How does Java garbage collection decide which objects can be reclaimed?

Explains GC roots, reachability, generational behavior, pauses, and why setting a reference to null is not a general cleanup strategy.

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The collector starts from GC roots such as active thread stacks, static fields, and certain JVM or native references, then traces objects reachable from them. Objects that are no longer reachable can have their heap memory reclaimed. Modern collectors usually exploit the observation that many objects die young, although the exact regions and algorithms depend on the selected collector. Reachability is the rule; reference counting is not Java's general garbage-collection model.

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Reachability-based collection, object lifetime reasoning, generational assumptions, and separation of memory from resource ownership.

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Garbage collection follows references from roots; it does not understand whether an object is still useful to the business.
The collector starts from GC roots such as active thread stacks, static fields, and certain JVM or native references, then traces objects reachable from them. Objects that are no longer reachable can have their heap memory reclaimed. Modern collectors usually exploit the observation that many objects die young, although the exact regions and algorithms depend on the selected collector. Reachability is the rule; reference counting is not Java's general garbage-collection model.
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A temporary object can be allocated frequently and still be cheap if it dies quickly, while one unintended reference from a static map can retain a large object graph indefinitely. Assigning a local variable to null rarely fixes the ownership problem because the compiler and runtime already understand when locals stop being live. Garbage collection also does not promptly close files, sockets, or database connections; those resources need deterministic ownership such as try-with-resources. is only a request and is not a normal application memory-management strategy.

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Describing Java GC as reference counting.
Using System.gc() as routine cleanup.
Expecting GC to close external resources.

How does Java garbage collection decide which objects can be reclaimed?

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