Why do all of the leading open source Java libraries have circular dependencies between their packages?

Question

Over the past few weeks, I have been studying Java package structure and dependency patterns. One common thread in the scriptures on this subject is a simple rule, according to which the package dependencies must constitute a directed acyclic graph (DAG). Author Robert Martin even formalized the Principle of Acyclic Dependencies (ADP), which states

The dependency structure between packages must be a directed acyclic graph (DAG). That is, there should not be cycles in the dependency structure.

Several Java libraries adhere to this simple rule. Namely, the Spring Framework (spring -core, spring -web, etc.) and Google Guava .

However, to my surprise, most of the leading open source Java projects do not !

The following open source projects have circular dependencies between packages:

• Netflix Hystrix (each package is part of a cycle!)
• AWS SDK
• Commons-lang
• Commons Collection
• Dagger
• Google gson
• Google guice
• Hibernate orm
• Hibernate validator
• Jackson core
• Jod time
• Play framework
• Junit
• Logback
• Jetty
• Aspectj
• Nettie
• java.util
• java.lang

Am I misunderstood the principle of software development? Or can developers opt out of this package organization?

References:

• Spring -core 4.2.3 [ GraphML ] [ JPG ]
• Google Guava [ GraphML ] [ JPG ]
• Hystrix [ GraphML ] [ JPG ]
• java.util [ GraphML ] [ JPG ]

Answer 1

I can confirm that after analyzing many java projects using JArchitect, many of them contain circular dependencies between packages, the reason is that many of them choose a "package by function" rather than a "batch" approach.

Here's a good article talking about the difference between the two approaches.

Take some packages from the JDK as an example.

These packages are developed using a function, the regular expression function is grouped in the java.util.regex package, which requires some security functions from the java.security package, and the security classes also need some regular expressions.