concurrentHashMap - juedaiyuer/researchNote GitHub Wiki

static final class HashEntry<K,V> {
	final K key;
	final int hash;
	volatile V value;
	final HashEntry<K,V> next;
}


// 定位段的方法

final Segment<K,V> segmentFor(int hash) {
	return segments[(hash >>> segmentShift) & segmentMask];  
} 


// ConcurrentHashMap数据成员
// segmentMask和segmentShift主要是为了定位段，参见上面的segmentFor方法
public class ConcurrentHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V>, Serializable {
	/** 
	* Mask value for indexing into segments. The upper bits of a 
	* key's hash code are used to choose the segment. 
	*/  
	final int segmentMask;  

	/** 
	* Shift value for indexing within segments. 
	*/  
	final int segmentShift;  

	/** 
	* The segments, each of which is a specialized hash table 
	*/  
	final Segment<K,V>[] segments;  
}

// Segment数据结构
static final class Segment<K,V> extends ReentrantLock implements Serializable {

	private static final long serialVersionUID = 2249069246763182397L;  
	
	/** 
	* The number of elements in this segment's region.
	* 用来协调修改和读取操作 
	*/ 

	transient volatile int count; 

	/** 
	* Number of updates that alter the size of the table. This is 
	* used during bulk-read methods to make sure they see a 
	* consistent snapshot: If modCounts change during a traversal 
	* of segments computing size or checking containsValue, then 
	* we might have an inconsistent view of state so (usually) 
	* must retry. 
	*
	* modCount统计段结构改变的次数，主要是为了检测对多个段进行遍历过程中某个段是否发生改变
	*/ 

	transient int modCount;  

	/** 
	* The table is rehashed when its size exceeds this threshold. 
	* (The value of this field is always <tt>(int)(capacity * 
	* loadFactor)</tt>.) 
	*/

	transient int threshold;  

	/** 
	* The per-segment table. 
	*/  

	transient volatile HashEntry<K,V>[] table;  

	/** 
	* The load factor for the hash table.  Even though this value 
	* is same for all segments, it is replicated to avoid needing 
	* links to outer object. 
	* @serial 
	*/  
	final float loadFactor;  
}