java-内存中缓存的线程安全实现

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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            LOGGER.info("CRL URI  no longer valid: " + crlUri);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;
    }




}

我有此类CachedCrlrepository,用于存储来自特定提供程序的CRL列表.我想知道我的实现是否是线程安全的,或者我在这里缺少什么.缓存用于Web服务,因此它是多线程的.

我对此特定方法表示怀疑

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;
        }

我认为这条线需要同步

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,运行GC后,缓存仍在内存中保留该条目.它从不执行这些代码行

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

解决方法:

通常,在期望大量流量和对对象的高并发访问(在本例中为crlCache)的情况下,不应使用同步Map.对于每个读取或写入线程,每个线程将在另一个线程之后等待,并且负载很重,线程计数将增加,最终服务器将崩溃.您可以查看ConcurrentHashMap.它旨在在这种情况下有效地工作.

您的第二点：

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

}

当前代码根本不需要,因为put方法已经同步.

对于最小的更改,请更换

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,但是有更好的选择. Google的Guava是一个非常强大且高效的缓存生成器.

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