Trandtional Approach:  While build and deploy a monolithic web applications as we need to have Java runtime environment like JVM. On top of that, we should also install our web servers like Tomcat, Jetty, Netty, inside our server or at our local.

• After these two components ready, then we need to package our applications into war or ear format 
• Then we need to deploy them manually into a server.
• Wth the help of runtime environment like JVM, our web applications will work where an end user can use them.

 

Modern Approach: With spring boot framework, the middle component, i.e. the servers are no longer required.

• Because this server component right now is embedded into the JAR of your web application or of your microservice, which means whenever you trying to use spring boot to build my microservices, it is automatically going to package my application along with the embedded server.

 

So as a developer, In Modern Approach, we don't need to worry about all the server installations and maintaining of all these servers configurations.

These self-contained JARs also called as self-contained JARs fat JARs or uber JARs  because they will have everything packaged, including all the dependencies, all the business logic along with the embedded server.