Springboot笔记<3> 组件注入注解@Conditional与@import

@Conditional

@Conditional是Spring4新提供的注解，它的作用是按照一定的条件进行判断，满足条件给容器注册bean。

创建ConfigConditional类和测试类ConfigConditionalTest

@Configuration
public class ConfigConditional {
    @Bean(name = "xiaoming")
    public Student getStudent1(){
        Student student = new Student();
        student.setName("xiaoming");
        student.setAge(18);
        return student;
    }
    @Bean(name = "xiaohong")
    public Student getStudent2(){
        Student student = new Student();
        student.setName("xiaohong");
        student.setAge(17);
        return student;
    }
}

测试类ConfigConditionalTest

输出结果为{xiaoming=Student(age=18, name=xiaoming), xiaohong=Student(age=17, name=xiaohong)}，说明两个Student实例被注入进容器。

public class ConfigConditionalTest {

    AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(ConfigConditional.class);

    @Test
    public void test1() {
        Map<String, Student> map = applicationContext.getBeansOfType(Student.class);
        System.out.println(map);
    }
}
//输出为{xiaoming=Student(age=18, name=xiaoming), xiaohong=Student(age=17, name=xiaohong)}

如果我想根据当前操作系统来注入Student实例，只有在非windows操作系统的情况下注入xiaoming。

这就需要我们用到@Conditional注解了。

@Conditional作用

Class<? extends Condition>[] value()说明@Conditional注解传入的是一个Class数组，存在多种条件类的情况。@Target({ElementType.TYPE, ElementType.METHOD})说明@Conditional可以作用在方法和类上，一个方法只能注入一个bean实例，所以@Conditional标注在方法上只能控制一个bean实例是否注入。标注在类上：一个类中可以注入很多实例，@Conditional标注在类上就决定了一批bean是否注入。

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {
   /**
    * All {@link Condition} classes that must {@linkplain Condition#matches match}
    * in order for the component to be registered.
    */
   Class<? extends Condition>[] value();

}

@Conditional举例

创建WindowsConfig，非windows系统，返回true。

public class WindowsConfig implements Condition {

    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        //获取ioc使用的beanFactory
        ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
        //获取类加载器
        ClassLoader classLoader = context.getClassLoader();
        //获取当前环境信息
        Environment environment = context.getEnvironment();
        //获取bean定义的注册类
        BeanDefinitionRegistry registry = context.getRegistry();
        //获得当前系统名
        String property = environment.getProperty("os.name");
        //不包含Windows则说明不是windows系统，返回true
        if (!property.contains("Windows")) {
            return true;
        }
        return false;
    }
}

@Conditional作用在方法上

@Configuration
public class ConfigConditional {
    @Bean(name = "xiaoming")
    @Conditional(WindowsConfig.class)
    public Student getStudent1(){
        Student student = new Student();
        student.setName("xiaoming");
        student.setAge(18);
        return student;
    }
    @Bean(name = "xiaohong")
    public Student getStudent2(){
        Student student = new Student();
        student.setName("xiaohong");
        student.setAge(17);
        return student;
    }
}

因为只在非windows系统注入xiaoming,因此输出为{xiaohong=Student(age=17, name=xiaohong)}。

@Conditional作用在类上

@Configuration
@Conditional(WindowsConfig.class)
public class ConfigConditional {
    @Bean(name = "xiaoming")
    public Student getStudent1(){
        Student student = new Student();
        student.setName("xiaoming");
        student.setAge(18);
        return student;
    }
    @Bean(name = "xiaohong")
    public Student getStudent2(){
        Student student = new Student();
        student.setName("xiaohong");
        student.setAge(17);
        return student;
    }
}

因为只在非windows系统注入xiaoming和xiaohong,因此输出为{}

@import

@Import注解用来帮助我们把一些需要定义为Bean的类导入到IOC容器里面。如果配置类在标准的springboot的包结构下，就是SpringbootApplication启动类在包的根目录下，配置类在子包下。就不需要使用@Import导入配置类，如果配置类在第三方的jar下，我们想要引入这个配置类，就需要@Import对其引入到工程中才能生效。因为这个时候配置类不再springboot默认的扫描范围内。

@Import源码注释里可以知道，value里面传递的Class会被Spring识别成为component组件。

@Import源码注释里面的一段说明，指出了@Import的用法：

1. 导入一个@Configuration类
2. 导入ImportSelector，ImportBeanDefinitionRegistrar的实现类
3. 导入一个普通的类

 /*
 *<p>Provides functionality equivalent to the {@code <import/>} element in Spring XML.
 * Allows for importing {@code @Configuration} classes, {@link ImportSelector} and
 * {@link ImportBeanDefinitionRegistrar} implementations, as well as regular component
 * classes (as of 4.2; analogous to {@link AnnotationConfigApplicationContext#register}).
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Import {

   /**
    * {@link Configuration @Configuration}, {@link ImportSelector},
    * {@link ImportBeanDefinitionRegistrar}, or regular component classes to import.
    */
   Class<?>[] value();

}

@Import引入普通类

@Import引入普通的类可以帮助我们把普通的类定义为Bean。@Import可以添加在@SpringBootApplication(启动类)、@Configuration(配置类)、@Component(组件类)对应的类上。

定义Teacher类

@Data
public class Teacher {
    private int age;
    private String name;
}

在springboot的启动类@Import Teacher类，这个类就可以被其他类当做bean来引用。

​ 输出结果Teacher(age=0, name=null)

@SpringBootApplication
@Import({Teacher.class})
public class SpringbootReviewApplication {

    public static void main(String[] args) {
        ConfigurableApplicationContext run = SpringApplication.run(SpringbootReviewApplication.class, args);
        Teacher teacher = run.getBean(Teacher.class);
        System.out.println(teacher);
    }
}
//输出结果Teacher(age=0, name=null)

@Import引入配置类(@Configuration修饰的类)

@Import除了可以把普通的类定义为Bean，@Import还可以引入一个@Configuration修饰的类(引入配置类)，从而把让配置类生效(配置类下的所有Bean添加到IOC容器里面去)如果配置类在标准的SpringBoot包结构下(SpringBootApplication启动类包的根目录下)。是不需要@Import导入配置类的，SpringBoot自动帮做了。上面的情况一般用于@Configuration配置类不在标准的SpringBoot包结构下面。所以一般在自定义starter的时候用到。

public class ImportDemo {
    public void doSomething () {
        System.out.println("ImportDemo.doSomething()");
    }
}

@Configuration
@Import({ImportDemo.class})
public class ImportConfig{
@Bean
    public Student returnOneStudent(){
        return new Student();
    }
}

@RestController
public class ImportDemoController {

    @Autowired
    private Student student;

    @Autowired
    private ImportDemo importDemo;

    @RequestMapping("/importDemo")
    public String demo() throws Exception {
        importDemo.doSomething();
        return "ImportDemo" + student.toString;
    }
}