一、Guava的设计思想###
之前一篇短文，简要的概括了一下GuavaCache具有的一些特性。例如像缓存淘汰、删除监听和缓存刷新等。这次主要写一些Guava Cache是怎样实现这些特性的。
GuavaCache的源码在 https://github.com/google/guava
GuavaCache的设计是类似与ConcurrentHashMap的，主要靠锁的细化，来减小并发，同时通过Hash算法来加快检索速度。但是GuavaCahce和ConcurrentHash不同的是GuavaCache要支持很多的Cache特性，所以设计上还是很比较复杂的。
二、源码的分析###
这里我们主要以LoadingCache为例子来分析GuavaCache的结构和实现，首先Wiki的例子是：
LoadingCache graphs = CacheBuilder.newBuilder()
.maximumSize(1000)
.expireAfterWrite(10, TimeUnit.MINUTES)
.removalListener(MY_LISTENER)
.build(
new CacheLoader() {
public Graph load(Key key) throws AnyException {
return createExpensiveGraph(key);
}
});
这里GuavaCache主要采用builder的模式，CacheBuilder的每一个方法都返回这个CacheBuilder知道build方法的调用。
那么我们先看一下CacheBuilder的各个方法：
/**
*
* 指定一个Cahce的大小上限，当Cache中的数据将要达到上限的时候淘汰掉不常用的。
* Specifies the maximum number of entries the cache may contain. Note that the cache may evict
* an entry before this limit is exceeded. As the cache size grows close to the maximum, the
* cache evicts entries that are less likely to be used again. For example, the cache may evict an
* entry because it hasn't been used recently or very often.
*
* When {@code size} is zero, elements will be evicted immediately after being loaded into the
* cache. This can be useful in testing, or to disable caching temporarily without a code change.
*
*
This feature cannot be used in conjunction with {@link #maximumWeight}.
*
* @param size the maximum size of the cache
* @return this {@code CacheBuilder} instance (for chaining)
* @throws IllegalArgumentException if {@code size} is negative
* @throws IllegalStateException if a maximum size or weight was already set
*/
public CacheBuilder maximumSize(long size) {
checkState(
this.maximumSize == UNSET_INT, "maximum size was already set to %s", this.maximumSize);
checkState(
this.maximumWeight == UNSET_INT,
"maximum weight was already set to %s",
this.maximumWeight);
checkState(this.weigher == null, "maximum size can not be combined with weigher");
checkArgument(size >= 0, "maximum size must not be negative");
this.maximumSize = size;
return this;
状态检测之后就是执行了一个赋值操作。
同理
public CacheBuilder expireAfterWrite(long duration, TimeUnit unit) {
checkState(
expireAfterWriteNanos == UNSET_INT,
"expireAfterWrite was already set to %s ns",
expireAfterWriteNanos);
checkArgument(duration >= 0, "duration cannot be negative: %s %s", duration, unit);
this.expireAfterWriteNanos = unit.toNanos(duration);
return this;
}
public CacheBuilder removalListener(
RemovalListener listener) {
checkState(this.removalListener == null);
// safely limiting the kinds of caches this can produce
@SuppressWarnings("unchecked")
CacheBuilder me = (CacheBuilder) this;
me.removalListener = checkNotNull(listener);
return me;
}
执行build方法：
public LoadingCache build(
CacheLoader loader) {
checkWeightWithWeigher();
return new LocalCache.LocalLoadingCache(this, loader);
}
这里主要返回一个LocalCache.LocalLoadingCache，这是LocalCache的一个内部类，到这里GuavaCahce真正的存储结构出现了，LocalLoadingCache继承了LocalManualCache实现了LoadingCache接口。实例化的时候，根据CacheBuilder构建了一个LocalCache，而LoadingCache和LocalManualCache只是在LocalCache上做了代理。
LocalLoadingCache( CacheBuilder builder, CacheLoader loader) { super(new LocalCache(builder, checkNotNull(loader)));}
private LocalManualCache(LocalCache localCache) { this.localCache = localCache;}
