One of the easiest and oldest ways to run a Java program which depends on an external library or framework is to download dependency JAR files, put them on the classpath and then run the program by creating a Main class with the main() method. This is simple but not as easy as you think, there are many challenges down the road e.g. you need to find the right version of JAR files and their dependencies e.g. Spring might have a dependency on other third-party libraries like Log4j. So, when the build tool like Maven and Gradle comes, everybody stopped downloading the JAR file manually.