src/test/java/org/apache/commons/beanutils2/converters/ClassReloader.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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
 *
 *      https://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.commons.beanutils2.converters;

import java.io.ByteArrayOutputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;

/**
 * A special classloader useful for testing j2ee-like scenarios.
 *
 * <p>
 * In some tests we want to be able to emulate "container" frameworks, where code runs in a hierarchy of class loaders, and certain classes may be loaded by
 * various class loaders in the hierarchy.
 * </p>
 *
 * <p>
 * Normally this is done by having certain jars or class-file-directories in the classpath of some class loaders but not others. This is quite difficult to
 * integrate with the build process for the unit tests though; compiling certain classes and having the output go into places that is not in the default
 * classpath for the unit tests would be a major pain.
 * </p>
 *
 * <p>
 * So this class takes a sneaky alternative approach: it can grab any class already loaded by a parent classloader and <em>reload</em> that class via this
 * classloader. The effect is exactly as if a class (or jar file) had been present in the classpath for a container's "shared" classloader <em>and</em> been
 * present in the component-specific classpath too, without any messing about with the way unit test code is compiled or executed.
 */
public class ClassReloader extends ClassLoader {

    public ClassReloader(final ClassLoader parent) {
        super(parent);
    }

    /**
     * Given a class already in the classpath of a parent classloader, reload that class via this classloader.
     */
    public Class<?> reload(final Class<?> clazz) throws FileNotFoundException, IOException {
        final String className = clazz.getName();
        final String classFile = className.replace('.', '/') + ".class";
        final ByteArrayOutputStream baos = new ByteArrayOutputStream();
        try (InputStream classStream = getParent().getResourceAsStream(classFile)) {

            if (classStream == null) {
                throw new FileNotFoundException(classFile);
            }

            final byte[] buf = new byte[1024];
            for (;;) {
                final int bytesRead = classStream.read(buf);
                if (bytesRead == -1) {
                    break;
                }
                baos.write(buf, 0, bytesRead);
            }
        }

        final byte[] classData = baos.toByteArray();

        // now we have the raw class data, let's turn it into a class
        final Class<?> newClass = defineClass(className, classData, 0, classData.length);
        resolveClass(newClass);
        return newClass;
    }
}