我必须使用Guava的 LoadingCache
用于将xpath表达式缓存为xpath值。
麻烦的是,不是所有的xpath都有值。大多数情况下,值是 null
.
代码片段。
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;
}
结果。
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)
如何避免调用 load()
当xpath值为空时?
如果你想区分一个空的 List<String>
和一个空的,因为 LoadingCache
不支持空值,您可以使用 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:如果你不熟悉 Optional
,还有就是 Javadoc的一页。维基以及Java 8的所有文档,现在包括一个等价的类。
TL;DR它是一个包装器,可以测试其内容的存在。
Optional<List<String>> optionalXpaths = cachedXpaths.getUnchecked(str);
if (optionalXpaths.isPresent()) {
List<String> xpaths = optionalXpaths.get();
// Do something with xpaths
}
或者是(但它更丑)。
List<String> xpaths = cachedXpaths.getUnchecked(str).orNull();
if (xpaths != null) {
// Do something with xpaths
}