debian安装的openjdk只包括jre,没有tools.jar,dt.jar，所以要安装openjdk完全版。
sudo apt-get install openjdk-8-jdk
若操作系统中安装多个版本的java，可以采用如下命令来选择具体的java:
sudo update-alternatives --config java
There are 2 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java 1061 auto mode
1 /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java 1061 manual mode
2 /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java 1051 manual mode
Press enter to keep the current choice[*], or type selection number:
配置maven,tomcat环境变量
