Source Code Cross Referenced for SPILoader.java in  » Test-Coverage » GroboUtils » net » sourceforge » groboutils » util » classes » v1 » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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Java Source Code / Java Documentation » Test Coverage » GroboUtils » net.sourceforge.groboutils.util.classes.v1 
Source Cross Referenced  Class Diagram Java Document (Java Doc) 


001:        /*
002:         *  @(#)SPILoader.java
003:         *
004:         * Copyright (C) 2002-2003 Matt Albrecht
005:         * groboclown@users.sourceforge.net
006:         * http://groboutils.sourceforge.net
007:         *
008:         *  Part of the GroboUtils package at:
009:         *  http://groboutils.sourceforge.net
010:         *
011:         *  Permission is hereby granted, free of charge, to any person obtaining a
012:         *  copy of this software and associated documentation files (the "Software"),
013:         *  to deal in the Software without restriction, including without limitation
014:         *  the rights to use, copy, modify, merge, publish, distribute, sublicense,
015:         *  and/or sell copies of the Software, and to permit persons to whom the 
016:         *  Software is furnished to do so, subject to the following conditions:
017:         *
018:         *  The above copyright notice and this permission notice shall be included in 
019:         *  all copies or substantial portions of the Software. 
020:         *
021:         *  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR 
022:         *  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, 
023:         *  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL 
024:         *  THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER 
025:         *  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
026:         *  FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER 
027:         *  DEALINGS IN THE SOFTWARE.
028:         */
029:        package net.sourceforge.groboutils.util.classes.v1;
030:
031:        import java.util.Properties;
032:        import java.util.Enumeration;
033:        import java.util.NoSuchElementException;
034:
035:        import java.net.URL;
036:
037:        import java.io.IOException;
038:        import java.io.InputStream;
039:
040:        import org.apache.log4j.Logger;
041:
042:        /**
043:         * Loads Service Provider Interface (SPI) classes from the given classloader
044:         * (if any).  This will search for files in the form
045:         * <tt>/META-INF/services/<i>classname</i></tt>.  The discovered file will be
046:         * parsed using <tt>java.util.Properties</tt> parsing method, but only the
047:         * keys will be recognized (note that if a line contains no ':' or '=', then
048:         * the line will be recognized as a key with an empty-string value).  The found
049:         * keys will be used as class names, constructed, and returned, using the
050:         * <tt>ClassLoadHelper</tt> class in this package.
051:         * <P>
052:         * This is intended to follow the SPI interface spec, partially described in
053:         * the JDK 1.4 documentation in the package docs for <tt>java.awt.im.spi</tt>.
054:         * <P>
055:         * Note that by using a Properties instance, this class is limited to only
056:         * one instance of a class type per META-INF service file.  This is assumed to
057:         * be an adequate restriction, since in most circumstances, there should only
058:         * be one instance of a service provider loaded per need.
059:         *
060:         * @author     Matt Albrecht <a href="mailto:groboclown@users.sourceforge.net">groboclown@users.sourceforge.net</a>
061:         * @since      June 28, 2002
062:         * @version    $Date: 2003/05/08 14:12:21 $
063:         */
064:        public class SPILoader {
065:            private static final Logger LOG = Logger.getLogger(SPILoader.class
066:                    .getName());
067:
068:            /**
069:             * List of all resource URLs to the class's SPI files.
070:             */
071:            private Enumeration spiUrls;
072:
073:            /**
074:             * Current SPI url's property (classname) keys.
075:             */
076:            private Enumeration propKeys = null;
077:
078:            private String nextKey = null;
079:
080:            private Class base;
081:
082:            private ClassLoadHelper clh;
083:
084:            /**
085:             * Use the context (thread) classloader or the system class loader.
086:             *
087:             * @param spiBase the base class that all loaded SPI classes must
088:             *      implement.  This is also used to define the name of the files
089:             *      to load from the META-INF directory.
090:             * @exception IOException if there is an I/O problem loading the
091:             *      initial set of resources.
