====
注入
====

1 什么是注入(Injection)

通过Spring工厂及配置文件，为所创建对象的属性即成员变量赋值。

2 为什么需要注入

/src/main/java/cn/tedu/injection/Employee.java：

public class Employee {
    private Integer id;
    private String name;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Employee{" +
            "id=" + id +
            ", name='" + name + '\'' +
            '}';
    }
}

/src/test/java/cn/tedu/injection/EmployeeTest.java：

class EmployeeTest {
    private Logger logger = Logger.getLogger(EmployeeTest.class);

    @org.junit.jupiter.api.Test
    void test() {
        ApplicationContext applicationContext =
            new ClassPathXmlApplicationContext("/applicationContext.xml");
        Employee employee = applicationContext.getBean("employee", Employee.class);
        employee.setId(139);      \ 通过set方法手
        employee.setName("关羽"); / 动为属性赋值
        logger.debug(employee);
    }
}

Employee{id=139, name='关羽'}

例程：Injection

如果Employee类的属性需要改变，则必须修改Java文件——耦合。

3 通过配置文件实现注入

/src/main/resources/applicationContext.xml：

...
<bean id="employee" class="cn.tedu.injection.Employee">
    <property name="id">     \_
        <value>139</value>     \ 与属性名
    </property>               _/ 严格一直
    <property name="name">   /
        <value>关羽</value>
    </property>
</bean>
...

/src/test/java/cn/tedu/injection/EmployeeTest.java：

class EmployeeTest {
    private Logger logger = Logger.getLogger(EmployeeTest.class);

    @org.junit.jupiter.api.Test
    void test() {
        ApplicationContext applicationContext =
            new ClassPathXmlApplicationContext("/applicationContext.xml");
        Employee employee = applicationContext.getBean("employee", Employee.class);
        /*
        employee.setId(139);      \ 无需手
        employee.setName("关羽"); / 动赋值
        */
        logger.debug(employee);
    }
}

Employee{id=139, name='关羽'}

例程：Injection

如果Employee类的属性需要改变，只需修改配置文件，Java代码不受影响——解耦。