Reason for writing online:
1. Problem analysis: Openjdk7 was installed at the beginning, and jdk1.6 was installed later. It is likely that the system is incompatible with jdk1.6.0 and jdk1.7.
Solution: uninstall jdk1.6.0, that is, delete rm -rf jdk1.6.0_45/ and remove the environment variable, the compilation is normal.
2.
If your environment variable CLASSPATH is set, the javap command would try to look in the CLASSPATH instead of the current dir. Thus no class is found.
You may try to unset CLASSPATH and build again.（帖子链接：https://groups.google.com/forum/#!msg/android-building/HoNt0qixSfI/65fETnZPw_IJ）（需要梯子才打开该链接）
Because there is only one openjdk in my computer, it is not the first case. So unset CLASSPATH before compiling is my workaround.