Up to Java 9, JDK's code base used multiple repositories (there are eight repositories in Java 9—root, corba, hotspot, jaxp, jaxws, jdk, langtools, and nashorn). Consolidating the JDK forest aims to combine the multiple repositories used by the JDK into a single repository.

As JDK's code base grew over the years, it was stored in separate repositories on purpose, for a separation of concerns. However, as the JDK evolved, the code base also developed interdependencies across different repositories.

The advantages offered by these multiple repositories have outgrown the disadvantages with their maintenance. For interdependent changesets, you can't perform a single commit to a repository. There have been cases where the code for even a single (and simple) bug fix spanned multiple repositories. In such cases, the commit can't be performed atomically. A common approach is to use the same bug ID across multiple repositories. But this is not mandated, since using the same bug ID is not mandated, commits for same bug across different repositories could be made using different bug IDs. This can lead to difficulty in tracking bug fixes.

Also, the individual repositories don't have independent development tracks and release cycles. Java has one main release cycle, which includes changes in all of these repositories. Therefore, it was high time to integrate the JDK code base into one repository, to ease its maintenance.