问题描述
packagecom.zcf.test;importjava.util.HashMap;importjava.util.Map;importjava.util.Random;publicclassThreadScopeShareData{privatestaticintdata=0;privatestaticMap<Thread,Integer>threadMap=newHashMap<Thread,Integer>();publicstaticvoidmain(String[]args){for(inti=0;i<2;i++){newThread(newRunnable(){@Overridepublicvoidrun(){intdata=newRandom().nextInt();System.out.println(Thread.currentThread().getName()+"hasputdata:"+data);threadMap.put(Thread.currentThread(),data);newA().get();newB().get();}}).start();;}}staticclassA{publicvoidget(){intdata=threadMap.get(Thread.currentThread());System.out.println("fromA"+Thread.currentThread().getName()+"getdata:"+data);}}staticclassB{publicvoidget(){intdata=threadMap.get(Thread.currentThread());System.out.println("fromB"+Thread.currentThread().getName()+"getdata:"+data);}}}有时会Thread-0hasputdata:1174772175Thread-1hasputdata:115649452fromAThread-0getdata:1174772175Exceptioninthread"Thread-1"java.lang.NullPointerExceptionfromBThread-0getdata:1174772175atcom.zcf.test.ThreadScopeShareData$A.get(ThreadScopeShareData.java:38)atcom.zcf.test.ThreadScopeShareData$1.run(ThreadScopeShareData.java:25)atjava.lang.Thread.run(UnknownSource)有时又不会，能详细说明一下原因吗？？？？
解决方案
解决方案二：
顶一下顶一下
解决方案三：
发代码用代码框，在表情左边
解决方案四：
thread2个，get4次，空指针异常了
解决方案五：
为什么会空指针异常呢？？求详细的解释
解决方案六：
新建两个线程，实际存在三个线程，Main线程执行完之后，threadMap自动回收。此时threadMap==null传入子线程的地址也就指向Null。
解决方案七：
ThreadMap不是main函数里的局部变量，为什么main执行完了之后会被回收呢？？？
解决方案八：
我试了在main函数的最后加了Thread.sleep(1000);还是会有时报这个错，报错的时候main线程还没有执行完。估计不是这个原因。
解决方案九：
线程安全。换成线程安全map。publicstaticConcurrentMap<Thread,Integer>map=newConcurrentHashMap<Thread,Integer>();
解决方案十：
由于HashMap是非线程安全的，两个线程同时写入值时，可能相互覆盖。报错中的空指针，是由于从map中取到的value为null，在自动解箱转为int时，抛出了空指针异常。当两个线程对象的hash值同时分配到map的同一个桶时，如下代码如果同时执行，只会保留后一个线程的值。测试的结果也验证了分析，报错时，hash后的值在同一个桶中。/***Addsanewentrywiththespecifiedkey,valueandhashcodetothe*specifiedbucket.Itistheresponsibilityofthismethodtoresizethe*tableifappropriate.**Subclassoverridesthistoalterthebehaviorofputmethod.*/voidaddEntry(inthash,Kkey,Vvalue,intbucketIndex){Entry<K,V>e=table[bucketIndex];table[bucketIndex]=newEntry<K,V>(hash,key,value,e);//同时执行，后执行的线程覆盖前面写入的if(size++>=threshold)resize(2*table.length);}
packagenet.jcip.test;importjava.util.HashMap;importjava.util.Map;importjava.util.Random;publicclassThreadScopeShareData{privatestaticMap<Thread,Integer>threadMap=newHashMap<Thread,Integer>();publicstaticvoidmain(String[]args){for(inti=0;i<2;i++){newThread(newRunnable(){@Overridepublicvoidrun(){intdata=newRandom().nextInt();System.out.println(indexFor(hash(Thread.currentThread().hashCode()),16));System.out.println(Thread.currentThread().getName()+"hasputdata:"+data);threadMap.put(Thread.currentThread(),data);newA().get();newB().get();}}).start();}}staticclassA{publicvoidget(){System.out.println(Thread.currentThread()+"threadMap="+threadMap);intdata=threadMap.get(Thread.currentThread());System.out.println("fromA"+Thread.currentThread().getName()+"getdata:"+data);}}staticclassB{publicvoidget(){intdata=threadMap.get(Thread.currentThread());System.out.println("fromB"+Thread.currentThread().getName()+"getdata:"+data);}}staticinthash(inth){h^=(h>>>20)^(h>>>12);returnh^(h>>>7)^(h>>>4);}/***Returnsindexforhashcodeh.*/staticintindexFor(inth,intlength){returnh&(length-1);}}
14Thread-1hasputdata:159371015214Thread-0hasputdata:1594094901Thread[Thread-0,5,main]threadMap={Thread[Thread-0,5,main]=1594094901}fromAThread-0getdata:1594094901Thread[Thread-1,5,main]threadMap={Thread[Thread-0,5,main]=1594094901}fromBThread-0getdata:1594094901Exceptioninthread"Thread-1"java.lang.NullPointerExceptionatnet.jcip.test.ThreadScopeShareData$A.get(ThreadScopeShareData.java:34)atnet.jcip.test.ThreadScopeShareData$1.run(ThreadScopeShareData.java:21)atjava.lang.Thread.run(UnknownSource)