用JAVA 源码角度看 客户端请求服务器流程中地址是域名情况下解析域名流程

1. 域名解析的入口点

• getaddrinfo 或 getAllByName 方法：在底层，Java 使用 getaddrinfo（在 Unix-like 系统中）或类似的系统调用来解析域名。在 Java 的 InetAddress 类中，getAllByName 方法是解析域名的入口点之一。它会调用 getAddressesFromNameService 方法来获取域名对应的 IP 地址列表。

2. getAddressesFromNameService 方法

• 调用 nameService.lookupAllHostAddr：此方法尝试通过注册的 NameService（如 PlatformNameService）来解析域名。PlatformNameService 是一个内部类，它实现了 NameService 接口，并调用 impl.lookupAllHostAddr 方法来执行实际的域名解析。
• 处理异常：如果解析失败，会抛出 UnknownHostException。对于 "localhost" 这样的特殊情况，会直接返回回环地址。
• 处理请求地址：如果指定了请求地址 (reqAddr)，则会检查解析结果中是否包含该地址，并进行相应的处理（如旋转数组）.

3. lookupAllHostAddr 方法

• 本地缓存和系统缓存：在某些实现中，可能会先检查本地缓存或系统缓存，以快速获取域名对应的 IP 地址。
• 调用底层系统调用：最终，会调用底层的系统调用（如 getaddrinfo）来从 DNS 服务器获取域名对应的 IP 地址列表。这个过程可能涉及与本地 DNS 服务器的通信，以及 DNS 服务器之间的查询转发.

4. plainConnect 和 plainConnect0 方法

• 权限检查：在 plainConnect 方法中，会先检查是否已经连接。如果未连接，则会进行权限检查，确保有权限连接到指定的 URL。
• 使用缓存：在 plainConnect0 方法中，会尝试从本地缓存中获取响应。如果缓存命中且满足条件（如不是 HTTPS 请求），则会直接使用缓存的响应，而不需要进行实际的网络连接。
• 代理选择：如果没有缓存命中，则会根据配置选择合适的代理（如实例代理或系统默认代理）。如果没有代理或代理连接失败，则会尝试直接连接。
• 创建 HTTP 客户端：通过 getNewHttpClient 方法创建一个新的 HTTP 客户端，并设置连接超时和读取超时等参数。
• 打开服务器连接：调用 openServer 方法来打开与远程服务器的连接。如果使用代理，则会通过代理服务器进行连接。

5. openServer 方法

• 安全检查：如果存在安全管理器，则会进行连接权限检查。
• 处理代理：如果使用代理，则会根据代理类型（如 HTTP 代理或 SOCKS 代理）进行相应的处理。对于 HTTP 代理，会调用 privilegedOpenServer 方法来通过代理服务器建立连接。
• 直接连接：如果没有代理或代理连接失败，则会直接连接到远程服务器。调用 super.openServer 方法来执行实际的连接操作.

总结

Java 客户端请求流程中解析域名的过程涉及多个层次的处理，从 InetAddress 类的高层方法到底层的系统调用。通过缓存、代理选择和权限检查等机制，Java 能够灵活地处理各种网络环境下的域名解析和连接请求。这个过程确保了客户端能够高效、安全地与远程服务器进行通信。

##实验代码

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;

public class HttpUrlConnectionDemo {

    public static void main(String[] args) {
        try {
            // 创建URL对象
            URL url = new URL("http://www.baidu.com");
            // 打开连接
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            // 设置请求方法为GET
            connection.setRequestMethod("GET");
            // 设置超时时间
            connection.setConnectTimeout(5000);
            connection.setReadTimeout(5000);

            // 获取响应码
            int responseCode = connection.getResponseCode();
            System.out.println("Response Code: " + responseCode);

            // 如果响应成功，则读取响应内容
            if (responseCode == HttpURLConnection.HTTP_OK) { // 200表示成功
                BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
                String inputLine;
                StringBuilder content = new StringBuilder();

                while ((inputLine = in.readLine()) != null) {
                    content.append(inputLine);
                }

                // 关闭流
                in.close();
                // 输出响应内容
                System.out.println("Response Content: " + content.toString());
            } else {
                System.out.println("GET request not worked");
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

