SpringFactoriesLoader类作用详解

public final class SpringFactoriesLoader {
    //文件位置，可以存在多个JAR文件中
    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
    //用来缓存MultiValueMap对象
    private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap<>();
    private SpringFactoriesLoader() {
    }
    /**
     * 根据给定的类型加载并实例化工厂的实现类
     */
    public static <T> List<T> loadFactories(Class<T> factoryType, @Nullable ClassLoader classLoader) {
        Assert.notNull(factoryType, "'factoryType' must not be null");
        //获取类加载器
        ClassLoader classLoaderToUse = classLoader;
        if (classLoaderToUse == null) {
            classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
        }
        //加载类的全限定名
        List<String> factoryImplementationNames = loadFactoryNames(factoryType, classLoaderToUse);
        if (logger.isTraceEnabled()) {
            logger.trace("Loaded [" + factoryType.getName() + "] names: " + factoryImplementationNames);
        }
        //创建一个存放对象的List
        List<T> result = new ArrayList<>(factoryImplementationNames.size());
        for (String factoryImplementationName : factoryImplementationNames) {
            //实例化Bean,并将Bean放入到List集合中
            result.add(instantiateFactory(factoryImplementationName, factoryType, classLoaderToUse));
        }
        //对List中的Bean进行排序
        AnnotationAwareOrderComparator.sort(result);
        return result;
    }
    /**
     * 根据给定的类型加载类路径的全限定名
     */
    public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
        //获取名称
        String factoryTypeName = factoryType.getName();
        //加载并获取所有META-INF/spring.factories中的value
        return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
    }
    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        //根据类加载器从缓存中获取，如果缓存中存在,就直接返回，如果不存在就去加载
        MultiValueMap<String, String> result = cache.get(classLoader);
        if (result != null) {
            return result;
        }
        try {
            //获取所有JAR及classpath路径下的META-INF/spring.factories的路径
            Enumeration<URL> urls = (classLoader != null ?
                    classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            result = new LinkedMultiValueMap<>();
            //遍历所有的META-INF/spring.factories的路径
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                UrlResource resource = new UrlResource(url);
                //将META-INF/spring.factories中的key value加载为Prpperties对象
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                for (Map.Entry<?, ?> entry : properties.entrySet()) {
                    //key名称
                    String factoryTypeName = ((String) entry.getKey()).trim();
                    for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
                        //以factoryTypeName为key,value为值放入map集合中
                        result.add(factoryTypeName, factoryImplementationName.trim());
                    }
                }
            }
            //放入到缓存中 
            cache.put(classLoader, result);
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load factories from location [" +
                    FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }
    //实例化Bean对象
    @SuppressWarnings("unchecked")
    private static <T> T instantiateFactory(String factoryImplementationName, Class<T> factoryType, ClassLoader classLoader) {
        try {
            Class<?> factoryImplementationClass = ClassUtils.forName(factoryImplementationName, classLoader);
            if (!factoryType.isAssignableFrom(factoryImplementationClass)) {
                throw new IllegalArgumentException(
                        "Class [" + factoryImplementationName + "] is not assignable to factory type [" + factoryType.getName() + "]");
            }
            return (T) ReflectionUtils.accessibleConstructor(factoryImplementationClass).newInstance();
        }
        catch (Throwable ex) {
            throw new IllegalArgumentException(
                "Unable to instantiate factory class [" + factoryImplementationName + "] for factory type [" + factoryType.getName() + "]",
                ex);
        }
    }
}

@SpringBootApplication
public class App{
    public static void main(String[] args) {
        //SpringApplication.run(App.class, args);
        List<String> names = SpringFactoriesLoader.loadFactoryNames(IStudentService.class, ClassUtils.getDefaultClassLoader());
        for (String name : names) {
            System.out.println(name);
        }
        System.out.println("===================================");
        List<IStudentService> iStudentServices = SpringFactoriesLoader.loadFactories(IStudentService.class, ClassUtils.getDefaultClassLoader());
        for (IStudentService iStudentService : iStudentServices) {
            System.out.println(iStudentService);
            System.out.println(iStudentService.game());
        }
    }
}