Garbage collection in Java is primarily managed automatically by the Garbage Collector (GC). The garbage collection mechanism involves the following key steps:
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Marking: The garbage collector first identifies all objects reachable from the root set (which typically includes global references, local variables, and input parameters within the stack frames of active threads). All reachable objects are considered active, while unreachable objects are deemed garbage.
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Sweeping: After the marking phase, the garbage collector removes all objects marked as garbage, freeing the memory they occupied. Specific methods may include directly deallocating memory or other techniques such as compaction or copying to optimize memory usage.
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Compacting (Optional): To prevent memory fragmentation, some garbage collectors perform memory compaction after removing unreachable objects. This step moves surviving objects to one end of the memory space, ensuring contiguous free memory for future allocations.
Common garbage collectors in Java include:
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Serial Garbage Collector: Suitable for small applications and single-processor environments. It pauses all application threads during garbage collection.
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Parallel Garbage Collector: Executes garbage collection in parallel across multiple processors, ideal for multi-core servers. It minimizes application pause times during collection.
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CMS Garbage Collector: Reduces pause times through concurrent marking and concurrent clearing phases, making it suitable for interactive applications.
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G1 Garbage Collector: Designed for server applications, it uses a partitioned heap approach to allow garbage collection to run concurrently with application threads while optimizing predictable pause times.
Different garbage collectors are tailored for various application types and scales. Developers should select the appropriate garbage collection strategy based on specific requirements.