java中ImageIO.write（）参数含义

请问ImageIO.write(image, "bmg", filWriterOne);括号里的三个参数分别代表说明意思？

image：图片对象
"bmg" ：图片格式
, filWriterOne 需要写入的地方
源码中有三个方法

  /**
     * Writes an image using the an arbitrary <code>ImageWriter</code>
     * that supports the given format to an
     * <code>ImageOutputStream</code>.  The image is written to the
     * <code>ImageOutputStream</code> starting at the current stream
     * pointer, overwriting existing stream data from that point
     * forward, if present.
     *
     * <p> This method <em>does not</em> close the provided
     * <code>ImageOutputStream</code> after the write operation has completed;
     * it is the responsibility of the caller to close the stream, if desired.
     *
     * @param im a <code>RenderedImage</code> to be written.
     * @param formatName a <code>String</code> containing the informal
     * name of the format.
     * @param output an <code>ImageOutputStream</code> to be written to.
     *
     * @return <code>false</code> if no appropriate writer is found.
     *
     * @exception IllegalArgumentException if any parameter is
     * <code>null</code>.
     * @exception IOException if an error occurs during writing.
     */
    public static boolean write(RenderedImage im,
                                String formatName,
                                ImageOutputStream output) throws IOException {
        if (im == null) {
            throw new IllegalArgumentException("im == null!");
        }
        if (formatName == null) {
            throw new IllegalArgumentException("formatName == null!");
        }
        if (output == null) {
            throw new IllegalArgumentException("output == null!");
        }

        return doWrite(im, getWriter(im, formatName), output);
    }

    /**
     * Writes an image using an arbitrary <code>ImageWriter</code>
     * that supports the given format to a <code>File</code>.  If
     * there is already a <code>File</code> present, its contents are
     * discarded.
     *
     * @param im a <code>RenderedImage</code> to be written.
     * @param formatName a <code>String</code> containing the informal
     * name of the format.
     * @param output a <code>File</code> to be written to.
     *
     * @return <code>false</code> if no appropriate writer is found.
     *
     * @exception IllegalArgumentException if any parameter is
     * <code>null</code>.
     * @exception IOException if an error occurs during writing.
     */
    public static boolean write(RenderedImage im,
                                String formatName,
                                File output) throws IOException {
        if (output == null) {
            throw new IllegalArgumentException("output == null!");
        }
        ImageOutputStream stream = null;

        ImageWriter writer = getWriter(im, formatName);
        if (writer == null) {
            /* Do not make changes in the file system if we have
             * no appropriate writer.
             */
            return false;
        }

        try {
            output.delete();
            stream = createImageOutputStream(output);
        } catch (IOException e) {
            throw new IIOException("Can't create output stream!", e);
        }

        try {
            return doWrite(im, writer, stream);
        } finally {
            stream.close();
        }
    }

    /**
     * Writes an image using an arbitrary <code>ImageWriter</code>
     * that supports the given format to an <code>OutputStream</code>.
     *
     * <p> This method <em>does not</em> close the provided
     * <code>OutputStream</code> after the write operation has completed;
     * it is the responsibility of the caller to close the stream, if desired.
     *
     * <p> The current cache settings from <code>getUseCache</code>and
     * <code>getCacheDirectory</code> will be used to control caching.
     *
     * @param im a <code>RenderedImage</code> to be written.
     * @param formatName a <code>String</code> containing the informal
     * name of the format.
     * @param output an <code>OutputStream</code> to be written to.
     *
     * @return <code>false</code> if no appropriate writer is found.
     *
     * @exception IllegalArgumentException if any parameter is
     * <code>null</code>.
     * @exception IOException if an error occurs during writing.
     */
    public static boolean write(RenderedImage im,
                                String formatName,
                                OutputStream output) throws IOException {
        if (output == null) {
            throw new IllegalArgumentException("output == null!");
        }
        ImageOutputStream stream = null;
        try {
            stream = createImageOutputStream(output);
        } catch (IOException e) {
            throw new IIOException("Can't create output stream!", e);
        }

        try {
            return doWrite(im, getWriter(im, formatName), stream);
        } finally {
            stream.close();
        }
    }