Java Sequence Generator examples
An example to show you how to create a thread safe sequence generator.
1. SequenceGenerator
package com.mkyong.concurrency.examples.sequence.generator;
public interface SequenceGenerator {
long getNext();
}
1.1 First try, read, add, write the value directly. Below method is not thread safe, multiple threads may get the same value at the same time.
package com.mkyong.concurrency.examples.sequence.generator;
public class UnSafeSequenceGenerator implements SequenceGenerator {
private long value = 1;
@Override
public long getNext() {
return value++;
}
}
1.2 To fix this, make the getNext()
as a synchronized
method.
package com.mkyong.concurrency.examples.sequence.generator;
public class SyncSequenceGenerator implements SequenceGenerator {
private long value = 1;
@Override
public synchronized long getNext() {
return value++;
}
}
1.3 The better solution is using the concurrent.atomic
classes, for example AtomicLong
package com.mkyong.concurrency.examples.sequence.generator;
import java.util.concurrent.atomic.AtomicLong;
public class AtomicSequenceGenerator implements SequenceGenerator {
private AtomicLong value = new AtomicLong(1);
@Override
public long getNext() {
return value.getAndIncrement();
}
}
2. Concurrent Access
Simulate a concurrent access environment to test the above sequence generator.
2.1. A Callable
task to access the sequence 10 time.
package com.mkyong.concurrency.examples.sequence;
import com.mkyong.concurrency.examples.sequence.generator.SequenceGenerator;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
public class PrintSequenceCallable implements Callable<List<Long>> {
private SequenceGenerator sequenceGenerator;
public PrintSequenceCallable(SequenceGenerator sequenceGenerator) {
this.sequenceGenerator = sequenceGenerator;
}
@Override
public List<Long> call() throws Exception {
List<Long> ids = new ArrayList<>();
for (int i = 1; i <= 10; i++) {
Thread.sleep(100); //take a nap
ids.add(sequenceGenerator.getNext());
}
return ids;
};
}
2.2 Start 3 threads to test the sequence generator.
package com.mkyong.concurrency.examples.sequence;
import com.mkyong.concurrency.examples.sequence.generator.SequenceGenerator;
import com.mkyong.concurrency.examples.sequence.generator.UnSafeSequenceGenerator;
import java.util.List;
import java.util.concurrent.*;
public class Main {
public static void main(String[] args) {
SequenceGenerator sequenceGenerator = new UnSafeSequenceGenerator();
//SequenceGenerator sequenceGenerator = new SyncSequenceGenerator();
//SequenceGenerator sequenceGenerator = new AtomicSequenceGenerator();
ExecutorService executor = Executors.newCachedThreadPool();
try {
// simulate 3 threads concurrent access the sequence generator
Callable<List<Long>> task1 = new PrintSequenceCallable(sequenceGenerator);
Callable<List<Long>> task2 = new PrintSequenceCallable(sequenceGenerator);
Callable<List<Long>> task3 = new PrintSequenceCallable(sequenceGenerator);
Future f1 = executor.submit(task1);
Future f2 = executor.submit(task2);
Future f3 = executor.submit(task3);
System.out.println(f1.get());
System.out.println(f2.get());
System.out.println(f3.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
} finally {
executor.shutdown();
}
}
}
Output
2.2.1 UnSafeSequenceGenerator
– Aka race condition, multiple threads is getting the same value, this is not what we want.
[3, 5, 6, 7, 8, 10, 12, 15, 16, 17]
[2, 4, 6, 7, 8, 9, 13, 15, 16, 17]
[1, 4, 6, 7, 8, 11, 14, 15, 16, 17]
2.2.2 SyncSequenceGenerator
– Thread safe.
[3, 6, 8, 10, 14, 17, 21, 24, 27, 29]
[1, 4, 9, 12, 15, 18, 20, 22, 25, 30]
[2, 5, 7, 11, 13, 16, 19, 23, 26, 28]
2.2.3 AtomicSequenceGenerator
– Thread safe.
[3, 6, 8, 12, 13, 18, 19, 22, 27, 29]
[2, 5, 7, 10, 14, 17, 20, 24, 26, 30]
[1, 4, 9, 11, 15, 16, 21, 23, 25, 28]
Both synchronized
and AtomicLong
are able to create a thread safe sequence generator. However, the synchronized
method is expensive, it will increase the performance cost, the recommended way is using the concurrent.atomic
classes like AtomicLong
, the atomic classes are designed for concurrent use.
It will restart the sequence if server got restarted. Right?
Hi In Multi-threaded environment Numbers are unique,but not in sequence. It should be in sequence in multi threaded environment too . how can you achieve that. for ex: 1,2,3,4,5,6….100