ServiceLoaderWrapper.java

/*
 * Copyright (C) 2018 The Android Open Source Project
 *
 * 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 androidx.test.internal.platform;

import android.os.StrictMode;
import androidx.annotation.RestrictTo;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.ServiceLoader;
import java.util.Set;

/**
 * Wrapper class for {@link ServiceLoader} that disables StrictMode.
 *
 * <p>Loading a service via ServiceLoader can result in disk I/O. Disk I/O on Android can be
 * restricted via StrictMode. This class disables disk access checking and then reenables StrictMode
 * when loading a service.
 *
 * @hide
 */
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP) // used by core, espresso, rule
public final class ServiceLoaderWrapper {

  private ServiceLoaderWrapper() {}

  /**
   * Loads the implementing classes for given service.
   *
   * @return List of implementing classes. Returns an empty list if no implementing classes were
   *     defined.
   */
  public static <T> List<T> loadService(Class<T> serviceClass) {
    StrictMode.ThreadPolicy oldPolicy = StrictMode.allowThreadDiskReads();

    // load using ServiceLoader
    List<T> services = new ArrayList<>();
    for (T element : ServiceLoader.load(serviceClass)) {
      services.add(element);
    }
    StrictMode.setThreadPolicy(oldPolicy);
    return services;
  }

  /** A factory for creating default implementations of service classes. */
  public interface Factory<T> {
    T create();
  }

  /**
   * A wrapper method around {@link #loadService(Class)} that strictly enforces there is only one
   * implementation of the service.
   *
   * @param serviceClass the service type class to load implementation for
   * @param defaultImplFactory the factory implementation for creating instances
   * @return the implementing service, or a new instance via defaultImplFactory.create() if no
   *     implementations are defined.
   * @throws IllegalStateException if more than one service implementations are found
   */
  public static <T> T loadSingleService(Class<T> serviceClass, Factory<T> defaultImplFactory) {
    T impl = loadSingleServiceOrNull(serviceClass);
    if (impl == null) {
      return defaultImplFactory.create();
    } else {
      return impl;
    }
  }

  /**
   * A wrapper method around {@link #loadService(Class)} that returns one implementation of the
   * service or {@code null} if no implementation is found.
   *
   * @param serviceClass the service type class to load implementation for
   * @return the implementing service or null if none is found.
   * @throws IllegalStateException if more than one service implementations are found
   */
  public static <T> T loadSingleServiceOrNull(Class<T> serviceClass) {
    List<T> impls = filter(ServiceLoaderWrapper.loadService(serviceClass));
    if (impls.isEmpty()) {
      return null;
    } else if (impls.size() == 1) {
      return impls.get(0);
    } else {
      String combinedImpls = null;
      for (T impl : impls) {
        combinedImpls = (combinedImpls != null) ? combinedImpls + ", " : ": ";
        combinedImpls += impl.getClass().getName();
      }

      throw new IllegalStateException(
          "Found more than one implementation for " + serviceClass.getName() + combinedImpls);
    }
  }

  @SuppressWarnings("unchecked")
  private static <T> List<T> filter(List<T> services) {
    Set<Class<?>> superseded = new HashSet<>();
    for (T service : services) {
      Class<? extends T> clazz = (Class<? extends T>) service.getClass();
      Supersedes supersedes = clazz.getAnnotation(Supersedes.class);
      if (supersedes != null) {
        superseded.add(supersedes.value());
      }
    }
    if (superseded.isEmpty()) {
      return services;
    } else {
      List<T> filtered = new ArrayList<>();
      for (T service : services) {
        if (!superseded.contains(service.getClass())) {
          filtered.add(service);
        }
      }
      return filtered;
    }
  }
}