Why is Java 8's `finalize()` method being invoked on a seemingly reachable object?

Understanding Finalize() Invocations in Java 8

In Java, the finalize() method is a legacy garbage collection hook that is rarely used in modern programming practices. However, in some cases, it can still be called unexpectedly, leading to confusion or even exceptions.

Unreachable Objects and Garbage Collection

In the context of the issue described, it is important to understand the concept of object reachability and garbage collection. Garbage collection is a process in Java that identifies and reclaims memory occupied by objects that are no longer reachable from any reference in the running program.

An object is reachable if it is referenced by any variable or object still in use. Conversely, an unreachable object is no longer pointed to by any active references. Normally, the JVM waits for an object to become unreachable before invoking its finalize() method.

Unusual Circumstances in Java 8

The issue described suggests that MIMEBodyPart's finalize() method is being called while the object is still reachable from the stack. It is unusual for such premature finalization to occur while an active instance method call is in progress.

Possible Explanation

One possible explanation is that the MIMEBodyPart object is not actually reachable from the stack in a way that Java's garbage collector considers. Even though it is referenced in a local variable, it may not be strongly reachable, meaning that there is no clear path of references from any live root object to the MIMEBodyPart.

Strengthening Reachability

To prevent unexpected finalization, it is recommended to ensure that strongly reachable references to objects remain intact throughout their intended use. In this case, one possible modification would be to store the MIMEBodyPart in a field of the parent object, which would make it strongly reachable.

Conclusion

Understanding the nuances of garbage collection and finalization is essential when dealing with complex Java code. The behavior described in this issue highlights the potential unintended consequences of using finalize() and the importance of maintaining clear reachability paths to objects.

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