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Class Loading and Class Loaders

Introduction

Class loading, within the plethora of environments that Ehcache can be running, could be complex. But with BigMemory Go, all class loading is done in a standard way in one utility class: ClassLoaderUtil.

Plugin Class Loading

Ehcache allows plugins for events and distribution. These are loaded and created as follows:

/**
 * Creates a new class instance. Logs errors along the way. Classes are loaded
 * using the Ehcache standard classloader.
 *
 * @param className a fully qualified class name
 * @return null if the instance cannot be loaded
 */
public static Object createNewInstance(String className) throws CacheException {

    Class clazz;
    Object newInstance;
    try {
    clazz = Class.forName(className, true, getStandardClassLoader());
    } catch (ClassNotFoundException e) {
    //try fallback
    try {
        clazz = Class.forName(className, true, getFallbackClassLoader());
    } catch (ClassNotFoundException ex) {
        throw new CacheException("Unable to load class " + className +
                     ". Initial cause was " + e.getMessage(), e);
    }
    }
    try {
    newInstance = clazz.newInstance();
    } catch (IllegalAccessException e) {
    throw new CacheException("Unable to load class " + className +
                 ". Initial cause was " + e.getMessage(), e);
    } catch (InstantiationException e) {
    throw new CacheException("Unable to load class " + className +
                 ". Initial cause was " + e.getMessage(), e);
    }
    return newInstance;
}

/**
 * Gets the ClassLoader that all classes in ehcache, and extensions,
 *  should use for classloading. All ClassLoading in Ehcache should use this one.
 * This is the only thing that seems to work for all of the class loading 
 * situations found in the wild.
 * @return the thread context class loader.
 */
public static ClassLoader getStandardClassLoader() {
    return Thread.currentThread().getContextClassLoader();
}

/**
 * Gets a fallback ClassLoader that all classes in ehcache, and
 * extensions, should use for classloading. This is used if the context class loader
 * does not work.
 * @return the ClassLoaderUtil.class.getClassLoader();
 */
public static ClassLoader getFallbackClassLoader() {
    return ClassLoaderUtil.class.getClassLoader();
}

If this does not work for some reason, a CacheException is thrown with a detailed error message.

Loading of ehcache.xml resources

If the configuration is otherwise unspecified, Ehcache looks for a configuration in the following order:

  • Thread.currentThread().getContextClassLoader().getResource("/ehcache.xml")
  • ConfigurationFactory.class.getResource("/ehcache.xml")
  • ConfigurationFactory.class.getResource("/ehcache-failsafe.xml")

Ehcache uses the first configuration found. Note the use of "/ehcache.xml" which requires that ehcache.xml be placed at the root of the classpath (i.e., not in any package).