Spring 基于注解的配置(二)(@Required,@Autowired, @Qualifier)

来源：互联网发布：网络常见的拓扑形式有编辑：程序博客网时间：2024/06/07 17:42

@Required

应用范围：bean属性的setter方法

作用：检查注解的Bean属性是否在XML中进行配置，若未进行配置则容器会抛出一个BeanInitializationException异常

使用方法：

方法一：

包函 <context:annotation-config />

<context:annotation-config />
<bean id="Hello"class="com.spring.demo.Hello">

<propertyname="id"value="12"/>
</bean>

Hello.java

package com.spring.demo;

import org.springframework.beans.factory.annotation.Required;

/**
* Created by ruanjianlin on 2017/9/27.
*/
public class Hello {
    privateString name;
    private int id;

    public Hello() {
    }

    publicHello(String name, intid) {
        this.name= name;
        this.id= id;
    }

    publicString getName() {
        returnname;
    }

    @Required
    public voidsetName(String name) {
        this.name= name;
    }

    public intgetId() {
        returnid;
    }

    public voidsetId(intid) {
        this.id= id;
    }
    public voidsay(){
        System.out.println("hello");
    }

    @Override
    publicString toString() {
        return"Hello{" +
                "name='"+ name+ '\''+
                ", id="+ id+
                '}';
    }
}

运行的结果：

Property 'name' is required for bean 'Hello'

方法二：

bean 配置文件包函“RequiredAnnotationBeanPostProcessor”

与@Autowired的联系：

1.@Autowired 的必要属性，建议使用@Required注解

2.每个类只能有一个构造器被标记为required=true(每个类有很多的构造器，默认的required=false)

3.如果因为找不到合适的bean或者含有多个Bean将会导致autowiring失败抛出异常，使用@Autowired(required=false)解决（主要用于构造方法和Bean setter方法上，也可以用于属性上）

Autowired是一种函数，可以对成员变量、方法和构造函数进行标注，来完成自动装配的工作，@Autowired标注可以放在成员变量上，也可以放在成员变量的set方法上。

这里必须明确：@Autowired是根据类型进行自动装配的，如果需要按名称进行装配，则需要配合@Qualifier使用

Spring 2.5 引入了 @Autowired注释，它可以对类成员变量、方法及构造函数进行标注，完成自动装配的工作。通过@Autowired的使用来消除set，get方法。

Spring遇到一个在 setter方法中使用的@Autowired注释，setter方法会在方法中视图执行 byType 自动连接

Setter方法中的@Autowired：

Computer.java

package com.spring.Autowire;

/**
* Created by ruanjianlin on 2017/10/6.
*/
public class Computer {
    publicComputer() {
        System.out.println("You get a new computer");
    }

    public voidplay(){
        System.out.println("The computer is working! ");
    }
}

Human.java

package com.spring.Autowire;

import org.springframework.beans.factory.annotation.Autowired;

/**
* Created by ruanjianlin on 2017/10/6.
*/
public class Human {

    privateComputercomputer;

    public Human() {
        System.out.println("The function of Human constructor is working!");
    }

    publicHuman(Computer computer) {
        this.computer= computer;
        System.out.println("The function of Human which has a parameter is working!");
    }

    publicComputergetComputer() {
        returncomputer;
    }

    @Autowired
    public voidsetComputer(Computer computer) {
        this.computer= computer;
        System.out.println("The function of setComputer is working!");
    }

    public voidplay(){
        computer.play();
    }
}

Autowired_Demo.xml配置：

<?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">

    <context:annotation-config/>
    <bean id="human"class="com.spring.Autowire.Human">

    </bean>
    <bean id="computer"class="com.spring.Autowire.Computer">

    </bean>
</beans>

输出结果：

The function of Human constructor is working!

You get a new computer

The function of setComputer is working!

The computer is working!

调用的方法：

Human()

Computer()

setComputer(Computer computer)

play()

构造函数中的@Autowired：

package com.spring.Autowire;

import org.springframework.beans.factory.annotation.Autowired;

/**
* Created by ruanjianlin on 2017/10/6.
*/
public class Human {

    privateComputercomputer;

    public Human() {
        System.out.println("The function of Human constructor is working!");
    }

    @Autowired
    publicHuman(Computer computer) {
        this.computer= computer;
        System.out.println("The function of Human which has a parameter is working!");
    }

    publicComputergetComputer() {
        returncomputer;
    }

    public voidsetComputer(Computer computer) {
        this.computer= computer;
        System.out.println("The function of setComputer is working!");
    }

    public voidplay(){
        computer.play();
    }
}

输出结果：

You get a new computer

The function of Human which has a parameter is working!

The computer is working!

