在Scala中使用番石榴的簡單緩存管理器參數化（泛型）類型錯誤

Question

我對Scala很新穎，仍然不太瞭解泛型。因此，我無法弄清楚爲什麼編譯器恨我類型不匹配錯誤。

我正在使用Google的Guava庫創建一些簡單的緩存，表示爲Scala的ConcurrentMap。我想跟蹤使用另一個緩存名稱映射到ConcurrentMap（緩存）的創建緩存。以下是我迄今爲止它編譯，但缺少高速緩存的跟蹤（我註釋掉失敗的位）：

import scala.collection.mutable.ConcurrentMap 

trait CacheManager { 

    def getCache[V <: AnyRef](
      cacheName: String, 
      cacheListener: Option[CacheListener] = None): ConcurrentMap[String, V] 

} 


import scala.collection.JavaConversions._ 
import com.google.common.collect.MapMaker 
import java.util.concurrent.{ConcurrentMap => JConcurrentMap, TimeUnit} 
import org.slf4j.LoggerFactory 
import com.google.common.cache.{RemovalNotification, RemovalListener, CacheBuilder} 
import scala.collection.mutable.ConcurrentMap 

class LocalCacheManager extends CacheManager { 

    private val logger = LoggerFactory.getLogger(classOf[LocalCacheManager]) 


    private val caches /*: ConcurrentMap[String, ConcurrentMap[String, _ <: AnyRef]]*/ = 
      asScalaConcurrentMap[String, ConcurrentMap[String, _ <: AnyRef]](
       new MapMaker().concurrencyLevel(4).makeMap[String, ConcurrentMap[String, _ <: AnyRef]]()) 

    def getCache[V <: AnyRef](cacheName: String, cacheListener: Option[CacheListener] = None) = { 
//  caches.getOrElseUpdate(cacheName, { 
      val cache = CacheBuilder.newBuilder() 
         .concurrencyLevel(4) 
         .softValues() 
         .expireAfterAccess(30, TimeUnit.MINUTES) 
         .build[String, V]() 
      asScalaConcurrentMap[String, V](cache.asMap()) 
//  }) 
    } 
} 

基本上，如果我嘗試添加番石榴緩存到緩存（通過註釋掉caches.getOrElseUpdate），那麼編譯器會抱怨有以下幾點：

error: type mismatch; 
found : scala.collection.mutable.ConcurrentMap[String,_$1] where type _$1 <: AnyRef 
required: scala.collection.mutable.ConcurrentMap[String,V] 
caches.getOrElseUpdate(cacheName, { 

Answer 1

檢索緩存時由於您提供的類型信息，這是沒有必要的嘗試和維持通配符輸入。將值輸入AnyRef然後在最後將類型轉換爲V要簡單得多。以下編譯和應該幫助。此外，不需要直接調用ascalaConcurrentMap，因爲它很好...隱式。

import scala.collection.JavaConversions._ 
import com.google.common.collect.MapMaker 
import java.util.concurrent.TimeUnit 
import com.google.common.cache.CacheBuilder 
import scala.collection.mutable.ConcurrentMap 

trait CacheListener // not sure what this is doing yet. 

trait CacheManager { 

    def getCache[V <: AnyRef](
      cacheName: String, 
      cacheListener: Option[CacheListener] = None): ConcurrentMap[String, V] 

} 

class LocalCacheManager extends CacheManager { 
    private val caches: ConcurrentMap[String, ConcurrentMap[String, AnyRef]] = 
       new MapMaker().concurrencyLevel(4).makeMap[String, ConcurrentMap[String, AnyRef]]() 
    def getCache[V <: AnyRef](cacheName: String, cacheListener: Option[CacheListener] = None) = 
     caches.getOrElseUpdate(cacheName, { 
       CacheBuilder.newBuilder() 
          .concurrencyLevel(4) 
          .softValues() 
          .expireAfterAccess(30, TimeUnit.MINUTES) 
          .asInstanceOf[CacheBuilder[String, AnyRef ]] 
          .build[String, AnyRef ]() 
          .asMap() 
       }).asInstanceOf[ConcurrentMap[String, V]] 
}