Spring定义bean的三种方式和自动注入

一、定义bean的三种途径：

首先编写Student和Teacher两个类

public class Student {  
  
    private String name;  
  
    private Teacher teacher;  
  
    public String getName() {  
        return name;  
    }  
  
    public void setName(String name) {  
        this.name = name;  
    }  
  
    public Teacher getTeacher() {  
        return teacher;  
    }  
  
    public void setTeacher(Teacher teacher) {  
        this.teacher = teacher;  
    }  
  
}

public class Teacher {  
  
    private String name;  
  
    public String getName() {  
        return name;  
    }  
  
    public void setName(String name) {  
        this.name = name;  
    }  
  
}

方法一：基于XML的bean定义（需要提供setter方法）

<?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">  
  
    <bean id="student" class="test.Student">  
        <property name="name" value="张三"/>  
        <property name="teacher" ref="teacher"/>  
    </bean>  
  
    <bean id="teacher" class="test.Teacher">  
        <property name="name" value="李四"/>  
    </bean>  
  
</beans>  
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public class Main {  
  
    public static void main(String args[]){  
        FileSystemXmlApplicationContext context=new FileSystemXmlApplicationContext("applicationContext.xml的绝对路径");  
        Student student= (Student) context.getBean("student");  
        Teacher teacher= (Teacher) context.getBean("teacher");  
        System.out.println("学生的姓名："+student.getName()+"。老师是"+student.getTeacher().getName());  
        System.out.println("老师的姓名："+teacher.getName());  
    }  
  
}

方法二：基于注解的bean定义（不需要提供setter方法）

Spring为此提供了四个注解，这些注解的作用与上面的XML定义bean效果一致，在于将组件交给Spring容器管理。组件的名称默认是类名（首字母变小写），也可以自己修改：

@Component：当对组件的层次难以定位的时候使用这个注解

@Controller：表示控制层的组件

@Service：表示业务逻辑层的组件

@Repository：表示数据访问层的组件

使用这些注解的时候还有一个地方需要注意，就是需要在applicationContext.xml中声明<contex:component-scan...>一项，指明Spring容器扫描组件的包目录。

@Component("teacher")  
public class Teacher {  
  
    @Value("李四")  
    private String name;  
  
    public String getName() {  
        return name;  
    }  
  
}  
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@Component("student")  
public class Student {  
  
    @Value("张三")  
    private String name;  
  
    @Resource  
    private Teacher teacher;  
  
    public String getName() {  
        return name;  
    }  
  
    public Teacher getTeacher() {  
        return teacher;  
    }  
  
}  
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<?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">  
  
    <!--扫描组件的包目录-->  
    <context:component-scan base-package="test"/>  
  
</beans>  
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public class Main {  
  
    public static void main(String args[]){  
        FileSystemXmlApplicationContext context=new FileSystemXmlApplicationContext("applicationContext.xml的绝对路径");  
        Student student= (Student) context.getBean("student");  
        Teacher teacher= (Teacher) context.getBean("teacher");  
        System.out.println("学生的姓名："+student.getName()+"。老师是"+student.getTeacher().getName());  
        System.out.println("老师的姓名："+teacher.getName());  
    }  
  
}

方法三：基于Java类的bean定义（需要提供setter方法）

@Configuration  
public class BeansConfiguration {  
  
    @Bean  
    public Student student(){  
        Student student=new Student();  
        student.setName("张三");  
        student.setTeacher(teacher());  
        return student;  
    }  
  
    @Bean  
    public Teacher teacher(){  
        Teacher teacher=new Teacher();  
        teacher.setName("李四");  
        return teacher;  
    }  
  
}  
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public class Main {  
  
    public static void main(String args[]){  
        AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext(BeansConfiguration.class);  
        Student student= (Student) context.getBean("student");  
        Teacher teacher= (Teacher) context.getBean("teacher");  
        System.out.println("学生的姓名："+student.getName()+"。老师是"+student.getTeacher().getName());  
        System.out.println("老师的姓名："+teacher.getName());  
    }  
  
}

二、Spring的自动注入

Spring提供了五种自动装配的类型

no：顾名思义，显式指明不使用Spring的自动装配功能

byName：根据属性和组件的名称匹配关系来实现bean的自动装配

byType：根据属性和组件的类型匹配关系来实现bean的自动装配，有多个适合类型的对象时装配失败

constructor：与byType类似是根据类型进行自动装配，但是要求待装配的bean有相应的构造函数

autodetect：利用Spring的自省机制判断使用byType或是constructor装配

基于XML的自动装配

<?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">  
  
    <bean id="student" class="test.Student" autowire="byName">  
        <property name="name" value="张三"/>  
    </bean>  
  
    <bean id="teacher" class="test.Teacher">  
        <property name="name" value="李四"/>  
    </bean>  
  
</beans>

这里我并没有显式为Student对象注入Teacher属性，而是使用autowired="byName"代替，这样一来Spring会帮我们处理这些细节，将名字是teacher的组件注入到Student对象中。

基于注解的自动装配

其实上面已经应用过了，这里再提一下@Resource和@Autowired的区别。@Resource默认是使用byName进行装配，@Autowired默认使用byType进行装配。

@Component("teacher")  
public class Teacher {  
  
    @Value("李四")  
    private String name;  
  
    public String getName() {  
        return name;  
    }  
  
    public void setName(String name) {  
        this.name = name;  
    }  
}  

 
@Component("student")  
public class Student {  
  
    @Value("张三")  
    private String name;  
  
    @Resource  
    private Teacher teacher;  
  
    public String getName() {  
        return name;  
    }  
  
    public void setName(String name) {  
        this.name = name;  
    }  
  
    public Teacher getTeacher() {  
        return teacher;  
    }  
  
    public void setTeacher(Teacher teacher) {  
        this.teacher = teacher;  
    }  
}

三、如何进行选择？

<bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">


<property name="driverClassName" value="com.mysql.jdbc.Driver"/>  
        <property name="url" value="jdbc:mysql://localhost:3306/study"/>  
        <property name="username" value="账号"/>  
        <property name="password" value="密码"/>  
</bean>

自动装配：一般我使用基于注解的自动装配。同样也是为了减少XML配置文件的“篇幅”。