Exception in thread "main" java.lang.NullPointerException at java.awt.Container.addImpl(Unknown Source) at java.awt.Container.add(Unknown Source) at javax.swing.JFrame.addImpl(Unknown Source) at java.awt.Container.add(Unknown Source) at Renwu.Login.<init>(Login.java:38) at Renwu.Login.main(Login.java:45) 大哥,这是啥意思,代码没出问题,mysql安装也没问题。