Here is my main class that initializes and starts 5 different Threads:
public class Server implements Runnable {
Server1 server1;
Thread server1Thread;
public Server() {}
@Override
public void run() {
server1 = new Server1();
server1Thread = new Thread(server1);
server1Thread.start();
}
public static void main(String[] args) {
for (int i = 0; i < 5; i++) {
Server s = new Server();
s.run();
}
}
}
Here is my Server1
Runnable:
import java.util.concurrent.ConcurrentHashMap;
public class Server1 implements Runnable {
private ConcurrentHashMap<Integer, Integer> storage= new ConcurrentHashMap<>();
public Server1() {}
@Override
public void run() {
synchronized (this){
for (int i = 0; i < 10; i++) {
storage.put(i, (int)(Math.random()*100));
}
for (int i : storage.keySet()) {
System.out.print("(" + i + "," + storage.get(i) + ") ");
}
System.out.println();
}
}
}
It puts in ConcurrentHashMap storage
keys from 0
to 9
and assigns them a random value between 0
and 100
. After that it prints it and prints new line at the end. I have user synchronized
block to make sure the thread itself access keys correctly but it prints something like this:
(0,8) (0,87) (1,60) (1,14) (2,20) (2,70) (3,5) (0,74) (0,42) (1,22) (4,96) (0,85) (1,97) (2,75) (3,68) (4,3) (5,49) (6,3) (7,9) (8,47) (9,52)
(3,2) (5,74) (2,86) (1,48) (3,5) (6,0) (4,0) (7,86) (4,22) (8,20) (2,17) (9,87)
(5,96) (5,15) (6,15) (6,92) (7,48) (8,93) (9,67)
(3,87) (7,43) (4,34) (5,48) (8,91) (9,64)
(6,84) (7,75) (8,47) (9,87)
which obviously means that some thread prints more that 10 keys that I assigned to it. How do I make every thread print exactly 10 keys and values that it is assigned to them and ensure concurrency here?
I am not sure how to test it.
Your threads don't share any internal state. They're working fine, but the output is interleaved.
For instance if you used a StringBuilder
to do the I/O in one operation, you should see correct output.
StringBuilder buff = new StringBuilder();
for (int i : storage.keySet()) {
buff.append("(" + i + "," + storage.get(i) + ") ");
}
System.out.println(buff);
There is no good reason for Server
to be Runnable
, or even to create any instances of it.
You do not share any of the maps. If you did, then you would also want to share a common lock, but this is not the usual way to use ConcurrentMap
.
All you had to do was synchronized (Server1.class)
as that is common across threads. Not the instance
Here is the verified output:
(0,75) (1,9) (2,61) (3,73) (4,55) (5,34) (6,34) (7,74) (8,41) (9,0)
(0,30) (1,42) (2,46) (3,66) (4,12) (5,17) (6,62) (7,59) (8,74) (9,4)
(0,50) (1,16) (2,29) (3,74) (4,68) (5,42) (6,33) (7,91) (8,25) (9,7)
(0,49) (1,10) (2,39) (3,94) (4,12) (5,55) (6,54) (7,89) (8,21) (9,75)
(0,77) (1,10) (2,37) (3,32) (4,73) (5,39) (6,64) (7,98) (8,96) (9,44)
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.