垃圾回收不只是"把没用的对象清掉"。真正要回答清楚 JVM GC，需要先说明三个问题：怎么判断垃圾、怎么回收垃圾、为什么要把堆分代。
判断垃圾主要靠可达性分析；回收垃圾常见算法有标记清除、标记整理、复制算法；分代回收则是基于大多数对象"朝生夕死"的经验，把新对象和老对象分开处理。
三种基础垃圾回收算法
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标记清除
标记整理
复制算法
这三种算法不是具体垃圾回收器，而是垃圾回收器背后的基本策略。
标记清除算法
标记清除分两步：先标记需要回收的对象，再清除这些对象占用的内存。
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从 GC Roots 做可达性分析
标记不可达对象
清除不可达对象
释放内存
它的优点是流程简单，标记和清除速度相对直接。缺点也很明显：清除后会产生内存碎片。
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存活对象和垃圾对象交错
清除垃圾对象
产生不连续空闲空间
碎片多了以后，即使总空闲内存不少，也可能找不到连续空间分配大对象。
| 优点 | 缺点 |
|---|---|
| 实现简单，清理速度较快 | 容易产生内存碎片 |
| 不需要移动大量对象 | 大对象分配可能受影响 |
标记整理算法
标记整理也会先标记对象，但清理时不是直接把垃圾对象挖掉，而是把存活对象向一端移动，再清理边界外的空间。
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标记存活对象
移动存活对象到一端
清理边界外空间
得到连续可用内存
标记整理解决了内存碎片问题，但对象移动会带来额外成本。
| 优点 | 缺点 |
|---|---|
| 回收后内存连续 | 移动对象成本较高 |
| 适合老年代 | 停顿时间可能变长 |
老年代中对象存活率通常较高，如果使用复制算法，会复制大量对象，成本较高。因此老年代收集器经常使用标记清除或标记整理思路。
复制算法
复制算法把内存分成两块，每次只使用其中一块。GC 时把存活对象复制到另一块，然后把原来的区域整体清空。
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发生 GC
复制存活对象到 To 区
清空 From 区
From 和 To 角色交换
复制算法的优势是清理后没有碎片，而且在垃圾对象多、存活对象少时效率很高。缺点是需要额外空间，内存利用率会下降。
| 优点 | 缺点 |
|---|---|
| 没有内存碎片 | 需要额外空闲区域 |
| 存活对象少时效率高 | 存活对象多时复制成本高 |
新生代对象大多存活时间短，所以复制算法非常适合新生代。
为什么要分代回收
JVM 分代回收基于一个经验判断：大多数对象生命周期很短，少数对象会存活很久。
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少数对象
新创建对象
是否很快变成垃圾
在新生代被回收
多次存活后晋升老年代
如果不分代，所有对象混在一起回收，每次 GC 都要扫描更大范围，成本会很高。分代后，新生代可以频繁、快速回收；老年代可以用更适合高存活率对象的算法。
堆的分代结构
传统分代模型中，堆主要分为新生代和老年代。新生代又分为 Eden、Survivor From、Survivor To。
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新生代
老年代
Eden
Survivor From
Survivor To
常见默认比例会看到新生代中 Eden 和两个 Survivor 约为 8:1:1，但现代 JVM 会受垃圾回收器、自适应策略和参数影响，不要把这个比例当成所有场景下绝对固定。
对象分配和晋升流程
普通对象通常先分配到 Eden 区。Eden 空间不足时触发 Minor GC，把 Eden 和 Survivor 中仍然存活的对象复制到另一个 Survivor 区；对象年龄增长到一定阈值，或者 Survivor 放不下，就可能晋升到老年代。
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放不下
否
是
新对象创建
优先进入 Eden
Eden 是否放得下
继续运行
触发 Minor GC
标记 Eden 和 Survivor 存活对象
复制到另一个 Survivor
对象年龄或空间是否达到晋升条件
继续留在新生代
晋升老年代
这里有几个常见晋升条件：
| 条件 | 说明 |
|---|---|
| 对象年龄达到阈值 | 多次 Minor GC 后仍存活 |
| Survivor 空间不足 | 放不下的存活对象提前进入老年代 |
| 大对象直接进入老年代 | 避免在新生代频繁复制大对象 |
| 动态年龄判断 | 某些年龄段对象总大小过大时提前晋升 |
晋升阈值常通过 -XX:MaxTenuringThreshold 控制，最大通常是 15，因为对象年龄在对象头中可用位数有限。
对象一定先进 Eden 吗
不一定。
"对象通常先进 Eden"是理解分代回收的主线，但真实 JVM 会有一些优化和例外。
| 情况 | 说明 |
|---|---|
| TLAB 分配 | 对象可能先分配到线程自己的 TLAB，本质上仍属于 Eden |
| 大对象 | 可能直接进入老年代，避免在新生代来回复制 |
| 长期存活对象 | 多次 Minor GC 后晋升老年代 |