那么LocalCache的构建是什么样的呢？
LocalCache(
CacheBuilder builder, @Nullable CacheLoader loader) {
//并发度,seg的个数
concurrencyLevel = Math.min(builder.getConcurrencyLevel(), MAX_SEGMENTS);
//key强弱关系
keyStrength = builder.getKeyStrength();
//value的强弱关系
valueStrength = builder.getValueStrength();
//比较器，类似于Object.equal
keyEquivalence = builder.getKeyEquivalence();
valueEquivalence = builder.getValueEquivalence();
//最大权重,weigher为null那么maxWeight=maxsize
maxWeight = builder.getMaximumWeight();
//entry的权重，用于淘汰策略
weigher = builder.getWeigher();
//lastAccess之后多长时间删除
expireAfterAccessNanos = builder.getExpireAfterAccessNanos();
//在写入后长时间之后删除
expireAfterWriteNanos = builder.getExpireAfterWriteNanos();
//刷新的时间间隔
refreshNanos = builder.getRefreshNanos();
//entry删除之后的Listener
removalListener = builder.getRemovalListener();
//删除监听的队列
removalNotificationQueue =
(removalListener == NullListener.INSTANCE)
? LocalCache.>discardingQueue()
: new ConcurrentLinkedQueue>();
//时钟
ticker = builder.getTicker(recordsTime());
//创建Entry的Factory
entryFactory = EntryFactory.getFactory(keyStrength, usesAccessEntries(), usesWriteEntries());
//缓存的状态统计器，用于统计缓存命中率等
globalStatsCounter = builder.getStatsCounterSupplier().get();
//加载数据的Loader
defaultLoader = loader;
//初始化HashTable的容量
int initialCapacity = Math.min(builder.getInitialCapacity(), MAXIMUM_CAPACITY);
//没有设置权重设置但是有maxsize的设置，那么需要减小容量的设置
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.
//类似于ConcurentHashMap
int segmentShift = 0;//seg的掩码
int segmentCount = 1;//seg的个数
//如果seg的个数事故小于并发度的
//初始化并发度为4,默认的maxWeight是-1，默认是不驱逐
while (segmentCount < concurrencyLevel && (!evictsBySize() || segmentCount * 20 <= maxWeight)) {
++segmentShift;
segmentCount <<= 1;
}
this.segmentShift = 32 - segmentShift;
segmentMask = segmentCount - 1;
this.segments = newSegmentArray(segmentCount);
int segmentCapacity = initialCapacity / segmentCount;
if (segmentCapacity * segmentCount < initialCapacity) {
++segmentCapacity;
}
int segmentSize = 1;
while (segmentSize < segmentCapacity) {
segmentSize <<= 1;
}
//默认不驱逐
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 {
//为每一个Segment进行初始化
for (int i = 0; i < this.segments.length; ++i) {
this.segments[i] =
createSegment(segmentSize, UNSET_INT, builder.getStatsCounterSupplier().get());
}
}
}
初始化的时候初始化一些配置等，可以看到和ConcurrentHashMap基本一致，但是引入了一些其他的概念。
那么回过头看一下，最关键的两个方法，首先是put方法：
@Override
public void put(K key, V value) {
localCache.put(key, value);
}
/**
* 代理到Segment的put方法
* @param key
* @param value
* @return
*/
@Override
public V put(K key, V value) {
checkNotNull(key);
checkNotNull(value);
int hash = hash(key);
return segmentFor(hash).put(key, hash, value, false);
}
@Nullable
V put(K key, int hash, V value, boolean onlyIfAbsent) {
//保证线程安全，加锁
lock();
try {
//获取当前的时间
long now = map.ticker.read();
//清除队列中的元素
preWriteCleanup(now);
//localCache的Count+1
int newCount = this.count + 1;
//扩容操作
if (newCount > this.threshold) { // ensure capacity
expand();
newCount = this.count + 1;
}
//获取当前Entry中的HashTable的Entry数组
AtomicReferenceArray> table = this.table;
//定位
int index = hash & (table.length() - 1);
//获取第一个元素
ReferenceEntry first = table.get(index);
//遍历整个Entry链表
// Look for an existing entry.
for (ReferenceEntry 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.
