src/org/clearsilver/FactoryLoader.java

/*
 * Copyright (C) 2010 Google Inc.
 *
 * Licensed 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.clearsilver;

import java.lang.reflect.Constructor;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 * This class holds static methods for getting and setting the CS and HDF
 * factory used throughout the Java Clearsilver Framework.
 * Clients are <strong>strongly encouraged</strong> to not use this class, and
 * instead directly inject {@link ClearsilverFactory} into the classes that
 * need to create {@link HDF} and {@link CS} instances.
 * For now, projects should set the {@link ClearsilverFactory} in FactoryLoader
 * and use the singleton accessor {@link #getClearsilverFactory()} if proper
 * dependency injection is not easy to implement.
 * <p>
 * Allows the default implementation to be the original JNI version without
 * requiring users that don't want to use the JNI version to have to link
 * it in.  The ClearsilverFactory object to use can be either passed into the
 * {@link #setClearsilverFactory} method or the class name can be specified
 * in the Java property {@code  org.clearsilver.defaultClearsilverFactory}.
 */
public final class FactoryLoader {
  private static final Logger logger =
      Logger.getLogger(FactoryLoader.class.getName());

  private static final String DEFAULT_CS_FACTORY_CLASS_PROPERTY_NAME =
      "org.clearsilver.defaultClearsilverFactory";
  private static final String DEFAULT_CS_FACTORY_CLASS_NAME =
      "org.clearsilver.jni.JniClearsilverFactory";

  // ClearsilverFactory to be used when constructing objects.  Allows
  // applications to subclass the CS and HDF objects used in Java Clearsilver
  private static ClearsilverFactory clearsilverFactory = null;

  // Read/Write lock for global factory pointer.
  private static final ReadWriteLock factoryLock = new ReentrantReadWriteLock();

  // Getters and setters
  /**
   * Get the {@link org.clearsilver.ClearsilverFactory} object to be used by
   * disparate parts of the application.
   */
  public static ClearsilverFactory getClearsilverFactory() {
    factoryLock.readLock().lock();
    if (clearsilverFactory == null) {
      factoryLock.readLock().unlock();
      factoryLock.writeLock().lock();
      try {
        if (clearsilverFactory == null) {
          clearsilverFactory = newDefaultClearsilverFactory();
        }
        factoryLock.readLock().lock();
      } finally {
        factoryLock.writeLock().unlock();
      }
    }
    ClearsilverFactory returned = clearsilverFactory;
    factoryLock.readLock().unlock();
    return returned;
  }

  /**
   * Set the {@link org.clearsilver.ClearsilverFactory} to be used by
   * the application. If parameter is {@code null}, then the default factory
   * implementation will be used the next time {@link #getClearsilverFactory()}
   * is called.
   *
   * @return the previous factory (may return {@code null})
   */
  public static ClearsilverFactory setClearsilverFactory(
      ClearsilverFactory clearsilverFactory) {
    factoryLock.writeLock().lock();
    try {
      ClearsilverFactory previousFactory = FactoryLoader.clearsilverFactory;
      FactoryLoader.clearsilverFactory = clearsilverFactory;
      return previousFactory;
    } finally {
      factoryLock.writeLock().unlock();
    }
  }

  private static ClearsilverFactory newDefaultClearsilverFactory() {
    String factoryClassName =
        System.getProperty(DEFAULT_CS_FACTORY_CLASS_PROPERTY_NAME,
            DEFAULT_CS_FACTORY_CLASS_NAME);
    try {
      ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
      Class<ClearsilverFactory> clazz =
          loadClass(factoryClassName, classLoader);
      Constructor<ClearsilverFactory> constructor = clazz.getConstructor();
      return constructor.newInstance();
    } catch (Exception e) {
      String errMsg = "Unable to load default ClearsilverFactory class: \"" +
          factoryClassName + "\"";
      logger.log(Level.SEVERE, errMsg, e);
      throw new RuntimeException(errMsg, e);
    }
  }

  private static Class<ClearsilverFactory> loadClass(String className,
      ClassLoader classLoader) throws ClassNotFoundException {
    return (Class<ClearsilverFactory>) Class.forName(className, true,
        classLoader);
  }
}