What already transfers
More than you would guess. Data engineering is software engineering applied to data, and the habits that make you a decent Java developer — testing, version control, reading someone else’s code, thinking about failure — are the habits that separate a data engineer from someone who writes notebooks.
Spark itself runs on the JVM. When a job fails with a memory error, you have a head start on everyone who arrived through analytics.
What you actually need to add
Distributed processing. Spark’s model — partitions, shuffles, lazy evaluation — is genuinely new. This is where most of your learning time goes.
Data modelling. Star schemas, slowly changing dimensions, and the question of what grain a table is at. Application developers rarely think this way.
Orchestration. Airflow or an equivalent: dependencies, retries, backfills, and what happens when yesterday’s run failed and nobody noticed.
SQL beyond CRUD. Window functions, query plans, and knowing why a query that worked on ten thousand rows dies on ten million.
A realistic timeline
Six to nine months while working full time. The first two months are SQL and Spark fundamentals, the middle stretch is projects, and the last two months are interview preparation running in parallel with applications.
Where people get stuck
Two places. The first is treating Spark like a faster pandas — it is not, and that misunderstanding shows up immediately in interviews. The second is having no portfolio project that involves failure handling, because interviewers ask what happens when a pipeline breaks at 3am, and “it hasn’t” is not an answer.