Comprehensive Java Backend Interview Review: Threads, Collections, Networking, HTTP, TCP, and MySQL

Interview Overview

Author introduces a recent Alibaba Java backend interview, summarizing around 20 fundamental questions on projects, Java collections, concurrency, networking, and MySQL.

Thread Pools

Used ScheduledThreadPool for periodic tasks; explained why it fits the scenario.

FixedThreadPool – fixed number of threads, excess tasks queued.

CachedThreadPool – virtually unlimited threads with SynchronousQueue for hand‑off.

SingleThreadExecutor – single thread, recreates on failure, preserves order.

SingleThreadScheduledExecutor – special case of ScheduledThreadPool with one thread.

Unique ID Generation

Adopted Snowflake algorithm to generate globally unique IDs instead of auto‑increment primary keys.

Other strategies: auto‑increment, UUID (not ideal for primary keys due to randomness and index fragmentation).

Open‑source alternatives: Meituan Leaf, Baidu UidGenerator.

Java List Implementations

Common List implementations: ArrayList, LinkedList, Vector, Stack.

Characteristics: ArrayList: dynamic array, fast random access, suitable for read‑heavy scenarios. LinkedList: doubly linked list, fast insert/delete, good for frequent modifications. Vector: thread‑safe version of ArrayList. Stack: LIFO structure based on Vector.

List vs Set

List is ordered and allows duplicates; Set is unordered and enforces uniqueness.

Use List when order or duplicate counts matter; use Set for deduplication and fast existence checks.

Common HTTP Status Codes

Explained the five classes of status codes (1xx–5xx) with typical examples such as 200 OK, 404 Not Found, 500 Internal Server Error, etc.

Flow Control and Congestion Control

Flow control uses sliding windows to prevent sender overload; congestion control employs slow start, congestion avoidance, fast retransmit, and fast recovery algorithms.

URL Request Execution Process

Browser parses URL, resolves IP via DNS hierarchy (cache → OS → local DNS → root → authoritative), then establishes TCP connection and requests the resource.

TCP Three‑Way Handshake

Detailed state transitions with packet flags and sequence numbers:

Client SYN → Server SYN‑ACK → Client ACK.

State diagram includes CLOSE, LISTEN, SYN‑SENT, SYN‑RCVD, ESTABLISHED.

HTTP vs HTTPS

HTTP transmits plaintext on port 80; HTTPS adds SSL/TLS encryption on port 443, providing confidentiality and integrity.

Database Indexes

Types: B+‑tree (InnoDB default), hash, full‑text.

B+‑tree stores only keys in non‑leaf nodes, resulting in fewer I/O operations, stable insert/delete performance, and linked leaf nodes for efficient range queries.

Transaction Isolation Issues

Identified dirty read, non‑repeatable read, and phantom read.

Isolation level mapping:

Dirty read → READ‑UNCOMMITTED.

Non‑repeatable read → REPEATABLE‑READ.

Phantom read → SERIALIZABLE.

MySQL InnoDB default is REPEATABLE‑READ, which largely prevents phantom reads via MVCC for snapshot reads and next‑key locks for current reads.

Interview Takeaways

Strengthen SQL knowledge and practice queries.

Review computer networking concepts more thoroughly.

Deepen understanding of Java concurrency.

Continuously ask “why” to solidify concepts.