//如果找到相应的元素
ValueReference valueReference = e.getValueReference();
//获取value
V entryValue = valueReference.get();
//如果entry的value为null，可能被GC掉了
if (entryValue == null) {
++modCount;
if (valueReference.isActive()) {
enqueueNotification( //减小锁时间的开销
key, hash, entryValue, valueReference.getWeight(), RemovalCause.COLLECTED);
//利用原来的key并且刷新value
setValue(e, key, value, now);//存储数据，并且将新增加的元素写入两个队列中
newCount = this.count; // count remains unchanged
} else {
setValue(e, key, value, now);//存储数据，并且将新增加的元素写入两个队列中
newCount = this.count + 1;
}
this.count = newCount; // write-volatile，保证内存可见性
//淘汰缓存
evictEntries(e);
return null;
} else if (onlyIfAbsent) {//原来的Entry中包含指定key的元素，所以读取一次，读取操作需要更新Access队列
// Mimic
// "if (!map.containsKey(key)) ...
// else return map.get(key);
recordLockedRead(e, now);
return entryValue;
} else {
//如果value不为null,那么更新value
// clobber existing entry, count remains unchanged
++modCount;
//将replace的Cause添加到队列中
enqueueNotification(
key, hash, entryValue, valueReference.getWeight(), RemovalCause.REPLACED);
setValue(e, key, value, now);//存储数据，并且将新增加的元素写入两个队列中
//数据的淘汰
evictEntries(e);
return entryValue;
}
}
}
//如果目标的entry不存在，那么新建entry
// Create a new entry.
++modCount;
ReferenceEntry newEntry = newEntry(key, hash, first);
setValue(newEntry, key, value, now);
table.set(index, newEntry);
newCount = this.count + 1;
this.count = newCount; // write-volatile
//淘汰多余的entry
evictEntries(newEntry);
return null;
} finally {
//解锁
unlock();
//处理刚刚的remove Cause
postWriteCleanup();
}
}
代码比较长，看上去是比较恶心的，注释写了一些，那么重点说几个注意的点：
- 加锁,和ConcurrentHashMap一样，加锁是为了保证线程安全。
- preWriteCleanup:在每一次做put之前都要清理一下，清理什么？看下代码：
@GuardedBy("this")
void preWriteCleanup(long now) {
runLockedCleanup(now);
}
void runLockedCleanup(long now) {
if (tryLock()) {
try {
drainReferenceQueues();
expireEntries(now); // calls drainRecencyQueue
readCount.set(0);
} finally {
unlock();
}
}
}
@GuardedBy("this")
void drainReferenceQueues() {
if (map.usesKeyReferences()) {
drainKeyReferenceQueue();
}
if (map.usesValueReferences()) {
drainValueReferenceQueue();
}
}
@GuardedBy("this")
void drainKeyReferenceQueue() {
Reference ref;
int i = 0;
while ((ref = keyReferenceQueue.poll()) != null) {
@SuppressWarnings("unchecked")
ReferenceEntry entry = (ReferenceEntry) ref;
map.reclaimKey(entry);
if (++i == DRAIN_MAX) {
break;
}
}
}
看上去可能有点懵，其实它要做的就是清空两个队列keyReferenceQueue和valueReferenceQueue，这两个队列是什么东西？其实是引用使用队列。
GuavaCache为了支持弱引用和软引用，引入了引用清空队列。同时将key和Value包装成了keyReference和valueReference。
在创建Entry的时候：
@GuardedBy("this")
ReferenceEntry newEntry(K key, int hash, @Nullable ReferenceEntry next) {
return map.entryFactory.newEntry(this, checkNotNull(key), hash, next);
}
利用map.entryFactory创建Entry。Factory的初始化是通过
entryFactory = EntryFactory.getFactory(keyStrength, usesAccessEntries(), usesWriteEntries());
实现的。keyStrength是我们在初始化时指定的引用强度。可选的有工厂有：
static final EntryFactory[] factories = {
STRONG,
STRONG_ACCESS,
STRONG_WRITE,
STRONG_ACCESS_WRITE,
WEAK,
WEAK_ACCESS,
WEAK_WRITE,
WEAK_ACCESS_WRITE,
};
通过相应的工厂创建对应的Entry，这里主要说一下WeakEntry:
WEAK {
@Override
ReferenceEntry newEntry(
Segment segment, K key, int hash, @Nullable ReferenceEntry next) {
return new WeakEntry(segment.keyReferenceQueue, key, hash, next);
}
},
/**
* Used for weakly-referenced keys.
