IO输入流中利用BufferedReader（缓存读取）以及BufferedWriter（缓存写入）进行文本复制 的问题

以下红色部分的问题能否有前辈可以指点下问题原因呢？非常感谢~~
@Test
    public void testBufferedReader() {
        BufferedReader br = null;
        BufferedWriter wr = null;
        try {
            File file = new File("file.txt");
            File file3 = new File("file3.txt");
            FileReader fr = new FileReader(file);
            FileWriter fw = new FileWriter(file3);
            br = new BufferedReader(fr);
            wr = new BufferedWriter(fw);
           

            String str;
            while((str = br.readLine()) != null) {
                wr.write(str);
                /*wr.flush();*///问题点：当我不添加wr.flush();语句时，程序就不能把file.txt中的内容复制到file3.txt当中；
            }
           

        } catch (IOException e) {
            e.printStackTrace();
        }finally {
      

        try {
            br.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
        }
    }