A JAL Japan Airlines Boeing 787-800, registration JA821J performing flight JL-21 from Tokyo Haneda (Japan) to Beijing (China) with 169 people on board, was enroute at FL400 about 150nm west of Tokyo when the captain's windshield cracked prompting the crew to descend to FL310 and return to Tokyo Haneda for a safe landing about 55 minutes after leaving FL400. A replacement Boeing 787-800 registration JA831J reaced Beijing with a delay of 4:15 hours.