JDK 对应数字版本号
J2SE 7 = 51 (0x33 hex),J2SE 6.0 = 50 (0x32 hex),J2SE 5.0 = 49 (0x31 hex),JDK 1.4 = 48 (0x30 hex),JDK 1.3 = 47 (0x2F hex),JDK 1.2 = 46 (0x2E hex),JDK 1.1 = 45 (0x2D hex).
