spring——Spring Bean属性注入——构造函数注入

亦凉 2024-03-31 17:23 111阅读 0赞

所谓 Bean 属性注入，简单点说就是将属性注入到 Bean 中的过程，而这属性既可以普通属性，也可以是一个对象(Bean)。  
Spring 主要通过以下 2 种方式实现属性注入：

*  构造函数注入
 *  setter 注入(又称设值注入)

###  ###

### 构造函数注入 ###

我们可以通过 Bean 的带参构造函数，以实现 Bean 的属性注入。

使用构造函数实现属性注入大致步骤如下：

1.  1.  在 Bean 中添加一个有参构造函数，构造函数内的每一个参数代表一个需要注入的属性；
    2.  在 Spring 的 XML 配置文件中，通过 <beans> 及其子元素 <bean> 对 Bean 进行定义；
    3.  在 <bean> 元素内使用 <constructor-arg> 元素，对构造函数内的属性进行赋值，Bean 的构造函数内有多少参数，就需要使用多少个 <constructor-arg> 元素。

项目依赖：

<?xml version="1.0" encoding="UTF-8"?>
    <project xmlns="http://maven.apache.org/POM/4.0.0"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
        <modelVersion>4.0.0</modelVersion>
    
        <groupId>org.example</groupId>
        <artifactId>ssw</artifactId>
        <version>1.0-SNAPSHOT</version>
    
        <properties>
            <maven.compiler.source>8</maven.compiler.source>
            <maven.compiler.target>8</maven.compiler.target>
        </properties>
    
        <dependencies>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-context</artifactId>
                <version>5.2.5.RELEASE</version>
            </dependency>
        </dependencies>
    
    </project>

![f20c0e2dc9ce828e66e745bece164d12.png][]

创建student类：

package org.example;
    
    import org.apache.commons.logging.Log;
    import org.apache.commons.logging.LogFactory;
    
    public class Student
    {
    
        private static final Log LOGGER = LogFactory.getLog(Student.class);
    
        private int id;
        private String name;
        private Grade grade;
    
        public Student(int id, String name, Grade grade)
        {
            LOGGER.info("正在执行 Course 的有参构造方法，参数分别为：id=" + id + ",name=" + name + ",grade=" + grade);
            this.id = id;
            this.name = name;
            this.grade = grade;
        }
    
        @Override
        public String toString()
        {
    
            return "Student{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", grade=" + grade +
                    '}';
        }
    }

![b2c0911b1951253f9ed15120c7513c96.png][]

创建grade类：

package org.example;
    
    import org.apache.commons.logging.Log;
    import org.apache.commons.logging.LogFactory;
    
    public class Grade
    {
    
        private static final Log LOGGER = LogFactory.getLog(Grade.class);
    
        private Integer gradeId;
        private String gradeName;
    
        public Grade(Integer gradeId, String gradeName)
        {
            LOGGER.info("正在执行 Course 的有参构造方法，参数分别为：gradeId=" + gradeId + ",gradeName=" + gradeName);
    
            this.gradeId = gradeId;
            this.gradeName = gradeName;
        }
        @Override
        public String toString()
        {
    
            return "Grade{" +
                    "gradeId=" + gradeId +
                    ", gradeName='" + gradeName + '\'' +
                    '}';
        }
    }

![1c13b86622faa23b0d1c6c2b8f25e1ed.png][]

创建bean：

<?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">
    
    
    
    
    
        <bean id="student" class="org.example.Student">
            <constructor-arg name="id" value="2"></constructor-arg>
            <constructor-arg name="name" value="李四"></constructor-arg>
            <constructor-arg name="grade" ref="grade"></constructor-arg>
        </bean>
    
        <bean id="grade" class="org.example.Grade">
            <constructor-arg name="gradeId" value="4"></constructor-arg>
            <constructor-arg name="gradeName" value="四年级"></constructor-arg>
        </bean>
    
    </beans>

![9cc8a758b642b65ff2a053e296445706.png][]

![b4c874197709641e79f44c0a31a89ba3.png][]

执行：

package org.example;
    
    import org.apache.commons.logging.Log;
    import org.apache.commons.logging.LogFactory;
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.support.ClassPathXmlApplicationContext;
    
    public class Main
    {
        private static final Log LOGGER = LogFactory.getLog(Main.class);
    
        public static void main(String[] args)
        {
            //获取 ApplicationContext 容器
            ApplicationContext context = new ClassPathXmlApplicationContext("bean.xml");
    
            //获取名为 student 的 Bean
            Student student = context.getBean("student", Student.class);
    
            //通过日志打印学生信息
            LOGGER.info(student.toString());
    
            System.out.println(student.toString());
    
        }
    
    }

![837553d8928b29df143d343860dd7ed8.png][]

执行结果：

![02ce9c557bc9623b9cb35d7c77b8b7ba.png][]

[f20c0e2dc9ce828e66e745bece164d12.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/dc17096b7edc4ff389753236e6cf178f.png
[b2c0911b1951253f9ed15120c7513c96.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/3cfbf2cfd3cd42588039a83480c45eb6.png
[1c13b86622faa23b0d1c6c2b8f25e1ed.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/c8f2181a3f3842febcf1bca78bd53636.png
[9cc8a758b642b65ff2a053e296445706.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/a6c59a753cae4b5b951476920fc0a3e4.png
[b4c874197709641e79f44c0a31a89ba3.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/5e75067336a24e2498d3923d14dc29cb.png
[837553d8928b29df143d343860dd7ed8.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/87883a365fb344f1980418a7bae32222.png
[02ce9c557bc9623b9cb35d7c77b8b7ba.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/2bb738a50350401aa3d74232b3378989.png