2. spring 应用之IOC

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我们知道Spring Framework 最重要的功能就是IoC (Inversion of Control ),也叫DI(dependency injection),这不是我说的,是官网这么说的,截图如下

spring官网说IoC,也叫DI,是同一个意思.我会在文章最后写一下自己的个人理解

首先复习一下spring的IOC相关应用

1.将对象交给spring管理

首先是测试类

public class UserService  {
    
    private String userName;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName1) {
        this.userName = userName1;
    }

}

然后是spring的配置文件applicationContext.xml

<?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
        https://www.springframework.org/schema/beans/spring-beans.xsd"
    default-autowire="no">

    
    <bean class="com.lusaisai.service.UserService" id="userService" >
        <!--此处name的值与set方法要一致-->
        <property name="userName" value="lusai"></property>
    </bean>
    
</beans>

最后是测试

public static void main(String[] args) {

        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        UserService userService = (UserService) context.getBean("userService");
   
　　　　　　System.out.println(userService.getUserName());

可以看到注入成功

2.依赖注入

首先是UserService依赖的对象UserDao

public interface UserDao {
    void test();
}

然后是它的实现类

public class UserDaoImpl implements UserDao {
    @Override
    public void test() {
        System.out.println("UserDaoImpl");
    }
}

在UserService增加依赖的对象UserDao和set方法,注意,这里需要UserService的空构造(不写的时候编译器会帮我们自动构建一个无参构造)

public class UserService  {

    private String userName;

    private UserDao userDao;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName1) {
        this.userName = userName1;
    }

    public UserDao getUserDao() {
        return userDao;
    }

    public void setUserDao(UserDao userDao) {
        this.userDao = userDao;
    }

    public void serviceTest(){
        userDao.test();
    }

}

然后是xml配置

<!--必须有空构造-->
    <bean class="com.lusaisai.service.UserService" id="userService" >
        <property name="userName" value="lusai"></property>
        <!--这里的ref指定下方bean标签配置对象的id-->
        <!--此处name的值与UserService的属性userDao的set方法名要一致-->
        <property name="userDao" ref="userDao"></property>
    </bean>

    <bean class="com.lusaisai.dao.UserDaoImpl" id="userDao" ></bean>

然后是测试

ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        UserService userService = (UserService) context.getBean("userService");
        System.out.println(userService.getUserName());
        userService.serviceTest();

看下结果

好了,注入成功

3.我们发现一个问题,我们已经在代码里写了UserDao,还有它的set和get方法,为啥还要在xml里用配置告诉spring应该如何注入呢,其实spring提供了自动装配的功能,如下图

我们可以看到default-autowire有5个可供选择

单独的bean标签里也有5个供选择,这里截个官网的图

下面我们来测试一下,首先是byName,我们把手动装配注释掉,添加上default-autowire="byName" 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
        https://www.springframework.org/schema/beans/spring-beans.xsd"
   default-autowire="byName" >

   <!--必须有空构造-->
   <bean class="com.lusaisai.service.UserService" id="userService" >
      <property name="userName" value="lusai"></property>
      <!--这里的ref指定下方bean标签配置对象的id-->
      <!--此处name的值与UserService的属性userDao的set方法名要一致-->
      <!--<property name="userDao" ref="userDao"></property>-->
   </bean>

   <bean class="com.lusaisai.dao.UserDaoImpl" id="userDao" ></bean>

</beans>

UserService中的代码没有改动,测试

ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        UserService userService = (UserService) context.getBean("userService");
        System.out.println(userService.getUserName());
        UserDao userDao = userService.getUserDao();
        System.out.println(userDao);

看下结果

好的,成功了,我们改一下set方法的名字

public void setUserDao1(UserDao userDao) {
        this.userDao = userDao;
    }

重新运行一次

可以看到,userDao没有注入进来, 我们把方法名复原再次修改属性名试一下

private String userName;

    private UserDao userDao2;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName1) {
        this.userName = userName1;
    }

    public UserDao getUserDao() {
        return userDao2;
    }

    public void setUserDao(UserDao userDao) {
        this.userDao2 = userDao;
    }

重新运行

可以看到,注入成功,因此byName的set方法名要和依赖对象的bean标签的id相同

接下来看下byType,我们将default-autowire="byType" 再加上一个同一个类不同id的bean标签

<?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
        https://www.springframework.org/schema/beans/spring-beans.xsd"
    default-autowire="byType" >

    <!--必须有空构造-->
    <bean class="com.lusaisai.service.UserService" id="userService" >
        <property name="userName" value="lusai"></property>
        <!--这里的ref指定下方bean标签配置对象的id-->
        <!--此处name的值与UserService的属性userDao的set方法名要一致-->
        <!--<property name="userDao" ref="userDao"></property>-->
    </bean>

