標準ThreadPoolExecutorとの動作比較

本実装はjava.util.concurrent.ThreadPoolExecutorの動作を拡張。主な差異:

• 標準スレッドプール：初期化時コアスレッド生成。キュー満杯で追加スレッド作成
• 本実装：要求時動的スレッド生成。タスク完了後全スレッド終了

利点：常駐スレッド削減によるメモリ節約（1スレッド≈1MB）とCPUコンテキスト切り替えコスト低減

基本実装例

public class DynamicThreadPool {
    private final AtomicInteger activeThreads = new AtomicInteger(0);
    private final Queue<Task> taskQueue = new ConcurrentLinkedQueue<>();
    private final int maxThreads;
    private final AtomicBoolean shutdownFlag = new AtomicBoolean(false);
    
    public void submit(Task task) {
        if (shutdownFlag.get()) throw new IllegalStateException("Pool shutdown");
        
        activeThreads.getAndUpdate(current -> 
            current < maxThreads ? current + 1 : current
        );
        
        if (activeThreads.get() <= maxThreads) {
            new Worker(task).start();
        } else {
            taskQueue.offer(task);
        }
    }
    
    private class Worker extends Thread {
        private final Task initialTask;
        
        Worker(Task task) {
            this.initialTask = task;
        }
        
        @Override
        public void run() {
            try {
                initialTask.execute();
            } finally {
                processNext();
            }
        }
        
        private void processNext() {
            Task next = taskQueue.poll();
            if (next != null) {
                new Worker(next).start();
            } else {
                activeThreads.decrementAndGet();
            }
        }
    }
    
    @FunctionalInterface
    public interface Task {
        void execute() throws Exception;
    }
}

ThreadPoolExecutorを利用した改良版

public class EnhancedThreadPool extends ThreadPoolExecutor {
    
    public EnhancedThreadPool(int maxThreads, String prefix) {
        super(maxThreads, maxThreads, 60, TimeUnit.SECONDS, 
              new LinkedBlockingQueue<>(),
              r -> new Thread(r, prefix + "-" + r.hashCode()));
        allowCoreThreadExpiration(true);
    }
    
    @Override
    public void allowCoreThreadTimeOut(boolean enable) {
        throw new UnsupportedOperationException();
    }
}

重要設定: allowCoreThreadTimeOut(true)でコアスレッドのアイドル終了を許可