Java的垃圾回收（GC）是自動(dòng)管理內(nèi)存的機(jī)制，通過(guò)回收不可達(dá)對(duì)象釋放堆內(nèi)存，減少內(nèi)存洩漏風(fēng)險(xiǎn)。 1. GC從根對(duì)象（如棧變量、活動(dòng)線程、靜態(tài)字段等）出發(fā)判斷對(duì)象可達(dá)性，無(wú)法到達(dá)的對(duì)像被標(biāo)記為垃圾。 2. 基於標(biāo)記-清除算法，標(biāo)記所有可達(dá)對(duì)象，清除未標(biāo)記對(duì)象。 3. 採(cǎi)用分代收集策略：新生代（Eden、S0、S1）頻繁執(zhí)行Minor GC；老年代執(zhí)行較少但耗時(shí)較長(zhǎng)的Major GC；Metaspace存儲(chǔ)類元數(shù)據(jù)。 4. JVM提供多種GC器：Serial GC適用於小型應(yīng)用；Parallel GC提升吞吐量；CMS降低停頓時(shí)間（Java 14起廢棄）；G1 GC適用於大堆並控制暫停時(shí)間；ZGC和Shenandoah支持超大堆且暫停時(shí)間極短。 5. GC觸發(fā)條件包括Eden區(qū)滿、晉升失敗或System.gc()調(diào)用（僅為建議）。 6. finalize()方法已棄用，WeakReference等引用類型可精細(xì)控制對(duì)像生命週期。總之，Java GC通過(guò)自動(dòng)回收不再引用的對(duì)象來(lái)管理內(nèi)存，開(kāi)發(fā)者無(wú)需手動(dòng)釋放，理解GC機(jī)制有助於優(yōu)化性能和排查問(wèn)題。

Garbage collection (GC) in Java is the automatic process of reclaiming memory by destroying unused or unreachable objects. It helps developers manage memory without manually allocating and deallocating it, reducing the risk of memory leaks and dangling pointers.

Java's garbage collection works primarily on the heap , where all objects are stored. The JVM (Java Virtual Machine) uses a garbage collector to identify and remove objects that are no longer reachable from any live thread or static reference.

Here's how it works in practice:

1. Object Reachability and the Root Set

Garbage collection starts by determining which objects are still reachable . An object is considered reachable if it can be accessed through a chain of references starting from root objects , such as:

• Local variables in the stack frames of active threads
• Active Java threads
• Static fields
• JNI (Java Native Interface) references

If an object cannot be reached from any of these roots, it's considered garbage and eligible for collection.

2. Mark-and-Sweep Algorithm (Conceptual Basis)

Most GC algorithms are based on the mark-and-sweep approach:

• Mark phase : The GC traverses all reachable objects starting from the root set and marks them as "alive."
• Sweep phase : The GC scans the heap and reclaims memory from unmarked (ie, unreachable) objects.

This basic method avoids deleting objects that are still in use.

3. Generational Garbage Collection

Java's heap is divided into generations based on the object lifetime observation (most objects die young):

• Young Generation : Where new objects are created.
  • Divided into: Eden space and two Survivor spaces (S0 and S1).
  • Minor GC occurs here frequently.
• Old (Tenured) Generation : Holds long-lived objects that have survived multiple GC cycles.
  • Major GC or Full GC runs here less frequently but takes longer.
• Metaspace (replaced PermGen in Java 8): Stores class metadata, not regular objects.

When an object survives several garbage collection cycles in the young generation, it gets promoted to the old generation.

4. Types of Garbage Collectors in JVM

Java provides different GC implementations optimized for various use cases:

• Serial GC : Simple, single-threaded collector. Best for small applications.
• Parallel GC (Throughput Collector) : Uses multiple threads for minor and major collections. Good for maximizing throughput.
• CMS (Concurrent Mark-Sweep) : Minimizes pause times by doing most of its work concurrently (deprecated as of Java 14).
• G1 GC (Garbage-First) : Designed for large heaps with predictable pause times. Divides heap into regions and prioritizes collection from regions with the most garbage.
• ZGC (Z Garbage Collector) : Scalable low-latency collector that can handle very large heaps (terabytes) with pause times under 10ms.
• Shenandoah : Another low-pause collector that does concurrent compaction.

5. How GC Is Triggered

Garbage collection is triggered automatically when:

• The Eden space fills up (triggers minor GC).
• Promotion from young to old generation fails due to lack of space (triggers major GC).
• Application calls System.gc() (hint only — not guaranteed to run).

Note: Calling System.gc() doesn't force GC; it only suggests that the JVM consider running it.

6. Finalization and Weak References

• Objects with finalizers ( finalize() method) may delay collection, as they are queued for finalization before being reclaimed (this method is deprecated since Java 9).
• Java also supports reference types like WeakReference , SoftReference , and PhantomReference that allow fine-grained control over object lifecycle and GC behavior.

In short, Java garbage collection automates memory management by tracking object reachability and reclaiming unused memory, using generational strategies and various GC algorithms to balance throughput, latency, and scalability. You don't need to free memory manually, but understanding GC helps write efficient code and troubleshoot performance issues.

Basically, if you stop referencing an object, the GC will eventually clean it up — when it decides it's the right time.

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