JDK對Http協議的Keep-Alive的支持，以JDK8為例

Http協議對keep-alive的支持

? keep-alive顧名思義就是保持連接的意思，在早期的HTTP/1.0中，每次http請求都要創建一個連接，而創建連接的過程需要消耗資源和時間，為了減少資源消耗，縮短響應時間，就需要重用連接。
? 在后來的HTTP/1.0中以及HTTP/1.1中，引入了重用連接的機制，就是在http請求頭中加入Connection: keep-alive來告訴對方這個請求響應完成后不要關閉，下一次咱們還用這個請求繼續交流。
? 協議規定HTTP/1.0如果想要保持長連接，需要在請求頭中加上Connection: keep-alive，而HTTP/1.1默認是支持長連接的，有沒有這個請求頭都行。另外，一般服務端都會設置keep-alive超時時間。超過指定的時間間隔，服務端就會主動關閉連接。同時服務端還會設置一個參數叫最大請求數，比如當最大請求數是300時，只要請求次數超過300次，即使還沒到超時時間，服務端也會主動關閉連接。如下圖所示為服務器返回的Responser Headers的值。

Reponse HeadersConnection: Keep-AliveContent-length: 35content-type: application/json;charset=UTF-8Keep-Alive: timeout=60,max=5

? 注意，當Connection:keep-alive存在時，下面的Keep-Alive: timeout=70, max=10才會生效。

? Http協議對keep-alive的支持是基于TCP連接的成功建立，而TCP協議是對Http透明的，即TCP協議的Keep-Alive與Http的Keep-Alive是無關的。

TCP協議中的keep-Alive

? TCP keepalive指的是TCP保活計時器（keepalive timer）。設想有這樣的情況：客戶已主動與服務器建立了TCP連接。但后來客戶端的主機突然出故障。顯然，服務器以后就不能再收到客戶發來的數據。因此，應當有措施使服務器不要再白白等待下去。這就是使用保活計時器。服務器每收到一次客戶的數據，就重新設置保活計時器，時間的設置通常是兩小時。若兩小時沒有收到客戶的數據，服務器就發送一個探測報文段，以后則每隔75秒發送一次。若一連發送10個探測報文段后仍無客戶的響應，服務器就認為客戶端出了故障，接著就關閉這個連接。

——摘自謝希仁《計算機網絡》

JDK對Http協議的keep-alive的支持

? JDK對Http協議的Keep-Alive的支持參考oracel官方說明https://docs.oracle.com/javase/8/docs/technotes/guides/net/http-keepalive.html。接下來結合jdk源碼分析：

1、HttpURLConnection（java.net.HttpURLConnection）使用長連接

? JDK自帶的HttpURLConnection，默認啟用keepAlive，支持HTTP / 1.1和HTTP / 1.0持久連接， 使用后的HttpURLConnection會放入緩存中供以后的同host:port的請求重用，底層的socket在keepAlive超時之前不會關閉。

? HttpURLConnection受以下system properties控制：
? http.keepAlive=（默認值：true），是否啟用keepAlive，如果設置為false，則HttpURLConnection不會緩存，使用完后會關閉socket連接。（可設置）
? http.maxConnections=（默認值：5），每個目標host緩存socket連接的最大數。（當http.keepAlive=false時為1，否則為5，下面結合源碼分析）

2、sun.net.www.http包下的HttpURLConnection和HttpClient長連接緩存

? sun.net.www.http.HttpURLConnection是java.net.HttpURLConnection的實現類，JDK自帶的HttpURLConnection底層使用JDK自帶的HttpClient發送http請求，java8的HttpClient（sun.net.www.http.HttpClient）存在諸多限制，在Jdk11中，java.net.HttpClient吸收第三方HttpClient的優點，性能肩比第三方HttpClient(okHttptClitn，apache Httpclient)。

