SpringBoot借助spring.factories文件跨模块实例化Bean

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.zhj.config.AutoConfiguration

org.springframework.boot.logging.LoggingSystemFactory=\
org.springframework.boot.logging.logback.LogbackLoggingSystem.Factory,\
org.springframework.boot.logging.log4j2.Log4J2LoggingSystem.Factory,\
org.springframework.boot.logging.java.JavaLoggingSystem.Factory

public final class SpringFactoriesLoader {
    
    // 文件位置
    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
    
    // 缓存
    private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap<>();
​
​
    private SpringFactoriesLoader() {
    }
​
​
    
    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);
        }
        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));
        }
        AnnotationAwareOrderComparator.sort(result);
        return result;
    }
    
    
    public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
        
        // 获取工厂类型名称
        String factoryTypeName = factoryType.getName();
        // 加载所有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中classpath路径下的META-INF/spring.factories
            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
            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就是接口的类名称
                    String factoryTypeName = ((String) entry.getKey()).trim();
                    for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
                        // 以factoryTypeName为key,实现类为value放入map集合中
                        result.add(factoryTypeName, factoryImplementationName.trim());
                    }
                }
            }
            // 加入缓存
            cache.put(classLoader, result);
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load factories from location [" +
                    FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }
    
    // 通过反射实例化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);
        }
    }
​
}