Android提供的LruCache类简介

摘自：http://blog.csdn.net/linghu_java/article/details/8574102

• packageandroid.util;
• importjava.util.LinkedHashMap;
• importjava.util.Map;
• /**
• *Acachethatholdsstrongreferencestoalimitednumberofvalues.Eachtime
• *avalueisaccessed,itismovedtotheheadofaqueue.Whenavalueis
• *addedtoafullcache,thevalueattheendofthatqueueisevictedandmay
• *becomeeligibleforgarbagecollection.
• *Cache保存一个强引用来限制内容数量，每当Item被访问的时候，此Item就会移动到队列的头部。
• * 当cache已满的时候加入新的item时，在队列尾部的item会被回收。
• *<p>Ifyourcachedvaluesholdresourcesthatneedtobeexplicitlyreleased,
• *override{@link#entryRemoved}.
• *如果你cache的某个值需要明确释放，重写entryRemoved()
• *<p>Ifacachemissshouldbecomputedondemandforthecorrespondingkeys,
• *override{@link#create}.Thissimplifiesthecallingcode,allowingitto
• *assumeavaluewillalwaysbereturned,evenwhenthere'sacachemiss.
• *如果key相对应的item丢掉啦，重写create().这简化了调用代码，即使丢失了也总会返回。
• *<p>Bydefault,thecachesizeismeasuredinthenumberofentries.Override
• *{@link#sizeOf}tosizethecacheindifferentunits.Forexample,thiscache
• *islimitedto4MiBofbitmaps:默认cache大小是测量的item的数量，重写sizeof计算不同item的
• * 大小。
• *<pre>{@code
• *intcacheSize=4*1024*1024;//4MiB
• *LruCache<String,Bitmap>bitmapCache=newLruCache<String,Bitmap>(cacheSize){
• *protectedintsizeOf(Stringkey,Bitmapvalue){
• *returnvalue.getByteCount();
• *}
• *}}</pre>
• *
• *<p>Thisclassisthread-safe.Performmultiplecacheoperationsatomicallyby
• *synchronizingonthecache:<pre>{@code
• *synchronized(cache){
• *if(cache.get(key)==null){
• *cache.put(key,value);
• *}
• *}}</pre>
• *
• *<p>Thisclassdoesnotallownulltobeusedasakeyorvalue.Areturn
• *valueofnullfrom{@link#get},{@link#put}or{@link#remove}is
• *unambiguous:thekeywasnotinthecache.
• * 不允许key或者value为null
• * 当get（），put（），remove（）返回值为null时，key相应的项不在cache中
• */
• publicclassLruCache<K,V>{
• privatefinalLinkedHashMap<K,V>map;
• /**Sizeofthiscacheinunits.Notnecessarilythenumberofelements.*/
• privateintsize; //已经存储的大小
• privateintmaxSize; //规定的最大存储空间
• privateintputCount; //put的次数
• privateintcreateCount; //create的次数
• privateintevictionCount; //回收的次数
• privateinthitCount; //命中的次数
• privateintmissCount; //丢失的次数
• /**
• *@parammaxSizeforcachesthatdonotoverride{@link#sizeOf},thisis
• *themaximumnumberofentriesinthecache.Forallothercaches,
• *thisisthemaximumsumofthesizesoftheentriesinthiscache.
• */
• publicLruCache(intmaxSize){
• if(maxSize<=0){
• thrownewIllegalArgumentException("maxSize<=0");
• }
• this.maxSize=maxSize;
• this.map=newLinkedHashMap<K,V>(0,0.75f,true);
• }
• /**
• *Returnsthevaluefor{@codekey}ifitexistsinthecacheorcanbe
• *createdby{@code#create}.Ifavaluewasreturned,itismovedtothe
• *headofthequeue.Thisreturnsnullifavalueisnotcachedandcannot
• *becreated.通过key返回相应的item，或者创建返回相应的item。相应的item会移动到队列的头部，
• * 如果item的value没有被cache或者不能被创建，则返回null。
• */
• publicfinalVget(Kkey){
• if(key==null){
• thrownewNullPointerException("key==null");
• }
• VmapValue;
• synchronized(this){
• mapValue=map.get(key);
• if(mapValue!=null){
• hitCount++; //命中
• returnmapValue;
• }
• missCount++; //丢失
• }
• /*
• *Attempttocreateavalue.Thismaytakealongtime,andthemap
• *maybedifferentwhencreate()returns.Ifaconflictingvaluewas
• *addedtothemapwhilecreate()wasworking,weleavethatvaluein
• *themapandreleasethecreatedvalue.
