http-client-实现-keepalive-源码探究

前几天在分享 ” 实现自己的 wget” 的时候，因为我们的请求是一次性的，http 头里设置的 Connection: Close。在HTTP/1.1 为了提升 HTTP 1.0 的网络性能，增加了 keepalive 的特性。浏览器在请求的时候都会加上 Connection: Keep-Alive 的头信息，是如何实现的呢？
我们知道在服务端（nginx）可以通过设置 keepalive_timeout 来控制连接保持时间，那么 http 连接的保持需要浏览器（客户端）支持吗？今天咱们一起来通过 java.net.HttpURLConnection 源码看看客户端是如何维护这些 http 连接的。

测试代码

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();}
}

解析返回的头信息

当客户端从服务端获取返回的字节流时

connection.getInputStream()

HttpClient会对返回的头信息进行解析，我简化了摘取了最重要的逻辑代码

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);
    }
    ...
}

是否需要保持长连接，是客户端申请，服务端决定，所以要以服务端返回的头信息为准。比如客户端发送的请求是 Connection: Keep-Alive，服务端返回的是Connection: Close 那也得以服务端为准。

客户端请求完成

当第一次执行时 bufferedInputStream.read(buf) 时，HttpClient会执行 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 长连接缓存

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);
        }
    }
    ...
}

这里涉及了 KeepAliveKey 和ClientVector

class KeepAliveKey {
    private String protocol = null;
    private String host = null;
    private int port = 0;
    private Object obj = null;
}

设计这个对象呢，是因为只有 protocol+host+port 才能确定为同一个连接。所以用 KeepAliveKey 作为 KeepAliveCache 的key。
ClientVector则是一个栈，每次有同一个域下的请求都入栈。

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()));
        }
    }
    ...
}

“断开”连接

connection.disconnect();

如果是保持长连接的，实际只是关闭了一些流，socket 并没有关闭。

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();
    }
}

连接的复用

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取的时候则出栈，出栈过程中如果该连接已经超时，则关闭与服务端的连接，继续执行出栈操作。

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;
        }
    }
    ...
}

这样就实现了客户端 http 连接的复用。

小结

存储结构如下

复用 tcp 的连接标准是 protocol+host+port，客户端连接与服务端维持的连接数也不宜过多，HttpURLConnection 默认只能存 5 个不同的连接，再多则直接断开连接（见上面 HttpClient#finished 方法），保持连接数过多对客户端和服务端都会增加不小的压力。
同时 KeepAliveCache 也每隔 5 秒钟扫描检测一次，清除过期的httpClient。