Next.js 15 officially launched at the end of October. The biggest change is the adjustment to caching strategy — from "cache by default" to "no cache by default". This change has far-reaching implications.

Cache Strategy Reversal

Next.js 14 的 fetch、Route Handlers、GET 函数默认缓存结果，需要手动 revalidate 或 noStore() 来禁用。Next.js 15 反了过来：

// Next.js 14：默认缓存
async function getData() {
  const res = await fetch("https://api.example.com/data");
  // 默认缓存！需要显式 revalidate
  return res.json();
}

// Next.js 15：默认不缓存
async function getData() {
  const res = await fetch("https://api.example.com/data");
  // 默认每次请求都获取新数据
  return res.json();
}

// 需要缓存时显式声明
async function getCachedData() {
  const res = await fetch("https://api.example.com/data", {
    next: { revalidate: 60 }, // 60 秒缓存
  });
  return res.json();
}

为什么这个改变重要

Next.js 14's default caching confused many new users: data not updating, difficult to debug, poor development experience. After the reversal, behavior is more intuitive.

When migrating our project, we only need to add revalidate config where caching is truly needed; everything else automatically becomes uncached.

Partial Prerendering (PPR) Experimentally Stable

PPR 允许同一个路由中混合静态和动态内容：

// app/product/[id]/page.tsx
import { Suspense } from "react";

// 静态部分：构建时生成，CDN 缓存
function ProductLayout({ children }: { children: React.ReactNode }) {
  return (
    <div className="product-page">
      <header>我的商城</header>       {/* 静态 */}
      <nav>分类导航</nav>             {/* 静态 */}
      {children}                      {/* 动态岛屿 */}
    </div>
  );
}

// 动态部分：每次请求实时渲染
async function ProductPrice({ id }: { id: string }) {
  const price = await getLivePrice(id);
  return <span className="price">{price}</span>;
}

export default function ProductPage({ params }: { params: { id: string } }) {
  return (
    <ProductLayout>
      <ProductInfo id={params.id} />          {/* 静态 */}
      <Suspense fallback={<PriceSkeleton />}>
        <ProductPrice id={params.id} />       {/* 动态 */}
      </Suspense>
      <Suspense fallback={<ReviewsSkeleton />}>
        <ProductReviews id={params.id} />     {/* 动态 */}
      </Suspense>
    </ProductLayout>
  );
}

The core value of PPR: static shell + dynamic holes. CDN returns the static HTML shell, and the dynamic parts are streamed in.

Turbopack Production Stable

Turbopack（Rust 编写的打包器）在 Next.js 15 中正式稳定：

# 开发模式
next dev --turbopack

# 构建时间对比（我们的项目，~800 页面）
# Webpack: ~120s
# Turbopack: ~35s

after() API

服务器操作完成后执行非阻塞任务：

// app/actions.ts
"use server";

import { after } from "next/server";

export async function submitOrder(formData: FormData) {
  // 关键路径：保存订单
  const order = await db.order.create({ data: { ... } });

  after(async () => {
    // 非关键路径：发送通知、更新统计数据等
    await sendOrderConfirmationEmail(order.id);
    await analytics.track("order_placed", { orderId: order.id });
    await cache.revalidate("order-stats");
  });

  return { orderId: order.id };
}

Functions inside after() do not block the response from returning; users see results faster.

Form Component Enhancements

// 直接用 <form> 调用 Server Action，不需要 JS
<form action={createTodo}>
  <input name="title" />
  <button type="submit">添加</button>
</form>

// 带 pending 状态
import { useFormStatus } from "react-dom";

function SubmitButton() {
  const { pending } = useFormStatus();
  return <button disabled={pending}>{pending ? "提交中..." : "提交"}</button>;
}

Migration Checklist

npx @next/codemod@canary upgrade

需要手动检查的点：

1. 依赖 fetch 默认缓存的代码，需要显式加 revalidate
2. unstable_cache API 改为 import { cache } from "react"
3. cookies() 和 headers() 现在是异步的
4. next/image 的默认配置有调整

Summary

• 缓存默认关闭：行为更直觉，需要缓存时显式声明
• PPR：静态壳 + 动态岛屿，兼顾性能和实时性
• Turbopack 正式稳定：开发构建速度提升 3-4 倍
• after() API：非阻塞副作用处理
• 迁移用官方 codemod 工具，降低手动修改成本