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29 Spring使用@Autowired注解自动装配

2017-08-23  本文已影响79人  笑Skr人啊

在上几篇 Spring自动装配 示例中,它会匹配当前Spring容器任何bean的属性自动装配。在大多数情况下,你可能只需要在特定的 bean 自动装配属性。
在Spring中,可以使用 @Autowired 注解通过setter方法,构造函数或字段自动装配Bean。此外,它可以在一个特定的bean属性自动装配。

请参见下面的完整的例子来演示如何使用@Autowired

1. Beans

一个 Customer bean 在bean配置文件中声明。稍后,您将使用 “@Autowired” 来自动装配一个Person bean。

package com.gp6.autoAssemblyBean.autowired;

public class Person {
    private String name;
    private String age;
    private String address;
    
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getAge() {
        return age;
    }
    public void setAge(String age) {
        this.age = age;
    }
    public String getAddress() {
        return address;
    }
    public void setAddress(String address) {
        this.address = address;
    }
}


package com.gp6.autoAssemblyBean.autowired;

import org.springframework.beans.factory.annotation.Autowired;

public class Customer {
    @Autowired  
    private Person person;
    
    private int type;
    private String action;
    
    public Person getPerson() {
        return person;
    }
    public void setPerson(Person person) {
        this.person = person;
    }
    public int getType() {
        return type;
    }
    public void setType(int type) {
        this.type = type;
    }
    public String getAction() {
        return action;
    }
    public void setAction(String action) {
        this.action = action;
    }
}

<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-2.5.xsd">

    <bean id="CustomerBean" class="com.gp6.autoAssemblyBean.autowired.Customer">
        <property name="action" value="buy" />
        <property name="type" value="1" />
        
        <!-- 放开下面这一行注释,才能将person的属性注入到CustomerBean -->
        <!-- <property name="person" ref="PersonBean" /> -->
    </bean>

    <bean id="PersonBean" class="com.gp6.autoAssemblyBean.autowired.Person">
        <property name="name" value="gp6" />
        <property name="address" value="address 123" />
        <property name="age" value="28" />
    </bean>
    
</beans>

package com.gp6.autoAssemblyBean.autowired;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Test {
    public static void main( String[] args ) {
        ApplicationContext context =  new ClassPathXmlApplicationContext("com/gp6/autoAssemblyBean/autowired/applicationContext.xml");

        Customer customer = (Customer)context.getBean("CustomerBean");
        
        System.out.println(customer.getPerson().getName());
        
    }
}


此时运行会报错,因为person属性未能注入到Customer中

2. 注册AutowiredAnnotationBeanPostProcessor

要启用@Autowired,必须注册“AutowiredAnnotationBeanPostProcessor',你可以用两种方式做到这一点:

添加 Spring 上下文和<context:annotation-config />在bean配置文件中。

<beans 
    //...
    xmlns:context="http://www.springframework.org/schema/context"
    //...
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-2.5.xsd">
    //...

    <context:annotation-config />
    //...
</beans>

下面是完整实例


<!-- 通过<context:annotation-config />注册AutowiredAnnotationBeanPostProcessor -->
<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-2.5.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-2.5.xsd">

    <context:annotation-config />
    
    <bean id="CustomerBean" class="com.gp6.autoAssemblyBean.autowired.registerAutowiredAnnotationBeanPostProcessor.first.Customer">
        <property name="action" value="buy" />
        <property name="type" value="1" />
    </bean>

    <bean id="PersonBean" class="com.gp6.autoAssemblyBean.autowired.registerAutowiredAnnotationBeanPostProcessor.first.Person">
        <property name="name" value="gp6First" />
        <property name="address" value="address 123" />
        <property name="age" value="28" />
    </bean>
    
</beans>

Customer,Person,Test类和上面完全一致,只是所在包不同,注意Test类中的配置文件地址

直接在bean配置文件包含“AutowiredAnnotationBeanPostProcessor”。

<!-- 直接包含 AutowiredAnnotationBeanPostProcessor -->
<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-2.5.xsd">

    
    <bean  class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor"/>

    <bean id="CustomerBean" class="com.gp6.autoAssemblyBean.autowired.registerAutowiredAnnotationBeanPostProcessor.second.Customer">
        <property name="action" value="buy" />
        <property name="type" value="1" />
    </bean>

