面试题与答案

PermGen (Permanent Generation) space was used to store metadata related to classes. In Java 8 and later, PermGen space is replaced by Metaspace, which is more flexible and avoids memory leaks related to class metadata.

Garbage collection tuning involves configuring the garbage collector to meet specific performance goals. It includes selecting the appropriate garbage collector algorithm and adjusting related parameters.

Garbage collection pause times refer to the periods during which application threads are stopped while the garbage collector performs its tasks. Minimizing pause times is essential for maintaining application responsiveness.

The G1 garbage collector is designed to provide high throughput and low-latency garbage collection. It divides the heap into regions and uses a series of Garbage-First (GF) queues to prioritize collection of regions with the most garbage.

The JVM uses a reachability analysis to determine if objects are reachable or not. Circular references are handled by the garbage collector, which identifies and collects objects that are no longer reachable.

The 'Reference' class is part of the java.lang.ref package and provides a way to create and manipulate references to objects. It is often used in advanced memory management scenarios and garbage collection.

The 'GC overhead limit exceeded' error occurs when the JVM spends too much time on garbage collection, exceeding a specified threshold (98% by default) of the total time. It indicates a potential performance issue.

The 'Card Table' is a data structure used by the G1 garbage collector to track changes in the heap. It allows the collector to efficiently identify and collect only the regions of the heap containing modified objects.

The 'CMSConcurrentAbortablePreclean' is a phase in the CMS (Concurrent Mark-Sweep) garbage collector that performs additional work on the Old Generation to prepare for the final mark phase. It is concurrent and aims to minimize pause times.

Shenandoah is a low-pause-time garbage collector introduced in Java. It uses advanced techniques to perform garbage collection concurrently with application threads, minimizing pause times and improving overall responsiveness.