如何不向Guava的LoadingCache添加空值？

Question

我必須使用Guava's LoadingCache用於緩存xpath值，其值爲key => xpath expression，值爲xpath values。

但麻煩是，並非所有的xpthes都有值。很多xpath值都有空值（如果要清晰的話，則爲null）。

下面是代碼片段：

private LoadingCache<String, List<String>> cachedXpaths = CacheBuilder.newBuilder() 
     .expireAfterWrite(3, TimeUnit.MINUTES) 
     .maximumSize(1000) 
     .concurrencyLevel(5) 
     .weakKeys() 
     .build(new CacheLoader<String, List<String>>() { 
      @Override 
      public List<String> load(String key) throws Exception { 
       return createListByKey(key); 
      } 
     }); 

private static List<String> createListByKey(String key) throws Exception { 
    List<String> values = null; 
    try { 
     values = instance.getXpathValues(key); 
    } catch (XPathExpressionException ignore) { 
    } 
    return values; 
} 

結果我：

testEncounterSection(com.epam.cdatest.section.EncountersSectionTest) Time elapsed: 0.002 sec <<< FAILURE! 
com.google.common.util.concurrent.UncheckedExecutionException: com.epam.cdatest.exceptions.XpathHasEmptyValueException 
    at com.epam.cdatest.parsers.XpathEvaluator.getXpathValues(XpathEvaluator.java:123) 
    at com.epam.cdatest.parsers.XpathEvaluator.createListByKey(XpathEvaluator.java:53) 
    at com.epam.cdatest.parsers.XpathEvaluator.access$000(XpathEvaluator.java:32) 
    at com.epam.cdatest.parsers.XpathEvaluator$1.load(XpathEvaluator.java:46) 
    at com.epam.cdatest.parsers.XpathEvaluator$1.load(XpathEvaluator.java:43) 

如何避免這種情況，調用load()當EPATH是空的？

Answer 1

如果你想要一個空List<String>和一個空的一個區分，因爲LoadingCache不支持空值，你可以使用一個LoadingCache<String, Optional<List<String>>>：

private LoadingCache<String, Optional<List<String>>> cachedXpaths = CacheBuilder.newBuilder() 
     .expireAfterWrite(3, TimeUnit.MINUTES) 
     .maximumSize(1000) 
     .concurrencyLevel(5) 
     .weakKeys() 
     .build(new CacheLoader<String, Optional<List<String>>>() { 
      @Override 
      public Optional<List<String>> load(String key) { 
       try { 
        // If getXpathValues() can also return null, use fromNullable() 
        return Optional.of(instance.getXpathValues(key)); 
       } catch (XPathExpressionException | XpathHasEmptyValueException ignore) { 
        // Maybe log something here as well 
        return Optional.absent(); 
       } 
      } 
     }); 

---

更新：如果你是不熟悉Optional，Javadoc，Wiki中的頁面，以及Java 8的所有文檔，它們現在包含一個等效的類。

TL; DR是可以對其內容的存在進行測試的包裝：

Optional<List<String>> optionalXpaths = cachedXpaths.getUnchecked(str); 
if (optionalXpaths.isPresent()) { 
    List<String> xpaths = optionalXpaths.get(); 
    // Do something with xpaths 
} 

或可選擇地（但它的醜陋）：

List<String> xpaths = cachedXpaths.getUnchecked(str).orNull(); 
if (xpaths != null) { 
    // Do something with xpaths 
}