    <bean class="com.lusaisai.dao.UserDaoImpl" id="userDao" ></bean>

    <bean class="com.lusaisai.dao.UserDaoImpl" id="userDao2" ></bean>

</beans>

userService代码

private String userName;

    private UserDao userDao;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName1) {
        this.userName = userName1;
    }

    public UserDao getUserDao() {
        return userDao;
    }

    public void setUserDao(UserDao userDao) {
        this.userDao = userDao;
    }

再运行一下结果

十月 17, 2019 11:53:15 下午 org.springframework.context.support.AbstractApplicationContext refresh
警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userService' defined in class path resource [applicationContext.xml]: Unsatisfied dependency expressed through bean property 'userDao'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.lusaisai.dao.UserDao' available: expected single matching bean but found 2: userDao,userDao2
Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userService' defined in class path resource [applicationContext.xml]: Unsatisfied dependency expressed through bean property 'userDao'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.lusaisai.dao.UserDao' available: expected single matching bean but found 2: userDao,userDao2
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1499)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1379)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:592)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515)
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:849)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:877)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:549)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:144)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:85)
    at com.lusaisai.test.Test.main(Test.java:16)
Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.lusaisai.dao.UserDao' available: expected single matching bean but found 2: userDao,userDao2
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveNotUnique(DependencyDescriptor.java:221)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1225)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1167)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1484)
    ... 13 more

报错信息,显示只要一个,但找到2个,我们注释掉一个

<!--<bean class="com.lusaisai.dao.UserDaoImpl" id="userDao" ></bean>-->

重新运行,看下结果

注入成功了,说明byType的自动装配方式中如果存在多个相同类型不同id的bean标签，则抛出异常,如果没有匹配的bean，则不自动装配

最后看下构造方法注入

首先

default-autowire="constructor"

然后在UserService中注释掉set方法,添加userDao的构造方法

private String userName;

    private UserDao userDao;

    public UserService(UserDao userDao) {
        this.userDao = userDao;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName1) {
        this.userName = userName1;
    }

    public UserDao getUserDao() {
        return userDao;
    }

,运行,看结果

注入成功了

我们把userDao的构造方法注释了,再运行看下结果

没注入成功,但也不报错

好,到此为止,我们的xml配置将对象交给spring管理就讲完了总结一下

可以看出,no和Default是不自动装配的,byName和byType是通过set方法自动装配的,同时要确保有空构造存在,我猜底层是用newInstance()实现的具体源码后面再看

byName是根据set方法名自动装配的,set方法名要和bean标签的id相对应,否则,注入不成功,但不会报错

byType是根据类型装配的,如果存在多个该属性类型的bean标签，则抛出异常,如果没有匹配的bean，则不自动装配

constructor是根据构造方法来装配的,如果容器中没有一个构造函数参数类型的bean，则不自动装配

可以看到这种xml格式其实是非常麻烦的,实际项目中我们一般通过注解来将对象交给spring管理

只需要将spring的配置文件按以下配置即可,加入包扫描,就能将包下的所有对象通过注解方式来注入

<?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
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
         http://www.springframework.org/schema/context/spring-context-3.0.xsd"
      >

    <context:component-scan base-package="com.lusaisai"></context:component-scan>

</beans>

我们来看例子,注意上面的spring配置文件中一个bean标签都没有了

这里加入了@Component注解,也可以用@Repository,@Service,@Controller注解,四个注解都可以将对象注入到spring容器进行管理,效果是一样的

@Component
public class UserDaoImpl implements UserDao {
    @Override
    public void test() {
        System.out.println("UserDaoImpl");
    }
}

这里在依赖的对象上加入了@autowire注解,我猜底层是通过filed.set调用的，所以不需要set方法,这个以后看源码了再讲

@Component
public class UserService  {

    private String userName="lusai";

    @Autowired
    private UserDao userDao;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName1) {
        this.userName = userName1;
    }

    public UserDao getUserDao() {
        return userDao;
    }

}

当然也可以将注解加在set方法上,如下

@Component
public class UserService  {

    private String userName="lusai";


    private UserDao userDao;

    @Autowired
    public void setUserDao(UserDao userDao) {
        this.userDao = userDao;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName1) {
        this.userName = userName1;
    }

    public UserDao getUserDao() {
        return userDao;
    }

测试一下

public static void main(String[] args) {

        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        UserService userService = (UserService) context.getBean("userService");
        System.out.println(userService.getUserName());
        UserDao userDao = userService.getUserDao();
        System.out.println(userDao);

