【浏览器】：缓存

目录
1. 别人是怎么玩的？
    2.1. 知乎
    2.2. 京东
2. 缓存作用？
3. 缓存分类？
4. 缓存原理？
    4.1. 核心词汇
    4.2. 总体流程
    4.3. 缓存控制
        4.3.1. Cache-Control
            4.3.1.1. No caching
            4.3.1.2. Cache but revalidate
            4.3.1.3. Expiration
            4.3.1.4. Cache-Control 配置建议
        4.3.2. Pragma   
    4.4. 缓存验证 
        4.4.1. LastModified
        4.4.2. ETags
    4.5 一种更新资源的方式
5. 用户行为与浏览器
    5.1. 地址栏访问
    5.2. F5 刷新
    5.2. Ctrl+F5 刷新
6. FAQ
    6.1. HTML Meta Tags and HTTP Headers？
    6.2. Pragma HTTP Headers (and why they don’t work)
    6.3. 如何清理浏览器缓存？
    6.4. Targets of caching operations？      
    6.5. Tomcat 如何生成 ETag？
    6.6. Tomcat 如何对待"If-None-Match"等请求头的？

Cache-Control: no-store

Cache-Control: no-cache

Cache-Control: max-age=31536000

package org.apache.catalina.webresources
public abstract class AbstractResource implements WebResource {
    @Override
    public final String getETag() {
        if (weakETag == null) {
            synchronized (this) {
                if (weakETag == null) {
                    long contentLength = getContentLength();
                    long lastModified = getLastModified();
                    if ((contentLength >= 0) || (lastModified >= 0)) {
                        weakETag = "W/\"" + contentLength + "-" +
                                   lastModified + "\"";
                    }
                }
            }
        }
        return weakETag;
    }
}

package org.apache.catalina.servlets;
public class DefaultServlet extends HttpServlet {
    /**
     * Check if the conditions specified in the optional If headers are
     * satisfied.
     *
     * @param request   The servlet request we are processing
     * @param response  The servlet response we are creating
     * @param resource  The resource
     * @return <code>true</code> if the resource meets all the specified
     *  conditions, and <code>false</code> if any of the conditions is not
     *  satisfied, in which case request processing is stopped
     * @throws IOException an IO error occurred
     */
    protected boolean checkIfHeaders(HttpServletRequest request,
                                     HttpServletResponse response,
                                     WebResource resource)
        throws IOException {

        return checkIfMatch(request, response, resource)
            && checkIfModifiedSince(request, response, resource)
            && checkIfNoneMatch(request, response, resource)
            && checkIfUnmodifiedSince(request, response, resource);

    }
    
    /**
     * Check if the if-match condition is satisfied.
     *
     * @param request   The servlet request we are processing
     * @param response  The servlet response we are creating
     * @param resource  The resource
     * @return <code>true</code> if the resource meets the specified condition,
     *  and <code>false</code> if the condition is not satisfied, in which case
     *  request processing is stopped
     * @throws IOException an IO error occurred
     */
    protected boolean checkIfMatch(HttpServletRequest request,
            HttpServletResponse response, WebResource resource)
            throws IOException {

        String eTag = resource.getETag();
        String headerValue = request.getHeader("If-Match");
        if (headerValue != null) {
            if (headerValue.indexOf('*') == -1) {

                StringTokenizer commaTokenizer = new StringTokenizer
                    (headerValue, ",");
                boolean conditionSatisfied = false;

                while (!conditionSatisfied && commaTokenizer.hasMoreTokens()) {
                    String currentToken = commaTokenizer.nextToken();
                    if (currentToken.trim().equals(eTag))
                        conditionSatisfied = true;
                }

                // If none of the given ETags match, 412 Precondition failed is
                // sent back
                if (!conditionSatisfied) {
                    response.sendError
                        (HttpServletResponse.SC_PRECONDITION_FAILED);
                    return false;
                }

            }
        }
        return true;
    }


    /**
     * Check if the if-modified-since condition is satisfied.
     *
     * @param request   The servlet request we are processing
     * @param response  The servlet response we are creating
     * @param resource  The resource
     * @return <code>true</code> if the resource meets the specified condition,
     *  and <code>false</code> if the condition is not satisfied, in which case
     *  request processing is stopped
     */
    protected boolean checkIfModifiedSince(HttpServletRequest request,
            HttpServletResponse response, WebResource resource) {
        try {
            long headerValue = request.getDateHeader("If-Modified-Since");
            long lastModified = resource.getLastModified();
            if (headerValue != -1) {

                // If an If-None-Match header has been specified, if modified since
                // is ignored.
                if ((request.getHeader("If-None-Match") == null)
                    && (lastModified < headerValue + 1000)) {
                    // The entity has not been modified since the date
                    // specified by the client. This is not an error case.
                    response.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                    response.setHeader("ETag", resource.getETag());

                    return false;
                }
            }
        } catch (IllegalArgumentException illegalArgument) {
            return true;
        }
        return true;
    }


    /**
     * Check if the if-none-match condition is satisfied.
     *
     * @param request   The servlet request we are processing
     * @param response  The servlet response we are creating
     * @param resource  The resource
     * @return <code>true</code> if the resource meets the specified condition,
     *  and <code>false</code> if the condition is not satisfied, in which case
     *  request processing is stopped
     * @throws IOException an IO error occurred
     */
    protected boolean checkIfNoneMatch(HttpServletRequest request,
            HttpServletResponse response, WebResource resource)
            throws IOException {

        String eTag = resource.getETag();
        String headerValue = request.getHeader("If-None-Match");
        if (headerValue != null) {

            boolean conditionSatisfied = false;

            if (!headerValue.equals("*")) {

                StringTokenizer commaTokenizer =
                    new StringTokenizer(headerValue, ",");

                while (!conditionSatisfied && commaTokenizer.hasMoreTokens()) {
                    String currentToken = commaTokenizer.nextToken();
                    if (currentToken.trim().equals(eTag))
                        conditionSatisfied = true;
                }

            } else {
                conditionSatisfied = true;
            }

            if (conditionSatisfied) {

                // For GET and HEAD, we should respond with
                // 304 Not Modified.
                // For every other method, 412 Precondition Failed is sent
                // back.
                if ( ("GET".equals(request.getMethod()))
                     || ("HEAD".equals(request.getMethod())) ) {
                    response.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                    response.setHeader("ETag", eTag);

                    return false;
                }
                response.sendError(HttpServletResponse.SC_PRECONDITION_FAILED);
                return false;
            }
        }
        return true;
    }

    /**
     * Check if the if-unmodified-since condition is satisfied.
     *
     * @param request   The servlet request we are processing
     * @param response  The servlet response we are creating
     * @param resource  The resource
     * @return <code>true</code> if the resource meets the specified condition,
     *  and <code>false</code> if the condition is not satisfied, in which case
     *  request processing is stopped
     * @throws IOException an IO error occurred
     */
    protected boolean checkIfUnmodifiedSince(HttpServletRequest request,
            HttpServletResponse response, WebResource resource)
            throws IOException {
        try {
            long lastModified = resource.getLastModified();
            long headerValue = request.getDateHeader("If-Unmodified-Since");
            if (headerValue != -1) {
                if ( lastModified >= (headerValue + 1000)) {
                    // The entity has not been modified since the date
                    // specified by the client. This is not an error case.
                    response.sendError(HttpServletResponse.SC_PRECONDITION_FAILED);
                    return false;
                }
            }
        } catch(IllegalArgumentException illegalArgument) {
            return true;
        }
        return true;
    }
    
}