*/
static class WeakEntry extends WeakReference implements ReferenceEntry {
WeakEntry(ReferenceQueue queue, K key, int hash, @Nullable ReferenceEntry next) {
super(key, queue);
this.hash = hash;
this.next = next;
}
@Override
public K getKey() {
return get();
}
/*
* It'd be nice to get these for free from AbstractReferenceEntry, but we're already extending
* WeakReference.
*/
// null access
@Override
public long getAccessTime() {
throw new UnsupportedOperationException();
}
@Override
public void setAccessTime(long time) {
throw new UnsupportedOperationException();
}
@Override
public ReferenceEntry getNextInAccessQueue() {
throw new UnsupportedOperationException();
}
@Override
public void setNextInAccessQueue(ReferenceEntry next) {
throw new UnsupportedOperationException();
}
@Override
public ReferenceEntry getPreviousInAccessQueue() {
throw new UnsupportedOperationException();
}
@Override
public void setPreviousInAccessQueue(ReferenceEntry previous) {
throw new UnsupportedOperationException();
}
// null write
@Override
public long getWriteTime() {
throw new UnsupportedOperationException();
}
@Override
public void setWriteTime(long time) {
throw new UnsupportedOperationException();
}
@Override
public ReferenceEntry getNextInWriteQueue() {
throw new UnsupportedOperationException();
}
@Override
public void setNextInWriteQueue(ReferenceEntry next) {
throw new UnsupportedOperationException();
}
@Override
public ReferenceEntry getPreviousInWriteQueue() {
throw new UnsupportedOperationException();
}
@Override
public void setPreviousInWriteQueue(ReferenceEntry previous) {
throw new UnsupportedOperationException();
}
// The code below is exactly the same for each entry type.
final int hash;
final ReferenceEntry next;
volatile ValueReference valueReference = unset();
@Override
public ValueReference getValueReference() {
return valueReference;
}
@Override
public void setValueReference(ValueReference valueReference) {
this.valueReference = valueReference;
}
@Override
public int getHash() {
return hash;
}
@Override
public ReferenceEntry getNext() {
return next;
}
}
WeakEntry继承了WeakReference实现了ReferenceEntry，也就是说这个引用是弱引用。WeakEntry引用的key和Value随时可能会被回收。构造的时候参数里面有ReferenceQueue
回到正题清理KeyReferenceQueue：
@GuardedBy("this")
void drainKeyReferenceQueue() {
Reference ref;
int i = 0;
while ((ref = keyReferenceQueue.poll()) != null) {
@SuppressWarnings("unchecked")
ReferenceEntry entry = (ReferenceEntry) ref;
map.reclaimKey(entry);
if (++i == DRAIN_MAX) {
break;
}
}
}
void reclaimKey(ReferenceEntry entry) {
int hash = entry.getHash();
segmentFor(hash).reclaimKey(entry, hash);
}
/**
* Removes an entry whose key has been garbage collected.