? 下面為sun.net.www.http.HttpURLConnection.HttpURLConnection類，有如下兩個方法返回HttpClient的實例和直接關閉socket的disconnect()方法。

public class HttpURLConnection extends java.net.HttpURLConnection {// subclass HttpsClient will overwrite & return an instance of HttpsClientprotected HttpClient getNewHttpClient(URL url, Proxy p, int connectTimeout)throws IOException {return HttpClient.New(url, p, connectTimeout, this);}// subclass HttpsClient will overwrite & return an instance of HttpsClientprotected HttpClient getNewHttpClient(URL url, Proxy p, int connectTimeout, boolean useCache)throws IOException {return HttpClient.New(url, p, connectTimeout, useCache, this);}/*** Disconnect from the server (public API)*/public void disconnect() {} }

? 下面為HttpClient類，parseHTTPHeader會解析header參數，判斷HttpURLConnection是否是長連接，如果是長連接會讀取keepAliveTimeout參數作為KeepAliveCache類里的長連接的緩存有效時間（默認為0）。并設置keepAliveConnections的值，如過為長連接值為5，否則為1。

? 完成獲取請求返回結果后或調用getInputStream().close()，JDK會清理連接并作為以后使用的連接緩存。具體邏輯為執行finished()方法，如果是短連接，直接關閉套接字（調用closeServer()方法），如果是長連接，則將這個長連接加到KeepAliveCache的緩存線程中（putInKeepAliveCache()方法）。

? 其中，protected static KeepAliveCache kac = new KeepAliveCache();代碼表面運行時只有全局只有一個緩存的線程類。

public class HttpClient extends NetworkClient {// whether this httpclient comes from the cacheprotected boolean cachedHttpClient = false;protected boolean inCache;/* where we cache currently open, persistent connections */protected static KeepAliveCache kac = new KeepAliveCache();private static boolean keepAliveProp = true;private boolean parseHTTPHeader(MessageHeader responses, ProgressSource pi, HttpURLConnection httpuc){keepAliveConnections = -1;keepAliveTimeout = 0;}public void finished() {if (reuse) /* will be reused */return;keepAliveConnections--;poster = null;if (keepAliveConnections > 0 && isKeepingAlive() &&!(serverOutput.checkError())) {/* This connection is keepingAlive && still valid.* Return it to the cache.*/putInKeepAliveCache();} else {closeServer();}}protected synchronized void putInKeepAliveCache() {if (inCache) {assert false : "Duplicate put to keep alive cache";return;}inCache = true;kac.put(url, null, this);} }

? KeepAliveCache繼承了HashMap，實現了Runnable接口。

? 它本身可以存儲不同的KeepAliveKey-ClientVector，KeepAliveKey是關于協議（一般為http），ip，port類，對應了一個socket連接。ClientVector實現了雙端隊列，存儲了HttpClient實例和idleStartTime（該HttpClient開始空閑的時間），最多可以存5個，對應HttpClient類的keepAliveConnections。ClientVector還有一個屬性nap，即緩存過期時間，在put()方法實例化時設置，new ClientVector(keepAliveTimeout > 0 ? keepAliveTimeout * 1000 : LIFETIME)

? KeepAliveCache也是一個線程類，run方法的邏輯是do-while循環檢測HashMap中緩存的長連接是否timeout，如果超時就清理，當HashMap為空時，線程完成自己的邏輯，執行完畢。

