为什么会有java.sql.SQLException: No suitable driver的错误?

java.sql.SQLException: No suitable driver
at java.sql.DriverManager.getConnection(Unknown Source)
at java.sql.DriverManager.getConnection(Unknown Source)
at y2javaee.xmal1.common.DBConnection.getConnection(DBConnection.java:31)
at y2javaee.xmal1.operation.UserBo.validUser(UserBo.java:27)
at y2javaee.xmal1.servlet.LoginServlet.doPost(LoginServlet.java:38)
at y2javaee.xmal1.servlet.LoginServlet.doGet(LoginServlet.java:25)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Unknown Source)
nulljava.lang.NullPointerException
at y2javaee.xmal1.operation.UserBo.validUser(UserBo.java:28)
at y2javaee.xmal1.servlet.LoginServlet.doPost(LoginServlet.java:38)
at y2javaee.xmal1.servlet.LoginServlet.doGet(LoginServlet.java:25)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Unknown Source)
我已经把3个sqlserver的驱动拷到/web-inf/lib下,在jre/lib/ext下也拷进去了,开发工具是eclipse import java.sql.*;public class DBConnection {

private static Connection conn; private static final String DRIVER_CLASS = "com.microsoft.jdbc.sqlserver.SQLServerDriver"; private static final String DATABASE_URL = "jdbc:microsoft:sqlsever://localhost:1433;DatabaseName=addressBook"; private static final String DATABASE_USRE = "sa"; private static final String DATABASE_PASSWORD = "sa";
public static Connection getConnection() { try {
Class.forName(DRIVER_CLASS);
conn = DriverManager.getConnection(DATABASE_URL,DATABASE_USRE,DATABASE_PASSWORD); } catch (Exception e) {
e.printStackTrace(); System.out.print(conn);
} return conn;
}
public static void closeConnection(){
try{
if(conn!=null)
conn.close();
}catch(Exception e){
e.printStackTrace();
}
}

public static void closeStatement(Statement stm){
try{
if(stm!=null)
stm.close();
}catch(Exception e){
e.printStackTrace();
}
}

public static void closeStatement(PreparedStatement ps){
try{
if(ps!=null)
ps.close();
}catch(Exception e){
e.printStackTrace();
}
}

public static void closeResultSet(ResultSet rs){
try{
if(rs!=null)
rs.close();
}catch(Exception e){
e.printStackTrace();
}
}
}