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The Async Http Client (AHC) classes.
There is a newer version: 3.0.0
/*
 * Copyright (c) 2010-2012 Sonatype, Inc. All rights reserved.
 *
 * This program is licensed to you under the Apache License Version 2.0,
 * and you may not use this file except in compliance with the Apache License Version 2.0.
 * You may obtain a copy of the Apache License Version 2.0 at http://www.apache.org/licenses/LICENSE-2.0.
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the Apache License Version 2.0 is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the Apache License Version 2.0 for the specific language governing permissions and limitations there under.
 */
package org.asynchttpclient.filter;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;

/**
 * A {@link org.asynchttpclient.filter.RequestFilter} throttles requests and block when the number of permits is reached, waiting for
 * the response to arrives before executing the next request.
 */
public class ThrottleRequestFilter implements RequestFilter {
    private final static Logger logger = LoggerFactory.getLogger(ThrottleRequestFilter.class);
    private final Semaphore available;
    private final int maxWait;

    public ThrottleRequestFilter(int maxConnections) {
        this(maxConnections, Integer.MAX_VALUE);
    }

    public ThrottleRequestFilter(int maxConnections, int maxWait) {
      this(maxConnections, maxWait, false);
    }

    public ThrottleRequestFilter(int maxConnections, int maxWait, boolean fair) {
      this.maxWait = maxWait;
      available = new Semaphore(maxConnections, fair);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public  FilterContext filter(FilterContext ctx) throws FilterException {

        try {
            if (logger.isDebugEnabled()) {
                logger.debug("Current Throttling Status {}", available.availablePermits());
            }
            if (!available.tryAcquire(maxWait, TimeUnit.MILLISECONDS)) {
                throw new FilterException(String.format("No slot available for processing Request %s with AsyncHandler %s",
                        ctx.getRequest(), ctx.getAsyncHandler()));
            }
        } catch (InterruptedException e) {
            throw new FilterException(String.format("Interrupted Request %s with AsyncHandler %s", ctx.getRequest(), ctx.getAsyncHandler()));
        }

        return new FilterContext.FilterContextBuilder<>(ctx).asyncHandler(new AsyncHandlerWrapper<>(ctx.getAsyncHandler(), available))
                .build();
    }
}