java 多线程的一个奇怪的问题

Question

先上代码：
packageMultithreading;
importjavax.xml.bind.DatatypeConverter;
importjava.io.File;
importjava.io.FileInputStream;
importjava.io.FileNotFoundException;
importjava.io.IOException;
importjava.security.DigestInputStream;
importjava.security.MessageDigest;
importjava.security.NoSuchAlgorithmException;
importjava.util.ArrayList;
importjava.util.List;
importjava.util.concurrent.TimeUnit;
publicclassThreadUsage{
publicstaticvoidmain(String[]args){
try{
Listfiles=newArrayList<>();
Listworkers=newArrayList<>();
//listfilesincurrentdirectory
for(Filefile:newFile(".").listFiles()){
if(file.isFile())
files.add(file.toString());
}
for(Stringfile:files){
DigestThreadworker=newDigestThread(file);
workers.add(worker);
worker.start();
}
for(DigestThreadworker:workers){
while(worker.digest==null);
//TimeUnit.SECONDS.sleep(2);
System.out.println(worker.fileName+":"+DatatypeConverter.printHexBinary(worker.digest));
}
}catch(Exceptione){
e.printStackTrace();
}
}
publicstaticclassDigestThreadextendsThread{
privateStringfileName;
privatebyte[]digest;
publicDigestThread(StringfileName){
this.fileName=fileName;
}
@Override
publicvoidrun(){
try{
//establishahashfunction
MessageDigestmessageDigest=MessageDigest.getInstance("SHA-256");
//usethestreamstofinishthereadandcomputationofeachbyte
FileInputStreamfileIn=newFileInputStream(fileName);
DigestInputStreamdigestIn=newDigestInputStream(fileIn,messageDigest);
while(digestIn.read()!=-1);
digestIn.close();
fileIn.close();
//finishthecomputation
digest=messageDigest.digest();
//printmessage
//System.out.println(fileName+":"+DatatypeConverter.printHexBinary(digest));
}catch(FileNotFoundExceptione){
e.printStackTrace();
}catch(NoSuchAlgorithmExceptione){
e.printStackTrace();
}catch(IOExceptione){
e.printStackTrace();
}
}
}
}
main中最后一个foreach循环，如果我选择while语句判断是否完成，运行->一直等待；如果我选择sleep2s，运行->每隔2s输出结果；
为什么while循环位卡住呢？每个线程计算完SHA，就赋值digest，这样不就可以退出循环了嘛？