@Required

應用範圍：bean屬性的setter方法

作用：檢查註解的Bean屬性是否在XML中進行配置，若未進行配置則容器會丟擲一個BeanInitializationException異常

使用方法：

方法一：

包函 <context:annotation-config />

<context:annotation-config /><bean id="Hello" class="com.spring.demo.Hello"><!-- <property name="name" value="ruanjianlin"/> -->

Hello.java

package com.spring.demo;import org.springframework.beans.factory.annotation.Required;/** * Created by ruanjianlin on 2017/9/27. */public class Hello {private String name;    private int id;    public Hello() {    }public Hello(String name, int

執行的結果：

Property 'name' is required for bean 'Hello'

方法二：

 bean 配置檔案包函“RequiredAnnotationBeanPostProcessor”

<bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor"/><bean id="Hello" class="com.spring.demo.Hello"><!-- <property name="name" value="ruanjianlin"/> --><property name="id" value="12" /></bean>

與@Autowired的聯絡：

[email protected] 的必要屬性，建議使用@Required註解

2.每個類只能有一個構造器被標記為required=true(每個類有很多的構造器，預設的required=false)

3.如果因為找不到合適的bean或者含有多個Bean將會導致autowiring失敗丟擲異常，使用@Autowired(required=false)解決（主要用於構造方法和Bean setter方法上，也可以用於屬性上）

Autowired是一種函式，可以對成員變數、方法和建構函式進行標註，來完成自動裝配的工作，@Autowired標註可以放在成員變數上，也可以放在成員變數的set方法上。

這裡必須明確：@Autowired是根據型別進行自動裝配的，如果需要按名稱進行裝配，則需要配合@Qualifier使用

Spring 2.5 引入了 @Autowired註釋，它可以對類成員變數、方法及建構函式進行標註，完成自動裝配的工作。 通過 @Autowired的使用來消除set ，get方法。

 Spring遇到一個在 setter方法中使用的 @Autowired註釋，setter方法會在方法中檢視執行 byType 自動連線

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 {privateComputer computer;    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!");}publicComputer getComputer() {returncomputer;}@Autowiredpublic 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>

輸出結果：

The function of  Human constructor is working!

You get a new computer

The function of setComputer is working!

The computer is working!

呼叫的方法：

Human()

Computer()

setComputer(Computer computer)

play()

建構函式中的@Autowired：

package com.spring.Autowire;import org.springframework.beans.factory.annotation.Autowired;/** * Created by ruanjianlin on 2017/10/6. */public class Human {privateComputer computer;    public Human() {        System.out.println("The function of  Human constructor is working!");}@AutowiredpublicHuman(Computer computer) {this.computer= computer;System.out.println("The function of  Human which has a parameter is working!");}publicComputer getComputer() {returncomputer;}public voidsetComputer(Computer computer) {this.computer= computer;System.out.println("The function of setComputer is working!");}public voidplay(){computer.play();}}

輸出結果：

You get a new computer

The function of  Human which has a parameter is working!

The computer is working!

執行的方法：

Human(Computer computer)

Computer()

play()

屬性中的@Autowired：

Human.java

package com.spring.Autowire;import org.springframework.beans.factory.annotation.Autowired;/** * Created by ruanjianlin on 2017/10/6. */public class Human {@AutowiredprivateComputer computer;    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!");}publicComputer getComputer() {returncomputer;}public voidsetComputer(Computer computer) {this.computer= computer;System.out.println("The function of setComputer is working!");}public voidplay(){computer.play();}}

輸出結果：

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'

執行的方法：

Human()

Computer()

play()

屬性上的新增@Autowired後getBean時並沒有呼叫setComputer(Computer computer)

原理解釋：

(1)Spring 通過一個 BeanPostProcessor 對 @Autowired進行解析，所以要讓 @Autowired起作用必須事先在 Spring容器中宣告 AutowiredAnnotationBeanPostProcessor Bean。

@Autowired在成員上的配置：Spring將直接採用 Java反射機制對Human中的computer 這個私有成員變數進行自動注入。所以對成員變數使用 @Autowired後，可將它的 setter方法（setComputer(Computer computer)

）從 Human中刪除。

(2) @Autowired對方法或建構函式進行標註：（按照本示例作解析）如果建構函式有一個形參，是computer，@Autowired尋找和它型別匹配的 Bean(byType)，將它作為Human (Computer computer)的引數來建立 Human Bean 此時不再呼叫setter方法，而@Autowired在setter上時會通過上述方式進行Bean的註冊，而不會呼叫帶引數的建構函式

（3）在使用Spring框架中@Autowired標籤時預設情況下使用@Autowired 註釋進行自動注入時，Spring容器中匹配的候選 Bean數目必須有且僅有一個。當找不到一個匹配的 Bean時，Spring 容器將拋BeanCreationException異常，並指出必須至少擁有一個匹配的 Bean。（因此可以使用 @Qualifier解除歧義）

@Qualifier的使用

@Autowired 可以對成員變數、方法以及建構函式進行註釋，而 @Qualifier 的標註物件是成員變數、方法入參、建構函式入參。

程式碼示例：

Sing介面類：

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 ManSingimplements Sing {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 WomanSingimplements Sing {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 {privateComputer computer;@Autowired    @Qualifier("manSing")privateSing sing;    public Human() {        System.out.println("The function of  Human constructor is working!");}@AutowiredpublicHuman(Computer computer) {this.computer= computer;System.out.println("The function of  Human which has a parameter is working!");}publicComputer getComputer() {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：

<?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>    <context:component-scanbase-package="com.spring"use-default-filters="false">        <context:include-filtertype="regex"expression="com.spring.Autowire.*"/>    </context:component-scan></beans>

上述為正確程式碼執行結果如下：

You get a new computer

The function of  Human which has a parameter is working!

The computer is working!

This is a man !

當我們不使用@Qualifier時報錯情況主要如下所示：

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

錯誤解析：由報錯資訊可知產生了歧義，系統無法決定 manSing,womanSing

關於BeanPostProcessor介面相關的內容以及該介面的原理及工作機制，本人目前處於初級階段，尚無法解析，在後期進行原始碼學習時將會重點探索，望感興趣的猿們可以自行探索。