*/
boolean reclaimKey(ReferenceEntry entry, int hash) {
lock();
try {
int newCount = count - 1;
AtomicReferenceArray> table = this.table;
int index = hash & (table.length() - 1);
ReferenceEntry first = table.get(index);
for (ReferenceEntry e = first; e != null; e = e.getNext()) {
if (e == entry) {
++modCount;
ReferenceEntry newFirst =
removeValueFromChain(
first,
e,
e.getKey(),
hash,
e.getValueReference().get(),
e.getValueReference(),
RemovalCause.COLLECTED);
newCount = this.count - 1;
table.set(index, newFirst);
this.count = newCount; // write-volatile
return true;
}
}
return false;
} finally {
unlock();
postWriteCleanup();
}
}
上面就是清理过程了，如果发现key或者value被GC了，那么会在put的时候触发清理。
3.setValue都干了什么？setValue其实是将value写入Entry，但是这是一个写操作，所以会刷新上一次写的时间，但是这是根据什么维护的呢？
/**
* Sets a new value of an entry. Adds newly created entries at the end of the access queue.
*/
@GuardedBy("this")
void setValue(ReferenceEntry entry, K key, V value, long now) {
ValueReference previous = entry.getValueReference();
int weight = map.weigher.weigh(key, value);
checkState(weight >= 0, "Weights must be non-negative");
ValueReference valueReference =
map.valueStrength.referenceValue(this, entry, value, weight);
entry.setValueReference(valueReference);
//写入队列
recordWrite(entry, weight, now);
previous.notifyNewValue(value);
}
/**
* Updates eviction metadata that {@code entry} was just written. This currently amounts to
* adding {@code entry} to relevant eviction lists.
*/
@GuardedBy("this")
void recordWrite(ReferenceEntry entry, int weight, long now) {
// we are already under lock, so drain the recency queue immediately
drainRecencyQueue();
totalWeight += weight;
if (map.recordsAccess()) {
entry.setAccessTime(now);
}
if (map.recordsWrite()) {
entry.setWriteTime(now);
}
accessQueue.add(entry);
writeQueue.add(entry);
}
其实GuavaCache会维护两个队列一个Write队列和一个Access队列，用这两个队列来实现最近读和最近写的清除操作，我们可以猜测这两个队列需要有序，同时也需要能快速定位元素。以Access队列为例：
/**
* A custom queue for managing access order. Note that this is tightly integrated with
* {@code ReferenceEntry}, upon which it reliese to perform its linking.
*
* Note that this entire implementation makes the assumption that all elements which are in the
* map are also in this queue, and that all elements not in the queue are not in the map.
*
*
The benefits of creating our own queue are that (1) we can replace elements in the middle of
* the queue as part of copyWriteEntry, and (2) the contains method is highly optimized for the
* current model.
*/
static final class AccessQueue extends AbstractQueue> {
final ReferenceEntry head =
new AbstractReferenceEntry() {
@Override
public long getAccessTime() {
return Long.MAX_VALUE;
}
@Override
public void setAccessTime(long time) {}
ReferenceEntry nextAccess = this;
@Override
public ReferenceEntry getNextInAccessQueue() {
return nextAccess;
}
@Override
public void setNextInAccessQueue(ReferenceEntry next) {
this.nextAccess = next;
}
ReferenceEntry previousAccess = this;
@Override
public ReferenceEntry getPreviousInAccessQueue() {
return previousAccess;
}
@Override
public void setPreviousInAccessQueue(ReferenceEntry previous) {
this.previousAccess = previous;
}
};
// implements Queue
@Override
public boolean offer(ReferenceEntry entry) {
// unlink
connectAccessOrder(entry.getPreviousInAccessQueue(), entry.getNextInAccessQueue());
// add to tail
connectAccessOrder(head.getPreviousInAccessQueue(), entry);
connectAccessOrder(entry, head);
return true;
}
@Override
public ReferenceEntry peek() {
ReferenceEntry next = head.getNextInAccessQueue();
return (next == head) ? null : next;
}
@Override
public ReferenceEntry poll() {
ReferenceEntry next = head.getNextInAccessQueue();
if (next == head) {
return null;
}
remove(next);
return next;
}
head.setNextInAccessQueue(head);
head.setPreviousInAccessQueue(head);
}
}
}
重点关注几个点：offer方法，offer主要做了几个事情：
1.将Entry和它的前节点后节点的关联断开，这样就需要Entry中维护它的前向和后向引用。
2.将新增加的节点加入到队列的尾部，寻找尾节点用了head.getPreviousInAccessQueue()。可以看出来是个环形队列。
3.将新增加的节点，或者新调整出来的节点设为尾部节点。
通过这几点，可以得知，最近更新的节点一定是在尾部的，head后面的节点一定是不活跃的，在每一次清除过期节点的时候一定清除head之后的超时的节点，这点可以通过poll进行验证。
Write队列也是同理。也就是每次写入操作都会更新元素的引用和写入的时间，并且更新元素在读写队列中的位置。我又一次感觉它挺牛逼的。
4.evictEntries(e)，item的淘汰，这个操作是在设置了Cache中能缓存最大条目的前提下触发的：
/**
* Performs eviction if the segment is over capacity. Avoids flushing the entire cache if the
* newest entry exceeds the maximum weight all on its own.
