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package net.rossillo.spring.web.mvc;

import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.TimeZone;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.web.method.HandlerMethod;
import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.handler.HandlerInterceptorAdapter;

/**
 * Provides a cache control handler interceptor to assign cache-control
 * headers to HTTP responses.
 * 
 * @author Scott Rossillo
 *
 */
public class CacheControlHandlerInterceptor extends HandlerInterceptorAdapter implements HandlerInterceptor {

	private static final String HEADER_EXPIRES = "Expires";
	private static final String HEADER_CACHE_CONTROL = "Cache-Control";
	
	private boolean useExpiresHeader = true;
	
	/**
	 * Creates a new cache control handler interceptor.
	 */
	public CacheControlHandlerInterceptor() {
		super();
	}
	
	/**
	 * Assigns a CacheControl header to the given response.
	 * 
	 * @param request the HttpServletRequest
	 * @param response the HttpServletResponse
	 * @param handler the handler for the given request
	 */
	protected final void assignCacheControlHeader(
			final HttpServletRequest request,
			final HttpServletResponse response, 
			final Object handler) {
		
		final CacheControl cacheControl = this.getCacheControl(request, response, handler);
		final String cacheControlHeader = this.createCacheControlHeader(cacheControl);
		
		if (cacheControlHeader != null) {
			response.setHeader(HEADER_CACHE_CONTROL, cacheControlHeader);
			if (useExpiresHeader) {
				response.setDateHeader(HEADER_EXPIRES, createExpiresHeader(cacheControl));
			}
		}
	}
	
	/**
	 * Returns cache control header value from the given {@link CacheControl}
	 * annotation.
	 * 
	 * @param cacheControl the CacheControl annotation from which to
	 * create the returned cache control header value
	 * 
	 * @return the cache control header value
	 */
	protected final String createCacheControlHeader(final CacheControl cacheControl) {

		final StringBuilder builder = new StringBuilder();

		if (cacheControl == null) {
			return null;
		}

		final CachePolicy[] policies = cacheControl.policy();

		if (cacheControl.maxAge() >= 0) {
			builder.append("max-age=").append(cacheControl.maxAge());
		}

		if (cacheControl.sharedMaxAge() >= 0) {
			if (builder.length() > 0) {
				builder.append(", ");
			}
			builder.append("s-maxage=").append(cacheControl.sharedMaxAge());
		}

		if (policies != null) {
			for (final CachePolicy policy : policies) {
				if (builder.length() > 0) {
					builder.append(", ");
				}
				builder.append(policy.policy());
			}
		}

		return (builder.length() > 0 ? builder.toString() : null);
	}
	
	/**
	 * Returns an expires header value generated from the given 
	 * {@link CacheControl} annotation.
	 * 
	 * @param cacheControl the CacheControl annotation from which to
	 * create the returned expires header value
	 * 
	 * @return the expires header value
	 */
	protected final long createExpiresHeader(final CacheControl cacheControl) {
		
		final Calendar expires = new GregorianCalendar(TimeZone.getTimeZone("GMT"));
		
		if (cacheControl.maxAge() >= 0) {
			expires.add(Calendar.SECOND, cacheControl.maxAge());
		}
		
		return expires.getTime().getTime();
	}
	
	/**
	 * Returns the {@link CacheControl} annotation specified for the
	 * given request, response and handler.
	 * 
	 * @param request the current HttpServletRequest
	 * @param response the current HttpServletResponse
	 * @param handler the current request handler
	 * 
	 * @return the CacheControl annotation specified by
	 * the given handler if present; null otherwise
	 */
	protected final CacheControl getCacheControl(
			final HttpServletRequest request,
			final HttpServletResponse response, 
			final Object handler) {
		
		if (handler == null || !(handler instanceof HandlerMethod)) {
			return null;
		}
		
		final HandlerMethod handlerMethod = (HandlerMethod) handler;
		CacheControl cacheControl = handlerMethod.getMethodAnnotation(CacheControl.class);
		
		if (cacheControl == null) {
			return handlerMethod.getBeanType().getAnnotation(CacheControl.class);
		}
		
		return cacheControl;
	}
	
	@Override
	public final boolean preHandle(
			final HttpServletRequest request,
			final HttpServletResponse response, 
			final Object handler) throws Exception {
		
		this.assignCacheControlHeader(request, response, handler);
		
		return super.preHandle(request, response, handler);
	}

	/**
	 * True to set an expires header when a {@link CacheControl} annotation is present
	 * on a handler; false otherwise.   Defaults to true.
	 * 
	 * @param useExpiresHeader true to set an expires header when a 
	 * CacheControl annotation is present on a handler; false otherwise
	 */
	public final void setUseExpiresHeader(final boolean useExpiresHeader) {
		this.useExpiresHeader = useExpiresHeader;
	}
	
}