##java源码

protected void plainConnect()  throws IOException {
        synchronized (this) {
            if (connected) {
                return;
            }
        }
        SocketPermission p = URLtoSocketPermission(this.url);
        if (p != null) {
            try {
                AccessController.doPrivilegedWithCombiner(
                    new PrivilegedExceptionAction<>() {
                        public Void run() throws IOException {
                            plainConnect0();
                            return null;
                        }
                    }, null, p
                );
            } catch (PrivilegedActionException e) {
                    throw (IOException) e.getException();
            }
        } else {
            // run without additional permission
            plainConnect0();
        }
    }

protected void plainConnect0()  throws IOException {
        // try to see if request can be served from local cache
        if (cacheHandler != null && getUseCaches()) {
            try {
                URI uri = ParseUtil.toURI(url);
                if (uri != null) {
                    cachedResponse = cacheHandler.get(uri, getRequestMethod(), getUserSetHeaders().getHeaders());
                    if ("https".equalsIgnoreCase(uri.getScheme())
                        && !(cachedResponse instanceof SecureCacheResponse)) {
                        cachedResponse = null;
                    }
                    if (logger.isLoggable(PlatformLogger.Level.FINEST)) {
                        logger.finest("Cache Request for " + uri + " / " + getRequestMethod());
                        logger.finest("From cache: " + (cachedResponse != null ? cachedResponse.toString() : "null"));
                    }
                    if (cachedResponse != null) {
                        cachedHeaders = mapToMessageHeader(cachedResponse.getHeaders());
                        cachedInputStream = cachedResponse.getBody();
                    }
                }
            } catch (IOException ioex) {
                // ignore and commence normal connection
            }
            if (cachedHeaders != null && cachedInputStream != null) {
                connected = true;
                return;
            } else {
                cachedResponse = null;
            }
        }
        try {
            /* Try to open connections using the following scheme,
             * return on the first one that's successful:
             * 1) if (instProxy != null)
             *        connect to instProxy; raise exception if failed
             * 2) else use system default ProxySelector
             * 3) else make a direct connection if ProxySelector is not present
             */

            if (instProxy == null) { // no instance Proxy is set
                /**
                 * Do we have to use a proxy?
                 */
                ProxySelector sel =
                    java.security.AccessController.doPrivileged(
                        new java.security.PrivilegedAction<>() {
                            public ProxySelector run() {
                                     return ProxySelector.getDefault();
                                 }
                             });
                if (sel != null) {
                    URI uri = sun.net.www.ParseUtil.toURI(url);
                    if (logger.isLoggable(PlatformLogger.Level.FINEST)) {
                        logger.finest("ProxySelector Request for " + uri);
                    }
                    Iterator<Proxy> it = sel.select(uri).iterator();
                    Proxy p;
                    while (it.hasNext()) {
                        p = it.next();
                        try {
                            if (!failedOnce) {
                                http = getNewHttpClient(url, p, connectTimeout);
                                http.setReadTimeout(readTimeout);
                            } else {
                                // make sure to construct new connection if first
                                // attempt failed
                                http = getNewHttpClient(url, p, connectTimeout, false);
                                http.setReadTimeout(readTimeout);
                            }
                            if (logger.isLoggable(PlatformLogger.Level.FINEST)) {
                                if (p != null) {
                                    logger.finest("Proxy used: " + p.toString());
                                }
                            }
                            break;
                        } catch (IOException ioex) {
                            if (p != Proxy.NO_PROXY) {
                                sel.connectFailed(uri, p.address(), ioex);
                                if (!it.hasNext()) {
                                    if (logger.isLoggable(PlatformLogger.Level.FINEST)) {
                                        logger.finest("Retrying with proxy: " + p.toString());
                                    }
                                    http = getNewHttpClient(url, p, connectTimeout, false);
                                    http.setReadTimeout(readTimeout);
                                    break;
                                }
                            } else {
                                throw ioex;
                            }
                            continue;
                        }
                    }
                } else {
                    // No proxy selector, create http client with no proxy
                    if (!failedOnce) {
                        http = getNewHttpClient(url, null, connectTimeout);
                        http.setReadTimeout(readTimeout);
                    } else {
                        // make sure to construct new connection if first
                        // attempt failed
                        http = getNewHttpClient(url, null, connectTimeout, false);
                        http.setReadTimeout(readTimeout);
                    }
                }
            } else {
                if (!failedOnce) {
                    http = getNewHttpClient(url, instProxy, connectTimeout);
                    http.setReadTimeout(readTimeout);
                } else {
                    // make sure to construct new connection if first
                    // attempt failed
                    http = getNewHttpClient(url, instProxy, connectTimeout, false);
                    http.setReadTimeout(readTimeout);
                }
            }

            ps = (PrintStream)http.getOutputStream();
        } catch (IOException e) {
            throw e;
        }
        // constructor to HTTP client calls openserver
        connected = true;
    }

protected HttpClient getNewHttpClient(URL url, Proxy p, int connectTimeout)
        throws IOException {
        return HttpClient.New(url, p, connectTimeout, this);
    }

