java/w11/Utils.java

import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Supplier;
import java.util.logging.Logger;

// ─────────────────────────────────────────────
// 必做1：UrlFormatException（Unchecked / Runtime）
// 选择 RuntimeException 理由：
//   URL 格式错误属于调用方编程错误（非法参数），
//   不应强迫上层 catch，符合 Unchecked 语义。
// ─────────────────────────────────────────────
class UrlFormatException extends RuntimeException {
    private final String invalidUrl;

    public UrlFormatException(String url) {
        // 保留根因信息，避免"异常包装丢失根因"问题
        super("URL 格式不合法，必须以 http:// 或 https:// 开头：" + url);
        this.invalidUrl = url;
    }

    /** 构造器2：包装底层异常，保留完整调用链 */
    public UrlFormatException(String url, Throwable cause) {
        super("URL 格式不合法：" + url, cause);
        this.invalidUrl = url;
    }

    public String getInvalidUrl() { return invalidUrl; }
}

// ─────────────────────────────────────────────
// 必做2：RetryUtils —— 指数退避重试
// wait = base * 2^attempt  (base = 500ms)
// ─────────────────────────────────────────────
class RetryUtils {
    private static final Logger log = Logger.getLogger(RetryUtils.class.getName());

    private static final long BASE_MS   = 500;   // 500ms 基准
    private static final int  MAX_RETRY = 4;     // 最多重试次数

    /**
     * 执行带指数退避重试的操作
     * @param task      要执行的任务，返回 true 表示成功
     * @param maxRetry  最大重试次数
     * @return true = 最终成功，false = 全部失败
     */
    public static boolean retry(Supplier<Boolean> task, int maxRetry) {
        for (int attempt = 0; attempt <= maxRetry; attempt++) {
            if (attempt > 0) {
                long waitMs = BASE_MS * (1L << attempt);   // base * 2^attempt
                log.info(String.format("第 %d 次重试，等待 %d ms ...", attempt, waitMs));
                try {
                    Thread.sleep(waitMs);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                    log.warning("重试等待被中断");
                    return false;
                }
            }
            try {
                if (task.get()) {
                    if (attempt > 0) log.info("第 " + attempt + " 次重试成功");
                    return true;
                }
                log.warning("第 " + attempt + " 次尝试失败（任务返回 false）");
            } catch (Exception e) {
                log.warning("第 " + attempt + " 次尝试抛出异常：" + e.getMessage());
            }
        }
        log.severe("已达最大重试次数 " + maxRetry + "，放弃。");
        return false;
    }

    /** 使用默认最大重试次数 */
    public static boolean retry(Supplier<Boolean> task) {
        return retry(task, MAX_RETRY);
    }

    /**
     * 选做思考题优化：
     * 如果某次失败是"永久失败"（如 404），则不应继续重试。
     * 通过 NonRetryableException 标记不可重试异常，
     * 捕获到后立即终止重试循环。
     */
    public static boolean retryWithFastFail(Supplier<Boolean> task, int maxRetry) {
        for (int attempt = 0; attempt <= maxRetry; attempt++) {
            if (attempt > 0) {
                long waitMs = BASE_MS * (1L << attempt);
                try { Thread.sleep(waitMs); }
                catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                    return false;
                }
            }
            try {
                if (task.get()) return true;
            } catch (NonRetryableException e) {
                // 选做思考题：404 / 永久失败 → 立即放弃，不再等待
                log.severe("不可重试异常，立即终止：" + e.getMessage());
                return false;
            } catch (Exception e) {
                log.warning("attempt " + attempt + " failed: " + e.getMessage());
            }
        }
        return false;
    }
}

// ─────────────────────────────────────────────
// 选做思考题辅助：不可重试异常（如 HTTP 404）
// ─────────────────────────────────────────────
class NonRetryableException extends RuntimeException {
    private final int httpStatus;

    public NonRetryableException(int httpStatus, String url) {
        super("HTTP " + httpStatus + " 永久失败，不应重试：" + url);
        this.httpStatus = httpStatus;
    }

    public int getHttpStatus() { return httpStatus; }
}

// ─────────────────────────────────────────────
// 选做：CircuitBreaker（断路器）
// 状态机：CLOSED → OPEN（连续失败 N 次）→ HALF_OPEN（冷却后）→ CLOSED
// ─────────────────────────────────────────────
class CircuitBreaker {
    private static final Logger log = Logger.getLogger(CircuitBreaker.class.getName());

    public enum State { CLOSED, OPEN, HALF_OPEN }

    private final int  failureThreshold;   // 连续失败多少次触发熔断
    private final long cooldownMs;         // 熔断后冷却时间

    private volatile State         state          = State.CLOSED;
    private final    AtomicInteger failureCount   = new AtomicInteger(0);
    private volatile long          openedAt       = 0;

    public CircuitBreaker(int failureThreshold, long cooldownMs) {
        this.failureThreshold = failureThreshold;
        this.cooldownMs       = cooldownMs;
    }

    /**
     * 执行一次操作，断路器保护：
     * - OPEN 状态直接拒绝（不发请求）
     * - 成功则重置计数器；失败则累加，超阈值触发熔断
     */
    public boolean call(Supplier<Boolean> task) {
        switch (getEffectiveState()) {
            case OPEN:
                log.warning("断路器 OPEN，请求被拒绝（冷却剩余 "
                    + (cooldownMs - (System.currentTimeMillis() - openedAt)) + "ms）");
                return false;

            case HALF_OPEN:
                // 半开状态：放一个试探请求
                log.info("断路器 HALF_OPEN，尝试探测...");
                if (execute(task)) {
                    reset();   // 探测成功，恢复 CLOSED
                    return true;
                }
                trip();        // 探测失败，重新熔断
                return false;

            default: // CLOSED
                if (execute(task)) {
                    failureCount.set(0);
                    return true;
                }
                if (failureCount.incrementAndGet() >= failureThreshold) {
                    trip();
                }
                return false;
        }
    }

    private boolean execute(Supplier<Boolean> task) {
        try { return Boolean.TRUE.equals(task.get()); }
        catch (Exception e) { return false; }
    }

    private State getEffectiveState() {
        if (state == State.OPEN
                && System.currentTimeMillis() - openedAt >= cooldownMs) {
            state = State.HALF_OPEN;
        }
        return state;
    }

    private void trip() {
        state    = State.OPEN;
        openedAt = System.currentTimeMillis();
        log.severe("断路器已熔断！连续失败 " + failureCount.get() + " 次。");
    }

    private void reset() {
        state = State.CLOSED;
        failureCount.set(0);
        log.info("断路器已恢复 CLOSED。");
    }

    public State getState() { return getEffectiveState(); }
}