*
* @param newest the most recently added entry
*/
@GuardedBy("this")
void evictEntries(ReferenceEntry newest) {
if (!map.evictsBySize()) {
return;
}
drainRecencyQueue();
// If the newest entry by itself is too heavy for the segment, don't bother evicting
// anything else, just that
if (newest.getValueReference().getWeight() > maxSegmentWeight) {
if (!removeEntry(newest, newest.getHash(), RemovalCause.SIZE)) {
throw new AssertionError();
}
}
while (totalWeight > maxSegmentWeight) {
ReferenceEntry e = getNextEvictable();
if (!removeEntry(e, e.getHash(), RemovalCause.SIZE)) {
throw new AssertionError();
}
}
}
这里主要做了几件事，首先判断是否开启淘汰，之后呢清理RecencyQueue，然后判断新增加的元素是否有很大的权重，如果是那么直接删掉，因为它太重了。最后判断是否权重已经大于上限，如果是的话那么我们就清除最近最少有使用的Entry，直到Weight小于上限。
// TODO(fry): instead implement this with an eviction head
@GuardedBy("this")
ReferenceEntry getNextEvictable() {
for (ReferenceEntry e : accessQueue) {
int weight = e.getValueReference().getWeight();
if (weight > 0) {
return e;
}
}
throw new AssertionError();
}
这里比较容易疑惑的是：Weight是啥？其实如果不做设置Weight都是1，Weight上限就是maxSize。但是Guava允许自己定义Weight，那么上限就是maxWeight了。这部分可以看上面初始化部分。
5.removeListener：removeListener可以看到，在元素被覆盖的时候后注册了一个事件，同时在finnally里面进行了一次清理：
/**
* Notifies listeners that an entry has been automatically removed due to expiration, eviction, or
* eligibility for garbage collection. This should be called every time expireEntries or
* evictEntry is called (once the lock is released).
*/
void processPendingNotifications() {
RemovalNotification notification;
while ((notification = removalNotificationQueue.poll()) != null) {
try {
removalListener.onRemoval(notification);
} catch (Throwable e) {
logger.log(Level.WARNING, "Exception thrown by removal listener", e);
}
}
}
可以看到为了减小put的开销，这里做了一个类似于异步的操作，并且在解锁之后做这样的操作来避免阻塞其他的put。
关于Guava的Put操作就分析完了，的确是够复杂的。下面看一下get部分：
// LoadingCache methods
//local cache的代理
@Override
public V get(K key) throws ExecutionException {
return localCache.getOrLoad(key);
}
/**
* 根据key获取value,如果获取不到进行load
* @param key
* @return
* @throws ExecutionException
*/
V getOrLoad(K key) throws ExecutionException {
return get(key, defaultLoader);
}
V get(K key, CacheLoader loader) throws ExecutionException {
int hash = hash(checkNotNull(key));//hash——>rehash
return segmentFor(hash).get(key, hash, loader);
}
// loading
//进行指定key对应的value的获取，读取不加锁
V get(K key, int hash, CacheLoader loader) throws ExecutionException {
//保证key-value不为null
checkNotNull(key);
checkNotNull(loader);
try {
if (count != 0) { // read-volatile volatile读会刷新缓存，尽量保证可见性,如果为0那么直接load
// don't call getLiveEntry, which would ignore loading values
ReferenceEntry e = getEntry(key, hash);
//如果对应的Entry不为Null,证明值还在
if (e != null) {
long now = map.ticker.read();//获取当前的时间，根据当前的时间进行Live的数据的读取
V value = getLiveValue(e, now);
//元素不为null的话可以不刷新
if (value != null) {
recordRead(e, now);//为entry增加accessTime,同时加入recencyQueue
statsCounter.recordHits(1);//更新当前的状态，增加为命中,可以用于计算命中率
//判断当前有没有到刷新的时机，如果没有的话那么返回原值。否则进行刷新
return scheduleRefresh(e, key, hash, value, now, loader);
