Spring Boot注解深入解析

1. @Configuration注解

@Configuration注解用于将一个类定义为Spring Boot的IOC容器入口。类中的某个方法如果使用@Bean注解，则该方法返回的对象会被注册为IOC容器中的Bean。需要注意的是，@Configuration注解本身并不影响Bean的注册，它主要是标记作用，配合@Bean注解使用。

@Configurationpublic class MyConfig {    @Bean("person2")    public Person person() {        return new Person();    }}public class MyTest {    @Test    public void test() {        ApplicationContext context = new AnnotationConfigApplicationContext(MyConfig.class);        Object obj = context.getBean("person2");        System.out.println(obj.toString());    }}public class Person {    private String name;    private String age;    public Person() {}    public Person(String name, String age) {        this.name = name;        this.age = age;    }    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; }    @Override    public String toString() {        return "Person{" +               "name='" + name + '\'' +               ", age='" + age + '\'' +               '}';    }}

2. @ComponentScan注解

@ComponentScan注解用于启用组件扫描功能，它可以自动发现包路径下所有标有@Component、@Service等注解的类。默认情况下，@ComponentScan会扫描当前类所在包及子包中的所有类。

@ComponentScan(value = "com.example.springbootapp")public class MyConfig {}public class MyTest {    @Test    public void test() {        ApplicationContext context = new AnnotationConfigApplicationContext(MyConfig.class);        System.out.println(context.getBeanDefinitionCount());        String[] beanNames = context.getBeanDefinitionNames();        System.out.println(Arrays.toString(beanNames).replaceAll("[|]", "").replaceAll(",", "\n"));    }}

3. @Scope注解

@Scope注解用于定义Bean的作用域。Spring Boot默认的作用域包括：

prototype：每次请求都会创建新的实例

singleton：只创建一个实例

request：每次请求创建一个实例

session：每次会话创建一个实例

@Configurationpublic class MyConfig {    @Scope("singleton")    @Bean("person2")    public Person person() {        return new Person();    }}public class MyTest {    @Test    public void test() {        ApplicationContext context = new AnnotationConfigApplicationContext(MyConfig.class);        Object obj1 = context.getBean("person2");        Object obj2 = context.getBean("person2");        System.out.println(obj1 == obj2); // 加了@Scope("prototype")表示原型，会重新拷贝    }}

4. @Lazy注解

@Lazy注解用于延迟初始化Bean。与@Lazy配合使用时，Bean在IOC容器初始化时不会立即加载，只有在首次通过context.getBean()获取时，才会进行初始化。

@Configurationpublic class MyConfig {    @Lazy    @Bean    public Person person() {        System.out.println("将对象添加到IOC容器中");        return new Person();    }}public class MyTest {    @Test    public void test() {        ApplicationContext context = new AnnotationConfigApplicationContext(MyConfig.class);        System.out.println("IOC容器创建完成");        context.getBean("person");    }}

5. @Conditional注解

@Conditional注解用于根据条件注册Bean。从Spring Boot 4开始引入，通过实现Condition接口，可以根据特定的条件判断来决定是否注册Bean。

@Configurationpublic class MyConfig {    @Conditional(WinConditional.class)    @Bean    public Person jason() {        System.out.println("将jason 添加到容器");        return new Person("jason", "18");    }    @Conditional(LinuxConditional.class)    @Bean    public Person jerry() {        System.out.println("jerry 添加到容器");        return new Person("jerry", "18");    }    @Bean    @Conditional(WinConditional.class)    public Person tom() {        System.out.println("tom 添加到容器");        return new Person("tom", "18");    }}public class LinuxConditional implements Condition {    @Override    public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {        ConfigurableListableBeanFactory beanFactory = conditionContext.getBeanFactory();        Environment environment = conditionContext.getEnvironment();        String osName = environment.getProperty("os.name");        System.out.println(osName);        return osName.contains("Linux");    }}public class WinConditional implements Condition {    @Override    public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {        ConfigurableListableBeanFactory beanFactory = conditionContext.getBeanFactory();        Environment environment = conditionContext.getEnvironment();        String osName = environment.getProperty("os.name");        System.out.println("-----------------------------------------" + osName);        return osName.contains("Win");    }}

6. @Import注解

@Import注解用于导入外部资源，支持导入XML文件、Properties文件、Excel文件等。可以通过指定value属性导入多个Bean。

@Configuration@Import(value = {Cat.class, MyImportSelector.class, MyImportDefinitionRegistrar.class})public class MyConfig {    @Bean    public Person person() {        return new Person();    }    @Bean    public MyFactoryBean monkey() {        return new MyFactoryBean();    }}public class MyFactoryBean implements FactoryBean {    @Override    public Monkey getObject() throws Exception {        return new Monkey();    }    @Override    public Class
    getObjectType() {        return Monkey.class;    }    @Override    public boolean isSingleton() {        return true;    }}

7. @Import的多种使用场景

通过@Import注解直接指定Bean：

@Import(value = {MyImportDefinitionRegistrar.class})public class MyConfig {}

通过实现ImportSelector接口：

public class MyImportSelector implements ImportSelector {    @Override    public String[] selectImports(AnnotationMetadata annotationMetadata) {        return new String[]{"com.example.springbootapp.entity.Company", "com.example.springbootapp.entity.Member"};    }}

通过自定义过滤器：

public class MyTypeFilter implements TypeFilter {    @Override    public boolean match(MetadataReader metadataReader, MetadataReaderFactory metadataReaderFactory) throws IOException {        Set
     
       annotationTypes = metadataReader.getAnnotationTypes();        boolean flag = false;        for (String str : annotationTypes) {            if (str.contains(Controller.class.getName()) || str.contains(Service.class.getName())) {                flag = true;                break;            }        }        return flag;    }}

通过这些注解的合理组合，可以实现对Spring Boot应用的精细化配置和管理。

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