Revert "fix potential bug when userFields Map may be accessed by more than one thread."

This reverts commit b81c5c629bf9032e7481d6d5138b6ca92d3f27ce.
Revert "Revert "fix potential bug when userFields Map may be accessed by more than one thread.""

This reverts commit 19d7827cac136980ea89a15fcbc7db06e08d230d.