guava collection/cache初探

写了上面一篇，看了点eventbus相关的guava代码后，发现里面用到了很多其他guava包里的方法，所以顺着看一下，比如之前用到的map都是guava自己的

Multimap：可以包含有几个重复Key的value，你可以put进入多个不同value但是相同的key，但是又不是让后面覆盖前面的内容。

Table：二维矩阵类似，有需要这类结构可以参考

Hash：还有一堆已经实现好了的哈希方法，如果有需要可以直接用

有些代码也可以从中参考

对于Cache首先写一个例子，一般缓存肯定要考虑的是怎么存，存多久，怎么过期，初始大小，最大大小等等

        LoadingCache<String, String> cache = CacheBuilder.newBuilder()
                .expireAfterWrite(1000, TimeUnit.MILLISECONDS)
                .expireAfterAccess(1000, TimeUnit.MILLISECONDS)
                .concurrencyLevel(8)
                .initialCapacity(100)
                .maximumSize(100)
                .weakKeys()
                .weakValues()
                .build(new CacheLoader<String, String>() {
                    @Override
                    public String load(String key) throws Exception {
                        return "test";
                    }
                });

其中参数大部分一看名字就知道，这里我觉得比较特殊的是这个，看注释就明白了

  @GwtIncompatible // java.lang.ref.WeakReference
  public CacheBuilder<K, V> weakKeys() {
    return setKeyStrength(Strength.WEAK);
  }

    WEAK {
      @Override
      <K, V> ValueReference<K, V> referenceValue(
          Segment<K, V> segment, ReferenceEntry<K, V> entry, V value, int weight) {
        return (weight == 1)
            ? new WeakValueReference<K, V>(segment.valueReferenceQueue, value, entry)
            : new WeightedWeakValueReference<K, V>(
                segment.valueReferenceQueue, value, entry, weight);
      }

      @Override
      Equivalence<Object> defaultEquivalence() {
        return Equivalence.identity();
      }
    };

put放入元素，跟一般的没多大区别，就是计算下hash，找到对应的index放进去，就是map的一般原理

  @Override
  public V put(K key, V value) {
    checkNotNull(key);
    checkNotNull(value);
    int hash = hash(key);
    return segmentFor(hash).put(key, hash, value, false);
  }

来看具体的put，起始也比较好懂

    @Nullable
    V put(K key, int hash, V value, boolean onlyIfAbsent) {
      lock();
      try {
        long now = map.ticker.read();
        preWriteCleanup(now);

        int newCount = this.count + 1;
        if (newCount > this.threshold) { // ensure capacity
          expand();
          newCount = this.count + 1;
        }

        AtomicReferenceArray<ReferenceEntry<K, V>> table = this.table;
        int index = hash & (table.length() - 1);
        ReferenceEntry<K, V> first = table.get(index);

        // Look for an existing entry.
        for (ReferenceEntry<K, V> e = first; e != null; e = e.getNext()) {
          K entryKey = e.getKey();
          if (e.getHash() == hash
              && entryKey != null
              && map.keyEquivalence.equivalent(key, entryKey)) {
            // We found an existing entry.

            ValueReference<K, V> valueReference = e.getValueReference();
            V entryValue = valueReference.get();

            if (entryValue == null) {
              ++modCount;
              if (valueReference.isActive()) {
                enqueueNotification(
                    key, hash, entryValue, valueReference.getWeight(), RemovalCause.COLLECTED);
                setValue(e, key, value, now);
                newCount = this.count; // count remains unchanged
              } else {
                setValue(e, key, value, now);
                newCount = this.count + 1;
              }
              this.count = newCount; // write-volatile
              evictEntries(e);
              return null;
            } else if (onlyIfAbsent) {
              // Mimic
              // "if (!map.containsKey(key)) ...
              // else return map.get(key);
              recordLockedRead(e, now);
              return entryValue;
            } else {
              // clobber existing entry, count remains unchanged
              ++modCount;
              enqueueNotification(
                  key, hash, entryValue, valueReference.getWeight(), RemovalCause.REPLACED);
              setValue(e, key, value, now);
              evictEntries(e);
              return entryValue;
            }
          }
        }

        // Create a new entry.
        ++modCount;
        ReferenceEntry<K, V> newEntry = newEntry(key, hash, first);
        setValue(newEntry, key, value, now);
        table.set(index, newEntry);
        newCount = this.count + 1;
        this.count = newCount; // write-volatile
        evictEntries(newEntry);
        return null;
      } finally {
        unlock();
        postWriteCleanup();
      }
    }

    void runLockedCleanup(long now) {
      if (tryLock()) {
        try {
          drainReferenceQueues();
          expireEntries(now); // calls drainRecencyQueue
          readCount.set(0);
        } finally {
          unlock();
        }
      }
    }

再看下get

  @Override
  public @Nullable V get(@Nullable Object key) {
    if (key == null) {
      return null;
    }
    int hash = hash(key);
    return segmentFor(hash).get(key, hash);
  }

    @Nullable
    V get(Object key, int hash) {
      try {
        if (count != 0) { // read-volatile
          long now = map.ticker.read();
          ReferenceEntry<K, V> e = getLiveEntry(key, hash, now);
          if (e == null) {
            return null;
          }

          V value = e.getValueReference().get();
          if (value != null) {
            recordRead(e, now);
            return scheduleRefresh(e, e.getKey(), hash, value, now, map.defaultLoader);
          }
          tryDrainReferenceQueues();
        }
        return null;
      } finally {
        postReadCleanup();
      }
    }

    /**
     * Records the relative order in which this read was performed by adding {@code entry} to the
     * recency queue. At write-time, or when the queue is full past the threshold, the queue will be
     * drained and the entries therein processed.
     *
     * <p>Note: locked reads should use {@link #recordLockedRead}.
     */
    void recordRead(ReferenceEntry<K, V> entry, long now) {
      if (map.recordsAccess()) {
        entry.setAccessTime(now);
      }
      recencyQueue.add(entry);
    }