    }

看下结果

可以看到,注入成功了

如果我们再给UserDao接口的另一个实现类也交给spring管理,会不会报错呢?如下

@Component
public class UserDaoImpl2 implements UserDao{
    @Override
    public void test() {
        System.out.println("UserDaoImpl2");
    }
}

运行一下看结果

Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.lusaisai.dao.UserDao' available: expected single matching bean but found 2: userDaoImpl,userDaoImpl2
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveNotUnique(DependencyDescriptor.java:221)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1225)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1167)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredMethodElement.inject(AutowiredAnnotationBeanPostProcessor.java:668)
    ... 15 more

果然报错了,和上面的报错信息差不多,说明同一个接口,最好不要写多个实现类,那我非要写多个实现类怎么办呢

我们可以用@Resource,并指定它的name属性的别名为类名的首字母小写,如下

@Component
public class UserService  {

    private String userName="lusai";


    @Resource(name = "userDaoImpl2")
    private UserDao userDao;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName1) {
        this.userName = userName1;
    }

    public UserDao getUserDao() {
        return userDao;
    }

}

运行一下,看看结果

成功了

总结:先来张网上的图

@Autowired与@Resource都可以用来装配bean. 都可以写在字段上,或写在setter方法上。两者如果都写在字段上，那么就不需要再写setter方法

@Autowired默认按类型装配（这个注解是属业spring的）,需要导入包org.springframework.beans.factory.annotation.Autowired 默认按照类型来进行装配

@Resource（这个注解属于java的），需要导入包javax.annotation.Resource。默认按照名称进行装配，名称可以通过name属性进行指定

最后,现在流行用javaconfig而不是xml来配置spring,这里贴一下javaconfig的代码

第一种:相当于写bean标签,如下,这里使用@bean注解将UserService 和UserDaoImpl对象交给spring管理

@Configuration
public class SpringConfig {

    @Bean
    public UserService userService(){
        return new UserService();
    }

    @Bean
    public UserDao userDao(){
        return new UserDaoImpl();
    }

}

这里注入依赖

public class UserService  {

    private String userName="lusai";

    @Autowired
    private UserDao userDao;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName1) {
        this.userName = userName1;
    }

    public UserDao getUserDao() {
        return userDao;
    }

}

这是测试demo

public static void main(String[] args) {

        AnnotationConfigApplicationContext annotationContext = new AnnotationConfigApplicationContext(SpringConfig.class);
        UserService userService = (UserService) annotationContext.getBean("userService");
        System.out.println(userService.getUserName());
        UserDao userDao = userService.getUserDao();
        System.out.println(userDao);

    }

看下结果

第二种:相当于扫描包,把上面的两个@bean注解注释了,开启扫描

@Configuration
@ComponentScan("com.lusaisai")
public class SpringConfig {

    /*@Bean
    public UserService userService(){
        return new UserService();
    }

    @Bean
    public UserDao userDao(){
        return new UserDaoImpl();
    }*/

}

在UserService 和UserDaoImpl对象上加上@Component,将他们交给spring管理,如下

@Component
public class UserDaoImpl implements UserDao {
    @Override
    public void test() {
        System.out.println("UserDaoImpl");
    }
}

@Component
public class UserService  {

    private String userName="lusai";

    @Autowired
    private UserDao userDao;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName1) {
        this.userName = userName1;
    }

    public UserDao getUserDao() {
        return userDao;
    }

}

重新测试

可以看到,注入成功了.

我们再提出一个问题 @Autowired与@Resource这两个注解和xml配置中的 default-autowire="byName" 和byType是不是用的相同的原理的呢,这里就后面看源码的时候再来解释吧

最后说一下,我自己对IOC和DI的理解:以上文中的UserService和UserDao为例:

1.UserService和UserDao这两个对象原来我们是自己new出来的,现在交给spring管理,这就叫控制反转,即IOC

2.UserService中有个属性UserDao,那我们可以说:UserService要使用UserDao，那么UserService就对UserDao产生了依赖,也就是UserService依赖UserDao,而UserDao存在于spring容器中,spring将容器中的UserDao对象交给UserService使用,这就叫依赖注入,即DI,

3.DI(依赖注入)其实就是IOC的另外一种说法,DI是由Martin Fowler 在2004年初的一篇论文中首次提出的。他总结道：控制的什么被反转了？就是获得依赖对象的方式反转了(获得依赖对象的方式从我们自己new改成交给spring)

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原文地址：https://www.cnblogs.com/lusaisai/p/11679088.html