examination/topics/gc-special/gc-deep-dive/fill_blank.json

{
  "topic": "gc-deep-dive",
  "type": "fill_blank",
  "schema_version": "1.0.0",
  "generated": "2026-09-03T00:00:00Z",
  "questions": [
    {
      "id": "fb-001",
      "type": "fill_blank",
      "difficulty": 1,
      "tags": ["gc", "memory", "jvm", "three-color-marking", "collections"],
      "question": "Python 默认采用的 GC 机制是___计数，而 Java 与 Go 采用的则是从 GC Roots 出发的___分析；Rust 则在编译期通过___所有权机制来管理内存。",
      "answer": ["引用计数", "可达性", "所有权"],
      "answer_rule": "all",
      "explanation": "Python 使用引用计数（配合周期检测器处理循环引用）；Java/Go 使用追踪式可达性分析；Rust 依赖编译期的所有权（ownership）和借用规则，无需运行时 GC。",
      "source": null,
      "related": []
    },
    {
      "id": "fb-002",
      "type": "fill_blank",
      "difficulty": 1,
      "tags": ["gc", "jvm", "memory", "collections"],
      "question": "Swift 采用的自动引用计数简称__，它属于___计数类回收机制而非追踪式 GC。",
      "answer": ["ARC", "引用"],
      "answer_rule": "all",
      "explanation": "Swift 的 ARC（Automatic Reference Counting）在编译期插入 retain/release，是一种引用计数式方案，与 Java/Go 的追踪式 GC 不同。",
      "source": null,
      "related": []
    },
    {
      "id": "fb-003",
      "type": "fill_blank",
      "difficulty": 2,
      "tags": ["gc", "jvm", "memory", "collections"],
      "question": "JVM 进行垃圾回收时所有业务线程必须暂停，这一现象缩写作 ___；STW 之外，GC 带来的主要副产品还包括额外的内存开销，通常约为堆的 ___%。",
      "answer": ["STW", "5%到20%"],
      "answer_rule": "all",
      "explanation": "STW = Stop-The-World，指 GC 期间暂停用户线程；追踪式 GC 通常额外占用 5%~20% 的堆内存（如对象头标记、卡片表、缓冲等）。",
      "source": null,
      "related": []
    },
    {
      "id": "fb-004",
      "type": "fill_blank",
      "difficulty": 2,
      "tags": ["gc", "memory", "collections"],
      "question": "针对内存碎片治理，标记-清除完成后可继续压缩以提高熵密度；另一种思路是___算法把存活对象复制到另一块连续区域；G1 等现代回收器则采用按 ___ 划分 Region 的自适应方式。",
      "answer": ["复制", "区域"],
      "answer_rule": "all",
      "explanation": "复制（copying）算法从一块空间复制存活对象到另一块以消除碎片；分代/区域式采用自适应的复制与晋升策略管理碎片。",
      "source": null,
      "related": []
    },
    {
      "id": "fb-005",
      "type": "fill_blank",
      "difficulty": 2,
      "tags": ["gc", "jvm", "memory", "collections"],
      "question": "经典 HotSpot 新生代内存被划分为三个区，默认占比为 Eden 占 80%、存活区 S0 与 S1 各占 ___%；对象经历一次 Minor GC 后年龄加 1，达到晋升阈值默认值 ___ 时被提升至老年代。",
      "answer": ["10%", "15"],
      "answer_rule": "all",
      "explanation": "默认 Eden(80%)/S0(10%)/S1(10%)，晋升阈值 -XX:MaxTenuringThreshold 默认 15，对象年龄达到即晋升到老年代。",
      "source": null,
      "related": []
    },
    {
      "id": "fb-006",
      "type": "fill_blank",
      "difficulty": 3,
      "tags": ["gc", "jvm", "memory", "collections"],
      "question": "CMS 垃圾回收器（旧）采用标记-清除，大致经历四个阶段；而 G1 不再沿用固定分区，而是把堆划分为等大小的 ___ 来承载新生代/老年代；ZGC 则使用 ___ 来快速记录对象状态以支撑极低的暂停。",
      "answer": ["Region", "染色指针"],
      "answer_rule": "all",
      "explanation": "G1（Garbage First）把堆抽象地划分成若干等大小 Region；ZGC 采用了染色指针（colored pointers）将部分状态编码在地址指针上，实现极低延迟。",
      "source": null,
      "related": []
    },
    {
      "id": "fb-007",
      "type": "fill_blank",
      "difficulty": 4,
      "tags": ["gc", "jvm", "memory", "three-color-marking", "collections"],
      "question": "三色标记法中：灰色表示对象已被扫描但其若干子对象还未遍历完，白色表示___候选，黑色表示已全部处理完成；若某存活对象在所有引用恢复之后仍被标成白色，则发生了漏标，需满足两个条件。",
      "answer": ["待回收", "漏标"],
      "answer_rule": "all",
      "explanation": "三色标记：白=未访问（潜在垃圾候选）、灰=已访问但子对象未完、黑=子对象已全处理。并发标记时需要满足条件之一（增量更新或 SATB）来避免漏标。",
      "source": null,
      "related": []
    },
    {
      "id": "fb-008",
      "type": "fill_blank",
      "difficulty": 4,
      "tags": ["gc", "jvm", "memory", "collections"],
      "question": "在增量更新的三色标记中，防止存活对象被误回收（漏标）需要同时满足两个条件：在垃圾收集器标记期间，某对象字段上的新指针必须记录到当前视图，并且该对象本身必须属于___（已被标记）集合。G1 默认采用基于栈的 ___ 快照来记录这些变化。",
      "answer": ["灰色", "SATB"],
      "answer_rule": "all",
      "explanation": "漏标的两条件：①对某字段写入一个新引用（引用替换）；②在抛弃的旧引用被记录前回收前，该对象已被标记过。G1 采用 SATB（增量式）保存开始时快照并记录变化，保证不漏标。",
      "source": null,
      "related": []
    },
    {
      "id": "fb-009",
      "type": "fill_blank",
      "difficulty": 5,
      "tags": ["gc", "jvm", "memory", "three-color-marking", "collections"],
      "question": "三色标记中的“漏标”指存活对象被错误当作白色从而被回收冻结的 bug。避免漏标需要两个条件同时成立：①某白色对象被（已被标记的）黑色对象引用（即写入了新引用）；②在该黑色对象的新引用被记录（额外扫描机会）之前，其旧引用已被收集器丢弃。若破坏其中任一条件即可避免漏标，CMS 使用___更新，G1 采用“起始于快照”（___）策略。",
      "answer": ["增量", "SATB"],
      "answer_rule": "all",
      "explanation": "防止漏标的两策略：增量更新（CMS，把被修改对象的灰色标记重新传播）与 SATB（Snapshot-at-the-Beginning，G1，记录并发开始的引用快照）。",
      "source": null,
      "related": []
    },
    {
      "id": "fb-010",
      "type": "fill_blank",
      "difficulty": 5,
      "tags": ["gc", "jvm", "memory", "three-color-marking", "collections"],
      "question": "进行可达性分析的垃圾回收时，标记阶段从若干被外部持有的根（Roots）出发，主要根类型包括：Java 虚拟机栈中帧局部的局部变量/参数、___中的静态对象、JNI 本地/全局引用、运行时常量池中的对象等。",
      "answer": ["方法区或堆中静态引用域"],
      "answer_rule": "all",
      "explanation": "GC Roots 的典型成员：栈帧局部变量表、方法区静态字段/引用、JNI 引用、运行时常量池里的字符串等。此处提供“方法区/堆静态引用”类答案。",
      "source": null,
      "related": []
    }
  ]
}