| Survivor 放不下 | 存活对象可能提前进入老年代 |
| 逃逸分析优化 | 对象可能被标量替换，不形成完整堆对象 |
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不能
是
否
可用
不可用
对象创建
JIT 是否能优化掉
可能标量替换
是否大对象
可能直接进入老年代
TLAB 是否可用
进入线程 TLAB
进入 Eden 公共区域
所以面试时更稳的表达是：绝大多数普通对象会优先在新生代 Eden 分配，但大对象、TLAB、逃逸分析、空间担保等机制会带来例外。
Minor GC、Major GC、Full GC、Mixed GC
这些名词也很容易混：
| 名称 | 回收范围 | 常见特点 |
|---|---|---|
| Minor GC | 新生代 | 触发频繁，停顿通常较短 |
| Major GC | 老年代 | 不同语境含义不完全一致，需要结合收集器 |
| Full GC | 整个堆和相关区域 | 停顿通常较长，应重点关注 |
| Mixed GC | 新生代加部分老年代 Region | G1 中的混合回收 |
Stop The World 简称 STW，意思是垃圾回收过程中应用线程暂停，等待 GC 线程完成某些关键阶段。
"GC 线程" "应用线程" "GC 线程" "应用线程" #mermaid-svg-S51CCvQBlTpOU1Yb{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;fill:#333;}@keyframes edge-animation-frame{from{stroke-dashoffset:0;}}@keyframes dash{to{stroke-dashoffset:0;}}#mermaid-svg-S51CCvQBlTpOU1Yb .edge-animation-slow{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 50s linear infinite;stroke-linecap:round;}#mermaid-svg-S51CCvQBlTpOU1Yb .edge-animation-fast{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 20s linear infinite;stroke-linecap:round;}#mermaid-svg-S51CCvQBlTpOU1Yb .error-icon{fill:#552222;}#mermaid-svg-S51CCvQBlTpOU1Yb .error-text{fill:#552222;stroke:#552222;}#mermaid-svg-S51CCvQBlTpOU1Yb .edge-thickness-normal{stroke-width:1px;}#mermaid-svg-S51CCvQBlTpOU1Yb .edge-thickness-thick{stroke-width:3.5px;}#mermaid-svg-S51CCvQBlTpOU1Yb .edge-pattern-solid{stroke-dasharray:0;}#mermaid-svg-S51CCvQBlTpOU1Yb .edge-thickness-invisible{stroke-width:0;fill:none;}#mermaid-svg-S51CCvQBlTpOU1Yb .edge-pattern-dashed{stroke-dasharray:3;}#mermaid-svg-S51CCvQBlTpOU1Yb .edge-pattern-dotted{stroke-dasharray:2;}#mermaid-svg-S51CCvQBlTpOU1Yb .marker{fill:#333333;stroke:#333333;}#mermaid-svg-S51CCvQBlTpOU1Yb .marker.cross{stroke:#333333;}#mermaid-svg-S51CCvQBlTpOU1Yb svg{font-family:"trebuchet ms",verdana,arial,sans-serif;font-size:16px;}#mermaid-svg-S51CCvQBlTpOU1Yb p{margin:0;}#mermaid-svg-S51CCvQBlTpOU1Yb .actor{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-S51CCvQBlTpOU1Yb text.actor>tspan{fill:black;stroke:none;}#mermaid-svg-S51CCvQBlTpOU1Yb .actor-line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);}#mermaid-svg-S51CCvQBlTpOU1Yb .innerArc{stroke-width:1.5;stroke-dasharray:none;}#mermaid-svg-S51CCvQBlTpOU1Yb .messageLine0{stroke-width:1.5;stroke-dasharray:none;stroke:#333;}#mermaid-svg-S51CCvQBlTpOU1Yb .messageLine1{stroke-width:1.5;stroke-dasharray:2,2;stroke:#333;}#mermaid-svg-S51CCvQBlTpOU1Yb #arrowhead path{fill:#333;stroke:#333;}#mermaid-svg-S51CCvQBlTpOU1Yb .sequenceNumber{fill:white;}#mermaid-svg-S51CCvQBlTpOU1Yb #sequencenumber{fill:#333;}#mermaid-svg-S51CCvQBlTpOU1Yb #crosshead path{fill:#333;stroke:#333;}#mermaid-svg-S51CCvQBlTpOU1Yb .messageText{fill:#333;stroke:none;}#mermaid-svg-S51CCvQBlTpOU1Yb .labelBox{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-S51CCvQBlTpOU1Yb .labelText,#mermaid-svg-S51CCvQBlTpOU1Yb .labelText>tspan{fill:black;stroke:none;}#mermaid-svg-S51CCvQBlTpOU1Yb .loopText,#mermaid-svg-S51CCvQBlTpOU1Yb .loopText>tspan{fill:black;stroke:none;}#mermaid-svg-S51CCvQBlTpOU1Yb .loopLine{stroke-width:2px;stroke-dasharray:2,2;stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);}#mermaid-svg-S51CCvQBlTpOU1Yb .note{stroke:#aaaa33;fill:#fff5ad;}#mermaid-svg-S51CCvQBlTpOU1Yb .noteText,#mermaid-svg-S51CCvQBlTpOU1Yb .noteText>tspan{fill:black;stroke:none;}#mermaid-svg-S51CCvQBlTpOU1Yb .activation0{fill:#f4f4f4;stroke:#666;}#mermaid-svg-S51CCvQBlTpOU1Yb .activation1{fill:#f4f4f4;stroke:#666;}#mermaid-svg-S51CCvQBlTpOU1Yb .activation2{fill:#f4f4f4;stroke:#666;}#mermaid-svg-S51CCvQBlTpOU1Yb .actorPopupMenu{position:absolute;}#mermaid-svg-S51CCvQBlTpOU1Yb .actorPopupMenuPanel{position:absolute;fill:#ECECFF;box-shadow:0px 8px 16px 0px rgba(0,0,0,0.2);filter:drop-shadow(3px 5px 2px rgb(0 0 0 / 0.4));}#mermaid-svg-S51CCvQBlTpOU1Yb .actor-man line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-S51CCvQBlTpOU1Yb .actor-man circle,#mermaid-svg-S51CCvQBlTpOU1Yb line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;stroke-width:2px;}#mermaid-svg-S51CCvQBlTpOU1Yb :root{--mermaid-font-family:"trebuchet ms",verdana,arial,sans-serif;} "正常执行业务""触发 STW""暂停""标记或复制对象""恢复应用线程""继续执行业务"
GC 优化很多时候就是在吞吐量、停顿时间和内存占用之间做取舍。
分代回收为什么高效
分代回收的核心是"用不同算法处理不同生命周期的对象"。
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对象存活率低
复制算法更合适
老年代
对象存活率高
标记清除或标记整理更合适
如果新生代每次 GC 后只有少量对象存活，复制算法只需要复制少量对象，剩下空间整体清空，速度很快。
如果老年代大多数对象都存活，复制算法就会变成大量搬运，成本很高，因此更适合标记整理或标记清除类算法。
面试回答模板
可以这样回答：
JVM 常见垃圾回收算法有标记清除、标记整理和复制算法。标记清除先标记垃圾对象再清除，速度较快但会产生内存碎片；标记整理会把存活对象向一端移动，再清理边界外空间，解决碎片问题但移动对象成本更高；复制算法把存活对象复制到另一块空间，再清空原空间，适合存活对象少的新生代。JVM 做分代回收是因为大多数对象生命周期很短，少数对象存活较久，所以新对象通常先进 Eden，Minor GC 后存活对象进入 Survivor，多次存活或 Survivor 放不下时晋升老年代。Minor GC 主要回收新生代，Full GC 通常回收范围更大、停顿更长，线上需要重点关注。
小结
垃圾回收算法和分代回收可以用一句话串起来：
新生代垃圾多、存活少，适合复制；老年代存活多、搬动贵，适合标记清除或标记整理。
理解这个取舍，比单纯背算法定义更重要。