Why Are finalize() Calls Unexpectedly Triggering Stream Closures in Java 8?

Understanding Finalize() Invocation in Java 8

In the transition from Java 7 to Java 8, message processing applications have occasionally encountered an exception related to stream closure. Investigation has revealed unexpected calls to finalize() on objects holding streams, leading to their closure during active read operations.

Initially puzzling, this behavior can be attributed to a poorly understood aspect of garbage collection. Even with references to an object present on the stack and during active method calls, an object can still be deemed unreachable. This happens when no subsequent code accesses its reference, rendering it "dead."

As seen in a simplified example, even during active method execution, an object can be finalized and garbage collected:

class FinalizeThis {
    @Override
    protected void finalize() {
        System.out.println("finalized!");
    }

    void loop() {
        for (int i = 0; i < 1,000,000,000; i++) {
            // Triggering GC with System.gc() doesn't guarantee garbage collection
        }
    }

    public static void main(String[] args) {
        new FinalizeThis().loop();
    }
}

In this example, the FinalizeThis object's reference remains in scope, yet it is unreachable due to the absence of further interaction with it. Consequently, it becomes eligible for finalization and subsequent garbage collection.

In the reported case, the MIMEBodyPart object may have been stored in a local variable with an m_ prefix, fulfilling the condition for potential unreachability. By modifying the object's storage location, as suggested in the comments, the finalization behavior ceased.

It is worth noting that this finalization issue can be circumvented by introducing a compilation directive. The -Xcomp flag forces method compilation before execution, allowing the compiler to perform reachability analysis and avoid unwarranted finalization calls.

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