    <bean id="PersonBean" class="com.gp6.autoAssemblyBean.autowired.registerAutowiredAnnotationBeanPostProcessor.second.Person">
        <property name="name" value="gp6Second" />
        <property name="address" value="address 123" />
        <property name="age" value="28" />
    </bean>
    
</beans> 

Customer,Person,Test类类和上面完全一致,只是所在包不同,注意Test类中的配置文件地址

3. @Autowired示例

现在,你可以通过 @Autowired 自动装配 bean,它可以在 setter 方法,构造函数或字段中使用。

package com.gp6.autoAssemblyBean.autowired.example;

import org.springframework.beans.factory.annotation.Autowired;

public class Customer {
    //1. @Autowired 字段
    @Autowired
    private Person person;
    
    private int type;
    private String action;
    
    //2. @Autowired 构造方法
    //@Autowired
    public Customer(Person person) {
        this.person = person;
    }
    
    public Person getPerson() {
        return person;
    }
    
    //3  @Autowired setter 方法
    //@Autowired
    public void setPerson(Person person) {
        this.person = person;
    }
    public int getType() {
        return type;
    }
    public void setType(int type) {
        this.type = type;
    }
    public String getAction() {
        return action;
    }
    public void setAction(String action) {
        this.action = action;
    }
}

上面的3种方法都会自动装配“PersonBean”到Customer的person属性。

Person,Test类类和上面完全一致,只是所在包不同,注意Test类中的配置文件地址

依赖检查

默认情况下,@Autowired将执行相关检查,以确保属性已经装配正常。当Spring无法找到匹配的Bean装配,它会抛出异常。要解决这个问题,可以通过 @Autowired 的“required”属性设置为false来禁用此检查功能。

package com.gp6.autoAssemblyBean.autowired.example;

import org.springframework.beans.factory.annotation.Autowired;

public class Customer {
    //1. @Autowired 字段
    @Autowired(required=false)
    private Person person;
    
    private int type;
    private String action;
    
    //2. @Autowired 构造方法
    //@Autowired(required=false)
    public Customer(Person person) {
        this.person = person;
    }
    
    public Person getPerson() {
        return person;
    }
    
    //3  @Autowired setter 方法
    //@Autowired(required=false)
    public void setPerson(Person person) {
        this.person = person;
    }
    public int getType() {
        return type;
    }
    public void setType(int type) {
        this.type = type;
    }
    public String getAction() {
        return action;
    }
    public void setAction(String action) {
        this.action = action;
    }
}

在上面的例子中,如果Spring不能找到一个匹配的Bean,person属性将不设定。

@Qualifier

@Qualifier注解我们用来控制bean应在字段上自动装配。例如,具有两个类似的 person bean 配置文件






<!-- 通过<context:annotation-config />注册AutowiredAnnotationBeanPostProcessor -->
<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-2.5.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-2.5.xsd">

    <context:annotation-config />
    
    <bean id="CustomerBean" class="com.gp6.autoAssemblyBean.autowired.qualifier.Customer">
        <property name="action" value="buy" />
        <property name="type" value="1" />
    </bean>

    <bean id="PersonBean1" class="com.gp6.autoAssemblyBean.autowired.qualifier.Person">
        <property name="name" value="gp6" />
        <property name="address" value="address 123" />
        <property name="age" value="28" />
    </bean>
    
    <bean id="PersonBean2" class="com.gp6.autoAssemblyBean.autowired.qualifier.Person">
        <property name="name" value="gp6" />
        <property name="address" value="address 123" />
        <property name="age" value="28" />
    </bean>
    
</beans>



Spring知道哪个 bean 应当装配?

为了解决这个问题,可以使用 @Qualifier 自动装配一个特定的 bean,例如,

package com.gp6.autoAssemblyBean.autowired.qualifier;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;

public class Customer {
    @Autowired
    @Qualifier("PersonBean1")
    private Person person;
    private int type;
    private String action;
    //getter and setter methods
}

这意味着,“PersonBean1” bean被自动装配到customer的person属性。阅读下面完整的例子 – Spring自动装配@Qualifier实例

总结

@Autowired注解非常灵活,功能强大,绝对比bean配置文件的“autowire”属性要更好。

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