}
//value为null,如果此时value正在刷新，那么此时等待刷新结果
ValueReference valueReference = e.getValueReference();
if (valueReference.isLoading()) {
return waitForLoadingValue(e, key, valueReference);
}
}
}
//如果取不到值，那么进行统一的加锁get
// at this point e is either null or expired;
return lockedGetOrLoad(key, hash, loader);
} catch (ExecutionException ee) {
Throwable cause = ee.getCause();
if (cause instanceof Error) {
throw new ExecutionError((Error) cause);
} else if (cause instanceof RuntimeException) {
throw new UncheckedExecutionException(cause);
}
throw ee;
} finally {
postReadCleanup();//每次Put和get之后都要进行一次Clean
}
}
get的实现和JDK1.6的ConcurrentHashMap思想一致，都是put加锁，但是get是用volatile保证。
这里主要做了几件事：
- 首先获取Entry，Entry不为null获取对应的Value，如果Value不为空，那么证明值还在，那么这时候判断一下是否要刷新直接返回了。否则判断目前引用是否在Loading，如果是就等待Loading结束。
- 如果取不到Entry或者Value为null 并且没有在Loading，那么这时候进行lockedGetOrLoad(),这是一个大活儿。
V lockedGetOrLoad(K key, int hash, CacheLoader loader) throws ExecutionException {
ReferenceEntry e;
ValueReference valueReference = null;
LoadingValueReference loadingValueReference = null;
boolean createNewEntry = true;
lock();//加锁，因为会改变数据结构
try {
// re-read ticker once inside the lock
long now = map.ticker.read();
preWriteCleanup(now);//清除引用队列,Acess队列和Write队列中过期的数据,这算是一次put操作
int newCount = this.count - 1;
AtomicReferenceArray> table = this.table;
int index = hash & (table.length() - 1);
ReferenceEntry first = table.get(index);
//定位目标元素
for (e = first; e != null; e = e.getNext()) {
K entryKey = e.getKey();
if (e.getHash() == hash
&& entryKey != null
&& map.keyEquivalence.equivalent(key, entryKey)) {
valueReference = e.getValueReference();
//如果目前处在loading状态，不创建新元素
if (valueReference.isLoading()) {
createNewEntry = false;
} else {
V value = valueReference.get();
if (value == null) { //可能被GC掉了，加入removeListener
enqueueNotification(
entryKey, hash, value, valueReference.getWeight(), RemovalCause.COLLECTED);
} else if (map.isExpired(e, now)) { //可能过期了
// This is a duplicate check, as preWriteCleanup already purged expired
// entries, but let's accomodate an incorrect expiration queue.
enqueueNotification(
entryKey, hash, value, valueReference.getWeight(), RemovalCause.EXPIRED);
} else {//目前就已经加载过了，返回
recordLockedRead(e, now);
statsCounter.recordHits(1);
// we were concurrent with loading; don't consider refresh
return value;
}
//删除在队列中相应的引用，因为后面要新创建
// immediately reuse invalid entries
writeQueue.remove(e);
accessQueue.remove(e);
this.count = newCount; // write-volatile
}
break;
}
}
//创建新的Entry,但是此时是没有值的
if (createNewEntry) {
loadingValueReference = new LoadingValueReference();
if (e == null) {
e = newEntry(key, hash, first);
e.setValueReference(loadingValueReference);
table.set(index, e);
} else {
e.setValueReference(loadingValueReference);
}
}
} finally {
unlock();
postWriteCleanup();
}
if (createNewEntry) {
try {
// Synchronizes on the entry to allow failing fast when a recursive load is
// detected. This may be circumvented when an entry is copied, but will fail fast most
// of the time.
