Copyright 2018 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 2018 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.concurrency.limits.servlet;

import com.netflix.concurrency.limits.Limiter;

import java.io.IOException;
import java.util.Optional;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

Servlet Filter that enforces concurrency limits on all requests into the servlet. 
See Also:
ServletLimiterBuilder/**
 * Servlet {@link Filter} that enforces concurrency limits on all requests into the servlet.
 * 
 * @see ServletLimiterBuilder
 */
public class ConcurrencyLimitServletFilter implements Filter {

    private static final int STATUS_TOO_MANY_REQUESTS = 429;
    private final Limiter<HttpServletRequest> limiter;

    public ConcurrencyLimitServletFilter(Limiter<HttpServletRequest> limiter) {
        this.limiter = limiter;
    }
    
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        
        Optional<Limiter.Listener> listener = limiter.acquire((HttpServletRequest)request);
        if (listener.isPresent()) {
            try {
                chain.doFilter(request, response);
                listener.get().onSuccess();
            } catch (Exception e) {
                listener.get().onIgnore();
                throw e;
            }
        } else {
            outputThrottleError((HttpServletResponse)response);
        }
    }

    protected void outputThrottleError(HttpServletResponse response) {
        try {
            response.setStatus(STATUS_TOO_MANY_REQUESTS);
            response.getWriter().print("Concurrency limit exceeded");
        } catch (IOException e) {
        }
    }
    
    @Override
    public void destroy() {
    }
}