spring自动装配bean有哪些方式？

 spring 配置文件中 <bean> 节点的 autowire 参数可以控制 bean 自动装配的方式

• default - 默认的方式和 "no" 方式一样
• no - 不自动装配，需要使用 <ref />节点或参数
• byName - 根据名称进行装配
• byType - 根据类型进行装配
• constructor - 根据构造函数进行装配

文档解释

Attribute : autowire
Controls whether bean properties are "autowired". This is an 
 automagical process in which bean references don't need to 
 be coded explicitly in the XML bean definition file, but rather 
 the Spring container works out dependencies. The effective 
 default is "no". There are 4 modes: 1. "no" The traditional 
 Spring default. No automagical wiring. Bean references must 
 be defined in the XML file via the <ref/> element (or "ref" 
 attribute). We recommend this in most cases as it makes 
 documentation more explicit. Note that this default mode also 
 allows for annotation-driven autowiring, if activated. "no" 
 refers to externally driven autowiring only, not affecting any 
 autowiring demands that the bean class itself expresses. 2. 
 "byName" Autowiring by property name. If a bean of class Cat 
 exposes a "dog" property, Spring will try to set this to the 
 value of the bean "dog" in the current container. If there is no 
 matching bean by name, nothing special happens. 3. "byType" 
 Autowiring if there is exactly one bean of the property type in 
 the container. If there is more than one, a fatal error is raised, 
 and you cannot use byType autowiring for that bean. If there is 
 none, nothing special happens. 4. "constructor" Analogous to 
 "byType" for constructor arguments. If there is not exactly one 
 bean of the constructor argument type in the bean factory, a 
 fatal error is raised. Note that explicit dependencies, i.e. 
 "property" and "constructor-arg" elements, always override 
 autowiring. Note: This attribute will not be inherited by child 
 bean definitions. Hence, it needs to be specified per concrete 
 bean definition. It can be shared through the 'default-autowire' 
 attribute at the 'beans' level and potentially inherited from 
 outer 'beans' defaults in case of nested 'beans' sections (e.g. 
 with different profiles).
 
Data Type : string
Default Value : default
Enumerated Values : 
    - default
    - no
    - byName
    - byType
    - constructor

代码示例

1、no 方式

spring 配置文件，使用 ref 参数注入 bean，必须要有对象的 setter 方法，这里即 Person 的 setFr 方法。

没有 <property name="fr" ref="fr"></property> 因没有注入 fr 属性，会报空指针错误。

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans 
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd">
        
    <bean id="person" class="constxiong.interview.assemble.Person" autowire="no">
        <property name="fr" ref="fr"></property>
    </bean>
    <bean id="fr" class="constxiong.interview.assemble.FishingRod"></bean>
        
</beans>

鱼竿 bean

package constxiong.interview.assemble;
 
 
/**
 * 鱼竿
 * @author ConstXiong
 * @date 2019-07-17 09:53:15
 */
public class FishingRod {
 
    /**
     * 被使用
     */
    public void used() {
        System.out.println("钓鱼...");
    }
}

人 bean

package constxiong.interview.assemble;
 
 
/**
 * 人
 * @author ConstXiong
 * @date 2019-07-17 09:54:56
 */
public class Person {
 
    private FishingRod fr;
    
    /**
     * 钓鱼
     */
    public void fish() {
        fr.used();
    }
    
    public void setFr(FishingRod fr) {
        this.fr = fr;
    }
    
}

测试代码

package constxiong.interview.assemble;
 
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
 
public class AssembleTest {
 
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("spring_assemble.xml");
        Person person = (Person)context.getBean("person");
        person.fish();
    }
    
}

2、byName 也是需要相应的 setter 方法才能注入

修改 spring 配置文件 autowire="byName"

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans 
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd">
        
    <bean id="person" class="constxiong.interview.assemble.Person" autowire="byName"></bean>
    <bean id="fr" class="constxiong.interview.assemble.FishingRod"></bean>
        
</beans>

3、byType 也是需要相应的 setter 方法才能注入

修改 spring 配置文件 autowire="byType"

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans 
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd">
        
    <bean id="person" class="constxiong.interview.assemble.Person" autowire="byType"></bean>
    <bean id="fr" class="constxiong.interview.assemble.FishingRod"></bean>
        
</beans>

其他不变

4、constructor 无需 setter 方法，需要通过 构造方法注入 bean

修改 spring 配置文件autowire="byType"

Person 类去除 setFr 方法，添加构造方法设置 fr 属性

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans 
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd">
        
    <bean id="person" class="constxiong.interview.assemble.Person" autowire="constructor"></bean>
    <bean id="fr" class="constxiong.interview.assemble.FishingRod"></bean>
        
</beans>

package constxiong.interview.assemble;
 
 
/**
 * 人
 * @author ConstXiong
 * @date 2019-07-17 09:54:56
 */
public class Person {
 
    private FishingRod fr;
    
    public Person(FishingRod fr) {
        this.fr = fr;
    }
    
    /**
     * 钓鱼
     */
    public void fish() {
        fr.used();
    }
    
}

1、2、3、4 的测试结果一致，打印

钓鱼...

原文链接
 

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