protected synchronized void openServer() throws IOException {

        SecurityManager security = System.getSecurityManager();

        if (security != null) {
            security.checkConnect(host, port);
        }

        if (keepingAlive) { // already opened
            return;
        }

        if (url.getProtocol().equals("http") ||
            url.getProtocol().equals("https") ) {

            if ((proxy != null) && (proxy.type() == Proxy.Type.HTTP)) {
                sun.net.www.URLConnection.setProxiedHost(host);
                privilegedOpenServer((InetSocketAddress) proxy.address());
                usingProxy = true;
                return;
            } else {
                // make direct connection
                openServer(host, port);
                usingProxy = false;
                return;
            }

        } else {
            /* we're opening some other kind of url, most likely an
             * ftp url.
             */
            if ((proxy != null) && (proxy.type() == Proxy.Type.HTTP)) {
                sun.net.www.URLConnection.setProxiedHost(host);
                privilegedOpenServer((InetSocketAddress) proxy.address());
                usingProxy = true;
                return;
            } else {
                // make direct connection
                super.openServer(host, port);
                usingProxy = false;
                return;
            }
        }
    }

public void openServer(String server, int port) throws IOException {
        serverSocket = doConnect(server, port);
        try {
            OutputStream out = serverSocket.getOutputStream();
            if (capture != null) {
                out = new HttpCaptureOutputStream(out, capture);
            }
            serverOutput = new PrintStream(
                new BufferedOutputStream(out),
                                         false, encoding);
        } catch (UnsupportedEncodingException e) {
            throw new InternalError(encoding+" encoding not found", e);
        }
        serverSocket.setTcpNoDelay(true);
    }

protected Socket doConnect (String server, int port)
    throws IOException, UnknownHostException {
        Socket s;
        if (proxy != null) {
            if (proxy.type() == Proxy.Type.SOCKS) {
                s = AccessController.doPrivileged(
                    new PrivilegedAction<>() {
                        public Socket run() {
                                       return new Socket(proxy);
                                   }});
            } else if (proxy.type() == Proxy.Type.DIRECT) {
                s = createSocket();
            } else {
                // Still connecting through a proxy
                // server & port will be the proxy address and port
                s = new Socket(Proxy.NO_PROXY);
            }
        } else {
            s = createSocket();
        }

        // Instance specific timeouts do have priority, that means
        // connectTimeout & readTimeout (-1 means not set)
        // Then global default timeouts
        // Then no timeout.
        if (connectTimeout >= 0) {
            s.connect(new InetSocketAddress(server, port), connectTimeout);
        } else {
            if (defaultConnectTimeout > 0) {
                s.connect(new InetSocketAddress(server, port), defaultConnectTimeout);
            } else {
                s.connect(new InetSocketAddress(server, port));
            }
        }
        if (readTimeout >= 0)
            s.setSoTimeout(readTimeout);
        else if (defaultSoTimeout > 0) {
            s.setSoTimeout(defaultSoTimeout);
        }
        return s;
    }

public static InetAddress[] getAllByName(String host)
        throws UnknownHostException {
        return getAllByName(host, null);
    }

private static InetAddress[] getAllByName(String host, InetAddress reqAddr)
        throws UnknownHostException {

        if (host == null || host.isEmpty()) {
            InetAddress[] ret = new InetAddress[1];
            ret[0] = impl.loopbackAddress();
            return ret;
        }

        validate(host);
        boolean ipv6Expected = false;
        if (host.charAt(0) == '[') {
            // This is supposed to be an IPv6 literal
            if (host.length() > 2 && host.charAt(host.length()-1) == ']') {
                host = host.substring(1, host.length() -1);
                ipv6Expected = true;
            } else {
                // This was supposed to be a IPv6 literal, but it's not
                throw invalidIPv6LiteralException(host, false);
            }
        }

