Copyright 2016 Netflix, Inc.
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.
/**
 * Copyright 2016 Netflix, Inc.
 *
 * 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 com.netflix.hystrix.util;

import java.lang.reflect.InvocationTargetException;
import java.util.concurrent.ThreadFactory;

public class PlatformSpecific {
    private final Platform platform;

    private enum Platform {
        STANDARD, APPENGINE_STANDARD, APPENGINE_FLEXIBLE
    }

    private static PlatformSpecific INSTANCE = new PlatformSpecific();

    private PlatformSpecific() {
        platform = determinePlatformReflectively();
    }

    public static boolean isAppEngineStandardEnvironment() {
        return INSTANCE.platform == Platform.APPENGINE_STANDARD;
    }

    public static boolean isAppEngine() {
        return INSTANCE.platform == Platform.APPENGINE_FLEXIBLE || INSTANCE.platform == Platform.APPENGINE_STANDARD;
    }

    /*
     * This detection mechanism is from Guava - specifically
     * http://docs.guava-libraries.googlecode.com/git/javadoc/src-html/com/google/common/util/concurrent/MoreExecutors.html#line.766
     * Added GAE_LONG_APP_ID check to detect only AppEngine Standard Environment
     */
    private static Platform determinePlatformReflectively() {
        if (System.getProperty("com.google.appengine.runtime.environment") == null) {
            return Platform.STANDARD;
        }
        // GAE_LONG_APP_ID is only set in the GAE Flexible Environment, where we want standard threading
        if (System.getenv("GAE_LONG_APP_ID") != null) {
            return Platform.APPENGINE_FLEXIBLE;
        }
        try {
            // If the current environment is null, we're not inside AppEngine.
            boolean isInsideAppengine = Class.forName("com.google.apphosting.api.ApiProxy")
                    .getMethod("getCurrentEnvironment")
                    .invoke(null) != null;
            return isInsideAppengine ? Platform.APPENGINE_STANDARD : Platform.STANDARD;
        } catch (ClassNotFoundException e) {
            // If ApiProxy doesn't exist, we're not on AppEngine at all.
            return Platform.STANDARD;
        } catch (InvocationTargetException e) {
            // If ApiProxy throws an exception, we're not in a proper AppEngine environment.
            return Platform.STANDARD;
        } catch (IllegalAccessException e) {
            // If the method isn't accessible, we're not on a supported version of AppEngine;
            return Platform.STANDARD;
        } catch (NoSuchMethodException e) {
            // If the method doesn't exist, we're not on a supported version of AppEngine;
            return Platform.STANDARD;
        }
    }

    public static ThreadFactory getAppEngineThreadFactory() {
        try {
            return (ThreadFactory) Class.forName("com.google.appengine.api.ThreadManager")
                    .getMethod("currentRequestThreadFactory")
                    .invoke(null);
        } catch (IllegalAccessException e) {
            throw new RuntimeException("Couldn't invoke ThreadManager.currentRequestThreadFactory", e);
        } catch (ClassNotFoundException e) {
            throw new RuntimeException("Couldn't invoke ThreadManager.currentRequestThreadFactory", e);
        } catch (NoSuchMethodException e) {
            throw new RuntimeException("Couldn't invoke ThreadManager.currentRequestThreadFactory", e);
        } catch (InvocationTargetException e) {
            throw new RuntimeException(e.getCause());
        }
    }
}