1. Prerequisites

Java
Gradle or Maven
SQL

2.General Development Skills

Learn GIT, create a few repositories on GitHub, share your code with other people
Know HTTP(S) protocol, request methods (GET, POST, PUT, PATCH, DELETE, OPTIONS)
Learn about implementation of a basic Authentication
Solid principles, etc

3.CLI Tools

args4j
JCommander
airline

4.Web Frameworks + Routers

Spring
Play Framework
Spark
Jersey
nanohttpd

5.Databases

-Relational
SQL Server
PostgreSQL
MariaDB
MySQL
Oracle

-Cloud Databases
CosmosDB
DynamoDB

-Search Engines
ElasticSearch
Solr
Sphinx

-NoSQL
MongoDB
Redis
Apache Cassandra
LiteDB
RavenDB
CouchDB

6.ORMs

Hibernate
Ebean
BelleORM

7.Caching

Caffeine
EHCache
Cache2k
Distributed Cache
-Java-Redis
-Java-Memcached
-Infinispan

8.Logging

-Log Frameworks
Zap
TinyLog
log4j

-Log Management System
ELK Stack
Sentry.io
Loggly.com
Tracer

9.Real-Time Communication

Socket.IO
atmosphere
webbit

10.API Clients

-REST
okhttp
retrofit
-GraphQL

11.Good to Know

Beanvalidation
bouncycastle
gson
Apache Shiro
JJWT
RxJava

12.Testing

-Unit, Behavior, Integration, Load Testing
JUnit
CitrusFramework
Gatling
Tsung
Mockito
Truth
Assertj
cucumber-jvm
cukes
hoverfly-java
rest-assured
jbehave
Mockserver

-E2E Testing
Selenium

13.Task Scheduling

Aurora
elasticjob
Sundial
cron-utils

14.MicroServices

-Message-Broker
RabbitMQ
Apache Kafka
ActiveMQ
Azure Service Bus

-Message-Bus
mbassador
rmq

-Frameworks
Apollo
lagom-framework
micronaut
eureka
helidon
armeria

-RPC
Protocol Buffers
gRPC-Java
thrift