        // Check and try to parse host string as an IP address literal
        if (IPAddressUtil.digit(host.charAt(0), 16) != -1
            || (host.charAt(0) == ':')) {
            byte[] addr = null;
            int numericZone = -1;
            String ifname = null;
            if (!ipv6Expected) {
                // check if it is IPv4 address only if host is not wrapped in '[]'
                try {
                    addr = IPAddressUtil.validateNumericFormatV4(host);
                } catch (IllegalArgumentException iae) {
                    var uhe = new UnknownHostException(host);
                    uhe.initCause(iae);
                    throw uhe;
                }
            }
            if (addr == null) {
                // Try to parse host string as an IPv6 literal
                // Check if a numeric or string zone id is present first
                int pos;
                if ((pos = host.indexOf('%')) != -1) {
                    numericZone = checkNumericZone(host);
                    if (numericZone == -1) { /* remainder of string must be an ifname */
                        ifname = host.substring(pos + 1);
                    }
                }
                if ((addr = IPAddressUtil.textToNumericFormatV6(host)) == null &&
                        (host.contains(":") || ipv6Expected)) {
                    throw invalidIPv6LiteralException(host, ipv6Expected);
                }
            }
            if(addr != null) {
                InetAddress[] ret = new InetAddress[1];
                if (addr.length == Inet4Address.INADDRSZ) {
                    if (numericZone != -1 || ifname != null) {
                        // IPv4-mapped address must not contain zone-id
                        throw new UnknownHostException(host + ": invalid IPv4-mapped address");
                    }
                    ret[0] = new Inet4Address(null, addr);
                } else {
                    if (ifname != null) {
                        ret[0] = new Inet6Address(null, addr, ifname);
                    } else {
                        ret[0] = new Inet6Address(null, addr, numericZone);
                    }
                }
                return ret;
            }
        } else if (ipv6Expected) {
            // We were expecting an IPv6 Literal since host string starts
            // and ends with square brackets, but we got something else.
            throw invalidIPv6LiteralException(host, true);
        }
        return getAllByName0(host, reqAddr, true, true);
    }

static InetAddress[] getAddressesFromNameService(String host, InetAddress reqAddr)
        throws UnknownHostException
    {
        InetAddress[] addresses = null;
        UnknownHostException ex = null;
        validate(host);

            try {
                addresses = nameService.lookupAllHostAddr(host);
            } catch (UnknownHostException uhe) {
                if (host.equalsIgnoreCase("localhost")) {
                    addresses = new InetAddress[] { impl.loopbackAddress() };
                }
                else {
                    ex = uhe;
                }
            }

        if (addresses == null) {
            throw ex == null ? new UnknownHostException(host) : ex;
        }

        // More to do?
        if (reqAddr != null && addresses.length > 1 && !addresses[0].equals(reqAddr)) {
            // Find it?
            int i = 1;
            for (; i < addresses.length; i++) {
                if (addresses[i].equals(reqAddr)) {
                    break;
                }
            }
            // Rotate
            if (i < addresses.length) {
                InetAddress tmp, tmp2 = reqAddr;
                for (int j = 0; j < i; j++) {
                    tmp = addresses[j];
                    addresses[j] = tmp2;
                    tmp2 = tmp;
                }
                addresses[i] = tmp2;
            }
        }

        return addresses;
    }

private static final class PlatformNameService implements NameService {

        public InetAddress[] lookupAllHostAddr(String host)
            throws UnknownHostException
        {
            return impl.lookupAllHostAddr(host);
        }

        public String getHostByAddr(byte[] addr)
            throws UnknownHostException
        {
            return impl.getHostByAddr(addr);
        }
    }

public native InetAddress[] lookupAllHostAddr(String hostname)
        throws UnknownHostException;

##openjdk17 对应解析域名的  C++源码。error = getaddrinfo(hostname, NULL, &hints, &res);

/*
 * Class:     java_net_Inet6AddressImpl
 * Method:    lookupAllHostAddr
 * Signature: (Ljava/lang/String;)[[B
 */
JNIEXPORT jobjectArray JNICALL
Java_java_net_Inet6AddressImpl_lookupAllHostAddr(JNIEnv *env, jobject this,
                                                 jstring host) {
    jobjectArray ret = NULL;
    const char *hostname;
    int error = 0;
    struct addrinfo hints, *res = NULL, *resNew = NULL, *last = NULL,
        *iterator;

    initInetAddressIDs(env);
    JNU_CHECK_EXCEPTION_RETURN(env, NULL);

    if (IS_NULL(host)) {
        JNU_ThrowNullPointerException(env, "host argument is null");
        return NULL;
    }
    hostname = JNU_GetStringPlatformChars(env, host, JNI_FALSE);
    CHECK_NULL_RETURN(hostname, NULL);