092:             */
093:            public SPILoader(Class spiBase) throws IOException {
094:                this (spiBase, null);
095:            }
096:
097:            /**
098:             * Create a new SPILoader, loading the service files from the given
099:             * class loader classpath.  If <tt>cl</tt> is <tt>null</tt>, then the
100:             * context (thread) class loader or system class loader will be used
101:             * instead.
102:             *
103:             * @param spiBase the base class that all loaded SPI classes must
104:             *      implement.  This is also used to define the name of the files
105:             *      to load from the META-INF directory.
106:             * @param cl classloader to load files from.
107:             * @exception IOException if there is an I/O problem loading the
108:             *      initial set of resources.
109:             */
110:            public SPILoader(Class spiBase, ClassLoader cl) throws IOException {
111:                if (spiBase == null) {
112:                    throw new IllegalArgumentException(
113:                            "spiBase cannot be null.");
114:                }
115:
116:                if (cl == null) {
117:                    this .clh = new ClassLoadHelper(spiBase);
118:                } else {
119:                    this .clh = new ClassLoadHelper(cl);
120:                }
121:
122:                String metaName = "/META-INF/services/" + spiBase.getName();
123:
124:                this .spiUrls = this .clh.getResources(metaName);
125:                if (this .spiUrls == null) {
126:                    throw new IOException("No URLs were discovered.");
127:                }
128:                this .base = spiBase;
129:            }
130:
131:            /**
132:             * Discovers if there is another provider class instance to load.
133:             *
134:             * @return <tt>true</tt> if there is another provider, otherwise
135:             *      <tt>false</tt>.
136:             */
137:            public boolean hasNext() throws IOException {
138:                while (this .nextKey == null) {
139:                    if (this .propKeys == null
140:                            || !this .propKeys.hasMoreElements()) {
141:                        if (!this .spiUrls.hasMoreElements()) {
142:                            LOG.debug("No more URLs to process");
143:                            return false;
144:                        }
145:                        URL url = (URL) this .spiUrls.nextElement();
146:                        LOG.debug("Processing next URL: " + url);
147:                        Properties props = new Properties();
148:                        LOG.debug("Opening URL stream");
149:                        InputStream is = url.openStream();
150:                        try {
151:                            props.load(is);
152:                        } finally {
153:                            is.close();
154:                        }
155:                        this .propKeys = props.keys();
156:                        LOG.debug("URL contains " + props.size()
157:                                + " class names");
158:                    } else {
159:                        this .nextKey = (String) this .propKeys.nextElement();
160:                        LOG.debug("Next classname is " + this .nextKey);
161:                    }
162:                }
163:                return true;
164:            }
165:
166:            /**
167:             * Returns a new instance of the next class name.  If the defined
168:             * next class is invalid, then an IOException is thrown.  The returned
169:             * object is guaranteed to be non-null and an instance of the constructor
170:             * baseClass.
171:             *
172:             * @return the next provider in the list of discovered SPI providers.
173:             * @exception IOException if there was an I/O error, or if the referenced
174:             *      resource defines an invalid class to load.
175:             * @exception NoSuchElementException if the enumeration is already at
176:             *      the end of the list.
177:             */
178:            public Object nextProvier() throws IOException {
179:                // ensure we have a next key to go to
180:                hasNext();
181:
182:                if (this .nextKey == null) {
183:                    // end of list
184:                    throw new NoSuchElementException("end of list");
185:                }
186:                String cn = this .nextKey;
187:
188:                // ensure we advance to the next key next iteration.
189:                this .nextKey = null;
190:
191:                Object o;
192:                try {
193:                    o = this .clh.createObject(cn);
194:                } catch (IllegalStateException ise) {
195:                    LOG.info("create object for type " + cn
196:                            + " threw exception.", ise);
197:                    throw new IOException(ise.getMessage());
198:                }
199:
200:                if (o == null) {
201:                    LOG
202:                            .info("create object for type " + cn
203:                                    + " returned null.");
204:                    throw new IOException(
205:                            "Could not create an instance of type " + cn);
206:                }
207:
208:                if (!this .base.isInstance(o)) {
209:                    throw new IOException("SPI defined class " + cn
210:                            + ", but expected class of type "
211:                            + this.base.getName());
212:                }
213:                return o;
214:            }
215:        }
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