MultiValueMap

org.springframework.util 包下的 MultiValueMap 接口

void add(K key, @Nullable V value);

看下 LinkedMultiValueMap 实现

@Override
public void add(K key, @Nullable V value) {
	List<V> values = this.targetMap.computeIfAbsent(key, k -> new LinkedList<>());
	values.add(value);
}

targetMap 在 LinkedMultiValueMap 构造方法的时候初始化了

private final Map<K, List<V>> targetMap;
public LinkedMultiValueMap() {
    this.targetMap = new LinkedHashMap<>();
}

介绍下 computeIfAbsent 当get(key) 是 null 的时候 我们 put 新的 value 如果不是 那么直接返回 get(key) 也就是 LinkedList

default V computeIfAbsent(K key,
         Function<? super K, ? extends V> mappingFunction) {
     Objects.requireNonNull(mappingFunction);
     V v;
     if ((v = get(key)) == null) {
         V newValue;
         if ((newValue = mappingFunction.apply(key)) != null) {
             put(key, newValue);
             return newValue;
         }
     }

     return v;
 }

举个例子

MultiValueMap<String,String> map = new LinkedMultiValueMap<>();
map.add("k","1");
map.add("k","2");

map 对应的 key 是 k， 对应的 value 是一个 LinkedList ，里面元素有 1 和 2

MultiValueMap 应用在 SpringFactoriesLoader 这个类检索 META-INF/spring.factories 文件

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
	MultiValueMap<String, String> result = cache.get(classLoader);
	if (result != null) {
		return result;
	}

	try {
		Enumeration<URL> urls = (classLoader != null ?
				classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
				ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
		result = new LinkedMultiValueMap<>();
		while (urls.hasMoreElements()) {
			URL url = urls.nextElement();
			UrlResource resource = new UrlResource(url);
			Properties properties = PropertiesLoaderUtils.loadProperties(resource);
			for (Map.Entry<?, ?> entry : properties.entrySet()) {
				String factoryTypeName = ((String) entry.getKey()).trim();
				for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
					result.add(factoryTypeName, factoryImplementationName.trim());
				}
			}
		}
		cache.put(classLoader, result);
		return result;
	}
	catch (IOException ex) {
		throw new IllegalArgumentException("Unable to load factories from location [" +
				FACTORIES_RESOURCE_LOCATION + "]", ex);
	}
}