eclipse下开发web项目获取项目路径怎么得不到？

public class FileTest {
     public static void main(String[] args) throws Exception {       

  System.out.println(Thread.currentThread().getContextClassLoader().getResource(""));     

  System.out.println(FileTest.class.getClassLoader().getResource(""));        

　　System.out.println(ClassLoader.getSystemResource(""));        
   System.out.println(FileTest.class.getResource(""));        
   System.out.println(FileTest.class.getResource("/")); //Class文件所在路径  
   System.out.println(new File("/").getAbsolutePath());        
   System.out.println(System.getProperty("user.dir"));    
  }
 }