概述

前面的两篇文章(Guava Cache系列之一：如何加载缓存和 Guava Cache系列之二：如何回收缓存)介绍了Guava Cache的使用，下面从源码来看一下Guava Cache的设计和实现
源码分析

构造函数

LocalCache(
      CacheBuilder builder, @Nullable CacheLoader loader) {
    //并发程度，根据我们传的参数和默认最大值中选取小者。
    //如果没有指定该参数的情况下，CacheBuilder将其置为UNSET_INT即为-1
    //getConcurrencyLevel方法获取时，如果为-1就返回默认值4
    //否则返回用户传入的参数
    concurrencyLevel = Math.min(builder.getConcurrencyLevel(), MAX_SEGMENTS);
    //键值的引用类型，没有指定的话，默认为强引用类型
    keyStrength = builder.getKeyStrength();
    valueStrength = builder.getValueStrength();
    //判断相同的方法，强引用类型就是Equivalence.equals()
    keyEquivalence = builder.getKeyEquivalence();
    valueEquivalence = builder.getValueEquivalence();

    maxWeight = builder.getMaximumWeight();
    weigher = builder.getWeigher();
    expireAfterAccessNanos = builder.getExpireAfterAccessNanos();
    expireAfterWriteNanos = builder.getExpireAfterWriteNanos();
    refreshNanos = builder.getRefreshNanos();
    //移除消息监听器
    removalListener = builder.getRemovalListener();
    //如果我们指定了移除消息监听器的话，会创建一个队列，临时保存移除的内容
    removalNotificationQueue = (removalListener == NullListener.INSTANCE)
        ? LocalCache.>discardingQueue()
        : new ConcurrentLinkedQueue>();

    ticker = builder.getTicker(recordsTime());
    //创建新的缓存内容（entry）的工厂，会根据引用类型选择对应的工厂
    entryFactory = EntryFactory.getFactory(keyStrength, usesAccessEntries(), usesWriteEntries());
    globalStatsCounter = builder.getStatsCounterSupplier().get();
    defaultLoader = loader;
    //初始化缓存容量，默认为16
    int initialCapacity = Math.min(builder.getInitialCapacity(), MAXIMUM_CAPACITY);
    if (evictsBySize() && !customWeigher()) {
      initialCapacity = Math.min(initialCapacity, (int) maxWeight);
    }

    // Find the lowest power-of-two segmentCount that exceeds concurrencyLevel, unless
    // maximumSize/Weight is specified in which case ensure that each segment gets at least 10
    // entries. The special casing for size-based eviction is only necessary because that eviction
    // happens per segment instead of globally, so too many segments compared to the maximum size
    // will result in random eviction behavior.
    int segmentShift = 0;
    int segmentCount = 1;
    //根据并发程度来计算segement数组的大小（大于等于concurrencyLevel的最小的2的幂，这里即为4）
    while (segmentCount < concurrencyLevel
           && (!evictsBySize() || segmentCount * 20 <= maxWeight)) {
      ++segmentShift;
      segmentCount <<= 1;
    }
    //这里的segmentShift和segmentMask用来打散entry，让缓存内容尽量均匀分布在每个segment下
    this.segmentShift = 32 - segmentShift;
    segmentMask = segmentCount - 1;
    //这里进行初始化segment数组，大小即为4
    this.segments = newSegmentArray(segmentCount);
    //每个segment的容量，总容量/segment的大小，向上取整，这里就是16/4=4
    int segmentCapacity = initialCapacity / segmentCount;
    if (segmentCapacity * segmentCount < initialCapacity) {
      ++segmentCapacity;
    }
    //这里计算每个Segment[i]下的table的大小
    int segmentSize = 1;
    //SegmentSize为小于segmentCapacity的最大的2的幂，这里为4
    while (segmentSize < segmentCapacity) {
      segmentSize <<= 1;
    }
    //初始化每个segment[i]
    //注：根据权重的方法使用较少，这里走else分支
    if (evictsBySize()) {
      // Ensure sum of segment max weights = overall max weights
      long maxSegmentWeight = maxWeight / segmentCount + 1;
      long remainder = maxWeight % segmentCount;
      for (int i = 0; i < this.segments.length; ++i) {
        if (i == remainder) {
          maxSegmentWeight--;
        }
        this.segments[i] =
            createSegment(segmentSize, maxSegmentWeight, builder.getStatsCounterSupplier().get());
      }
    } else {
      for (int i = 0; i < this.segments.length; ++i) {
        this.segments[i] =
            createSegment(segmentSize, UNSET_INT, builder.getStatsCounterSupplier().get());
      }
    }
  }
成员变量简介如下：
Guava Cache系列之三：源码分析

概述

源码分析

构造函数

get

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