Backward compatibility has always been a major goal for Java. Usually, migration to a new Java version is mostly trivial from the developer perspective. The module system and the modularized JDK arguably represent the biggest change to the whole Java platform since its inception. Even so, backward compatibility is a top priority.
Migrating an existing application to Java 9 is best approached as a two-step process. This chapter focuses on migrating existing code to build and run on Java 9, without migrating code to modules. The next chapter dives into migrating code to modules, providing strategies to accomplish this.
Why migrate to Java 9 when you don’t anticipate using its flagship feature, the module system? An upgrade to Java 9 also gives access to the other features that are part of Java 9. Think of new APIs, tools, and performance improvements.
Whether you go all the way to modules or leave it at the first step depends. Is the application likely to see lots of extensions and new features? In that case, reaping the benefits of modularity may justify the cost of taking the second step. When an application is in maintenance mode and only has to run on Java 9, it makes sense to only take the first step, as described in this chapter.
For library maintainers, the question isn’t if Java 9 support is necessary, but when. Migrating a library to Java 9 and modules raises different concerns than migrating applications. In Chapter 10, we address ...