spring之自动装配的三种方式

springIOC容器可以自动装配Bean，需要做的仅仅是在<bean>中的autowire属性里指定自动装配的方式。

ByType：根据类型自动装配。若IOC容器中有多个目标Bean类型一致的bean，在这种情况下，spring将无法判定那个bean最适合该属性，所以不能执行自动装配。

ByName：根据名称自动装配，必须将目标Bean的名称和属性名设置完全相同。

contructor：通过构造器自动装配，当Bean中存在多个构造器时，此种自动装配方式将会很复杂，不推荐使用，

Car.java

package com.gong.spring.beans.autowire;

public class Car {
    
    public Car() {
    }

    public Car(String name) {
        this.name = name;
    }
    private String name;

    public String getName() {
        return name;
    }

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

    @Override
    public String toString() {
        return "Car [name=" + name + "]";
    }
    
}

Address.java

package com.gong.spring.beans.autowire;

public class Address {
    private String street;

    public String getStreet() {
        return street;
    }

    public void setStreet(String street) {
        this.street = street;
    }

    @Override
    public String toString() {
        return "Address [street=" + street + "]";
    }
    
}

Student.java

package com.gong.spring.beans.autowire;
import java.util.List;
import java.util.Map;

public class Student {
    
    private String name;
    private int age;
    private double score;
    private Car car;
    private Address address;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public double getScore() {
        return score;
    }
    public void setScore(double score) {
        this.score = score;
    }
    public Car getCar() {
        return car;
    }
    public void setCar(Car car) {
        this.car = car;
    }
    public Address getAddress() {
        return address;
    }
    public void setAddress(Address address) {
        this.address = address;
    }
    @Override
    public String toString() {
        return "Student [name=" + name + ", age=" + age + ", score=" + score + ", car=" + car + ", address=" + address
                + "]";
    }
    
    
}

beans-autowire.xml

<?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:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="car" class="com.gong.spring.beans.autowire.Car" p:name="baoma"></bean>
    <bean id="address" class="com.gong.spring.beans.autowire.Address" p:street="络南街道"></bean>
    <bean id="address2" class="com.gong.spring.beans.autowire.Address" p:street="珞狮街道"></bean>
    <!-- 
    <bean id="student" class="com.gong.spring.beans.autowire.Student" p:name="tom"
    p:age="12" p:score="99.00" p:address-ref="address" p:car-ref="car"></bean>
     -->
    <!-- 使用byName时，要装配的bean的名字要和类文件中的setter方法中的名字保持一致，若没有匹配的，则不装配 
    <bean id="student" class="com.gong.spring.beans.autowire.Student" p:name="tom"
    p:age="12" p:score="99.00" autowire="byName"></bean>
    -->
    <!-- byType根据bean的类型和当前Bean的属性自动装配，若IOC容器中有一个以上的bean可以匹配，则会抛出异常 -->
    <bean id="student" class="com.gong.spring.beans.autowire.Student" p:name="tom"
    p:age="12" p:score="99.00" autowire="byName"></bean>
</beans>

Main.java

package com.gong.spring.beans.autowire;

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

public class Main {
    public static void main(String[] args) {
        //1.创建spring的IOC容器对象
        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans-autowire.xml");
        //2.从容器中获取Bean实例
        Student student = (Student) ctx.getBean("student"); 
        System.out.println(student.toString());
    }
}

XML配置里自动装配的缺点：

（1）自动装配将会装配Bean的所有属性，然而只需要装配个别属性时，自动装配就不够灵活；

（2）要么根据类型，要么根据名称自动装配，不能两者兼顾；

（3）一般而言，在实际项目中很少使用自动装配，因为与自动装配功能带来的好处相比，明确清晰的配置方式更有说服力；