/*   Copyright 2004 The Apache Software Foundation
 *
 *   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.apache.xmlbeans.impl.schema;

import org.apache.xmlbeans.SchemaTypeSystem;

/**
 * This class is the hook which causes the SchemaTypeSystem to be loaded when
 * a generated class is used.  It isn't used by the runtime directly, instead
 * this class is used by the schema compiler as a template class.  By using a
 * template class, the SchemaTypeSystemImpl can create all the binary files
 * required without needing to rely on javac.  The generated source still
 * requires a java compiler.
 *
 * @see SchemaTypeSystemImpl#save(org.apache.xmlbeans.Filer)
 */
//
// !!! It's important that there never NEVER be any references to this class because
// !!! the static initializer will fail.  This class must only be used as a class file.
// !!! If this scares you, turn back now !!!
//
// !!! If you modify this class, you will have to run bootstrap.
// !!! If this scares you, turn back now !!!
//
public class TypeSystemHolder
{
    private TypeSystemHolder() { }

    public static final SchemaTypeSystem typeSystem = loadTypeSystem();

    // Commenting out this line has the effect of not loading all components in a
    // typesystem upfront, but just as they are needed, which may improve
    // performance significantly
    //static { typeSystem.resolve(); }

    private static final SchemaTypeSystem loadTypeSystem()
    {
        try
        {
            // The fact that we are using TypeSystemHolder.class.getClassLoader()
            // to load the SchemaTypeSystem class means that the TypeSystemHolder's
            // classloader MUST be a descendant of the XmlBeans classloader, even
            // though it can be different.
            // In other words, each Schema type has only one copy per VM, not one
            // copy per copy of XmlBeans in use, therefore this implies that
            // there either must be one copy of XmlBeans in the VM or copies in
            // different classloaders will each need a different copy of one
            // Schema type, which seems a reasonable restriction, but needs to be
            // understood nevertheless
            return (SchemaTypeSystem)
                Class.forName(
                    "org.apache.xmlbeans.impl.schema.SchemaTypeSystemImpl",
                    true, TypeSystemHolder.class.getClassLoader())
                .getConstructor(new Class[] { Class.class })
                .newInstance(new java.lang.Object[] { TypeSystemHolder.class });
        }
        catch (ClassNotFoundException e)
        {
            throw new RuntimeException("Cannot load org.apache.xmlbeans.impl.SchemaTypeSystemImpl: make sure xbean.jar is on the classpath.", e);
        }
        catch (Exception e)
        {
            throw new RuntimeException("Could not instantiate SchemaTypeSystemImpl (" + e.toString() + "): is the version of xbean.jar correct?", e);
        }
    }
}