垃圾回收算法是思想，垃圾回收器是实现。面试里问"有哪些垃圾回收器"，不是让你背一串名字，而是看你能不能说清楚：不同收集器分别追求什么目标，适合什么场景，G1 为什么成为现代 JVM 的重要默认选择。
一句话概括：Serial 追求简单，Parallel 追求吞吐量，CMS 追求低停顿，G1 通过 Region 化和可预测停顿在吞吐量与延迟之间做平衡。
常见垃圾回收器全景
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Serial 系列
Parallel 系列
CMS 系列
G1
Serial
Serial Old
Parallel Scavenge
Parallel Old
ParNew
CMS
| 收集器 | 主要作用区域 | 核心特点 | 适合场景 |
|---|---|---|---|
| Serial | 新生代 | 单线程，复制算法，STW | 小堆、客户端程序 |
| Serial Old | 老年代 | 单线程，标记整理，STW | 小堆或兜底 |
| Parallel Scavenge | 新生代 | 多线程，关注吞吐量 | 后台计算、批处理 |
| Parallel Old | 老年代 | 多线程，标记整理 | 吞吐量优先 |
| ParNew | 新生代 | 多线程，常与 CMS 搭配 | 低停顿老年代方案的一部分 |
| CMS | 老年代 | 并发标记清除，低停顿 | 对响应时间敏感的老系统 |
| G1 | 新生代和老年代 | Region 化，可预测停顿 | 大堆、服务端应用 |
JDK 8 在服务端模式下常见默认选择是 Parallel GC；JDK 9 之后 G1 成为默认垃圾回收器。CMS 已经在较新的 JDK 中退出历史舞台，维护新系统时更常关注 G1、ZGC、Shenandoah 等方案。
并行和并发不要混
垃圾回收器里经常出现两个词：并行和并发。它们不是一回事。
| 概念 | 关注点 | 例子 |
|---|---|---|
| 并行 | 多个 GC 线程同时干活 | Parallel GC 多个线程一起回收 |
| 并发 | GC 线程和应用线程同时运行 | CMS 或 G1 的并发标记阶段 |
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应用线程暂停
多个 GC 线程一起回收
并发
应用线程继续运行
GC 线程同时做部分标记或清理
所以 Parallel 的重点是"多个 GC 线程一起工作"，但它仍然会 STW。CMS 和 G1 的一些阶段可以和应用线程并发执行，但也不是全程无停顿，初始标记、重新标记、对象复制等阶段仍然可能暂停应用线程。
Serial 收集器
Serial 是最简单的垃圾回收器。它使用单个 GC 线程完成回收，回收时所有应用线程暂停。
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它的优势是简单、额外开销小；缺点是停顿明显，不适合大堆和高并发服务端应用。
Parallel 收集器
Parallel 系列使用多条 GC 线程并行回收，目标是提高吞吐量。
吞吐量可以简单理解为：
text
应用运行时间 / (应用运行时间 + GC 时间)Parallel 适合后台任务、批处理、计算型任务等"更关心总体处理效率，而不是单次停顿极短"的场景。
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Parallel 的关键词是"并行"和"吞吐量"，不是"完全无停顿"。它仍然会 Stop The World，只是回收阶段可以用多线程缩短总回收时间。
CMS 收集器
CMS 全称 Concurrent Mark Sweep，目标是降低老年代垃圾回收停顿时间。
它的核心流程包括初始标记、并发标记、重新标记、并发清除。
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并发标记
重新标记
并发清除
| 阶段 | 是否 STW | 说明 |
|---|---|---|
| 初始标记 | 是 | 标记 GC Roots 直接关联对象 |
| 并发标记 | 否 | 和应用线程并发执行，继续追踪引用链 |
| 重新标记 | 是 | 修正并发期间引用变化导致的标记偏差 |
| 并发清除 | 否 | 清理不可达对象 |
CMS 的优点是停顿时间短；缺点是使用标记清除算法，会产生内存碎片，并且并发阶段会和应用线程抢 CPU。如果并发清理赶不上对象分配速度，还可能触发更重的 Full GC。
现在新项目一般不再优先选择 CMS，但理解 CMS 有助于理解"低停顿收集器为什么要把部分阶段并发化"。
G1 的核心思想
G1 全称 Garbage First。它不再把堆简单看成连续的新生代和老年代，而是把堆切成多个大小相等的 Region。每个 Region 可以根据需要扮演 Eden、Survivor、Old 或 Humongous。
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Region 1
