概述

Spring中依賴注入dependency injection(DI)一般來說有兩種形式： 1）基於xml的依賴注入， 2）基於註解的依賴注入。 基於xml的依賴注入方式通常又可以分為：1）建構函式方式注入。2）setter方式注入。

環境

Spring4.3.9, JDK1.8  pom新增spring依賴如下：

<dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-context</artifactId>
        <version>4.3.9.RELEASE</version>
</dependency>
 

一個包就夠了。 測試專案結構： 目的：將HelloService的例項HelloServiceImpl注入到HelloPrinter中並在Application中通過容器取出來使用。 程式碼如下： HelloService介面

public interface HelloService {
	String sayHello();
}

HelloService實現

public class HelloServiceImpl implements HelloService {

	public String sayHello() {
		return "hello";
	}
	
}

HelloPrinter類（未完成版本，在這裡使用不同的方法依賴注入）

public class HelloPrinter {

	private HelloService helloService;
	
	public void print() {
		System.out.println(helloService.sayHello());
	}
	
}
 

Application類

public class Application {
	
	public static void main(String[] args) {
		ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
		HelloPrinter printer = (HelloPrinter) context.getBean("helloPrinter");
		printer.print();
	}
}

依賴注入

建構函式方式

單個引數

HelloPrinter類：

public class HelloPrinter {

	private HelloService helloService;
	
	public HelloPrinter(HelloService helloService) {
		this.helloService = helloService;
	}
	
	public void print() {
		System.out.println(helloService.sayHello());
	}
	
}
 

添加了建構函式，同時通過建構函式初始化helloService

spring.xml配置（以下helloService的配置省略）：

<?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">

    <bean id="helloService" class="io.spring.hello.HelloServiceImpl" />
    
    <bean id="helloPrinter" class="io.spring.hello.HelloPrinter">
        <constructor-arg ref="helloService" />
    </bean>

</beans>

首先使用<bean id="xx" class="xxx" />的方式將helloService的例項加入容器。 再使用上面的<constructor-arg>注入 執行輸出：

hello

多個引數

1. 按照順序注入 HelloPrinter類：

public class HelloPrinter {

	private HelloService helloService;
	
	public HelloPrinter(HelloService helloService, String s1, String s2, int id) {
		this.helloService = helloService;
		System.out.println(s1 + s2 + id);
	}
	
	public void print() {
		System.out.println(helloService.sayHello());
	}
	
}

xml配置：

<bean id="helloService" class="io.spring.hello.HelloServiceImpl" />
    
<bean id="helloPrinter" class="io.spring.hello.HelloPrinter">
    <constructor-arg ref="helloService" />
    <constructor-arg value="aaa"/>
    <constructor-arg value="bbb"/>
    <constructor-arg value="123"/>
</bean>

執行輸出：

aaabbb123
hello

2. 按照型別注入 HelloPrinter

public class HelloPrinter {

	private HelloService helloService;
	
	public HelloPrinter(HelloService helloService, String s, int id) {
		this.helloService = helloService;
		System.out.println(s + id);
	}
	
	public void print() {
		System.out.println(helloService.sayHello());
	}
	
}

xml配置

<bean id="helloService" class="io.spring.hello.HelloServiceImpl" />

<bean id="helloPrinter" class="io.spring.hello.HelloPrinter">
    <constructor-arg type="int" value="123"/>
    <constructor-arg type="io.spring.hello.HelloService" ref="helloService" />
    <constructor-arg type="java.lang.String" value="aaa"/>
</bean>

即使順序不同也可以正確注入。 如果有多個相同的型別，則按照實際注入的順序和構造器中引數型別的順序決定。（相同的型別先到先得） 執行輸出：

aaa123
hello

3. 按照name注入

HelloPrinter

public class HelloPrinter {

	private HelloService helloService;
	
	public HelloPrinter(HelloService helloService, String s, int id) {
		this.helloService = helloService;
		System.out.println(s + id);
	}
	
	public void print() {
		System.out.println(helloService.sayHello());
	}
	
}

xml

<bean id="helloPrinter" class="io.spring.hello.HelloPrinter">
    <constructor-arg name="id" value="123"/>
    <constructor-arg name="helloService" ref="helloService" />
    <constructor-arg name="s" value="aaa"/>
</bean>

指定構造器中引數的name 輸出：

aaa123
hello

4. 按照index注入
HelloPrinter

public class HelloPrinter {

	private HelloService helloService;
	
	public HelloPrinter(HelloService helloService, String s, int id) {
		this.helloService = helloService;
		System.out.println(s + id);
	}
	
	public void print() {
		System.out.println(helloService.sayHello());
	}
	
}

xml配置：

<bean id="helloPrinter" class="io.spring.hello.HelloPrinter">
    <constructor-arg index="2" value="123"/>
    <constructor-arg index="0" ref="helloService" />
    <constructor-arg index="1" value="aaa"/>
</bean>

index從0開始，分別對應構造器中的引數順序。

setter注入

HelloPrinter

public class HelloPrinter {

	private HelloService helloService;
	
	private String s;
	
	private int id;
	
	public void setHelloService(HelloService helloService) {
		this.helloService = helloService;
	}

	public void setS(String s) {
		this.s = s;
	}

	public void setId(int id) {
		this.id = id;
	}

	public void print() {
		System.out.println(helloService.sayHello() + s + id);
	}
	
}

xml配置：

<bean id="helloPrinter" class="io.spring.hello.HelloPrinter">
    <property name="helloService" ref="helloService" />
    <property name="s" value="hhhh" />
    <property name="id" value="123123" />
</bean>

執行輸出：

hellohhhh123123

注意：使用構造器注入方式不能出現迴圈的依賴注入 例：

<bean id="helloService" class="io.spring.hello.HelloServiceImpl">
    <constructor-arg name="helloPrinter" ref="helloPrinter" />
</bean>

<bean id="helloPrinter" class="io.spring.hello.HelloPrinter">
    <constructor-arg name="helloService" ref="helloService" />
</bean>

spring會報錯

Error creating bean with name 'helloService': Requested bean is currently in creation: Is there an unresolvable circular reference?