摘要：?jiǎn)栴}描述最近通知應(yīng)用在近三個(gè)月內(nèi)出現(xiàn)過(guò)次緩存的問(wèn)題，第一次在重啟之后一直沒(méi)有出現(xiàn)過(guò)問(wèn)題，所以也沒(méi)有去重視，但是最近又出現(xiàn)過(guò)一次，看來(lái)很有必要徹底排查一次具體的錯(cuò)誤日志如下具體表現(xiàn)就是出現(xiàn)此異常之后連續(xù)的出現(xiàn)大量此異常

問(wèn)題描述
最近通知應(yīng)用在近三個(gè)月內(nèi)出現(xiàn)過(guò)2次DNS緩存的問(wèn)題，第一次在重啟之后一直沒(méi)有出現(xiàn)過(guò)問(wèn)題，所以也沒(méi)有去重視，但是最近又出現(xiàn)過(guò)一次，看來(lái)很有必要徹底排查一次；具體的錯(cuò)誤日志如下：

2018-03-16 18:53:59,501 ERROR [DefaultMessageListenerContainer-1] (com.bill99.asap.service.CryptoClient.seal(CryptoClient.java:34))- null
java.lang.NullPointerException
    at java.net.InetAddress$Cache.put(InetAddress.java:779) ~[?:1.7.0_79]
    at java.net.InetAddress.cacheAddresses(InetAddress.java:858) ~[?:1.7.0_79]
    at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1334) ~[?:1.7.0_79]
    at java.net.InetAddress.getAllByName0(InetAddress.java:1248) ~[?:1.7.0_79]
    at java.net.InetAddress.getAllByName(InetAddress.java:1164) ~[?:1.7.0_79]
    at java.net.InetAddress.getAllByName(InetAddress.java:1098) ~[?:1.7.0_79]
    at java.net.InetAddress.getByName(InetAddress.java:1048) ~[?:1.7.0_79]
    at java.net.InetSocketAddress.(InetSocketAddress.java:220) ~[?:1.7.0_79]
    at sun.net.NetworkClient.doConnect(NetworkClient.java:180) ~[?:1.7.0_79]
    at sun.net.www.http.HttpClient.openServer(HttpClient.java:432) ~[?:1.7.0_79]
    at sun.net.www.http.HttpClient.openServer(HttpClient.java:527) ~[?:1.7.0_79]
    at sun.net.www.http.HttpClient.(HttpClient.java:211) ~[?:1.7.0_79]
    at sun.net.www.http.HttpClient.New(HttpClient.java:308) ~[?:1.7.0_79]
    at sun.net.www.http.HttpClient.New(HttpClient.java:326) ~[?:1.7.0_79]
    at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:997) ~[?:1.7.0_79]
    at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:933) ~[?:1.7.0_79]
    at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:851) ~[?:1.7.0_79]
    at sun.net.www.protocol.http.HttpURLConnection.getOutputStream(HttpURLConnection.java:1092) ~[?:1.7.0_79]
    at org.springframework.ws.transport.http.HttpUrlConnection.getRequestOutputStream(HttpUrlConnection.java:81) ~[spring-ws-core.jar:1.5.6]
    at org.springframework.ws.transport.AbstractSenderConnection$RequestTransportOutputStream.createOutputStream(AbstractSenderConnection.java:101) ~[spring-ws-core.jar:1.5.6]
    at org.springframework.ws.transport.TransportOutputStream.getOutputStream(TransportOutputStream.java:41) ~[spring-ws-core.jar:1.5.6]
    at org.springframework.ws.transport.TransportOutputStream.write(TransportOutputStream.java:60) ~[spring-ws-core.jar:1.5.6]

