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/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements. See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership. The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied. See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package org.apache.neethi.util;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import java.net.URL;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * This class handles looking up service providers on the class path.
 * It implements the system described in:
 *
 * JAR
 * File Specification Under Service Provider. Note that this
 * interface is very similar to the one they describe which seems to
 * be missing in the JDK.
 *
 */
public final class Service {

    // Remember providers we have looked up before.
    static Map> instanceMap = new HashMap>();
    
    private Service() {
        //not constructed
    }
    
    @SuppressWarnings("unchecked")
    private static  List cast(List p) {
        return (List)p;
    }

    /**
     * Returns an iterator where each element should implement the
     * interface (or subclass the baseclass) described by cls.  The
     * Classes are found by searching the classpath for service files
     * named: 'META-INF/services/<fully qualified classname> that list
     * fully qualified classnames of classes that implement the
     * service files classes interface.  These classes must have
     * default constructors.
     *
     * @param cls The class/interface to search for providers of.
     * @return The list of providers that implement the cls
     */
    public static synchronized  List providers(Class cls) {

        String serviceFile = "META-INF/services/" + cls.getName();

        List l = cast(instanceMap.get(serviceFile));
        if (l != null) {
            return l;
        }

        l = new ArrayList();
        instanceMap.put(serviceFile, l);

        ClassLoader cl = null;
        try {
            cl = cls.getClassLoader();
        } catch (SecurityException se) {
            // Ooops! can't get his class loader.
        }
        // Can always request your own class loader. But it might be 'null'.
        if (cl == null) {
            cl = Service.class.getClassLoader();
        }
        if (cl == null) {
            cl = ClassLoader.getSystemClassLoader();
        }

        // No class loader so we can't find 'serviceFile'.
        if (cl == null) {
            return l;
        }

        Enumeration e;
        try {
            e = cl.getResources(serviceFile);
        } catch (IOException ioe) {
            return l;
        }

        while (e.hasMoreElements()) {
            InputStream is = null;
            try {
                URL u = e.nextElement();

                is = u.openStream();
                
                Reader         r  = new InputStreamReader(is, "UTF-8");
                BufferedReader br = new BufferedReader(r);

                String line = br.readLine();
                while (line != null) {
                    try {
                        // First strip any comment...
                        int idx = line.indexOf('#');
                        if (idx != -1) {
                            line = line.substring(0, idx);
                        }

                        // Trim whitespace.
                        line = line.trim();

                        // If nothing left then loop around...
                        if (line.length() == 0) {
                            line = br.readLine();
                            continue;
                        }

                        // Try and load the class 
                        Object obj = cl.loadClass(line).newInstance();
                        // stick it into our vector...
                        l.add(cls.cast(obj));
                    } catch (Exception ex) {
                        // Just try the next line
                    }
                    line = br.readLine();
                }
            } catch (Exception ex) {
                // Just try the next file...
            } catch (LinkageError le) {
                // Just try the next file...
            } finally {
                try {
                    if (is != null) {
                        is.close();
                    }
                } catch (IOException ex) {
                    //ignore
                }
            }
        }
        return l;
    }
    
}