前幾天在分享"實作自己的wget"的時候，因為我們的請求是一次性的，http 頭裡設定的 Connection: Close HTTP/1.1 HTTP 1.0 keepalive Connection: Keep-Alive
的頭資訊，是如何實作的呢？
我們知道在服務端（nginx）可以通過設定
keepalive_timeout
http
java.net.HttpURLConnection
http
測試代碼
package net.mengkang.demo;
import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
public class Demo {
public static void main(String[] args) throws IOException {
test();
test();
}
private static void test() throws IOException {
URL url = new URL("http://static.mengkang.net/upload/image/2019/0921/1569075837628814.jpeg");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestProperty("Charset", "UTF-8");
connection.setRequestProperty("Connection", "Keep-Alive");
connection.setRequestMethod("GET");
connection.connect();
BufferedInputStream bufferedInputStream = new BufferedInputStream(connection.getInputStream());
File file = new File("./xxx.jpeg");
OutputStream out = new FileOutputStream(file);
int size;
byte[] buf = new byte[1024];
while ((size = bufferedInputStream.read(buf)) != -1) {
out.write(buf, 0, size);
}
connection.disconnect();
}
} 解析傳回的頭資訊
當用戶端從服務端擷取傳回的位元組流時
connection.getInputStream() HttpClient
會對傳回的頭資訊進行解析，我簡化了摘取了最重要的邏輯代碼
private boolean parseHTTPHeader(MessageHeader var1, ProgressSource var2, HttpURLConnection var3) throws IOException {
String var15 = var1.findValue("Connection");
...
if (var15 != null && var15.toLowerCase(Locale.US).equals("keep-alive")) {
HeaderParser var11 = new HeaderParser(var1.findValue("Keep-Alive"));
this.keepAliveConnections = var11.findInt("max", this.usingProxy ? 50 : 5);
this.keepAliveTimeout = var11.findInt("timeout", this.usingProxy ? 60 : 5);
}
...
} 是否需要保持長連接配接，是用戶端申請，服務端決定，是以要以服務端傳回的頭資訊為準。比如用戶端發送的請求是
Connection: Keep-Alive
，服務端傳回的是
Connection: Close
那也得以服務端為準。
用戶端請求完成
當第一次執行時
bufferedInputStream.read(buf)
時，
HttpClient
會執行
finished()
方法
public void finished() {
if (!this.reuse) {
--this.keepAliveConnections;
this.poster = null;
if (this.keepAliveConnections > 0 && this.isKeepingAlive() && !this.serverOutput.checkError()) {
this.putInKeepAliveCache();
} else {
this.closeServer();
}
}
} 加入到 http 長連接配接緩存
protected static KeepAliveCache kac = new KeepAliveCache();
protected synchronized void putInKeepAliveCache() {
if (this.inCache) {
assert false : "Duplicate put to keep alive cache";
} else {
this.inCache = true;
kac.put(this.url, (Object)null, this);
}
} public class KeepAliveCache extends HashMap<KeepAliveKey, ClientVector> implements Runnable {
...
public synchronized void put(URL var1, Object var2, HttpClient var3) {
KeepAliveKey var5 = new KeepAliveKey(var1, var2); // var2 null
ClientVector var6 = (ClientVector)super.get(var5);
if (var6 == null) {
int var7 = var3.getKeepAliveTimeout();
var6 = new ClientVector(var7 > 0 ? var7 * 1000 : 5000);
var6.put(var3);
super.put(var5, var6);
} else {
var6.put(var3);
}
}
...
} 這裡涉及了
KeepAliveKey
和
ClientVector
class KeepAliveKey {
private String protocol = null;
private String host = null;
private int port = 0;
private Object obj = null;
} 設計這個對象呢，是因為隻有
protocol
+
host
port
才能确定為同一個連接配接。是以用
KeepAliveKey
作為
KeepAliveCache
的
key
。
ClientVector
則是一個棧，每次有同一個域下的請求都入棧。
class ClientVector extends Stack<KeepAliveEntry> {
private static final long serialVersionUID = -8680532108106489459L;
int nap;
ClientVector(int var1) {
this.nap = var1;
}
synchronized void put(HttpClient var1) {
if (this.size() >= KeepAliveCache.getMaxConnections()) {
var1.closeServer();
} else {
this.push(new KeepAliveEntry(var1, System.currentTimeMillis()));
}
}
...
} “斷開”連接配接
connection.disconnect(); 如果是保持長連接配接的，實際隻是關閉了一些流，socket 并沒有關閉。
public void disconnect() {
...
boolean var2 = var1.isKeepingAlive();
if (var2) {
var1.closeIdleConnection();
}
...
} public void closeIdleConnection() {
HttpClient var1 = kac.get(this.url, (Object)null);
if (var1 != null) {
var1.closeServer();
}
} 連接配接的複用
public static HttpClient New(URL var0, Proxy var1, int var2, boolean var3, HttpURLConnection var4) throws IOException {
...
HttpClient var5 = null;
if (var3) {
var5 = kac.get(var0, (Object)null);
...
}
if (var5 == null) {
var5 = new HttpClient(var0, var1, var2);
} else {
...
var5.url = var0;
}
return var5;
} public class KeepAliveCache extends HashMap<KeepAliveKey, ClientVector> implements Runnable {
...
public synchronized HttpClient get(URL var1, Object var2) {
KeepAliveKey var3 = new KeepAliveKey(var1, var2);
ClientVector var4 = (ClientVector)super.get(var3);
return var4 == null ? null : var4.get();
}
...
} ClientVector
取的時候則出棧，出棧過程中如果該連接配接已經逾時，則關閉與服務端的連接配接，繼續執行出棧操作。
class ClientVector extends Stack<KeepAliveEntry> {
private static final long serialVersionUID = -8680532108106489459L;
int nap;
ClientVector(int var1) {
this.nap = var1;
}
synchronized HttpClient get() {
if (this.empty()) {
return null;
} else {
HttpClient var1 = null;
long var2 = System.currentTimeMillis();
do {
KeepAliveEntry var4 = (KeepAliveEntry)this.pop();
if (var2 - var4.idleStartTime > (long)this.nap) {
var4.hc.closeServer();
} else {
var1 = var4.hc;
}
} while(var1 == null && !this.empty());
return var1;
}
}
...
} 這樣就實作了用戶端
http
連接配接的複用。
小結
存儲結構如下
複用
tcp
的連接配接标準是
protocol
host
port
，用戶端連接配接與服務端維持的連接配接數也不宜過多，
HttpURLConnection
預設隻能存5個不同的連接配接，再多則直接斷開連接配接（見上面
HttpClient#finished
方法），保持連接配接數過多對用戶端和服務端都會增加不小的壓力。
同時
KeepAliveCache
也每隔5秒鐘掃描檢測一次，清除過期的
httpClient
![TCP保活（TCP keepalive）[圖]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)