运行的方法：

Human(Computer computer)

Computer()

play()

属性中的@Autowired：

Human.java

package com.spring.Autowire;

import org.springframework.beans.factory.annotation.Autowired;

/**
* Created by ruanjianlin on 2017/10/6.
*/
public class Human {
    @Autowired
    privateComputercomputer;

    public Human() {
        System.out.println("The function of Human constructor is working!");
    }

    publicHuman(Computer computer) {
        this.computer= computer;
        System.out.println("The function of Human which has a parameter is working!");
    }

    publicComputergetComputer() {
        returncomputer;
    }

    public voidsetComputer(Computer computer) {
        this.computer= computer;
        System.out.println("The function of setComputer is working!");
    }

    public voidplay(){
        computer.play();
    }
}

输出结果：

The function of Human constructor is working!

You get a new computer

The computer is working!

Disconnected from the target VM, address: '127.0.0.1:54693', transport: 'socket'

运行的方法：

Human()

Computer()

play()

属性上的添加@Autowired后getBean时并没有调用setComputer(Computer computer)

原理解释：

(1)Spring 通过一个 BeanPostProcessor 对 @Autowired进行解析，所以要让@Autowired起作用必须事先在Spring容器中声明AutowiredAnnotationBeanPostProcessor Bean。

@Autowired在成员上的配置：Spring将直接采用 Java反射机制对Human中的computer 这个私有成员变量进行自动注入。所以对成员变量使用 @Autowired后，可将它的 setter方法（setComputer(Computer computer)

）从 Human中删除。

(2) @Autowired对方法或构造函数进行标注：（按照本示例作解析）如果构造函数有一个形参，是computer，@Autowired寻找和它类型匹配的Bean(byType)，将它作为Human (Computer computer)的参数来创建 Human Bean 此时不再调用setter方法，而@Autowired在setter上时会通过上述方式进行Bean的注册，而不会调用带参数的构造函数

（3）在使用Spring框架中@Autowired标签时默认情况下使用@Autowired注释进行自动注入时，Spring容器中匹配的候选Bean数目必须有且仅有一个。当找不到一个匹配的 Bean时，Spring容器将抛BeanCreationException异常，并指出必须至少拥有一个匹配的 Bean。（因此可以使用 @Qualifier解除歧义）

@Qualifier的使用

@Autowired 可以对成员变量、方法以及构造函数进行注释，而 @Qualifier 的标注对象是成员变量、方法入参、构造函数入参。

代码示例：

Sing接口类：

package com.spring.Autowire;

import org.springframework.stereotype.Component;

/**
* Created by ruanjianlin on 2017/10/7.
*/
@Component("sing")
public interface Sing {
public voidsing();
}

ManSing.java

package com.spring.Autowire;

import org.springframework.stereotype.Component;

/**
* Created by ruanjianlin on 2017/10/7.
*/
@Component("manSing")
public class ManSingimplementsSing {
    public voidsing() {
        System.out.println("This is a man!");
    }
}

WomanSing.java

package com.spring.Autowire;

import org.springframework.stereotype.Component;

/**
* Created by ruanjianlin on 2017/10/7.
*/
@Component("womanSing")
public class WomanSingimplementsSing {

    public voidsing() {
        System.out.println("This is a woman!");
    }
}

Human.java

package com.spring.Autowire;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;

/**
* Created by ruanjianlin on 2017/10/6.
*/
public class Human {

    privateComputercomputer;
    @Autowired
    @Qualifier("manSing")
    privateSingsing;

    public Human() {
        System.out.println("The function of Human constructor is working!");
    }

    @Autowired
    publicHuman(Computer computer) {
        this.computer= computer;
        System.out.println("The function of Human which has a parameter is working!");
    }

    publicComputergetComputer() {
        returncomputer;
    }

    public voidsetComputer(Computer computer) {
        this.computer= computer;
        System.out.println("The function of setComputer is working!");
    }

    public voidsetSing(Sing sing) {
        this.sing= sing;
    }

    public voidplay(){
        computer.play();
        sing.sing();
    }
}

Autowired_Demo.xml配置：

上述为正确代码运行结果如下：

You get a new computer

The function of Human which has a parameter is working!

The computer is working!

This is a man !

当我们不使用@Qualifier时报错情况主要如下所示：

org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.spring.Autowire.Sing com.spring.Autowire.Human.sing; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.spring.Autowire.Sing] is defined: expected single matching bean but found 2: manSing,womanSing

错误解析：由报错信息可知产生了歧义，系统无法决定 manSing,womanSing

关于BeanPostProcessor接口相关的内容以及该接口的原理及工作机制，本人目前处于初级阶段，尚无法解析，在后期进行源码学习时将会重点探索，望感兴趣的猿们可以自行探索。

阅读全文

0 0