Reference Docs — Where You Look Things Up at 11 PM IST

When a Stack Overflow answer is 8 years old and mentions Java 8, you need a hierarchy of trust. This file gives you the lookup order for any question. Bookmark all of these; disable notifications for all of these.

The Lookup Hierarchy

For any Java question, check sources in this order until answered:

  1. Official JavaDoc / JDK docs — always first for stdlib

  2. Official framework docs (Spring, Hibernate, JUnit) — for framework-specific

  3. JEP text — for language/JVM features (openjdk.org/jeps/)

  4. Baeldung / Reflectoring / thorben-janssen — secondary tutorial sources

  5. Stack Overflow with year filter — only accept answers from last 3 years unless it’s a canonical old answer

  6. GitHub issues in the actual project — for bugs, edge cases

  7. AI (Claude / ChatGPT) — last, and only to explain something you’ve already partially researched

Tier 1: Official Sources

Source

URL

What for

JDK 21 API Docs

docs.oracle.com/en/java/javase/21/docs/api/

Every method in every stdlib class

JDK 25 API Docs

docs.oracle.com/en/java/javase/25/docs/api/

Latest LTS as of Sept 2025

Java Language Spec (JLS)

docs.oracle.com/javase/specs/jls/se21/html/

The rules-lawyer source

Java Virtual Machine Spec (JVMS)

docs.oracle.com/javase/specs/jvms/se21/html/

Bytecode, class file format

JEP Index

openjdk.org/jeps/0

All JEPs, filter by status

Spring Framework Reference

docs.spring.io/spring-framework/reference/

The primary Spring source

Spring Boot Reference

docs.spring.io/spring-boot/docs/current/reference/htmlsingle/

Boot-specific autoconfig, actuators

Spring Data JPA Reference

docs.spring.io/spring-data/jpa/reference/

Repository patterns, query derivation

Spring Security Reference

docs.spring.io/spring-security/reference/

Auth, filters, OAuth2

Hibernate ORM User Guide

docs.jboss.org/hibernate/orm/current/userguide/html_single/Hibernate_User_Guide.html

Deeper than Spring Data alone

JUnit 5 User Guide

junit.org/junit5/docs/current/user-guide/

Assertions, extensions, parameterized

Mockito docs

site.mockito.org

Mocking API

Micrometer docs

docs.micrometer.io

Metrics; the observability primitive

OpenTelemetry Java

opentelemetry.io/docs/languages/java/

Tracing, in 2026 the default

Apache Kafka docs

kafka.apache.org/documentation/

The bin/ scripts, KIPs

PostgreSQL 16 docs

postgresql.org/docs/16/

SQL truth

Bookmark these in a browser folder called “Java Refs” and pin the folder. When you’re stuck at 11 PM IST, muscle memory should take you here before Google.

Tier 2: Trusted Tutorial Sites

Site

Focus

Verdict

baeldung.com

Spring, Java how-to

#1 Google result for most Spring queries. Article quality varies wildly — verify with official docs. Older articles (pre-2022) sometimes wrong for Spring Boot 3.x.

reflectoring.io

Architecture, testing, hexagonal

Long-form and thoughtful. Author Tom Hombergs also wrote “Get Your Hands Dirty on Clean Architecture.”

thorben-janssen.com

Hibernate & JPA depth

The single best free Hibernate resource. Newsletter is worth subscribing to.

jenkov.com

Older Java fundamentals

Aging (2015-2020 vintage) but concept explanations still solid. Use for classic OOP/collections topics.

martinfowler.com

Architecture bliki

Slow-cadence, dense. PoEAA patterns explained.

arnoldgalovics.com

Java performance, Hibernate

Practical benchmark write-ups.

piotrminkowski.com

Spring Cloud, K8s

Working examples with GitHub repos.

stackabuse.com

General Java tutorials

Uneven, but good for niche how-tos.

howtodoinjava.com

Java + Spring how-to

India-based, high volume, sometimes shallow. Use as tie-breaker between Baeldung and official.

Tier 3: When Docs Aren’t Enough

Source

When to use

Stack Overflow

Google “[error message] site:stackoverflow.com”. Add year filter: after:2023.

GitHub issues

Search repo:spring-projects/spring-boot <error> or repo:openjdk/jdk. Often has better answers than SO for framework quirks.

JetBrains YouTrack

For IntelliJ bugs: youtrack.jetbrains.com/issues/IDEA

r/java, r/springboot

For “is this idiomatic” questions. Not for error-hunting.

Discord: Java official

Real-time help for weird bugs.

AI as Reference — The Rule

AI (Claude, ChatGPT, Gemini, Cursor) hallucinates method signatures, deprecated APIs, and non-existent Spring annotations with confidence. In Java, this is worse than in Python because Java is more precise — you catch AI lies at compile time, which wastes cycles.

Rule for months 1-8: Do NOT ask AI “how do I X in Java?” as your first move. Read the JavaDoc / Spring reference first. Ask AI only after you have a partial answer, to clarify. See 11_tools_setup/07_ai_agent_workflow_discipline.md.

The Docs You Do NOT Need to Read Cover-to-Cover

  • JLS / JVMS — dip in when a JEP references a section. Don’t read linearly.

  • Spring Framework Reference (2000+ pages) — read the chapter for the feature you’re using.

  • Hibernate User Guide (long) — same.

Reference docs are a random-access data structure. Treat them that way. The urge to “read the whole Spring docs” is procrastination.

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