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this root hex is already configured in conf. here it is declared again. Should it be possible for this value declared in one place instead of two different files？

This is to ensure that the main-net node does not need to change the configuration file.

There is a varible method close and a method with the same name in this class, maybe you should use different names to distinguish them.

No changes for now.

In which case shall it throw an Exception?

Such as too many open files, not enough disk space, etc.

here it uses Double-Checked Locking to ensure that only one instance is created in a multi-threaded environment. However, this code suffers from concurrency issues. Double-Checked Locking is not always reliable in Java because it depends on the memory model and compiler optimizations.
A safer implementation of the singleton pattern uses static inner classes in a way that is thread-safe and efficient。
private static class MerkleTreeHolder { private static final MerkleTree instance = new MerkleTree(); } public static MerkleTree getInstance() { return MerkleTreeHolder.instance; }

This class has member variables and can not be used for concurrency, need to modify the usage scenarios while labeling them as not being able to be concurrent.