[Interview with Experts]——Java Garbage Collection

1. What is Java garbage collection (Garbage Collection)? Why is it needed?

Java garbage collection is the process of automatically managing memory to free memory by recycling objects that are no longer referenced by the program. Its main purpose is to avoid memory leaks and improve program performance.

2. How does garbage collection work in Java? Please briefly explain how garbage collection works.

Java garbage collection works by identifying and recycling objects that are no longer referenced. The main algorithms include mark-clear, mark-organize, generational recycling, etc. The mark-and-sweep algorithm first marks objects that are no longer referenced and then clears them.

3. What are garbage objects (Garbage Objects)? How to determine if an object can be garbage collected?

Garbage objects are objects that are no longer referenced by the program. Determine whether an object can be garbage collected through reference counting or reachability analysis.

4.What are the types of Java garbage collectors? Can you briefly describe how each of them works?

Java garbage collectors include Serial, Parallel, CMS, G1, ZGC, Shenandoah, etc. Each recycler has different working principles and applicable scenarios.

5. What are strong references, soft references, weak references and virtual references? What is their role in garbage collection?

Strong reference is the most common reference type. When an object is referenced by a strong reference, the garbage collector will not reclaim it. Soft references, weak references, and phantom references are weaker reference types that allow objects to be recycled more easily.

6. What is the "Stop-the-World" event of garbage collection? What impact will it have on application performance?

A "Stop-the-World" event occurs when all threads of an application are suspended while garbage collection is occurring. This can have a temporary impact on application performance.

7. Why does a memory leak (Memory Leak) occur? How to avoid memory leaks?

A memory leak occurs when objects in an application cannot be garbage collected, resulting in increasing memory consumption. It usually occurs when holding a reference to an object that is no longer needed. Methods to avoid memory leaks include releasing references promptly and using tools for memory analysis.

8. How to manually trigger Java garbage collection? What is the function of System.gc()?

To manually trigger garbage collection, you can use the System.gc() method, but it is not recommended to use it frequently because garbage collection should be automatic.

9. Do you understand the impact of the modular system in Java 9 and later versions on garbage collection?

Java 9 and later versions introduce a modular system, replacing the permanent generation with metaspace to provide more flexible memory management.

10.What is Permanent Generation? Why is it replaced by Metaspace in Java 8 and later versions?

Permanent Generation (Permanent Generation) is a part of Java heap memory, used to store metadata information of classes and methods, string constant pools, static variables, etc. In Java 8 and previous versions, the permanent generation is a fixed-size memory area whose size is allocated when the application is started and is not dynamically expanded or reclaimed. This means that if the application dynamically loads a large number of classes or generates a large number of string constants, it may cause insufficient memory in the permanent generation and trigger an OutOfMemoryError. In order to solve some problems of the permanent generation, Java 8 introduced metaspace. The advantages are as follows:
Dynamic memory management: Metaspace dynamically allocates memory and is no longer limited by the fixed size of the permanent generation. This means that the application is less likely to crash due to metadata overflow.
Automatic garbage collection: Metaspace memory is automatically managed by the garbage collector. There is no need to manually adjust the permanent generation size or worry about permanent generation memory leaks.
Performance improvements: Application performance may improve because the metaspace is managed more efficiently and regular permanent generation garbage collection (such as Full GC) is no longer required.
Better class unloading: Class unloading is easier to implement in the metaspace, thus better supporting dynamic class loading and unloading, such as common use cases in OSGi and application servers.

11.Have you ever encountered OutOfMemoryError? Can you share your experience on how to diagnose and resolve this issue?

OutOfMemoryError is a common error when Java applications are running, usually caused by memory exhaustion. Diagnosing and resolving this problem usually requires looking at stack traces and memory usage to find the cause of the memory overflow.

12. Why use the GC logs of the garbage collector (Garbage Collection Logs)? How can I analyze these logs to optimize performance?

The GC log records the activities of the garbage collector, including pause times and memory usage. Analyzing GC logs can help identify performance bottlenecks and optimize garbage collection.

13. Characteristics of G1 (Garbage-First), Serial and CMS collectors. What are the characteristics of garbage collectors? What are the advantages?

