N.J. Stat. Ann. § 40A:9-115

The appointment of an undersheriff shall be by writing under the hand and seal of the sheriff. Every undersheriff, before he assumes his office, shall take and subscribe before a judge of the Superior Court, an oath that he will well and faithfully, impartially and justly execute the office of undersheriff, according to the best of his ability and judgment. His appointment, with the certificate of his oath indorsed thereon and attested by the judge, shall be filed in the office of the county clerk. Nothing in this section shall prevent the sheriff at his pleasure from removing an undersheriff.