该篇博客主要阐述使用注解来配置Bean以及装配Bean(Bean与Bean之间关联关系),通过注解可以大大减少使用XML一大堆的配置代码,方便开发
注意:该篇博客阅读时必须理解清楚什么是配置Bean什么是装配Bean
- 配置Bean:将Bean装载到IoC容器中(注册)
- 装配Bean:Bean与Bean之间的依赖注入
博客主要阐述
1、基于注解配置Bean
2、基于注解装配Bean
一、基于注解配置Bean
作用:就是将Bean装到IoC容器中
1、为自动检测标注Bean
Spring能从classpath下自动扫描、侦测、实例化具有特定注解的组件,这些特定注解的组件如下
- @Component——通用的构造型注解
- @Controller——标识控制层(web层)
- @Repository——标识持久层(Dao层)
- @Service——标识业务层(Service层)
在目前的Spring版本中,@Controller、@Repository、@Service这三个注解与@Component基本等效,可以混用,但是从命名上可以看出三个注解分别代表哪一层。所以虽然目前三个注解与@Component是等效的,但Spring之后版本或许会为这三个注解带来不一样的新意,所以最好是使用这三个注解来标注每一个层
2、Component用法
Person.java
package com.linjie.springidol;
import org.springframework.stereotype.Component;
/**
* @author LinJie
* Component用法
*/
@Component("person")
public class Person {
public void add(Person p) {
System.out.println("Person.....");
}
}
测试类
package com.linjie.springidol;
import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
/**
* @author LinJie
*
*/
public class SpringTest {
@Test
public void test() {
ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
Person person = (Person) context.getBean("person");
System.out.println(person);
person.add(person);
}
}
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"
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 definitions here -->
<!--base-package:指定Spring IoC容器 扫描的包 -->
<context:component-scan base-package="com.linjie.springidol">
</context:component-scan>
</beans>
结果
运行的结果就很清楚了,@Component为我们完成了Bean的注册和装配,我们就不再需要使用XML中的< bean>元素了。其实就相当于
<bean id="person" class="com.linjie.springidol.Person">
1、在applicationContext我们只需要用:
<context:component-scan base-package="com.linjie.springidol"></context:component-scan>
就能够使用注解了,假如你无法实现的话,可能是你未导入spring-aop-4.3.10.RELEASE.jar 具体原因可转战笔者这篇博客:https://blog.csdn.net/w_linux/article/details/80098853
2、要注意需要在applicationContext中引用context约束,如果你没有相关插件引入可以用以下方法将context约束模板引入
进入该路径spring-framework-4.3.10.RELEASE-dist\spring-framework-4.3.10.RELEASE\docs\spring-framework-reference\html,找到xsd-configuration.html,搜索the context schema,将模板导入到自己的applicationContext.xml中即可
3、base-package:指定一个需要扫描的父类包,Spring容器将会扫描这个父类包及其子类包中的所有类,当需要扫描多个包(没有继承关系)时,可以使用逗号分隔
4、@Component所标注的Person类,如果不像上面例子中@Component(“person”),而只是@Component,其Bean的ID默认为无限定类名,即首字母小写。Person Bean的ID就是person,当然显示命名ID即@Component(“person”)
5、resource-pattern只扫描指定的类而非基包下的所有类,这里dao是springidol的子包
<context:component-scan
base-package="com.linjie.springidol"
resource-pattern="dao/*.class"
></context:component-scan>
6、context:component-scan的两个子节点
- context:exclude-filter:子节点指定排除哪些指定表达式的组件(排除)
context:include-filter:子节点指定包含哪些表达式的组件 (只包含),该子节点需要use-default-filters=”false”配合使用
<context:component-scan base-package="com.linjie.springidol"> <context:exclude-filter type="annotation" expression="org.springframework.stereotype.Service"/> <!--<context:include-filter type="annotation" expression="org.springframework.stereotype.Service"/> --> </context:component-scan>
type属性值:
7、一个简单例子
项目目录(一个父包三个子包)
UserView.java(Controller)
package com.linjie.springidol.view;
import org.springframework.stereotype.Controller;
/**
* @author LinJie
* View层
*/
@Controller
public class UserView {
public void show() {
System.out.println("view1...");
}
}
UserService.java(Service)
package com.linjie.springidol.service;
import org.springframework.stereotype.Service;
/**
* @author LinJie
* Service层
*/
@Service
public class UserService {
public void show() {
System.out.println("service....");
}
}
UserDao.java(Repository)
package com.linjie.springidol.dao;
import org.springframework.stereotype.Repository;
/**
* @author LinJie
* Dao层
*/
@Repository
public class UserDao {
public void show() {
System.out.println("dao...");
}
}
Person.java(Component)
package com.linjie.springidol;
import org.springframework.stereotype.Component;
/**
* @author LinJie
* Component用法
*/
@Component("person")
public class Person {
public void show() {
System.out.println("Person.....");
}
}
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"
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 definitions here -->
<context:component-scan
base-package="com.linjie.springidol">
</context:component-scan>
</beans>
测试类
package com.linjie.springidol;
import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import com.linjie.springidol.dao.UserDao;
import com.linjie.springidol.service.UserService;
import com.linjie.springidol.view.UserView;
public class SpringTest {
@Test
public void test() {
ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
Person person = (Person) context.getBean("person");
person.show();
UserDao userDao = (UserDao) context.getBean("userDao");
userDao.show();
