Spring 三种依赖注入方式详解

-- pom.xml
<!-- https://mvnrepository.com/artifact/org.springframework/spring-context -->
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-webmvc</artifactId>
    <version>5.1.8.RELEASE</version>
</dependency>
-- web.xml
<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd" >

<web-app>
    <display-name>Archetype Created Web Application</display-name>
    
    <filter>
        <filter-name>CharacterEncodingFilter</filter-name>
        <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
        <init-param>
            <param-name>encoding</param-name>
            <param-value>UTF-8</param-value>
        </init-param>
        <init-param>
            <param-name>forceEncoding</param-name>
            <param-value>true</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>CharacterEncodingFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

</web-app>
-- applicationContext.xml
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:p="http://www.springframework.org/schema/p"
       xmlns:mvc="http://www.springframework.org/schema/mvc"
       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-3.0.xsd
      http://www.springframework.org/schema/context
      http://www.springframework.org/schema/context/spring-context.xsd
      http://www.springframework.org/schema/mvc
      http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd">
    <context:component-scan base-package="cywen.demo.fw" />
    <mvc:annotation-driven />
</beans>

• field通过@Autowired注入，没有循环依赖的问题

@Component
public class IndexService1 {
    @Autowired
    private IndexService2 indexService2;
}

@Component
public class IndexService2 {
    @Autowired
    private IndexService1 indexService1;
}

• setter通过@Autowired注入，没有循环依赖的问题

@Component
public class IndexService1 {
    private IndexService2 indexService2;
    @Autowired
    public void setIndexService2(IndexService2 indexService2) {
        this.indexService2 = indexService2;
    }
}

@Component
public class IndexService2 {
    private IndexService1 indexService1;
    @Autowired
    public void setIndexService1(IndexService1 indexService1){
        this.indexService1 = indexService1;
    }
}

– constructor注入依赖，下例中项目启动失败，抛出循环依赖的异常

@Component
public class IndexService1 {
    private IndexService2 indexService2;
    @Autowired
    public IndexService1(IndexService2 indexService2) {
        this.indexService2 = indexService2;
    }
}

@Component
public class IndexService2 {
    private IndexService1 indexService1;
    @Autowired
    public IndexService2(IndexService1 indexService1) {
        this.indexService1 = indexService1;
    }
}

• 总结

  如果使用构造器注入，在spring项目启动的时候，就会抛出：BeanCurrentlyInCreationException：Requested bean is currently in creation: Is there an unresolvable circular reference？从而提醒你避免循环依赖

  所以，综合考虑，还是属性上添加@Autowired更好一些，主要原因是代码简洁