import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicBoolean;

public class ThreadPoolWarmUpDemo {

    public static void main(String[] args) throws Exception {
        System.out.println("========== 未開啟預熱，測試開始 ==========");
        testWithoutWarmUp();

        System.out.println("\n========== 開啟預熱后，測試開始 ==========");
        testWithWarmUp();
    }

    /** 用于模擬“第一次慢，后面快” */
    private static final AtomicBoolean firstRun = new AtomicBoolean(true);

    /** 模擬核心業(yè)務邏輯 */
    private static void mockBizWork() {
        try {
            if (firstRun.compareAndSet(true, false)) {
                // 只在第一次執(zhí)行時慢
                Thread.sleep(50);
            } else {
                // 后續(xù)任務很快
                Thread.sleep(2);
            }
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }

    /** 創(chuàng)建一個全新的線程池（關鍵） */
    private static ThreadPoolExecutor newExecutor() {
        return new ThreadPoolExecutor(
                4, 4,
                60, TimeUnit.SECONDS,
                new LinkedBlockingQueue<>()
        );
    }

    /** 未開啟線程池預熱 */
    public static void testWithoutWarmUp() throws Exception {
        ThreadPoolExecutor executor = newExecutor();
        firstRun.set(true);

        long start = System.currentTimeMillis();

        Future<Long> future = executor.submit(() -> {
            long s = System.currentTimeMillis();
            mockBizWork();
            return System.currentTimeMillis() - s;
        });

        long firstCost = future.get();
        long totalCost = System.currentTimeMillis() - start;

        System.out.println("第一次任務耗時 = " + firstCost + " ms");
        System.out.println("整體耗時（含線程創(chuàng)建）= " + totalCost + " ms");

        executor.shutdown();
    }

    /** 開啟預熱 */
    public static void testWithWarmUp() throws Exception {
        ThreadPoolExecutor executor = newExecutor();
        firstRun.set(true);

        System.out.println(">>> 開始預熱核心線程...");
        executor.prestartAllCoreThreads();

        // 預熱任務：吃掉“第一次慢”
        for (int i = 0; i < 4; i++) {
            executor.submit(ThreadPoolWarmUpDemo::mockBizWork).get();
        }

        System.out.println(">>> 預熱完成！");

        long start = System.currentTimeMillis();

        Future<Long> future = executor.submit(() -> {
            long s = System.currentTimeMillis();
            mockBizWork();
            return System.currentTimeMillis() - s;
        });

        long cost = future.get();
        long totalCost = System.currentTimeMillis() - start;

        System.out.println("第一次任務耗時（已預熱）= " + cost + " ms");
        System.out.println("整體耗時（線程已就緒）= " + totalCost + " ms");

        executor.shutdown();
    }
}

executor.prestartAllCoreThreads();
executor.submit(warmTask);