客户端与服务器之间通信收不到信息——readLine()

写服务器端和客户端之间通信，结果一直读取不到信息，在https://blog.csdn.net/yiluxiangqian7715/article/details/50173573

上找到了原因：使用BufferedReader的readLine()方法来读取信息，但是没有遇见行结束符“\n”，“\r”或“\r\n”还是会继续读取（即阻塞了），所以要想读取到信息，只需要在写入的时候加上行结束符。

BufferedReader的readLine方法源码如下：

    /**
     * Reads a line of text.  A line is considered to be terminated by any one
     * of a line feed ('\n'), a carriage return ('\r'), or a carriage return
     * followed immediately by a linefeed.
     *
     * @return     A String containing the contents of the line, not including
     *             any line-termination characters, or null if the end of the
     *             stream has been reached
     *
     * @exception  IOException  If an I/O error occurs
     *
     * @see java.nio.file.Files#readAllLines
     */
    public String readLine() throws IOException {
        return readLine(false);
    }

类似的还有RandomAccessFile的readLine():

    /**
     * Reads the next line of text from this file.  This method successively
     * reads bytes from the file, starting at the current file pointer,
     * until it reaches a line terminator or the end
     * of the file.  Each byte is converted into a character by taking the
     * byte's value for the lower eight bits of the character and setting the
     * high eight bits of the character to zero.  This method does not,
     * therefore, support the full Unicode character set.
     *
     * <p> A line of text is terminated by a carriage-return character
     * ({@code '\u005Cr'}), a newline character ({@code '\u005Cn'}), a
     * carriage-return character immediately followed by a newline character,
     * or the end of the file.  Line-terminating characters are discarded and
     * are not included as part of the string returned.
     *
     * <p> This method blocks until a newline character is read, a carriage
     * return and the byte following it are read (to see if it is a newline),
     * the end of the file is reached, or an exception is thrown.
     *
     * @return     the next line of text from this file, or null if end
     *             of file is encountered before even one byte is read.
     * @exception  IOException  if an I/O error occurs.
     */

    public final String readLine() throws IOException {
        StringBuffer input = new StringBuffer();
        int c = -1;
        boolean eol = false;

        while (!eol) {
            switch (c = read()) {
            case -1:
            case '\n':
                eol = true;
                break;
            case '\r':
                eol = true;
                long cur = getFilePointer();
                if ((read()) != '\n') {
                    seek(cur);
                }
                break;
            default:
                input.append((char)c);
                break;
            }
        }

        if ((c == -1) && (input.length() == 0)) {
            return null;
        }
        return input.toString();
    }