1、在开启java服务器程序时候，web.xml中一直有各种各样错误，查找解决方案好久，都没有靠谱的，头大。忽然看到一条，司马当成活马医：http://*.com/questions/13437569/what-does-this-web-xml-error-mean，其中一条解决方案如下：
replace
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_5.xsd">
with
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee;http://java.sun.com/xml/ns/j2ee/web-app_2_5.xsd">
The solution is that you have to put semicolon between URLs
I am sure you will not get the error again :)
很好，try it and all errors gone. solved perfectly.
2、
Caused by: java.lang.IllegalStateException: Target host must not be null, or set in parameters.
报错的原因是因为没有在加url中加“http://”，加上http://就是解决方案。
3、
org.apache.http.conn.HttpHostConnectException: Connection to http://localhost:8080 refused
or Caused by: java.net.ConnectException: failed to connect to /192.168.1.108 (port 8080): connect failed: ENETUNREACH (Network is unreachable)
or Caused by: java.net.ConnectException: failed to connect to /127.0.0.1 (port 8080): connect failed: ECONNREFUSED (Connection refused)
这个问题可能原因有（1）Android平台的网络没开（我就死在这半天）（2）Manifest.xml的internet权限没有添加。（3）系统防火墙没有关闭。（4）url书写错误。
4、eclipese连接MySQL出错，出错位置在“Class.forName("com.mysql.jdbc.Driver")”
解决方法为：将mysql-connector-java-5.0.3-bin.jar导入eclipse中就可以了。