G1: G1 is a generational garbage collector that divides the heap memory into the young generation and the old generation and adopts different recycling strategies. It has advantages when dealing with large heaps of memory. It is often used in applications with predictable low pause times, such as large web applications.
Serial: The Serial collector is a single-threaded garbage collector, which means that it can only use one CPU core to perform garbage collection operations. Suitable for single-threaded applications or small applications, not suitable for multi-core CPU applications. It is mostly used in mobile devices or embedded systems.
CMS: The CMS collector is a concurrent collector that allows garbage collection to be executed concurrently with application threads, reducing pause times. Suitable for lower garbage collection pause times. More common in large applications that require low latency.

14.What are the best practices for Java garbage collection? What performance tuning techniques can be used?

Best practices for Java garbage collection include avoiding the creation of unnecessary objects, releasing object references in a timely manner, choosing an appropriate garbage collector, and monitoring your application's memory usage. Performance tuning can be achieved by adjusting garbage collector parameters, heap size, and other factors.

15. Do you know the tools and libraries related to Java garbage collection?

Java garbage collection related tools and libraries include VisualVM, JConsole, Jvisualvm, MAT (Memory Analyzer Tool), etc. They can be used to monitor and analyze memory usage and solve performance problems.

16. What are the types of garbage collection algorithms? What are their respective advantages and disadvantages?

Garbage collection algorithms include mark-sweep, mark-compact, generational collection, etc. Each algorithm has its own advantages and disadvantages. For example, mark-and-sweep fragments memory, while mark-and-compact defragments memory to reduce fragmentation. Generational recycling divides the memory into the young generation and the old generation according to the life cycle of the object, and adopts different recycling strategies respectively.

17.What is object life cycle in Java? How does it affect garbage collection?

The life cycle of an object refers to the time period after it is created until it is garbage collected. Objects with a short life cycle are usually allocated in the young generation, while objects with a long life cycle are allocated in the old generation. This generational strategy helps improve the efficiency of garbage collection because objects in the young generation are easier to collect.

18.How does Java's garbage collector select objects to be recycled?

Java's garbage collector uses reachability analysis to determine which objects can still be reached. Starting from the root object (such as stack, static variables, registers, etc.), reachable objects are marked through the reference chain between objects, and unmarked objects will be recycled.

19.What is a Reference Queue? How to use it to handle weak and phantom references?

Reference queues are mechanisms for managing weak and phantom references. When objects with weak references or virtual references are recycled, they will be put into the reference queue, and the application can poll the queue to obtain the recycled objects and perform corresponding cleanup work.

20.What are the advantages and differences of the metaspace introduced in Java 9 compared to the permanent generation?

Metaspace was introduced after Java 8 and is used to store class metadata. Compared with the permanent generation, the metaspace has dynamically allocated memory, better performance, and fewer memory restrictions. It can dynamically expand based on the needs of the application, while the permanent generation requires a certain amount of memory to be allocated at startup.

21.What is the finalization of an object? How does it relate to garbage collection?

Finalization is a special method finalize() of the object. When the object is garbage collected, this method will be called to perform cleaning operations. However, finalization is not a reliable way to release resources because there is no guarantee when it will be executed. Therefore, it is recommended to use more reliable resource management methods such as try-with-resources.

22. Please explain how Java's garbage collector Concurrent Marking and Concurrent Sweeping work.

Concurrent marking allows the garbage collector to mark reachable objects while the application is running without stopping the application. Concurrent cleanup clears unreachable objects after marking is completed, which is also performed while the application is running.

23. What is a garbage collection memory leak? Can you give an example?

Garbage collection memory leaks refer to objects in the application being accidentally retained and unable to be garbage collected, resulting in memory leaks. For example, improperly closing a file or network connection can prevent related resources from being released.

24.What is the relationship between class loaders and garbage collection in Java?

Class loaders are responsible for loading classes and their metadata. When a class is no longer referenced, the class loader releases the reference to the class, making the class unreachable and eventually garbage collected.

25. How to tune the garbage collection performance of Java applications? Can provide some common performance tuning suggestions.

Performance tuning can include choosing an appropriate garbage collector, adjusting the heap size, avoiding unnecessary object allocations, reducing memory leaks, etc. Using monitoring tools and analyzing GC logs can help identify performance issues and tune them.