The project modules are specified as modules of the build system. Whether we can follow the straightforward way of having a single project or multiple projects; for example, motivated by system tests, we will build and execute them as part of the build process.

A good build system needs to ship certain features. The main task of it is to compile the sources and package the binaries as artifacts. Required dependencies are also resolved and used for compilation or packaged, respectively. There are several scopes where dependencies are required, such as during compilation, testing, or runtime. Different scope definition specify whether dependencies are shipped with the artifact.

The project should be built in a reliable, reproducible way. Several builds with identical project contents and build configuration must produce the same results. This is important for implementing Continuous Delivery (CD) pipelines, which enable reproducible builds. That said the build system must be able to run on a Continuous Integration (CI) server, such as Jenkins or TeamCity. This requires the software to ship a command-line interface, especially for Unix-based systems. Chapter 6, Application Development Workflows, will show the motivations behind Continuous Delivery.

The build system will be used by software engineers working on various environments and operating systems, which should be supported as well. For JVM-based build systems this portability is usually given. It may be the case that projects have specific requirements such as native code that needs to be built on specific environments. For Java enterprise applications, however, this is usually not the case.

In general the build process should run as fast as possible. Booting up and configuring the build system should not require much time. The longer the build takes the higher the turnaround times and the slower the feedback engineers get from the build pipeline. In Chapter 4, Lightweight Java EE, we will cover more of this topic.

At the time of writing, Apache Maven is the most used build system well known to the majority of Java developers.

Maven is a Java-based build system configured by XML. It's projects are defined by a so-called project object model (POM). Maven makes use of a convention over configuration approach that minimizes the required configuration. The default configuration is well suited for Java applications.

Another build tool with high usage is Gradle. Gradle is a build tool that offers a Groovy-based Domain-Specific Language (DSL) to configure fully extensible and scriptable project builds. Since Groovy is a full programming language Gradle build scripts are naturally powerful and flexible.

Both Gradle and Maven include a sophisticated dependency management and are well suited to build Java-based projects. There are certainly still other build systems, such as SBT, however, Gradle and Maven are, by far, the most-used ones and will be covered in the following section.