onlyoffice 在线预览的java示例代码有吗？

是的，OnlyOffice官方提供了Java示例代码。您可以参考以下示例代码实现在线预览：

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;

public class OnlyOfficeOnlinePreview {
    private static final String DOCUMENT_SERVER_URL = "http://documentserver/";
    private static final String DOCUMENT_KEY = "document-key";

    public static void main(String[] args) {
        String url = DOCUMENT_SERVER_URL + "/ConvertService.ashx" + "?url=" + DOCUMENT_SERVER_URL + "/WebEditors/" + DOCUMENT_KEY + "/doc.docx" + "&outputtype=pdf";

        try {
            URL obj = new URL(url);
            HttpURLConnection con = (HttpURLConnection) obj.openConnection();

            con.setRequestMethod("GET");

            BufferedReader in = new BufferedReader(new InputStreamReader(con.getInputStream()));
            String inputLine;
            StringBuilder response = new StringBuilder();

            while ((inputLine = in.readLine()) != null) {
                response.append(inputLine);
            }

            in.close();

            System.out.println(response.toString());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

其中，`DOCUMENT_SERVER_URL`是OnlyOffice Document Server的URL，`DOCUMENT_KEY`是文档的唯一标识符。在示例代码中，我们使用了HTTP GET请求获取PDF格式的预览文件。您可以根据需要调整代码实现其他功能。