setProperty()为何不能往指定文件中写东西呢？！

Question

try{
      java.io.File file = new java.io.File( new java.io.File(".").
                                               getCanonicalPath() +
                                               "\\branch.properties" );
   
         java.util.Properties pro = new java.util.Properties();
         java.io.FileInputStream in = new java.io.FileInputStream(file);
         pro.load(in);
 in.close();
         pro.setProperty("a","aaaa ok");
         pro.setProperty("b","bbbbb ok");         java.io.FileOutputStream out = new java.io.FileOutputStream(file);
         pro.save(out,"add by lza ");
         out.close();   
    }catch(Exception e){}