Spring is, by far, the most popular framework for application development in the Java ecosystem. By a wide margin. The reason is actually quite simple - it does a lot of things right, and it's getting better and better with each release.
There aren't a lot of technologies with the kind of wide reach, stability and maturity of Spring, that still move fast and innovate. That's a really tough balance to hit, and Spring has been hitting it for over a decade.
Today, Spring, along with Boot, is a polished, modern and highly expressive framework, easily passing the Twitter test. That's where you can actually fit useful functionality into a single tweet :)
So, the popularity of the Spring ecosystem is well deserved. If you're working in Java, you're very likely doing work with Spring.
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