guava cache 存储的数据结构2之ReferenceEntry实现类 --- 3

上一节提到缓存中的实体类是ReferenceEntry{K, V>（key可以是任意的类型，value是ValueReference{K, V>），但是它只是个接口，下图是他的实现



创建方式

这个是在Segment类里，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 : 默认是Strength.STRONG，还有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,
};
//提供具体的实体类
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 ：所有的方法都是抛出异常

public ValueReference<K, V> getValueReference() {
throw new UnsupportedOperationException();
}
public void setValueReference(ValueReference<K, V> valueReference) {
throw new UnsupportedOperationException();
}
...//都是抛出异常,所以这个类没有实际意义

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 : 访问的实体类,访问时间，前一个和后一个访问的节点

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

1.2 StrongWriteEntry ：写入的实体类,写入时间，前一个和后一个写入的节点

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 的所有属性都有

WeakEntry : 除了hash,next,valueReference 其余方法都是抛出异常

这个实现WeakReference,采用了弱引用，再没有引用时即会被垃圾回收

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();
}
.....
}

其余几个方法和Strong的相对应