Why Are My Strongly Reachable Java 8 Objects Being Finalized Prematurely?

"Finalize() Called on Strongly Reachable Objects in Java 8"

Issue:

An application developed with Java 7 and recently upgraded to Java 8 experiences occasional exceptions indicating that a stream has been closed prematurely. Investigations reveal that the finalizer thread is prematurely invoking finalize() on an object holding the stream, which triggers the closure.

Background:

The code structure involves a MIME writer (MIMEWriter), a MIME body part (MIMEBodyPart), and an input stream (InflaterInputStream) representing an attached file. MIMEBodyPart extends HTTPMessage, which includes a close() method that closes the underlying stream. Additionally, HTTPMessage has a finalize() method that attempts to call close() on the stream if still open.

Sequence of Events:

1. MIMEWriter writes headers for the attachment part.
2. MIMEBodyPart writes the body content using IOUtil.copy, which copies chunks from the input stream to the output stream.
3. IOUtil.copy attempts to read a chunk but encounters the closed stream, triggering the exception.

Cause:

The MIMEBodyPart.finalize() method is invoked prematurely by the finalizer thread while IOUtil.copy is actively running. Java 8 introduced garbage collection optimizations that allow objects to be finalized even if they are still referenced by local variables or active method calls.

The MIMEBodyPart object is indeed reachable from the stack frame of MIMEBodyPart.writeBodyPartContent, implying that the JVM should not attempt to finalize it. However, since the reference to MIMEBodyPart within the IOUtil.copy loop is not actively used, it becomes unreachable and eligible for garbage collection and finalization.

Consequences:

The premature finalization may lead to incorrect behavior and potential data loss.

Solution:

The recommended approach is to revisit the home-grown library and eliminate the use of finalize() methods. Since Java Mail's MIME library did not exhibit the issue, it could serve as an alternative.

Alternative Conjecture:

Another possible explanation involves the InflaterInputStream. If the MIMEBodyPart.finalize() method is invoked during a non-interruptible operation within InflaterInputStream, it could disrupt the stream and trigger the exception. However, this hypothesis requires further investigation.

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