spring 框架中的 bean 自动装配

Question

我正在尝试从 Pro Spring 5 本书中学习 Spring。

这是一个我在自动装配时不理解的例子：

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="fooOne" class="com.apress.prospring5.ch3.xml.Foo"/>
    <bean id="barOne" class="com.apress.prospring5.ch3.xml.Bar"/>

    <bean id="targetByName" autowire="byName" class="com.apress.prospring5.ch3.xml.Target"
        lazy-init="true"/>

    <bean id="targetByType" autowire="byType" class="com.apress.prospring5.ch3.xml.Target"
        lazy-init="true"/>

    <bean id="targetConstructor" autowire="constructor" 
        class="com.apress.prospring5.ch3.xml.Target" lazy-init="true"/>
</beans>

目标Class

package com.apress.prospring5.ch3.xml;

import org.springframework.context.support.GenericXmlApplicationContext;

public class Target {
    private Foo fooOne;
    private Foo fooTwo;
    private Bar bar;

    public Target() {
    }

    public Target(Foo foo) {
        System.out.println("Target(Foo) called");
    }

    public Target(Foo foo, Bar bar) {
        System.out.println("Target(Foo, Bar) called");
    }

    public void setFooOne(Foo fooOne) {
        this.fooOne = fooOne;
        System.out.println("Property fooOne set");
    }

    public void setFooTwo(Foo foo) {
        this.fooTwo = foo;
        System.out.println("Property fooTwo set");
    }

    public void setBar(Bar bar) {
        this.bar = bar;
        System.out.println("Property bar set");
    }

    public static void main(String... args) {
        GenericXmlApplicationContext ctx = new GenericXmlApplicationContext();
        ctx.load("classpath:spring/app-context-03.xml");
        ctx.refresh();

        Target t = null;

        System.out.println("Using byName:\n");
        t = (Target) ctx.getBean("targetByName");

        System.out.println("\nUsing byType:\n");
        t = (Target) ctx.getBean("targetByType");

        System.out.println("\nUsing constructor:\n");
        t = (Target) ctx.getBean("targetConstructor");

        ctx.close();

    }
}

富Class

package com.apress.prospring5.ch3.xml;

public class Foo {

}

酒吧Class

package com.apress.prospring5.ch3.xml;

public class Bar {

}

没听懂的地方：

<bean id="targetByName" autowire="byName" class="com.apress.prospring5.ch3.xml.Target"
        lazy-init="true"/>

如何在知道我们没有在 bean 定义中使用任何属性或构造函数注入的情况下注入目标属性 (fooOne,fooTwo,bar)？

通常我们应该有这样的东西：

 <property name = "fooOne">
         <bean id = "fooOne" class = "com.apress.prospring5.ch3.xml.Foo"/>
      </property>

Answer 1

<bean id="targetByName" autowire="byName" class="com.apress.prospring5.ch3.xml.Target"
lazy-init="true"/>

因为它将自动连线模式声明为“byName”，它具有以下行为（取自docs）：

Autowiring by property name. Spring looks for a bean with the same name as the property that needs to be autowired. For example, if a bean definition is set to autowire by name and it contains a master property (that is, it has a setMaster(..) method), Spring looks for a bean definition named master and uses it to set the property.

也就是说是setter注入

回到你的例子，因为 Target 有以下 setters ， spring 将执行以下注入：

public class Target {

    // Find a bean which name is "fooOne" , and call this setter to inject 
    public void setFooOne(Foo fooOne) {}

    // Find a bean which name is "fooTwo" , and call this setter to inject (As no beans called fooTwo in your example , it will be null) 
    public void setFooTwo(Foo foo) {}

    //Find a bean which name is "bar" , and call this setter to inject (As no beans called bar in your example  , it will be null)   
    public void setBar(Bar bar) {}      
}

当然，如果 bean 的类型与 setter 参数的类型不匹配，则会发生异常。

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