  Main goal of the JUNO experiment is to determine the neutrino mass ordering
using a 20kt liquid-scintillator detector. Its key feature is an excellent
energy resolution of at least 3 % at 1 MeV, for which its instruments need to
meet a certain quality and thus have to be fully characterized. More than
20,000 20-inch PMTs have been received and assessed by JUNO after a detailed
testing program which began in 2017 and elapsed for about four years. Based on
this mass characterization and a set of specific requirements, a good quality
of all accepted PMTs could be ascertained. This paper presents the performed
testing procedure with the designed testing systems as well as the statistical
characteristics of all 20-inch PMTs intended to be used in the JUNO experiment,
covering more than fifteen performance parameters including the photocathode
uniformity. This constitutes the largest sample of 20-inch PMTs ever produced
and studied in detail to date, i.e. 15,000 of the newly developed 20-inch
MCP-PMTs from Northern Night Vision Technology Co. (NNVT) and 5,000 of dynode
PMTs from Hamamatsu Photonics K. K.(HPK).
