Threads - Java Network Programming - page 60

Returning Information from a Thread
One of the hardest things for programmers accustomed to traditional, single-threaded
procedural models to grasp when moving to a multithreaded environment is how to
return information from a thread. Getting information out of a finished thread is one
of the most commonly misunderstood aspects of multithreaded programming. The
run() method and the start() method don't return any values. For example, suppose
that instead of simply printing out the SHA-256 digest, as in Examples 3-1 and 3-2 , the
digest thread needs to return the digest to the main thread of execution. Most people's
first reaction is to store the result in a field and provide a getter method, as shown in
Examples 3-3 and 3-4 . Example 3-3 is a Thread subclass that calculates a digest for a
specified file. Example 3-4 is a simple command-line user interface that receives file‐
names and spawns threads to calculate digests for them.
Example 3-3. A thread that uses an accessor method to return the result
import java.io.* ;
import java.security.* ;
public class ReturnDigest extends Thread {
private String filename ;
private byte [] digest ;
public ReturnDigest ( String filename ) {
this . filename = filename ;
}
@Override
public void run () {
try {
FileInputStream in = new FileInputStream ( filename );
MessageDigest sha = MessageDigest . getInstance ( "SHA-256" );
DigestInputStream din = new DigestInputStream ( in , sha );
while ( din . read () != - 1 ) ; // read entire file
din . close ();
digest = sha . digest ();
} catch ( IOException ex ) {
System . err . println ( ex );
} catch ( NoSuchAlgorithmException ex ) {
System . err . println ( ex );
}
}
public byte [] getDigest () {
return digest ;
}
}