Region 2
Region 3
Region 4
Region 5
Eden
Survivor
Old
Humongous
Free
G1 的关键点有三个：
- 堆被拆成多个 Region，而不是只按物理连续区域划分新生代和老年代。
- G1 会估算每个 Region 的回收价值，优先回收垃圾最多、收益最高的 Region。
- 可以通过停顿时间目标，让 JVM 尽量在指定时间内选择合适数量的 Region 回收。
Humongous Region 用来存放大对象。通常对象大小超过 Region 的一半时，就可能被当作大对象处理。
G1 的回收过程
G1 常见回收过程可以分为 Young GC、并发标记、Mixed GC。
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否
是
对象分配到 Eden Region
Eden 是否不足
Young GC
存活对象复制到 Survivor 或 Old
老年代占用是否达到阈值
并发标记
重新标记
统计 Region 回收价值
Mixed GC
回收 Eden、Survivor 和部分 Old Region
Young GC
Young GC 主要回收 Eden Region。存活对象会被复制到 Survivor Region，年龄较大或空间不足时进入 Old Region。
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存活对象
Survivor Region
Old Region
垃圾对象直接清理
这个过程需要 STW，因为对象复制和引用修正必须保证一致性。
并发标记
当老年代占用达到一定阈值后，G1 会启动并发标记，找出老年代中哪些对象存活，并统计各 Region 的回收价值。
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并发标记
重新标记
清理统计
并发标记阶段大部分可以和应用线程一起运行，但初始标记和重新标记仍然需要短暂停顿。
Mixed GC
Mixed GC 不只回收新生代，还会选择一部分回收价值高的 Old Region 一起回收。
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Eden Region
Survivor Region
Old Region
选择垃圾比例高的 Old Region
Mixed GC 回收集合
这就是 Garbage First 名字的来源：优先回收垃圾收益更高的区域。
G1 为什么适合服务端应用
G1 的优势不是"永远最快"，而是更适合大堆和可控停顿目标。
| 能力 | 说明 |
|---|---|
| Region 化 | 回收可以不必每次覆盖整个老年代 |
| 复制整理 | 回收后减少碎片 |
| 可预测停顿 | 根据停顿目标选择回收集合 |
| 兼顾吞吐和延迟 | 比传统低停顿方案更均衡 |
常见参数：
bash
-XX:+UseG1GC
-XX:MaxGCPauseMillis=200
-XX:InitiatingHeapOccupancyPercent=45MaxGCPauseMillis 是目标，不是硬保证。如果对象分配速度太快、堆设置不合理、Mixed GC 回收跟不上，G1 仍然可能出现长停顿甚至 Full GC。
G1 也不是万能的
G1 常见风险包括：
| 风险 | 说明 |
|---|---|
| 大对象过多 | Humongous Region 分配和回收压力变大 |
| 堆太小 | Region 选择空间不足，GC 更频繁 |
| 分配速度过快 | 并发标记或回收跟不上应用分配 |
| 停顿目标过低 | JVM 为满足目标每次回收太少，导致回收效率下降 |
调 G1 时，不要只盯一个参数。要结合 GC 日志，看 Young GC 频率、Mixed GC 效果、老年代增长趋势、Full GC 原因和停顿分布。
面试回答模板
可以这样回答：
JVM 常见垃圾回收器有 Serial、Parallel、CMS 和 G1。Serial 是单线程收集器，简单但停顿明显；Parallel 使用多线程回收，目标是提高吞吐量；CMS 主要用于老年代，采用并发标记清除，目标是降低停顿，但会产生内存碎片，并且在较新 JDK 中已经移除。G1 是面向服务端的垃圾回收器，它把堆划分成多个 Region，每个 Region 可以作为 Eden、Survivor、Old 或 Humongous。G1 会根据 Region 的垃圾比例和停顿时间目标，优先回收价值高的 Region。它的流程包括 Young GC、并发标记和 Mixed GC。G1 适合大堆和需要控制停顿的场景，但停顿目标不是硬保证，如果回收速度赶不上分配速度，仍然可能触发 Full GC。
小结
垃圾回收器可以按目标来记：
Serial 要简单，Parallel 要吞吐，CMS 要低停顿，G1 要在大堆场景下兼顾停顿和吞吐。
能把 G1 的 Region、Young GC、并发标记、Mixed GC 讲清楚，GC 收集器这组问题就比较稳了。