/*
 * 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.phoenix.util;

import java.util.concurrent.Callable;

import org.apache.phoenix.call.CallWrapper;

/**
 * Executes {@code Callable}s using a context classloader that is set up to load classes from
 * Phoenix.
 * <p/>
 * Loading HBase configuration settings and endpoint coprocessor classes is done via the context
 * classloader of the calling thread. When Phoenix is being run via a JDBC-enabled GUI, the
 * driver is often loaded dynamically and executed via multiple threads, which makes it difficult
 * or impossible to predict the state of the classloader hierarchy in the current thread. This
 * class is intended to get around that, to ensure that the same classloader used to load Phoenix
 * classes is set as the context classloader for specific calls.
 */
public class PhoenixContextExecutor {

    // We cache the class loader because calls to Class.getClassLoader are relatively expensive
    private static final ClassLoader CACHED_CLASSLOADER = PhoenixContextExecutor.class.getClassLoader();

    private static class CurrentContextWrapper implements CallWrapper {
        private ClassLoader saveCcl;

        @Override
        public void before() {
            saveCcl = Thread.currentThread().getContextClassLoader();
            Thread.currentThread().setContextClassLoader(CACHED_CLASSLOADER);
        }

        @Override
        public void after() {
            Thread.currentThread().setContextClassLoader(saveCcl);

        };
    }

    public static CallWrapper inContext() {
        return new CurrentContextWrapper();
    }

    /**
     * Execute an operation (synchronously) using the context classloader used to load this class,
     * instead of the currently-set context classloader of the current thread. This allows loading
     * dynamically-loaded classes and configuration files using the same classloader used to
     * load the rest of the JDBC driver.
     * <p/>
     * The context classloader of the current thread is reset to its original value after the
     * callable has been executed.
     *
     * @param target the callable to be executed
     * @return the return value from the callable
     */
    public static <T> T call(Callable<T> target) throws Exception {
        ClassLoader saveCcl = Thread.currentThread().getContextClassLoader();
        try {
            Thread.currentThread().setContextClassLoader(CACHED_CLASSLOADER);
            return target.call();
        } finally {
            Thread.currentThread().setContextClassLoader(saveCcl);
        }
    }

    /**
     * Same as {@link #call(java.util.concurrent.Callable)}, but doesn't throw checked exceptions.
     *
     * @param target the callable to be executed
     * @return the return value from the callable
     * @throws Exception any exception thrown by the underlying callable
     */
    public static <T> T callWithoutPropagation(Callable<T> target) {
        try {
            return call(target);
        } catch (RuntimeException e) {
            throw e;
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

}