線程內存緩存的安全實現

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      LOGGER.info("CRL URI no longer valid: " + crlUri); 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




} 

我有這個類CachedCrlrepository存儲來自特定提供者的CRL列表。我想知道我的實現是線程安全的還是我在這裏丟失了某些東西。緩存用於Web服務，因此它是多線程的。

我在這個特殊的方法

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; 
     } 

我覺得這一行需要同步

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

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

} 

另外一個問題，我看到的是，一個GC運行緩存後仍然有疑問內存中的條目。它從來沒有執行這些行代碼

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

Answer 1

通常你不應該使用在你期待擁擠的交通和你的對象，在這種情況下是crlCache高併發訪問的情況下同步地圖。對於每一個讀取或寫入的線程都會在另一個線程之後等待，並且在負載較重的情況下，線程數量會很高，最終服務器會崩潰。你可以看看ConcurrentHashMap。其目的是在這種情況下有效地工作。

你的第二點：

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

} 

是根本不需要與當前的代碼put方法已經同步。

對於最小的變化更換

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

與

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

最後，使用SoftReference是好的，但有更好的選擇。來自google的Guava是一個非常強大且高效的緩存構建器。