guava cache 存儲的數據結構2之ReferenceEntry實現類 --- 3

上一節提到緩存中的實體類是ReferenceEntry{K, V>（key可以是任意的類型，value是ValueReference{K, V>），但是它只是個接口，下圖是他的實現

• 創建方式

  這個是在Segment類裏，map 是LocalCache.

     ReferenceEntry<K, V> newEntry(K key, int hash, @Nullable ReferenceEntry<K, V> next) {
      return map.entryFactory.newEntry(this, checkNotNull(key), hash, next);
    }

  而entryFactory = EntryFactory.getFactory(keyStrength, usesAccessEntries(), usesWriteEntries());  

   1. keyStrength : 默認是Strength.STRONG，還有SOFT和WEAK

   enum EntryFactory {
    STRONG -- new StrongEntry<K, V>(key, hash, next),
    .....

    static final EntryFactory[] factories = {
      STRONG, STRONG_ACCESS, STRONG_WRITE, STRONG_ACCESS_WRITE,
      WEAK, WEAK_ACCESS, WEAK_WRITE, WEAK_ACCESS_WRITE,
    };
    //提供具體的實體類
    static EntryFactory getFactory(Strength keyStrength, boolean usesAccessQueue,
        boolean usesWriteQueue) {
      int flags = ((keyStrength == Strength.WEAK) ? 4: 0)
          | (usesAccessQueue ? 1: 0)
          | (usesWriteQueue ? 2: 0);
      return factories[flags];
    }

• AbstractReferenceEntry ：所有的方法都是拋出異常

    public ValueReference<K, V> getValueReference() {
      throw new UnsupportedOperationException();
    }
    public void setValueReference(ValueReference<K, V> valueReference) {
      throw new UnsupportedOperationException();
    }
    ...//都是拋出異常,所以這個類沒有實際意義

     1.  StrongEntry : 提供key,value,hash,nxt

      final K key;
      final int hash;
      final ReferenceEntry<K, V> next;//就是每次插入的
      volatile ValueReference<K, V> valueReference = unset();

       1.1 StrongAccessEntry : 訪問的實體類,訪問時間，前一個和後一個訪問的節點

           volatile long accessTime = Long.MAX_VALUE
           ReferenceEntry<K, V> nextAccess = nullEntry();
           ReferenceEntry<K, V> previousAccess = nullEntry();

       1.2 StrongWriteEntry ：寫入的實體類,寫入時間，前一個和後一個寫入的節點

           volatile long writeTime = Long.MAX_VALUE;
           ReferenceEntry<K, V> nextWrite = nullEntry();
           ReferenceEntry<K, V> previousWrite = nullEntry();

       1.3  StrongAccessWriteEntry : 以上的 1.1和1.2 的所有屬性都有

• WeakEntry : 除了hash,next,valueReference 其餘方法都是拋出異常

  這個實現WeakReference,採用了弱引用，再沒有引用時即會被垃圾回收

      class WeakEntry<K, V> extends WeakReference<K> implements ReferenceEntry<K, V> {
           final int hash;
           final ReferenceEntry<K, V> next;
           volatile ValueReference<K, V> valueReference = unset();
           public void setPreviousInWriteQueue(ReferenceEntry<K, V> previous) {
               throw new UnsupportedOperationException();
           }
           .....
      }

     其餘幾個方法和Strong的相對應