内存中缓存的线程安全实现

Question

import java.io.IOException;
import java.lang.ref.SoftReference;
import java.net.URI;
import java.security.cert.CRLException;
import java.security.cert.CertificateException;
import java.security.cert.X509CRL;
import java.security.cert.X509Certificate;
import java.util.Collections;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

import javax.naming.NamingException;

import org.joda.time.DateTime;
import org.kp.oppr.esb.logger.Logger;
import org.springframework.beans.factory.annotation.Autowired;

public class CachedCrlRepository {

    private static final Logger LOGGER = new Logger("CachedCrlRepository");

    private final Map<URI, SoftReference<X509CRL>> crlCache = Collections
            .synchronizedMap(new HashMap<URI, SoftReference<X509CRL>>());;

    private static int DEFAULT_CACHE_AGING_HOURS;

    @Autowired
    private DgtlSgntrValidator validator;

    @Autowired
    private  CrlRepository crlRepository;

    public X509CRL findCrl(URI crlUri, X509Certificate issuerCertificate,
            Date validationDate) throws DigitalValdiationException,
            CertificateException, CRLException, IOException, NamingException {
        SoftReference<X509CRL> crlRef = this.crlCache.get(crlUri);
        if (null == crlRef) {
            LOGGER.info("Key CRL URI : " + crlUri +  "  not found in the cache " );
            return refreshCrl(crlUri, issuerCertificate, validationDate);
        }
        X509CRL crl = crlRef.get();
        if (null == crl) {
            LOGGER.info("CRL Entry garbage collected: " + crlUri);
            return refreshCrl(crlUri, issuerCertificate, validationDate);
        }
        if (validationDate.after(crl.getNextUpdate())) {
            LOGGER.info("CRL URI  no longer valid: " + crlUri);
            LOGGER.info("CRL validation date: " + validationDate + " is after CRL next update date: " + crl.getNextUpdate());
            return refreshCrl(crlUri, issuerCertificate, validationDate);
        }

        Date thisUpdate = crl.getThisUpdate();
        LOGGER.info("This update " + thisUpdate);

        /*
         * The PKI the nextUpdate CRL extension indicates 7 days. The
         * actual CRL refresh rate is every 3 hours. So it's a bit dangerous to
         * only base the CRL cache refresh strategy on the nextUpdate field as
         * indicated by the CRL.
         */

        DateTime cacheMaturityDateTime = new DateTime(thisUpdate)
                .plusHours(DEFAULT_CACHE_AGING_HOURS);
        LOGGER.info("Cache maturity Date Time " + cacheMaturityDateTime);
        if (validationDate.after(cacheMaturityDateTime.toDate())) {
            LOGGER.info("Validation date: "  + validationDate + " is after cache maturity date: " + cacheMaturityDateTime.toDate());
            return refreshCrl(crlUri, issuerCertificate, validationDate);
        }
        LOGGER.info("using cached CRL: " + crlUri);
        return crl;
    }

    public static int getDEFAULT_CACHE_AGING_HOURS() {
        return DEFAULT_CACHE_AGING_HOURS;
    }

    public static void setDEFAULT_CACHE_AGING_HOURS(int dEFAULT_CACHE_AGING_HOURS) {
        DEFAULT_CACHE_AGING_HOURS = dEFAULT_CACHE_AGING_HOURS;
    }

    private X509CRL refreshCrl(URI crlUri, X509Certificate issuerCertificate,
            Date validationDate) throws DigitalValdiationException,
            CertificateException, CRLException, IOException, NamingException {
        X509CRL crl = crlRepository.downloadCRL(crlUri.toString());
        this.crlCache.put(crlUri, new SoftReference<X509CRL>(crl));
        return crl;
    }




}

我有这个类 CachedCrlrepository 存储来自特定提供者的 CRL 列表。我想知道我的实现是否是线程安全的，或者我在这里遗漏了一些东西。缓存用于 Web 服务，因此它是多线程的。

我对这种特殊方法的怀疑

private X509CRL refreshCrl(URI crlUri, X509Certificate issuerCertificate,
                Date validationDate) throws DigitalValdiationException,
                CertificateException, CRLException, IOException, NamingException {
            X509CRL crl = crlRepository.downloadCRL(crlUri.toString());
            this.crlCache.put(crlUri, new SoftReference<X509CRL>(crl));
            return crl;
        }

我认为这个特定的行需要同步

this.crlCache.put(crlUri, new SoftReference<X509CRL>(crl));

 synchronized(this)
{
this.crlCache.put(crlUri, new SoftReference<X509CRL>(crl));

}

我看到的另一个问题是，在运行 GC 之后，缓存中仍然有该条目在内存中。它从不执行这行代码

if (null == crl) {
            LOGGER.info("CRL Entry garbage collected: " + crlUri);
            return refreshCrl(crlUri, issuerCertificate, validationDate);
        } 

Answer 1

一般情况下，如果您期望对象（在这种情况下为crlCache）出现大量流量和高并发访问，则不应使用同步映射。对于每一个读或写线程都会在另一个之后等待并且负载很重，你的线程数会很高，最终你的服务器会崩溃。您可以查看ConcurrentHashMap。它旨在在这种情况下高效工作。

你的第二点：

synchronized(this)
{
this.crlCache.put(crlUri, new SoftReference<X509CRL>(crl));

}

当前代码完全不需要，因为 put 方法已经同步。

为了最小的变化替换

private final Map<URI, SoftReference<X509CRL>> crlCache = Collections
            .synchronizedMap(new HashMap<URI, SoftReference<X509CRL>>());;

与

private final ConcurrentHashMap<URI, SoftReference<X509CRL>> crlCache = new ConcurrentHashMap<URI, SoftReference<X509CRL>>();

最后，使用SoftReference 很好，但还有更好的选择。来自 google 的Guava 是一个非常强大且高效的缓存构建器。