<bean id="CustomerBean" class="com.andrew.SpringInnerBeans.Customer">
    <constructor-arg>
        <bean class="com.andrew.SpringInnerBeans.Person">
            <property name="name" value="Andrew" />
            <property name="address" value="Address" />
            <property name="age" value="27" />
        </bean>
    </constructor-arg>
</bean>

这样，Spring将首先构建Person bean，并使用一个arg构造函数创建Customer bean作为new Customer(person)。

Answer 3

这是XML文件设置的问题。您没有提供构造函数参数，而是告诉Spring实例化具有某些属性的新对象。

看到这个堆栈溢出帖子： Does Spring require all beans to have a default constructor?

您必须提供构造函数参数，以便通知Spring您希望将构造函数与参数一起使用。

即： <bean id="CustomerBean" class="com.andrew.SpringInnerBeans.Customer"> <property name="person"> <bean class="com.andrew.SpringInnerBeans.Person"> <constructor-arg index="0" value="Andrew"/> <constructor-arg index="1" value="Address"/> <constructor-arg index="2" value="27"/> </bean> </property> </bean>

这将告诉Spring你想要使用提供的构造函数来接受参数而不是no-arg构造函数。从技术上讲，您根本不需要提供默认构造函数。

Answer 4

  

为什么在运行时，当我设置的bean不需要无参数构造函数时，它会失败？

您设置的bean 需要一个无参数构造函数。我不是Spring专家（事实上一直困在他们身上）但是你要求Spring做的事情：

• 使用no-arg构造函数创建客户对象
• 使用no-arg构造函数创建Person对象
• 使用setter
在person对象中设置属性
• 再次使用setters
将客户对象中的人物属性设置为上面创建的人物对象

freakman's answer展示了如何使用构造函数参数创建Person对象，我想你也可以类似地创建客户对象。那么你不应该需要无法治疗的人。