Why are Strongly Reachable Java Objects Finalized in Java 8?

Finalization of Strongly Reachable Objects in Java 8

Java 7 to Java 8 upgrade may occasionally result in an exception indicating stream closure during read operations. This is caused by the finalization thread calling finalize() on the object holding the stream, subsequently closing it.

Considering the provided code, the finalize() method in HTTPMessage, extended by MIMEBodyPart, invokes close() which closes the associated stream. During execution of MIMEWriter.writePart(), IOUtils.copy reads chunks from the input stream until depletion.

The issue arises when HTTPMessage.finalize() is executed while IOUtils.copy is running, leading to the Stream closed exception. While the MIMEBodyPart object is reachable from the stack frame, theJVM calls finalize(). This behavior is perplexing.

Potential Explanation

Even with references to objects in local stack variables and active method calls, objects can be finalized and garbage collected if deemed "unreachable." For an object to be unreachable, no subsequent code interacts with its reference.

In this specific case, the MimeBodyPart object may be stored in a local variable. Since no subsequent code in IOUtils.copy references the MimeBodyPart object, the JVM might deem it unreachable, triggering its finalization.

Examples and Code Modifications

An example is provided to demonstrate how objects can be finalized during active method calls, even with references on the stack.

Additionally, by modifying the code to hold a reference to the FinalizeThis object in a field instead of a local variable, finalization is observed.

Conclusion

It's recommended to avoid using finalize(), as best practices dictate. However, it's possible for Java 8 to finalize reachable objects under certain circumstances. This should be considered when reviewing code and troubleshooting exceptions related to object finalization.

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