sdks/java/core/src/test/java/org/apache/beam/sdk/testing/InterceptingUrlClassLoader.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
 *
 *     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.beam.sdk.testing;

import java.io.IOException;
import java.util.function.Predicate;
import org.apache.beam.vendor.guava.v20_0.com.google.common.base.Predicates;
import org.apache.beam.vendor.guava.v20_0.com.google.common.collect.Sets;
import org.apache.beam.vendor.guava.v20_0.com.google.common.io.ByteStreams;

/**
 * A classloader that intercepts loading of specifically named classes. This classloader copies the
 * original classes definition and is useful for testing code which needs to validate usage with
 * multiple classloaders..
 */
public class InterceptingUrlClassLoader extends ClassLoader {
  private final Predicate<String> test;

  public InterceptingUrlClassLoader(final ClassLoader parent, final String... ownedClasses) {
    this(parent, Predicates.in(Sets.newHashSet(ownedClasses))::apply);
  }

  public InterceptingUrlClassLoader(final ClassLoader parent, final Predicate<String> test) {
    super(parent);
    this.test = test;
  }

  @Override
  public Class<?> loadClass(final String name) throws ClassNotFoundException {
    final Class<?> alreadyLoaded = super.findLoadedClass(name);
    if (alreadyLoaded != null) {
      return alreadyLoaded;
    }

    if (name != null && test.test(name)) {
      try {
        final String classAsResource = name.replace('.', '/') + ".class";
        final byte[] classBytes =
            ByteStreams.toByteArray(getParent().getResourceAsStream(classAsResource));
        return defineClass(name, classBytes, 0, classBytes.length);
      } catch (final IOException e) {
        throw new RuntimeException(e);
      }
    }
    return getParent().loadClass(name);
  }
}