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    <p>The XMLReader goes through a series of steps to identify which drive to use. Quoting the docs</p> <ul> <li>If the system property org.xml.sax.driver has a value, that is used as an XMLReader class name. </li> <li>The JAR "Services API" is used to look for a class name in the META-INF/services/org.xml.sax.driver file in jarfiles available to the runtime. </li> <li>SAX parser distributions are strongly encouraged to provide a default XMLReader class name that will take effect only when previous options (on this list) are not successful. </li> <li>Finally, if ParserFactory.makeParser() can return a system default SAX1 parser, that parser is wrapped in a ParserAdapter. (This is a migration aid for SAX1 environments, where the org.xml.sax.parser system property will often be usable.)</li> </ul> <p>Looking at the code for the <a href="https://github.com/amazonwebservices/aws-sdk-for-java/blob/master/src/main/java/com/amazonaws/services/s3/model/transform/XmlResponsesSaxParser.java" rel="noreferrer">AWS SDK</a> ...</p> <pre><code>public XmlResponsesSaxParser() throws AmazonClientException { // Ensure we can load the XML Reader. try { xr = XMLReaderFactory.createXMLReader(); } catch (SAXException e) { // oops, lets try doing this (needed in 1.4) System.setProperty("org.xml.sax.driver", "org.apache.crimson.parser.XMLReaderImpl"); try { // Try once more... xr = XMLReaderFactory.createXMLReader(); } catch (SAXException e2) { throw new AmazonClientException("Couldn't initialize a sax driver for the XMLReader"); } } } </code></pre> <p>There are a couple of things I don't like about that code.</p> <ol> <li>The root cause of SaxException e is eaten up.</li> <li>The root cause of SaxException e2 is also eaten up. The least the code should do is print a warning mentioning the root cause.</li> <li>Using System.setProperty() inside level framework code can cause some hard to debug issues. </li> </ol> <p>These points make it harder to debug the issue. The best educated guess I can make is that the crimson parser is accessible in one class loading path but absent in the other. A conclusive way to find the problem would be to set a breakpoint on the code that tries to instantiate the reader and find what the underlying root cause is.</p>
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    1. USDeepak Bala
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    1. COYour final paragraph is correct, but that doesn't help me, really. I've indeed debugged this, and can see that sometimes the calls to createXMLReader fail. But I don't know why it fails- especially why it fails and succeeds in the same place, depending on other wars which are loaded. As I wrote in the question- I have a fix which causes all my AmazonS3Clients to be initialized, but it doesn't feel like the best solution for this phenomena.
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