Ehcache(2.9.x) - API Developer Guide, Class Loading

Posted 2020-08-07 huey

About Class Loading

Class loading, within the plethora of environments that Ehcache can be running, could be complex. But with Ehcache, 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).