Top Java Interview Questions Every Backend Engineer Should Master

Opening Introduction

Briefly introduce your work experience, responsibilities, and main projects to ease interview pressure.

Highlight a project or platform you are most proud of, focusing on the technical details you handled.

Java Multithreading

Thread pool principles, reasons for creating a pool, and creation methods.

Thread lifecycle and situations that cause dead threads.

Thread safety issues, how to achieve it, and implementation methods.

Core parameters of a thread pool and how to size it reasonably.

Use cases and principles of volatile and ThreadLocal.

When can ThreadLocal cause OOM and why?

Differences between synchronized and volatile, lock granularity, deadlock simulation, atomicity and visibility.

JVM Topics

JVM memory model, garbage collection mechanisms and principles.

Differences between Minor GC and Full GC, trigger conditions, and algorithms used.

Various class loaders in the JVM and their purposes.

Parent delegation model, its operation, and benefits.

Scenarios where breaking the parent delegation model is needed.

Common JVM tuning methods with specific parameter adjustments.

Overview of JVM memory layout, class loading, GC algorithms, collectors, and class file structure.

Advanced Java Topics

Implementation principles and use cases of red‑black trees.

What NIO is and suitable scenarios.

Improvements from Java 8 to Java 9.

Internal data structure of HashMap and its implementation (including comparisons with ConcurrentHashMap, Hashtable, etc.).

Purpose, implementation, and performance considerations of reflection.

Custom annotation scenarios and implementation.

Differences among List, Map, ArrayList, LinkedList, and Vector.

Spring Framework

Implementation principles and scenarios of Spring AOP.

Scope and lifecycle of Spring beans.

Improvements introduced by Spring Boot over Spring and by Spring 5 over Spring 4.

How to create a custom Spring Boot starter.

What Spring IoC is and its advantages.

Principles of Spring MVC, dynamic proxies, reflection, AOP, and transaction isolation levels.

Middleware

Complete call chain of a Dubbo invocation.

Load balancing strategies supported by Dubbo.

How to limit concurrent requests on a Dubbo provider.

Configuration methods supported by Dubbo at startup.

Overview of various message‑queue products and their pros/cons.

How message middleware ensures consistency and implements retry mechanisms.

Introduction to Spring Cloud circuit‑breaker mechanisms.

Comparison between Spring Cloud and Dubbo and appropriate usage scenarios.

Database Topics

Lock mechanisms: row lock, table lock, exclusive lock, shared lock.

Business scenarios and implementation of optimistic locking.

Transaction concepts, distributed transactions, common solutions, and two‑phase/three‑phase commit.

MySQL binlog recording modes and their trade‑offs.

MySQL lock types: pessimistic, optimistic, exclusive, shared, table‑level, row‑level.

Principles of distributed transactions, synchronous/asynchronous, blocking/non‑blocking.

Transaction isolation levels, MySQL defaults, Spring and JDBC transaction implementations, nested and distributed transactions.

SQL parsing and execution process, including row‑to‑column transformations.

Redis

Why Redis is fast and issues with multithreading.

Supported data structures in Redis.

Problems related to Redis skip‑list.

How a single‑threaded Redis achieves high concurrency.

Implementing distributed locks with Redis.

Atomicity of Redis distributed lock operations and internal implementation.

Other Topics

Source code you have read and detailed questions that may be asked to assess depth of understanding.

Challenging problems encountered in projects and how you solved them.

These high‑frequency interview questions are organized to help candidates prepare effectively for Java positions.