• * 如果丢失了就试图创建一个item
• */
• VcreatedValue=create(key);
• if(createdValue==null){
• returnnull;
• }
• synchronized(this){
• createCount++;//创建++
• mapValue=map.put(key,createdValue);
• if(mapValue!=null){
• //Therewasaconflictsoundothatlastput
• //如果前面存在oldValue，那么撤销put（）
• map.put(key,mapValue);
• }else{
• size+=safeSizeOf(key,createdValue);
• }
• }
• if(mapValue!=null){
• entryRemoved(false,key,createdValue,mapValue);
• returnmapValue;
• }else{
• trimToSize(maxSize);
• returncreatedValue;
• }
• }
• /**
• *Caches{@codevalue}for{@codekey}.Thevalueismovedtotheheadof
• *thequeue.
• *
• *@returnthepreviousvaluemappedby{@codekey}.
• */
• publicfinalVput(Kkey,Vvalue){
• if(key==null||value==null){
• thrownewNullPointerException("key==null||value==null");
• }
• Vprevious;
• synchronized(this){
• putCount++;
• size+=safeSizeOf(key,value);
• previous=map.put(key,value);
• if(previous!=null){ //返回的先前的value值
• size-=safeSizeOf(key,previous);
• }
• }
• if(previous!=null){
• entryRemoved(false,key,previous,value);
• }
• trimToSize(maxSize);
• returnprevious;
• }
• /**
• *@parammaxSizethemaximumsizeofthecachebeforereturning.Maybe-1
• *toevicteven0-sizedelements.
• * 清空cache空间
• */
• privatevoidtrimToSize(intmaxSize){
• while(true){
• Kkey;
• Vvalue;
• synchronized(this){
• if(size<0||(map.isEmpty()&&size!=0)){
• thrownewIllegalStateException(getClass().getName()
• +".sizeOf()isreportinginconsistentresults!");
• }
• if(size<=maxSize){
• break;
• }
• Map.Entry<K,V>toEvict=map.eldest();
• if(toEvict==null){
• break;
• }
• key=toEvict.getKey();
• value=toEvict.getValue();
• map.remove(key);
• size-=safeSizeOf(key,value);
• evictionCount++;
• }
• entryRemoved(true,key,value,null);
• }
• }
• /**
• *Removestheentryfor{@codekey}ifitexists.
• *删除key相应的cache项，返回相应的value
• *@returnthepreviousvaluemappedby{@codekey}.
• */
• publicfinalVremove(Kkey){
• if(key==null){
• thrownewNullPointerException("key==null");
• }
• Vprevious;
• synchronized(this){
• previous=map.remove(key);
• if(previous!=null){
• size-=safeSizeOf(key,previous);
• }
• }
• if(previous!=null){
• entryRemoved(false,key,previous,null);
• }
• returnprevious;
• }
• /**
• *Calledforentriesthathavebeenevictedorremoved.Thismethodis
• *invokedwhenavalueisevictedtomakespace,removedbyacallto
• *{@link#remove},orreplacedbyacallto{@link#put}.Thedefault
• *implementationdoesnothing.
• *当item被回收或者删掉时调用。改方法当value被回收释放存储空间时被remove调用，
• * 或者替换item值时put调用，默认实现什么都没做。
• *<p>Themethodiscalledwithoutsynchronization:otherthreadsmay
• *accessthecachewhilethismethodisexecuting.
• *
• *@paramevictedtrueiftheentryisbeingremovedtomakespace,false
• *iftheremovalwascausedbya{@link#put}or{@link#remove}.
• * true---为释放空间被删除；false---put或remove导致
• *@paramnewValuethenewvaluefor{@codekey},ifitexists.Ifnon-null,
• *thisremovalwascausedbya{@link#put}.Otherwiseitwascausedby
• *anevictionora{@link#remove}.
