Javaシーケンスジェネレータの例

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

TOC

Javaシーケンスジェネレータの例

Java Sequence Generatorの例

1. SequenceGenerator

2. 同時アクセス

ソースコードをダウンロード

参考文献

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