27 Spring由构造方法自动装配

在Spring中，可以使用“通过构造自动装配”，实际上是按构造函数的参数类型自动装配。 这意味着，如果一个bean的数据类型与其他bean的构造器参数的数据类型是相同的，那么将自动装配。
下面看看Spring构造函数自动装配的一个完整例子。

1. Beans

这里有两个 beans, 分别是：developer 和 language

package com.gp6.autoAssemblyBean.constructor;

public class Language {
    private String name;

    public String getName() {
        return name;
    }

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

package com.gp6.autoAssemblyBean.constructor;

public class Developer {
    private Language language;
    
    //构造函数
    public Developer(Language language) {
        this.language = language;
    }

    public Language getLanguage() {
        return language;
    }

    public void setLanguage(Language language) {
        this.language = language;
    }
}

<!-- 通过ref手动装配-->
<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-2.5.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-2.5.xsd">

    <!-- 由于Developer中有构造函数,此方法不适用 -->
    <!-- <bean id="developer" class="com.gp6.autoAssemblyBean.constructor.Developer" >
        <property name="language" ref="language" />
    </bean>
    
    <bean id="language" class="com.gp6.autoAssemblyBean.constructor.Language" >
        <property name="name" value="chinese" /> 
    </bean>
     -->

    <bean id="developer" class="com.gp6.autoAssemblyBean.constructor.Developer">
        <constructor-arg>
            <ref bean="language" />
        </constructor-arg>
    </bean>
        
    <bean id="language" class="com.gp6.autoAssemblyBean.constructor.Language" >
        <property name="name" value="Java" />
    </bean>
    
</beans>  

package com.gp6.autoAssemblyBean.constructor;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Test {
    public static void main( String[] args ) {
        ApplicationContext context =  new ClassPathXmlApplicationContext("com/gp6/autoAssemblyBean/constructor/applicationContext.xml");

        Developer developer = (Developer)context.getBean("developer");
        
        System.out.println(developer.getLanguage().getName());
        
    }
}

随着自动装配用构造函数启用后，你可以不设置构造器属性。Spring会找到兼容的数据类型，并自动装配它。

<!-- 通过类型Constructor装配 -->
<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-2.5.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-2.5.xsd">

    <bean id="developer" class="com.gp6.autoAssemblyBean.constructor.Developer" autowire="constructor" />
        
    <bean id="language" class="com.gp6.autoAssemblyBean.constructor.Language" >
        <property name="name" value="Java" />
    </bean>
    
</beans>