Service.java

/**
 * *****************************************************************************
 *
 * Copyright (c) 2004-2009 Oracle Corporation.
 *
 * All rights reserved. This program and the accompanying materials are made
 * available under the terms of the Eclipse Public License v1.0 which
 * accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 * 
 * Kohsuke Kawaguchi
 *
 *
 ******************************************************************************
 */
package hudson.util;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URL;
import java.util.Collection;
import java.util.Enumeration;
import java.util.List;
import java.util.ArrayList;
import java.util.logging.Level;
import java.util.logging.Logger;
import static java.util.logging.Level.WARNING;

/**
 * Load classes by looking up <tt>META-INF/services</tt>.
 *
 * @author Kohsuke Kawaguchi
 */
public class Service {

    private static final Logger LOGGER = Logger.getLogger(Service.class.getName());

    /**
     * Poorman's clone of JDK6 ServiceLoader.
     */
    public static <T> List<T> loadInstances(ClassLoader classLoader, Class<T> type) throws IOException {
        List<T> result = new ArrayList<T>();

        final Enumeration<URL> e = classLoader.getResources("META-INF/services/" + type.getName());
        BufferedReader configFile = null;
        try {
            while (e.hasMoreElements()) {
                URL url = e.nextElement();
                configFile = new BufferedReader(new InputStreamReader(url.openStream(), "UTF-8"));
                String line;
                while ((line = configFile.readLine()) != null) {
                    line = line.trim();
                    if (line.startsWith("#") || line.length() == 0) {
                        continue;
                    }

                    try {
                        Class<?> t = classLoader.loadClass(line);
                        if (!type.isAssignableFrom(t)) {
                            // invalid type
                            continue;
                        }
                        result.add(type.cast(t.newInstance()));
                    } catch (ClassNotFoundException x) {
                        LOGGER.log(WARNING, "Failed to load " + line, x);
                    } catch (InstantiationException x) {
                        LOGGER.log(WARNING, "Failed to load " + line, x);
                    } catch (IllegalAccessException x) {
                        LOGGER.log(WARNING, "Failed to load " + line, x);
                    }
                }
            }
        } finally {
            IOUtils.closeQuietly(configFile);
        }

        return result;
    }

    /**
     * Look up <tt>META-INF/service/<i>SPICLASSNAME</i></tt> from the
     * classloader and all the discovered classes into the given collection.
     */
    public static <T> void load(Class<T> spi, ClassLoader cl, Collection<Class<? extends T>> result) {
        try {
            Enumeration<URL> e = cl.getResources("META-INF/services/" + spi.getName());
            while (e.hasMoreElements()) {
                BufferedReader r = null;
                URL url = e.nextElement();
                try {
                    r = new BufferedReader(new InputStreamReader(url.openStream(), "UTF-8"));
                    String line;
                    while ((line = r.readLine()) != null) {
                        // comment line
                        if (line.startsWith("#")) {
                            continue;
                        }
                        line = line.trim();
                        if (line.length() == 0) {
                            // empty line. ignore.
                            continue;
                        }
                        try {
                            result.add(cl.loadClass(line).asSubclass(spi));
                        } catch (ClassNotFoundException x) {
                            LOGGER.log(Level.WARNING, "Failed to load " + line, x);
                        }
                    }
                } catch (IOException x) {
                    LOGGER.log(Level.WARNING, "Failed to load " + url, x);
                } finally {
                    IOUtils.closeQuietly(r);
                }
            }
        } catch (IOException x) {
            LOGGER.log(Level.WARNING, "Failed to look up service providers for " + spi, x);
        }
    }
}