具體表現(xiàn)就是出現(xiàn)此異常之后連續(xù)的出現(xiàn)大量此異常，同時(shí)系統(tǒng)節(jié)點(diǎn)不可用；

問(wèn)題分析
1.既然InetAddress$Cache.put報(bào)空指針，那就具體看一下源代碼：

if (policy != InetAddressCachePolicy.FOREVER) {
     // As we iterate in insertion order we can
     // terminate when a non-expired entry is found.
     LinkedList expired = new LinkedList<>();
     long now = System.currentTimeMillis();
     for (String key :  ) {
            CacheEntry entry = cache.get(key);
 
            if (entry.expiration >= 0 && entry.expiration < now) {
                    expired.add(key);
            } else {
                    break;
            }
     }
 
     for (String key : expired) {
            cache.remove(key);
     }
}

報(bào)空指針的的地方就是entry.expiration，也就是說(shuō)從cache取出來(lái)的entry為null，可以查看cache寫(xiě)入的地方：

CacheEntry entry = new CacheEntry(addresses, expiration);
cache.put(host, entry);

每次都是new一個(gè)CacheEntry然后再put到cache中，不會(huì)寫(xiě)入null進(jìn)去；此時(shí)猜測(cè)是多線(xiàn)程引發(fā)的問(wèn)題，cache.keySet()在遍歷的時(shí)候同時(shí)也進(jìn)行了remove操作，導(dǎo)致cache.get(key)到一個(gè)空值，查看源代碼可以發(fā)現(xiàn)一共有兩次對(duì)cache進(jìn)行remove的地方，分別是put方法和get方法，put方法代碼如上，每次在遍歷的時(shí)候檢測(cè)是否過(guò)期，然后統(tǒng)一進(jìn)行remove操作；還有一處就是get方法，代碼如下：

public CacheEntry get(String host) {
       int policy = getPolicy();
       if (policy == InetAddressCachePolicy.NEVER) {
             return null;
       }
       CacheEntry entry = cache.get(host);
 
       // check if entry has expired
       if (entry != null && policy != InetAddressCachePolicy.FOREVER) {
             if (entry.expiration >= 0 && entry.expiration < System.currentTimeMillis()) {
                    cache.remove(host);
                    entry = null;
              }
       }
       return entry;
 }

類(lèi)似put方法也是每次在get的時(shí)候進(jìn)行有效期檢測(cè)，然后進(jìn)行remove操作；
所以如果出現(xiàn)多線(xiàn)程問(wèn)題大概就是：1.同時(shí)調(diào)用put，get方法，2.多個(gè)線(xiàn)程都調(diào)用put方法；繼續(xù)查看源碼調(diào)用put和get的地方，一共有三處分別是：

private static void cacheInitIfNeeded() {
        assert Thread.holdsLock(addressCache);
        if (addressCacheInit) {
            return;
        }
        unknown_array = new InetAddress[1];
        unknown_array[0] = impl.anyLocalAddress();
 
        addressCache.put(impl.anyLocalAddress().getHostName(),
                         unknown_array);
 
        addressCacheInit = true;
    }
 
    /*
     * Cache the given hostname and addresses.
     */
    private static void cacheAddresses(String hostname,
                                       InetAddress[] addresses,
                                       boolean success) {
        hostname = hostname.toLowerCase();
        synchronized (addressCache) {
            cacheInitIfNeeded();
            if (success) {
                addressCache.put(hostname, addresses);
            } else {
                negativeCache.put(hostname, addresses);
            }
        }
    }
 
    /*
     * Lookup hostname in cache (positive & negative cache). If
     * found return addresses, null if not found.
     */
    private static InetAddress[] getCachedAddresses(String hostname) {
        hostname = hostname.toLowerCase();
 
        // search both positive & negative caches
 
        synchronized (addressCache) {
            cacheInitIfNeeded();
 
            CacheEntry entry = addressCache.get(hostname);
            if (entry == null) {
                entry = negativeCache.get(hostname);
            }
 
            if (entry != null) {
                return entry.addresses;
            }
        }
 
