DocumentBuilder.parse似乎随机地从HTTP请求中跳过了返回的XML InputStream的开头

Question

I have the following code to send a HTTP request, receive the response (which is in the form of an XML) and parse it:

public Document getDocumentElementFromDatabase() {
    // this URL is actually built dynamically from a query, but for this example I just use one of the possible resulting URLs
    String url = "http://musicbrainz.org/ws/2/recording?query=%22Thunderstruck%22+AND+artistname%3A%222Cellos%22";

    try {
        // sleep between successive requests to avoid flooding the server
        Thread.sleep(1000);
        HttpURLConnection connection = runQuery(url);
        InputStream stream = connection.getInputStream();
        if (stream != null) {
            BufferedInputStream buff = new BufferedInputStream(stream);
            return DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(buff);
        }
    }

    // I've grouped exception handling for this example
    catch (ParserConfigurationException | InterruptedException | SAXException | IOException e) {
        e.printStackTrace();
    }

    finally {
        if (connection != null) connection.disconnect();
    }

    return null;
}

private void runQuery(String url) throws MalformedURLException, IOException {
    HttpURLConnection connection = (HttpURLConnection) new URL(url).openConnection();
    connection.setRequestProperty("User-Agent", "MyAppName/1.0 ( myemail@email.email )");
    return connection;
}

This code gets called multiple times and sometimes I get the following error:

[Fatal Error] :1:1: Content is not allowed in prolog.

org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; Content is not allowed in prolog.

at com.sun.org.apache.xerces.internal.parsers.DOMParser.parse(Unknown Source)

at com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderImpl.parse(Unknown Source)

at javax.xml.parsers.DocumentBuilder.parse(Unknown Source)

...

If I try to access the URL in say Chrome, I get a valid XML response every time, no matter how many times I reload. What's more, this same issue does not seem to appear when I run the exact same code on my laptop.

After a bit of tinkering, I tried printing the InputStream s directly as strings (using method 4 from this link ), rather than parsing them, and I noticed that sometimes the response in fact did not have the expected XML header ( <?xml version="1.0" encoding="UTF-8" standalone="yes"?> ), but other times it did.

My guess is I'm doing something wrong with the streams, but I can't figure out what.

Answer 1

I have found the problem. The site seemed to sometimes return a JSON response instead of an XML, which caused the parser to freak out. I've added the following line to runQuery :

connection.setRequestProperty("Accept", "application/xml");

and I can now successfully run the code without errors.