? KeepAliveCache提供put()方法，運行時KeepAliveCache線程類全局只有一個，沒有緩存的線程類時，在put()方法中創建該緩存類keepAliveTimer，并設置長連接的緩存時間。

public class KeepAliveCacheextends HashMap<KeepAliveKey, ClientVector>implements Runnable {/* Sleeps for an alloted timeout, then checks for timed out connections.* Errs on the side of caution (leave connections idle for a relatively* short time).*/@Overridepublic void run() {do {try {Thread.sleep(LIFETIME);} catch (InterruptedException e) {}// Remove all outdated HttpClients.synchronized (this) {long currentTime = System.currentTimeMillis();List<KeepAliveKey> keysToRemove = new ArrayList<>();for (KeepAliveKey key : keySet()) {ClientVector v = get(key);synchronized (v) {KeepAliveEntry e = v.peek();while (e != null) {if ((currentTime - e.idleStartTime) > v.nap) {v.poll();e.hc.closeServer();} else {break;}e = v.peek();}if (v.isEmpty()) {keysToRemove.add(key);}}}for (KeepAliveKey key : keysToRemove) {removeVector(key);}}} while (!isEmpty());}/*** Register this URL and HttpClient (that supports keep-alive) with the cache* @param url The URL contains info about the host and port* @param http The HttpClient to be cached*/public synchronized void put(final URL url, Object obj, HttpClient http) {boolean startThread = (keepAliveTimer == null);if (!startThread) {if (!keepAliveTimer.isAlive()) {startThread = true;}}if (startThread) {clear();/* Unfortunately, we can't always believe the keep-alive timeout we got* back from the server. If I'm connected through a Netscape proxy* to a server that sent me a keep-alive* time of 15 sec, the proxy unilaterally terminates my connection* The robustness to get around this is in HttpClient.parseHTTP()*/final KeepAliveCache cache = this;AccessController.doPrivileged(new PrivilegedAction<>() {public Void run() {keepAliveTimer = InnocuousThread.newSystemThread("Keep-Alive-Timer", cache);keepAliveTimer.setDaemon(true);keepAliveTimer.setPriority(Thread.MAX_PRIORITY - 2);keepAliveTimer.start();return null;}});}KeepAliveKey key = new KeepAliveKey(url, obj);ClientVector v = super.get(key);if (v == null) {int keepAliveTimeout = http.getKeepAliveTimeout();v = new ClientVector(keepAliveTimeout > 0 ?keepAliveTimeout * 1000 : LIFETIME);v.put(http);super.put(key, v);} else {v.put(http);}}class ClientVector extends ArrayDeque<KeepAliveEntry> {// sleep time in milliseconds, before cache clearint nap;ClientVector(int nap) {this.nap = nap;}}class KeepAliveKey {private String protocol = null;private String host = null;private int port = 0;private Object obj = null; // additional key, such as socketfactory/*** Constructor* @param url the URL containing the protocol, host and port information*/public KeepAliveKey(URL url, Object obj) {this.protocol = url.getProtocol();this.host = url.getHost();this.port = url.getPort();this.obj = obj;}}class KeepAliveEntry {HttpClient hc;long idleStartTime;KeepAliveEntry(HttpClient hc, long idleStartTime) {this.hc = hc;this.idleStartTime = idleStartTime;}} }

3、HttpURLConnection使用短連接

3.1）使用短連接，不緩存長連接

? 設置System.setProperty(“http.keepAlive”, ”false”);將整個 APP 的 http 長連接支持關掉。不會啟動新的線程去緩存HttpClient連接，在HttpClient類的finished方法里不執行緩存連接的操作，直接關閉socket連接。

3.2）客戶端不使用keep-alive功能

? HttpURLConnection的實例獲取到服務方的數據后直接關閉HttClient(關閉socket)，不緩存如下所示：

//第一種，Header指定短連接 httpConn.setRequestProperty("Connection", "close"); //第二種，請求完后直接關閉socket httpURLConnection.disconnect();

3.3）服務端長連接關閉

? 返回的Response Header中包含Connection:close即可。

參考另外幾位大佬們的文章：
http://www.itersblog.com/archives/3.html
https://blog.csdn.net/u012216753/article/details/78084327
https://blog.csdn.net/tianshouzhi/article/details/103922842?utm_medium=distribute.pc_relevant.none-task-blog-baidujs_title-5&spm=1001.2101.3001.4242

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