UserService userService = (UserService) context.getBean("userService");
userService.show();
UserView userView = (UserView) context.getBean("userView");
userView.show();
}
}
结果
二、基于注解装配Bean
作用:Bean与Bean之间的关联关系建立(前提:已经装配Bean到IoC容器中)
元素会自动注册AutowiredAnnotationBeanPostProcessor实例(一个Bean后置处理器),该实例可以自动装配具有@Autowired、@Resource、@Inject注解的属性
Autowired
1. @Autowired注解自动装配具有兼容类型的单个Bean属性
2. 构造器、普通字段(访问控制符没有影响)、一切具有参数的方法都可以应用
普通字段:在UserService中调用UserDao
UserService.java
package com.linjie.springidol.service;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import com.linjie.springidol.dao.UserDao;
/**
* @author LinJie
* Service层
*/
@Service
public class UserService {
@Autowired
private UserDao userDao;
public void show() {
System.out.println("service....");
userDao.show();
}
}
构造器:在UserService中调用UserDao
package com.linjie.springidol.service;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import com.linjie.springidol.dao.UserDao;
/**
* @author LinJie
* Service层
*/
@Service
public class UserService {
private UserDao userDao;
@Autowired
public UserService(UserDao userDao) {
super();
this.userDao = userDao;
}
public void show() {
System.out.println("service....");
userDao.show();
}
}
有参数的方法上:在UserService中调用UserDao
package com.linjie.springidol.service;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import com.linjie.springidol.dao.UserDao;
/**
* @author LinJie
* Service层
*/
@Service
public class UserService {
private UserDao userDao;
@Autowired
public void setUserService(UserDao userDao) {
this.userDao = userDao;
}
public void show() {
System.out.println("service....");
userDao.show();
}
}
3. 加上@Autowired意思就是会去IoC容器里找是否有与其被注解的Bean类型兼容的Bean(通过@Component等注册Bean的注解所装饰的才会在IoC容器里)
4. 所有使用@Autowired注解的属性都需要被设置(即通过@Component等注解配置到IoC容器中),否则会报异常,如果没有配置这个Bean,可以设置@Autowired注解的required属性为false,但该Bean就是null:@Autowired(required=false),但是注意当使用构造器装配时,只有一个构造器可以将@Autowired的required属性设置为true。其他使用@Autowired注解所标注的构造器只能将required属性设置为false,当使用@Autowired标注多个构造器时,Spring就会从所有满足装配条件构造器中选择入参最多的那个构造器
UserDao未被配置
若此时调用UserDao则报错
org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.linjie.springidol.dao.UserDao' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {}
使用@Autowired(required=false)
package com.linjie.springidol.service;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import com.linjie.springidol.dao.UserDao;
/**
* @author LinJie
* Service层
*/
@Service
public class UserService {
private UserDao userDao;
@Autowired(required=false)
public void setUserService(UserDao userDao) {
this.userDao = userDao;
}
public void show() {
System.out.println("service....");
System.out.println("userDao :"+userDao);
userDao.show();
}
}
显示结果(无法实现调用show(),因为userDao为null)
5. 限定歧义性的依赖:可能会有多个Bean都满足装配条件,并且都可以装配到属性或参数中(采用@Qualifier)
场景
假设有两个Bean都实现了Instrument接口
分析
在这种场景下,@Autowired注解没办法选择哪一个Bean才是它需要的
解决
采用Spring的@Qualifier
用法
public void setInstrument(@Qualifier("small1")Instrument instrument){}
或者
@Qualifier("small1")
public void setInstrument(Instrument instrument) {}
代码演示
Instrument.java(接口)
package com.linjie.springidol;
public interface Instrument {
public void play();
}
small1.java(实现类1)
package com.linjie.springidol;
import org.springframework.stereotype.Component;
@Component
public class small1 implements Instrument{
@Override
public void play() {
System.out.println("small1");
}
}
small2.java(实现类2)
package com.linjie.springidol;
import org.springframework.stereotype.Component;
@Component
public class small2 implements Instrument{
@Override
public void play() {
System.out.println("small2");
}
}
big.java(调用IoC中的Bean)
package com.linjie.springidol;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Component;
@Component("b")
public class big {
private Instrument instrument;
@Autowired
public void setInstrument(@Qualifier("small1")Instrument instrument) {
this.instrument = instrument;
}
public void show() {
System.out.println(instrument);
instrument.play();
}
}
测试类
package com.linjie.springidol;
import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import com.linjie.springidol.dao.UserDao;
import com.linjie.springidol.service.UserService;
import com.linjie.springidol.view.UserView;
public class SpringTest {
@Test
public void test() {
ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
big b = (big) context.getBean("b");
b.show();
}
}
结果(可以看到通过Qualifier来指定使用small1 Bean)
6、@Autowired可以应用在数组类型的属性上,此时Spring将会把所有匹配的Bean都进行自动装配
7、@Autowired可以应用在集合属性上,此时Spring读取集合的类型信息,然后自动装配所有与之兼容的Bean
8、@Autowired用在java.util.Map上,若Map的键值为String,那么Spring将自动装配与之Map值类型兼容的Bean,此时Bean的名称作为键值
9、关于@Resource、@Inject与@Autowired相类似,所以不多做阐述
参考
《Spring IN ACTION》