SpringBoot @Been和@Import

@Bean

在启动类中编写

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@SpringBootApplication
public class Demo16Application {
    public static void main(String[] args) {
        ApplicationContext run = SpringApplication.run(Demo16Application.class, args);
        LoaderOptions bean = run.getBean(LoaderOptions.class);
        System.out.println(bean);
    }
    @Bean
    public LoaderOptions getLoaderOptions(){
        return new LoaderOptions();
    }
}

在配置类中编写

在启动类包中创建config文件夹接着在创建配置类

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@Configuration
public class MyConfig {
    @Bean
    public LoaderOptions getLoaderOptions(){
        return new LoaderOptions();
    }
}

@Import

当启动类中的@ComponentScan找不到配置文件时可以用@Import导入

常规用法

在启动类中添加@Import(MyConfig.class),就可以导入配置类,MyConfig为配置类的名字
配置类中的信息与上文的一致,只是路径发生改变,导致@SpringBootApplication无法找到该配置类

ImportSelector接口

新建SelectImport文件在实现ImportSelector,接着在添加配置类的路径

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public class SelectImport implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata importingClassMetadata) {
        return new String[]{"com.example.config.MyConfig"};
    }
}

启动类中修改@Import中的参数@Import(SelectImport.class)

引入resources中imports文件

在resources文件夹中创建common.imports文件,imports中的信息为配置类的路径,可存放多个配置类
接着将该文件中的信息导入导SelectImport类中

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public class SelectImport implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata importingClassMetadata) {
        List<String> imports = new ArrayList<>();
        InputStream resourceAsStream = SelectImport.class.getClassLoader().getResourceAsStream("common.imports");
        BufferedReader br = new BufferedReader(new InputStreamReader(resourceAsStream));
        String line = null;
        try{
            while((line = br.readLine())!= null){
                imports.add(line);
            }
        }catch (IOException e){
            throw new RuntimeException(e);
        }finally {
            if (br!=null){
                try{
                    br.close();
                }catch (IOException e){
                    throw new RuntimeException(e);
                }
            }
        }
        return imports.toArray(new String[0]);
    }
}

注册条件

@ConditionalOnProperty

配置文件中存在对应的属性，才声明该bean
prefix为Property文件中的student类,name为student中的属性名
当没有Properties的数据传入该bean就不会将该bean带入ioc容器中

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@Configuration
public class MyConfig {
    @Bean
    @ConditionalOnProperty(prefix = "student",name={"name","id"})
    @ConfigurationProperties(prefix = "student")
    public Student getStudent(){
        return new Student();
    }
}

@ConditionalOnMissingBean

当不存在当前类型的bean时，才声明该bean
当容器中有Teacher时Student不创建,有前后之分

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@Configuration
public class MyConfig {
    @Bean
    public Teacher getTeacher(){
        return new Teacher();
    }
    @Bean
    @ConditionalOnMissingBean(Teacher.class)
    @ConfigurationProperties(prefix = "student")
    public Student getStudent(){
        return new Student();
    }
}

@ConditionalOnClass

当前环境存在指定的这个类时，才声明该bean
name未指定类的全类名

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public class MyConfig {
    @Bean
    @ConditionalOnClass(name="org.springframework.web.servlet.DispatcherServlet")
    public Teacher getTeacher(){
        return new Teacher();
    }
    @Bean
    @ConditionalOnMissingBean(Teacher.class)
    @ConfigurationProperties(prefix = "student")
    public Student getStudent(){
        return new Student();
    }
}