synchronized (e) {
return loadSync(key, hash, loadingValueReference, loader);
}
} finally {
statsCounter.recordMisses(1);
}
} else {
// The entry already exists. Wait for loading.
return waitForLoadingValue(e, key, valueReference);
}
}
首先说一下为什么加锁，加锁的原因有两个：
- load算是一个写操作，改变数据结构，需要加锁。
- 为了避免缓存击穿，加锁一个防止缓存击穿的发生，当然是JVm级别的不是分布式级别的。
因为是写所以要进行preWriteCleanup，根据key定位一下Entry，如果能定位到，那么判断是否在Loading，如果是的话不创建新的Entry并且等待Loading结束。如果不是那么判断value是否为null和是否过期，如果是的话都要进行创建新Entry，如果都不是证明value是加载过了，那么更新下Access队列然后返回。
接下来清除一下Access和Write队列的元素，创建新的Entry。这里比较有意思：
// at most one of loadSync/loadAsync may be called for any given LoadingValueReference
//同步刷新
V loadSync(
K key,
int hash,
LoadingValueReference loadingValueReference,
CacheLoader loader)
throws ExecutionException {
ListenableFuture loadingFuture = loadingValueReference.loadFuture(key, loader);
return getAndRecordStats(key, hash, loadingValueReference, loadingFuture);
}
这里创建了一个loadingReference，这也就是之前看到的判断是否在Loading。如果是Loading状态那么表面有一个线程正在更新Cache，其他的线程等待就可以了。
这里可以看到其实也支持异步的刷新：
ListenableFuture loadAsync(
final K key,
final int hash,
final LoadingValueReference loadingValueReference,
CacheLoader loader) {
final ListenableFuture loadingFuture = loadingValueReference.loadFuture(key, loader);
loadingFuture.addListener(
new Runnable() {
@Override
public void run() {
try {
getAndRecordStats(key, hash, loadingValueReference, loadingFuture);
} catch (Throwable t) {
logger.log(Level.WARNING, "Exception thrown during refresh", t);
loadingValueReference.setException(t);
}
}
},
directExecutor());
return loadingFuture;
}
后面更新的逻辑就不贴了。
从上面我们可以看到，对于每一次get都会去进行Access队列的更新，同时对于多线程的更新只会引起一个线程去load数据，对于不存在的数据，get时也会进行一次load操作。同时通过同步操作解决了缓存击穿的问题。不得不说GuavaCache设计的很巧妙。
其实Guava还有一个比较好玩的东西，asMap(),我们感觉GuavaCache像Map，但是还不完全是Map，那么就提供了一个方法以Map的视图去展现。
看下asMap()
@Override
public ConcurrentMap asMap() {
return localCache;
}
其实就是localCache返回了，返回类型是ConcurrentMap，那么我们看看localCache的继承结构：
@GwtCompatible(emulated = true)
class LocalCache extends AbstractMap implements ConcurrentMap {
果然和Map关系大大的，也就是说，LocalCache本身是个ConcurrentMap，但是对于LocalCache的这些map方法我们是调用不到的，因为我们只能用LoadingCache嘛。通过asMap我们能得到LocalCache，但是我们不能使用除了Map接口之外的方法，也就是说我们不能使用自动加载等一系列的功能。
正如官方Wiki说的：
至此所有的核心源码分析完了，觉得有点恶心，源码这东西就要静下来细细的看，收获会很大。
由于文章比较长，如果有什么问题还请赐教。最后，祝自己这个苦逼码农圣诞快乐。