spring in action学习笔记一:DI(Dependency Injection)依赖注入之CI(Constructor Injection)构造器注入

一:这里先说一下DI(Dependency Injection)依赖注入有种表现形式:一种是CI(Constructor Injection)构造方法注入,另一种是SI(Set Injection) set 注入。这篇随笔讲的是第一种构造方法注入(Constructor Injection).

其实DI(Dependency Injection)依赖注入你不妨反过来读:注入依赖也就是把"依赖"注入到一个对象中去。那么何为"依赖"呢?依赖就是讲一个对象初始化或者将实例化的时候需要另一个对象,我们把另一个对象成为"依赖"。如构造方法注入(Constructor Injection)就是把依赖当做构造方法的参数注入进去的。如:

//Injection B to A. 我们把B称为依赖。一般情况下B是一个接口,这符合面向接口编程的规范。

public A (B b){

}

二:使用依赖注入的好处是什么?

  1.DI(Dependency Injection)最大的好处是decoupling(解耦)。如上面的代码把B注入给A如果B是一个接口或者抽象类。只要任何一个类实现或者继承B都可以注入给A.因此可以达到解耦的目的。

  2. 依赖注入给谁?        

dependencies injected into the objects that need them.(依赖注入给那些需要他们的对象。)

3.在spring中怎么让依赖传入到被依赖的对象(看上面的代码:怎么把B传入到A中)?

 在这里我引用一下spring in action中的一段文字:(注:我下面的示例代码基本跟spring in action 的一样)

The big question here is,how can you give SlayDragonQuest to BraveKnight ? And how can you give a PrintStream to SlayDragonQuest?

The act of creating associations between application components is commonly referred to as wiring.

spring通过配置配置文件,配置bean的形式,完成装配,又装配实现把依赖注入给需要他们的对象中的。

【我这里还想说明一下配置文件中的一个bean其实就对应一个javabean,也就是一个pojo对象,bean就是javabean的缩写形式而已】

示例代码的目录结构如下图所示:

spring in action学习笔记一:DI(Dependency Injection)依赖注入之CI(Constructor Injection)构造器注入

  上面中的带两个圈的配置文件想说明两个问题:

一个是:new ClassPathXmlApplicationContext("spring-julysecond.xml");//注意这个.xml文件是在src目录下。

    new ClassPathXmlApplicationContext("com/config/spring-julythird.xml")//注意这个.xml文件在com.cofig这个文件下。

另一个问题是DI的解耦问题:spring-julysecond.xml文件中注入的是SlayDragonQuest,spring-julythird.xml注入的是SearchHillQuest.在不改变代码的情况下看传入的Quest的实现类不同,从而打出的结果不同。

spring-julysecond.xml的代码如下:

 <?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">
<!--this is constructor injection-->
<!--the act of creating associations between application components is commonly referred to
as wiring-->
<bean id="knight" class="com.qls.impl.BraveKnight">
<!-- collaborators and configuration for this bean go here -->
<constructor-arg ref="query"></constructor-arg>
</bean>
<bean id="query" class="com.qls.impl.SlayDragonQuest">
<!-- collaborators and configuration for this bean go here -->
<constructor-arg value="#{T(System).out}"/>
</bean>
<!-- more bean definitions go here -->
</beans>

//spring-julythird.xml的代码如下:

 <?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">
<!--this is constructor injection-->
<!--the act of creating associations between application components is commonly referred to
as wiring-->
<bean id="knight" class="com.qls.impl.BraveKnight">
<!-- collaborators and configuration for this bean go here -->
<constructor-arg ref="query"></constructor-arg>
</bean>
<bean id="query" class="com.qls.impl.SearchHillQuest">
<constructor-arg value="#{T(System).out}"/>
</bean>
<!-- more bean definitions go here -->
</beans>

//Query接口的代码如下:

 package com.qls.inter;

 /**
* Created by ${秦林森} on 2017/6/2.
*/
public interface Query {
void query();
void embark();
}

//Knight的代码如下:

 package com.qls.inter;

 /**
* Created by ${秦林森} on 2017/6/2.
*/
public interface Knight {
void embarkOnQuest();
}

//Query接口的实现类的代码如下:

//首先是:SlayDragonQuest的代码如下:

 package com.qls.impl;

 import com.qls.inter.Query;

 import java.io.PrintStream;

 /**
* Created by ${秦林森} on 2017/6/2.
*/
public class SlayDragonQuest implements Query{
private PrintStream stream; public SlayDragonQuest(PrintStream stream) {
this.stream = stream;
}
@Override
public void embark(){
stream.println("Embarking on quest to slay the dragon");
stream.println("ouyangfeng is the most beautiful woman in the world.");
}
@Override
public void query(){
System.out.println("slay dragon quest");
}
}

//其次是SearchHillQuest的代码如下:

 package com.qls.impl;

 import com.qls.inter.Query;

 import java.io.PrintStream;

 /**
* Created by ${秦林森} on 2017/6/3.
*/
public class SearchHillQuest implements Query{
private PrintStream stream; public SearchHillQuest(PrintStream stream) {
this.stream = stream;
} @Override
public void query() { } @Override
public void embark() {
stream.println("the hero search the hill where the damsel was missing");
}
}

//BraveKnight的代码如下:

 package com.qls.impl;

 import com.qls.inter.Knight;
import com.qls.inter.Query;
import org.springframework.context.annotation.Configuration; /**
* Created by ${秦林森} on 2017/6/2.
*/ public class BraveKnight implements Knight{
private Query query; public BraveKnight(Query query) {
this.query = query;
} @Override
public void embarkOnQuest() {
query.embark();
}
}

//测试类的代码如下:

 package com.qls.test;

 import com.qls.inter.Knight;
import org.springframework.context.support.ClassPathXmlApplicationContext; /**
* Created by ${秦林森} on 2017/6/2.
*/
public class Test {
public static void main(String[] args) {
/**
* 加载配置文件的方式:在src下直接写配置文件的名即可,
* 不直接在src下要把前面的包名加上。包名的前面可加也可以不加斜杠/
*/
// ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("/com/config/spring-julythird.xml");
// ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("com/config/spring-julythird.xml");
ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("spring-julysecond.xml");
Knight knight = context.getBean(Knight.class);
knight.embarkOnQuest();
context.close(); }
}
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