com.microsoft.sqlserver.jdbc.SQLServerException: 到主机 的 TCP/IP 连接失败。

package sqlserver2005;

import java.sql.Connection;
import java.sql.DriverManager;

public class TestJDBCTOSQLSERVER2005 {
private final String DBDRIVER="com.microsoft.sqlserver.jdbc.SQLServerDriver";
private final String DBURL="jdbc:sqlserver://localhost:1433;DatabaseName=bbs;";
private final String DBUSER="sa";
private final String DBPASSWORD="sa";
private Connection myconn=null;

public TestJDBCTOSQLSERVER2005()
{
try
{
Class.forName(DBDRIVER);
myconn=DriverManager.getConnection(DBURL, DBUSER, DBPASSWORD);
}
catch(Exception e)
{
System.out.print(e);
}
}
//取得数据库链接
public Connection getConnection()
{
return myconn;
}
//关闭数据库
public void close()
{
try
{
myconn.close();
}
catch(Exception e)
{

}
}

public static void main(String [] args){

System.out.println("The main is ok");
TestJDBCTOSQLSERVER2005 tt=new TestJDBCTOSQLSERVER2005();
System.out.println("The main is down");
}

}