• */
• protectedvoidentryRemoved(booleanevicted,Kkey,VoldValue,VnewValue){}
• /**
• *Calledafteracachemisstocomputeavalueforthecorrespondingkey.
• *Returnsthecomputedvalueornullifnovaluecanbecomputed.The
• *defaultimplementationreturnsnull.
• *当某Item丢失时会调用到，返回计算的相应的value或者null
• *<p>Themethodiscalledwithoutsynchronization:otherthreadsmay
• *accessthecachewhilethismethodisexecuting.
• *
• *<p>Ifavaluefor{@codekey}existsinthecachewhenthismethod
• *returns,thecreatedvaluewillbereleasedwith{@link#entryRemoved}
• *anddiscarded.Thiscanoccurwhenmultiplethreadsrequestthesamekey
• *atthesametime(causingmultiplevaluestobecreated),orwhenone
• *threadcalls{@link#put}whileanotheriscreatingavalueforthesame
• *key.
• */
• protectedVcreate(Kkey){
• returnnull;
• }
• privateintsafeSizeOf(Kkey,Vvalue){
• intresult=sizeOf(key,value);
• if(result<0){
• thrownewIllegalStateException("Negativesize:"+key+"="+value);
• }
• returnresult;
• }
• /**
• *Returnsthesizeoftheentryfor{@codekey}and{@codevalue}in
• *user-definedunits.Thedefaultimplementationreturns1sothatsize
• *isthenumberofentriesandmaxsizeisthemaximumnumberofentries.
• *返回用户定义的item的大小，默认返回1代表item的数量，最大size就是最大item值
• *<p>Anentry'ssizemustnotchangewhileitisinthecache.
• */
• protectedintsizeOf(Kkey,Vvalue){
• return1;
• }
• /**
• *Clearthecache,calling{@link#entryRemoved}oneachremovedentry.
• * 清空cacke
• */
• publicfinalvoidevictAll(){
• trimToSize(-1);//-1willevict0-sizedelements
• }
• /**
• *Forcachesthatdonotoverride{@link#sizeOf},thisreturnsthenumber
• *ofentriesinthecache.Forallothercaches,thisreturnsthesumof
• *thesizesoftheentriesinthiscache.
• */
• publicsynchronizedfinalintsize(){
• returnsize;
• }
• /**
• *Forcachesthatdonotoverride{@link#sizeOf},thisreturnsthemaximum
• *numberofentriesinthecache.Forallothercaches,thisreturnsthe
• *maximumsumofthesizesoftheentriesinthiscache.
• */
• publicsynchronizedfinalintmaxSize(){
• returnmaxSize;
• }
• /**
• *Returnsthenumberoftimes{@link#get}returnedavaluethatwas
• *alreadypresentinthecache.
• */
• publicsynchronizedfinalinthitCount(){
• returnhitCount;
• }
• /**
• *Returnsthenumberoftimes{@link#get}returnednullorrequiredanew
• *valuetobecreated.
• */
• publicsynchronizedfinalintmissCount(){
• returnmissCount;
• }
• /**
• *Returnsthenumberoftimes{@link#create(Object)}returnedavalue.
• */
• publicsynchronizedfinalintcreateCount(){
• returncreateCount;
• }
• /**
• *Returnsthenumberoftimes{@link#put}wascalled.
• */
• publicsynchronizedfinalintputCount(){
• returnputCount;
• }
• /**
• *Returnsthenumberofvaluesthathavebeenevicted.
• * 返回被回收的数量
• */
• publicsynchronizedfinalintevictionCount(){
• returnevictionCount;
• }
• /**
• *Returnsacopyofthecurrentcontentsofthecache,orderedfromleast
• *recentlyaccessedtomostrecentlyaccessed.返回当前cache的副本，从最近最少访问到最多访问
• */
• publicsynchronizedfinalMap<K,V>snapshot(){
• returnnewLinkedHashMap<K,V>(map);
• }
• @OverridepublicsynchronizedfinalStringtoString(){
• intaccesses=hitCount+missCount;
• inthitPercent=accesses!=0?(100*hitCount/accesses):0;
• returnString.format("LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]",
• maxSize,hitCount,missCount,hitPercent);
• }
• }