为ThreadLocal定制的ThreadLocalMap-优快云博客

ThreadLocalMap是java.lang.ThreadLocal的静态内部类。ThreadLocalMap表面看起来至少实现类似hashmap的功能，但是仔细分析它却有下面的属性。
首先，它只是尽量的让value保持在哈希值的位置或后面，
其次，它提供了对不同key（这里是ThreadLocal）对象的但hashCode相同的存储的支持。 最后，它对Key的引用是弱引用。这个和WeakHashMap一致。但这个和hashMap有点不一样，hashMap要使用用弱引用的话， 一般是对value进行弱引用的，而不是key. 也许有一天我们在自己的程序中需要类似ThreadLocalMap的功能。 以下是从ThreadLocalMap简化而来的代码： 其中Key对应JDK中的ThreadLocal classKey{ finalinthashCode=nextHashCode(); staticAtomicIntegernextHashCode= newAtomicInteger(); staticfinalintHASH_INCREMENT=0x61c88647; staticintnextHashCode(){ returnnextHashCode.getAndAdd(HASH_INCREMENT); } } classThreadLocalMap{ staticclassEntryextendsWeakReference<Key>{ Objectvalue; Entry(Keyk,Objectv){ super(k); value=v; } } //初始值，必须是2的n次方 privatestaticfinalintINITIAL_CAPACITY=16; /** *Thetable,resizedasnecessary. *table.lengthMUSTalwaysbeapoweroftwo. */ privateEntry[]table; privateintsize=0; /** *Thenextsizevalueatwhichtoresize. */ privateintthreshold;//Defaultto0 /** *Settheresizethresholdtomaintainatworsta2/3loadfactor. */ privatevoidsetThreshold(intlen){ threshold=len*2/3; } privatestaticintnextIndex(inti,intlen){ return((i+1<len)?i+1:0); } privatestaticintprevIndex(inti,intlen){ return((i-1>=0)?i-1:len-1); } /** *Constructanewmapinitiallycontaining(firstKey,firstValue). *ThreadLocalMapsareconstructedlazily,soweonlycreate *onewhenwehaveatleastoneentrytoputinit. */ ThreadLocalMap(KeyfirstKey,ObjectfirstValue){ table=newEntry[INITIAL_CAPACITY]; inti=firstKey.hashCode&(INITIAL_CAPACITY-1); table=newEntry(firstKey,firstValue); size=1; setThreshold(INITIAL_CAPACITY); } privateEntrygetEntry(Keykey){ inti=key.hashCode&(table.length-1); Entrye=table[i]; if(e!=null&&e.get()==key) returne; else returngetEntryAfterMiss(key,i,e); } privateEntrygetEntryAfterMiss(Keykey,inti,Entrye){ Entry[]tab=table[i]; intlen=tab.length; while(e!=null){ Keyk=e.get(); if(k==key) returne; if(k==null) expungeStaleEntry(i); else i=nextIndex(i,len); e=tab[i]; } returnnull; } privatevoidset(Keykey,Objectvalue){ Entry[]tab=table; intlen=tab.length; inti=key.hashCode&(len-1); for(Entrye=tab[i];e!=null;e=tab[i=nextIndex(i,len)]){ Keyk=e.get(); if(k==key){ e.value=value; return; } if(k==null){ replaceStaleEntry(key,value,i); return; } } tab=newEntry(key,value); intsz=++size; if(!cleanSomeSlots(i,sz)&&sz>=threshold) rehash(); } privatevoidremove(Keykey){ Entry[]tab=table; intlen=tab.length; inti=key.hashCode&(len-1); for(Entrye=tab; e!=null; e=tab[i=nextIndex(i,len)]){ if(e.get()==key){ e.clear(); expungeStaleEntry(i); return; } } } privatevoidreplaceStaleEntry(Keykey,Objectvalue, intstaleSlot){ Entry[]tab=table; intlen=tab.length; Entrye; intslotToExpunge=staleSlot; for(inti=prevIndex(staleSlot,len); (e=tab)!=null; i=prevIndex(i,len)) if(e.get()==null) slotToExpunge=i; for(inti=nextIndex(staleSlot,len); (e=tab)!=null; i=nextIndex(i,len)){ Keyk=e.get(); if(k==key){ e.value=value; tab=tab[staleSlot]; tab[staleSlot]=e; //Startexpungeatprecedingstaleentryifitexists if(slotToExpunge==staleSlot) slotToExpunge=i; cleanSomeSlots(expungeStaleEntry(slotToExpunge),len); return; } if(k==null&&slotToExpunge==staleSlot) slotToExpunge=i; } //Ifkeynotfound,putnewentryinstaleslot tab[staleSlot].value=null; tab[staleSlot]=newEntry(key,value); //Ifthereareanyotherstaleentriesinrun,expungethem if(slotToExpunge!=staleSlot) cleanSomeSlots(expungeStaleEntry(slotToExpunge),len); } /**擦除staleSlot的Entry。 *调整所有不在哈希映射位置的value的位置，让它尽量在哈希映射的位置 */ privateintexpungeStaleEntry(intstaleSlot){ Entry[]tab=table; intlen=tab.length; tab[staleSlot].value=null; tab[staleSlot]=null; size--; //Rehashuntilweencounternull Entrye; inti; for(i=nextIndex(staleSlot,len); (e=tab)!=null; i=nextIndex(i,len)){ Keyk=e.get(); if(k==null){ e.value=null; tab=null; size--; }else{ inth=k.hashCode&(len-1); if(h!=i){ tab=null; //UnlikeKnuth6.4AlgorithmR,wemustscanuntil //nullbecausemultipleentriescouldhavebeenstale. while(tab[h]!=null) h=nextIndex(h,len); tab[h]=e; } } } returni; } privatebooleancleanSomeSlots(inti,intn){ booleanremoved=false; Entry[]tab=table; intlen=tab.length; do{ i=nextIndex(i,len); Entrye=tab; if(e!=null&&e.get()==null){ n=len; removed=true; i=expungeStaleEntry(i); } }while((n>>>=1)!=0); returnremoved; } privatevoidrehash(){ expungeStaleEntries(); //Uselowerthresholdfordoublingtoavoidhysteresis if(size>=threshold-threshold/4) resize(); } privatevoidresize(){ Entry[]oldTab=table; intoldLen=oldTab.length; intnewLen=oldLen*2; Entry[]newTab=newEntry[newLen]; intcount=0; for(intj=0;j<oldLen;++j){ Entrye=oldTab[j]; if(e!=null){ Keyk=e.get(); if(k==null){ e.value=null;//HelptheGC }else{ inth=k.hashCode&(newLen-1); while(newTab[h]!=null) h=nextIndex(h,newLen); newTab[h]=e; count++; } } } setThreshold(newLen); size=count; table=newTab; } privatevoidexpungeStaleEntries(){ Entry[]tab=table; intlen=tab.length; for(intj=0;j<len;j++){ Entrye=tab[j]; if(e!=null&&e.get()==null) expungeStaleEntry(j); } } }