import jdk.internal.util.xml.XMLStreamWriter; //导入依赖的package包/类
public XMLStreamWriterImpl(OutputStream os, String encoding)
    throws XMLStreamException
{
    Charset cs = null;
    if (encoding == null) {
        _encoding = XMLStreamWriter.DEFAULT_ENCODING;
    } else {
        try {
            cs = getCharset(encoding);
        } catch (UnsupportedEncodingException e) {
            throw new XMLStreamException(e);
        }

        this._encoding = encoding;
    }

    _writer = new XMLWriter(os, encoding, cs);
}
 

import jdk.internal.util.xml.XMLStreamWriter; //导入依赖的package包/类
/**
 * Write the XML Declaration. Note that the encoding parameter does not set
 * the actual encoding of the underlying output. That must be set when the
 * instance of the XMLStreamWriter is created
 *
 * @param encoding encoding of the xml declaration
 * @param version version of the xml document
 * @param standalone indicate if the xml document is standalone
 * @throws XMLStreamException If given encoding does not match encoding of the
 * underlying stream
 */
public void writeStartDocument(String encoding, String version, String standalone)
    throws XMLStreamException
{
    if (_state > 0) {
        throw new XMLStreamException("XML declaration must be as the first line in the XML document.");
    }
    _state = STATE_XML_DECL;
    String enc = encoding;
    if (enc == null) {
        enc = _encoding;
    } else {
        //check if the encoding is supported
        try {
            getCharset(encoding);
        } catch (UnsupportedEncodingException e) {
            throw new XMLStreamException(e);
        }
    }

    if (version == null) {
        version = XMLStreamWriter.DEFAULT_XML_VERSION;
    }

    _writer.write("<?xml version=\"");
    _writer.write(version);
    _writer.write(DOUBLEQUOT);

    if (enc != null) {
        _writer.write(" encoding=\"");
        _writer.write(enc);
        _writer.write(DOUBLEQUOT);
    }

    if (standalone != null) {
        _writer.write(" standalone=\"");
        _writer.write(standalone);
        _writer.write(DOUBLEQUOT);
    }
    _writer.write("?>");
    writeLineSeparator();
}