Java Notes - Page 2

Reflection inspects and manipulates classes at runtime. It powers most frameworks and should be rare in application code.

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Java can still leak: any object the program keeps reachable but no longer needs. Here is how to find and fix them.

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The collector reclaims objects that can no longer be reached. Understanding reachability explains both leaks and pauses.

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Where each piece of data actually lives, and what runs out when each area is exhausted.

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Classes are found, verified, prepared and initialised on first use, by a hierarchy of loaders that delegate upwards.

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The class loader, the runtime data areas and the execution engine, and how a class file becomes running machine code.

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The rules that decide when a write by one thread becomes visible to another, expressed as the happens-before relationship.

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Purpose built collections for shared access, which are both safer and far faster than wrapping an ordinary one in a lock.

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A Future is a placeholder for a result that is not ready yet. CompletableFuture adds composition, so results can be chained without blocking.

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An executor separates submitting work from running it, so threads are reused instead of created per task.

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The two failures that define concurrency: results that depend on timing, and threads that wait for each other forever.

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Explicit locks offer timeouts, fairness and multiple conditions, and the atomic classes give lock free updates for single variables.