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Contructor 注入

演示怎么给构造函数注入参数,使用指定的构造函数创建对象。

Customer

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package com.xtuer.beans;
public class Customer {
private String name;
private int age;
private String telephone;
public Customer(String name, int age) {
this.name = name;
this.age = age;
}
public Customer(String name, String telephone, int age) {
this.name = name;
this.telephone = telephone;
this.age = age;
}
public Customer(String name, int age, String telephone) {
this.name = name;
this.telephone = telephone;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getTelephone() {
return telephone;
}
public void setTelephone(String telephone) {
this.telephone = telephone;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
}

spring-beans.xml

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<?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="customer" class="com.xtuer.beans.Customer">
<constructor-arg value="Alice"/>
<constructor-arg value="40"/>
<constructor-arg value="1234567"/>
</bean>
</beans>

InjectionTest

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import com.xtuer.beans.Customer;
import com.xtuer.util.CommonUtils;
import org.junit.BeforeClass;
import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class InjectionTest {
private static ApplicationContext context;
@BeforeClass
public static void setup() {
context = new ClassPathXmlApplicationContext("spring-beans.xml");
}
@Test
public void test() {
Customer user = context.getBean("customer", Customer.class);
CommonUtils.output(user);
}
}

输出

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{
"name" : "Alice",
"age" : 1234567,
"telephone" : "40"
}

age 为什么是 1234567? 不是我们想要的!

构造函数的断定时有歧义

Customer 有 3 个构造函数
  • public Customer(String name, int age)
  • public Customer(String name, String telephone, int age)
  • public Customer(String name, int age, String telephone)

上面 Bean 的定义使用有 3 个参数的构造函数来创建对象(因为有 3 个 <constructor-arg>),找到了 2 个这样的构造函数,用了第一个语法上符合条件的构造函数来创建对象。但是在我们看来 40 应该是 age,最后 age 却是 1234567,与期望的不一样,也就是构造函数的断定上出现了歧义。如果构造函数有歧义的时候,可以指定构造函数的参数类型,用于确定要使用的构造函数:

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<bean id="customer" class="com.xtuer.beans.Customer">
<constructor-arg type="java.lang.String" value="Alice"/>
<constructor-arg type="int" value="40"/>
<constructor-arg type="java.lang.String" value="1234567"/>
</bean>

这次就能明确指定要使用的构造函数是 Customer(String name, int age, String telephone)