        // not found
        return null;
    }

cacheInitIfNeeded只在cacheAddresses和getCachedAddresses方法中被調(diào)用，用來(lái)檢測(cè)cache是否已經(jīng)被初始化了；而另外兩個(gè)方法都加了對(duì)象鎖addressCache，所以不會(huì)多線(xiàn)程問(wèn)題；

2.猜測(cè)外部直接調(diào)用了addressCache，沒(méi)有使用內(nèi)部提供的方法
查看源碼可以發(fā)現(xiàn)addressCache本身是私有屬性，也不存在對(duì)外的訪問(wèn)方法

private static Cache addressCache = new Cache(Cache.Type.Positive);

那業(yè)務(wù)代碼中應(yīng)該也不能直接使用，除非使用反射的方式，隨手搜了一下全局代碼查看關(guān)鍵字”addressCache”，搜到了類(lèi)似如下代碼：

static{
      Class clazz = java.net.InetAddress.class;  
      final Field cacheField = clazz.getDeclaredField("addressCache");  
      cacheField.setAccessible(true);  
      final Object o = cacheField.get(clazz);  
      Class clazz2 = o.getClass();  
      final Field cacheMapField = clazz2.getDeclaredField("cache");  
      cacheMapField.setAccessible(true);  
      final Map cacheMap = (Map)cacheMapField.get(o); 
}

通過(guò)反射的方式獲取了addressCache對(duì)象，然后又獲取了cache對(duì)象（cache是一個(gè)LinkedHashMap）,同時(shí)提供了一個(gè)類(lèi)似如下的方法：

    public class TEst {
 
    public static void main(String[] args) throws IOException, InterruptedException {
        final LinkedHashMap map = new LinkedHashMap<>();
        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 2000; i++) {
                    map.put(new Random().nextInt(1000), new HH(new Random(100).nextInt()));
                }
            }
        }).start();
        for (int i = 0; i < 100; i++) {
            new Thread(new Runnable() {
 
                @Override
                public void run() {
                    for (int i = 0; i < 500; i++) {
                        map.remove(new Random().nextInt(1000));
                    }
                }
            }).start();
        }
        Thread.sleep(2000);
        System.out.println("size=" + map.keySet().size() + "," + map.keySet());
        for (Integer s : map.keySet()) {
            System.out.println(map.get(s));
        }
    }
}
 
class HH {
    private int k;
 
    public HH(int k) {
        this.k = k;
    }
 
    public int getK() {
        return k;
    }
 
    public void setK(int k) {
        this.k = k;
    }
}

模擬單線(xiàn)程put操作，業(yè)務(wù)端會(huì)有多條線(xiàn)程同時(shí)remove操作，執(zhí)行看輸出結(jié)果(可以執(zhí)行多次看結(jié)果)：

size=0,[121, 517, 208]
null
null
null

可以發(fā)現(xiàn)會(huì)出現(xiàn)猜測(cè)的情況，HashMap中的size屬性本身不是線(xiàn)程安全的，所以多線(xiàn)程的情況下有可能出現(xiàn)0，這樣導(dǎo)致get方法獲取都為null，當(dāng)然HashMap還有很多其他的多線(xiàn)程問(wèn)題，因?yàn)镠ashMap也不是為多線(xiàn)程準(zhǔn)備的，至此大概了解了原因。

問(wèn)題解決
給反射獲取的cache對(duì)象加上和cacheAddresses方法同樣的鎖，或者直接不在業(yè)務(wù)代碼中處理cache對(duì)象；可以借鑒一下阿里在github開(kāi)源的操作dns緩存的項(xiàng)目：https://github.com/alibaba/ja...

總結(jié)
本次排查問(wèn)題花了一些時(shí)間在排查是不是jdk提供的類(lèi)是不是有bug，這其實(shí)是有些浪費(fèi)時(shí)間的；還有就是在排查問(wèn)題中不要放過(guò)任何一種可能往往問(wèn)題就發(fā)生在那些理所當(dāng)然的地方。