Spring-ImportSelector接口功能使用案例

public interface ImportSelector {
	//返回类的字符串数组，也就是要创建的bean的className，比如userService.class.getName()
    //被创建的bean是在其他bean(@Component、@Service等注解修饰的Bean)创建之前创建的
	String[] selectImports(AnnotationMetadata importingClassMetadata);
}

//定义一个业务类
public class UserServiceTest {
	public String getUserName(){
		return "测试";
	}
}
//实现ImportSelector接口
public class MyImportSelect implements ImportSelector {
	@Override
	public String[] selectImports(AnnotationMetadata importingClassMetadata) {
		//返回要注册到Spring容器的Class集合
		return new String[]{UserServiceTest.class.getName()};
	}
}
//配置类
@Configuration
@Import(MyImportSelect.class) //导入我们实现ImportSelector的类。
public class AppConfigClassTest {
}

public class MainTest {
	public static void main(String[] args) {
		AnnotationConfigApplicationContext AnnotationConfigApplicationContext = new AnnotationConfigApplicationContext(AppConfigClassTest.class);
		//这里我们不能通过“userServiceTest”来获取bean,因为这个bean的name不是userServiceTest，而是userServiceTest.getClass().getName()
        //因为bean的别名成功器，只是针对像注解@Service等注解，会生成一个首字母小写的BeanName
		UserServiceTest userServiceTest = AnnotationConfigApplicationContext.getBean(UserServiceTest.class);
		System.out.println(((UserServiceTest)userServiceTest).getUserName());
	}
}

//这里就直接跳到ConfigurationClassPostProcessor处理@Import注解的逻辑上
private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass, 
                Collection<SourceClass> importCandidates, Predicate<String> exclusionFilter, boolean checkForCircularImports) {
    //如果importCandidates为空直接return,为什么会有这个，因为下面代码可能会递归调用processImports，就比如Import一个类，这个类也带了@Import注解，那就会在调用一次processImports方法
    if (importCandidates.isEmpty()) {
        return;
    }
    for (SourceClass candidate : importCandidates) {
        if (candidate.isAssignable(ImportSelector.class)) {
        	//1、import的类，实现了ImportSelector接口
            Class<?> candidateClass = candidate.loadClass();
            //利用反射Class实例化对象，这个对象不是代理对象不要搞混了。
            ImportSelector selector = ParserStrategyUtils.instantiateClass(candidateClass, ImportSelector.class,
                    this.environment, this.resourceLoader, this.registry);
            Predicate<String> selectorFilter = selector.getExclusionFilter();
            if (selectorFilter != null) {
                exclusionFilter = exclusionFilter.or(selectorFilter);
            }
            if (selector instanceof DeferredImportSelector) {
                this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
            } else {
            	//调用ImportSelector接口里面的selectImports方法，拿到返回值Class集合。在通过递归的方式嗲用processImports挨个解析每一个Class
                String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
                //转成SourceClass集合
                Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames, exclusionFilter);
                //再次调用processImports方法
                processImports(configClass, currentSourceClass, importSourceClasses, exclusionFilter, false);
            }
        } else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
           。。。。。。
        } else {
            //3、ImportSelector和ImportBeanDefinitionRegistrar都没有实现
            this.importStack.registerImport(currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
            //进一步解析其他注解，比如@Component @Import等最后会把configClass注册到Spring容器中。
            processConfigurationClass(candidate.asConfigClass(configClass), exclusionFilter);
        }
    }
}

//创建实例对象
static <T> T instantiateClass(Class<?> clazz, Class<T> assignableTo, Environment environment, ResourceLoader resourceLoader, BeanDefinitionRegistry registry) {
	ClassLoader classLoader = 
	       (registry instanceof ConfigurableBeanFactory ? ((ConfigurableBeanFactory) registry).getBeanClassLoader() : resourceLoader.getClassLoader());
	T instance = (T) createInstance(clazz, environment, resourceLoader, registry, classLoader);
	ParserStrategyUtils.invokeAwareMethods(instance, environment, resourceLoader, registry, classLoader);
	return instance;
}
//调用createInstance方法创建实例对象
private static Object createInstance(Class<?> clazz, Environment environment, ResourceLoader resourceLoader, BeanDefinitionRegistry registry, @Nullable ClassLoader classLoader) {
	Constructor<?>[] constructors = clazz.getDeclaredConstructors();
	。。。。。。。
	return BeanUtils.instantiateClass(clazz);//通过Bean的工具类生成实例对象
}