SpringMVC 源码解析AbstractCachingViewResolver自定义缓存

￡神魔★判官ぃ 2022-06-05 02:28 113阅读 0赞

AbstractCachingViewResolver中使用了两个Map用于缓存View视图对象，一个是ConcurrentHashMap在线程安全的前提下提供了较好的并发访问能力，效率较高，另一个是LinkedHashMap保证了值的有序性，同时它有方法是删除最前保存的值，removeEldestEntry(),返回true时表示达到了最大空间。删除值。返回false就是没有达到上线,最常使用的ConcurrentHashMap获取缓存数据，当操作缓存时同时操作ConcurrentHashMap、LinkedHashMap，两者结合起来非常方便，值得我们学习与使用。

/** The maximum number of entries in the cache */
    	private volatile int cacheLimit = DEFAULT_CACHE_LIMIT;
    
    	/** Whether we should refrain from resolving views again if unresolved once */
    	private boolean cacheUnresolved = true;
    
    	/** Fast access cache for Views, returning already cached instances without a global lock */
    	private final Map<Object, View> viewAccessCache = new ConcurrentHashMap<Object, View>(DEFAULT_CACHE_LIMIT);
    
    	/** Map from view key to View instance, synchronized for View creation */
    	@SuppressWarnings("serial")
    	private final Map<Object, View> viewCreationCache =
    			new LinkedHashMap<Object, View>(DEFAULT_CACHE_LIMIT, 0.75f, true) {
    				@Override
    				protected boolean removeEldestEntry(Map.Entry<Object, View> eldest) {
    					if (size() > getCacheLimit()) {
    						viewAccessCache.remove(eldest.getKey());
    						return true;
    					}
    					else {
    						return false;
    					}
    				}
    			};
     public View resolveViewName(String viewName, Locale locale) throws Exception {
    		if (!isCache()) {
    			return createView(viewName, locale);
    		}
    		else {
    			Object cacheKey = getCacheKey(viewName, locale);
    			View view = this.viewAccessCache.get(cacheKey);
    			if (view == null) {
    				synchronized (this.viewCreationCache) {
    					view = this.viewCreationCache.get(cacheKey);
    					if (view == null) {
    						// Ask the subclass to create the View object.
    						view = createView(viewName, locale);
    						if (view == null && this.cacheUnresolved) {
    							view = UNRESOLVED_VIEW;
    						}
    						if (view != null) {
    							this.viewAccessCache.put(cacheKey, view);
    							this.viewCreationCache.put(cacheKey, view);
    							if (logger.isTraceEnabled()) {
    								logger.trace("Cached view [" + cacheKey + "]");
    							}
    						}
    					}
    				}
    			}
    			return (view != UNRESOLVED_VIEW ? view : null);
    		}
    	}