Java Sequence Generatorの例
スレッドセーフシーケンスジェネレーターの作成方法を示す例。
1. SequenceGenerator
SequenceGenerator.java
package com.example.concurrency.examples.sequence.generator; public interface SequenceGenerator { long getNext(); }
1.1 First try, read, add, write the value directly. 以下のメソッドはスレッドセーフではありません。複数のスレッドが同時に同じ値を取得する場合があります。
UnSafeSequenceGenerator.java
package com.example.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.
SyncSequenceGenerator.java
package com.example.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
AtomicSequenceGenerator.java
package com.example.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. 同時アクセス
同時アクセス環境をシミュレートして、上記のシーケンスジェネレーターをテストします。
2.1. シーケンスに10回アクセスするためのCallable
タスク。
PrintSequenceCallable.java
package com.example.concurrency.examples.sequence; import com.example.concurrency.examples.sequence.generator.SequenceGenerator; import java.util.ArrayList; import java.util.List; import java.util.concurrent.Callable; public class PrintSequenceCallable implements Callable> { private SequenceGenerator sequenceGenerator; public PrintSequenceCallable(SequenceGenerator sequenceGenerator) { this.sequenceGenerator = sequenceGenerator; } @Override public List
call() throws Exception { List 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.
Main.java
package com.example.concurrency.examples.sequence; import com.example.concurrency.examples.sequence.generator.SequenceGenerator; import com.example.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> task1 = new PrintSequenceCallable(sequenceGenerator); Callable
> task2 = new PrintSequenceCallable(sequenceGenerator); Callable
> 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(); } } }
出力
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]
synchronized
とAtomicLong
はどちらも、スレッドセーフなシーケンスジェネレーターを作成できます。 ただし、synchronized
メソッドは高価であり、パフォーマンスコストが増加します。推奨される方法は、AtomicLong
のようなconcurrent.atomic
クラスを使用することです。アトミッククラスは、同時使用するように設計されています。
ソースコードをダウンロード
$ git clone https://github.com/example/java-concurrency.git