DescriptorReader.java

package com.ctp.cdi.query.meta.unit;

import static java.lang.Thread.currentThread;

import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.util.Collections;
import java.util.Enumeration;
import java.util.LinkedList;
import java.util.List;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;

import org.xml.sax.InputSource;
import org.xml.sax.SAXException;

abstract class DescriptorReader {
    
    private final DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    
    List<Descriptor> readAllFromClassPath(String resource) throws IOException {
        List<Descriptor> result = new LinkedList<Descriptor>();
        Enumeration<URL> urls = classLoader().getResources(resource);
        while (urls.hasMoreElements()) {
            URL u = urls.nextElement();
            result.add(readFromUrl(u));
        }
        return Collections.unmodifiableList(result);
    }
    
    Descriptor readFromClassPath(String resource) throws IOException {
        return readFromUrl(classLoader().getResource(resource));
    }
    
    Descriptor readFromUrl(URL url) throws IOException {
        if (!exists(url)) {
            throw new IllegalArgumentException("URL does not exist: " + url);
        }
        InputStream stream = url.openStream();
        try {
            DocumentBuilder builder = factory.newDocumentBuilder();
            return new Descriptor(builder.parse(new InputSource(stream)), url);
        } catch (SAXException e) {
            throw new RuntimeException("Failed reading XML document", e);
        } catch (ParserConfigurationException e) {
            throw new RuntimeException("Failed reading XML document", e);
        } finally {
            stream.close();
        }
    }
    
    Descriptor read(String baseUrl, String resource) throws IOException {
        try {
            URL url = new URL(baseUrl + resource);
            return readFromUrl(url);
        } catch (IllegalArgumentException e) {
            return readFromClassPath(resource);
        }
    }
    
    String extractBaseUrl(URL fileUrl, String resource) {
        String file = fileUrl.toString();
        return file.substring(0, file.length() - resource.length());
    }

    ClassLoader classLoader() {
        return currentThread().getContextClassLoader();
    }
    
    boolean exists(URL url) {
        try {
            return url != null && url.openConnection() != null && url.openConnection().getContentLength() > 0;
        } catch (IOException e) {
            return false;
        }
    }

}