    // try once, with our static buffer
    memset(&hints, 0, sizeof(hints));
    hints.ai_flags = AI_CANONNAME;
    hints.ai_family = AF_UNSPEC;

    error = getaddrinfo(hostname, NULL, &hints, &res);

    if (error) {
#if defined(MACOSX)
        // if getaddrinfo fails try getifaddrs
        ret = lookupIfLocalhost(env, hostname, JNI_TRUE);
        if (ret != NULL || (*env)->ExceptionCheck(env)) {
            goto cleanupAndReturn;
        }
#endif
        // report error
        NET_ThrowUnknownHostExceptionWithGaiError(env, hostname, error);
        goto cleanupAndReturn;
    } else {
        int i = 0, inetCount = 0, inet6Count = 0, inetIndex = 0,
            inet6Index = 0, originalIndex = 0;
        int addressPreference =
            (*env)->GetStaticIntField(env, ia_class, ia_preferIPv6AddressID);;
        iterator = res;
        while (iterator != NULL) {
            // skip duplicates
            int skip = 0;
            struct addrinfo *iteratorNew = resNew;
            while (iteratorNew != NULL) {
                if (iterator->ai_family == iteratorNew->ai_family &&
                    iterator->ai_addrlen == iteratorNew->ai_addrlen) {
                    if (iteratorNew->ai_family == AF_INET) { /* AF_INET */
                        struct sockaddr_in *addr1, *addr2;
                        addr1 = (struct sockaddr_in *)iterator->ai_addr;
                        addr2 = (struct sockaddr_in *)iteratorNew->ai_addr;
                        if (addr1->sin_addr.s_addr == addr2->sin_addr.s_addr) {
                            skip = 1;
                            break;
                        }
                    } else {
                        int t;
                        struct sockaddr_in6 *addr1, *addr2;
                        addr1 = (struct sockaddr_in6 *)iterator->ai_addr;
                        addr2 = (struct sockaddr_in6 *)iteratorNew->ai_addr;

                        for (t = 0; t < 16; t++) {
                            if (addr1->sin6_addr.s6_addr[t] !=
                                addr2->sin6_addr.s6_addr[t]) {
                                break;
                            }
                        }
                        if (t < 16) {
                            iteratorNew = iteratorNew->ai_next;
                            continue;
                        } else {
                            skip = 1;
                            break;
                        }
                    }
                } else if (iterator->ai_family != AF_INET &&
                           iterator->ai_family != AF_INET6) {
                    // we can't handle other family types
                    skip = 1;
                    break;
                }
                iteratorNew = iteratorNew->ai_next;
            }

            if (!skip) {
                struct addrinfo *next
                    = (struct addrinfo *)malloc(sizeof(struct addrinfo));
                if (!next) {
                    JNU_ThrowOutOfMemoryError(env, "Native heap allocation failed");
                    ret = NULL;
                    goto cleanupAndReturn;
                }
                memcpy(next, iterator, sizeof(struct addrinfo));
                next->ai_next = NULL;
                if (resNew == NULL) {
                    resNew = next;
                } else {
                    last->ai_next = next;
                }
                last = next;
                i++;
                if (iterator->ai_family == AF_INET) {
                    inetCount++;
                } else if (iterator->ai_family == AF_INET6) {
                    inet6Count++;
                }
            }
            iterator = iterator->ai_next;
        }

        // allocate array - at this point i contains the number of addresses
        ret = (*env)->NewObjectArray(env, i, ia_class, NULL);
        if (IS_NULL(ret)) {
            /* we may have memory to free at the end of this */
            goto cleanupAndReturn;
        }

        if (addressPreference == java_net_InetAddress_PREFER_IPV6_VALUE) {
            inetIndex = inet6Count;
            inet6Index = 0;
        } else if (addressPreference == java_net_InetAddress_PREFER_IPV4_VALUE) {
            inetIndex = 0;
            inet6Index = inetCount;
        } else if (addressPreference == java_net_InetAddress_PREFER_SYSTEM_VALUE) {
            inetIndex = inet6Index = originalIndex = 0;
        }

        iterator = resNew;
        while (iterator != NULL) {
            if (iterator->ai_family == AF_INET) {
                jobject iaObj = (*env)->NewObject(env, ia4_class, ia4_ctrID);
                if (IS_NULL(iaObj)) {
                    ret = NULL;
                    goto cleanupAndReturn;
                }
                setInetAddress_addr(env, iaObj, ntohl(((struct sockaddr_in*)iterator->ai_addr)->sin_addr.s_addr));
                if ((*env)->ExceptionCheck(env))
                    goto cleanupAndReturn;
                setInetAddress_hostName(env, iaObj, host);
                if ((*env)->ExceptionCheck(env))
                    goto cleanupAndReturn;
                (*env)->SetObjectArrayElement(env, ret, (inetIndex | originalIndex), iaObj);
                inetIndex++;
            } else if (iterator->ai_family == AF_INET6) {
                jint scope = 0;
                jboolean ret1;
                jobject iaObj = (*env)->NewObject(env, ia6_class, ia6_ctrID);
                if (IS_NULL(iaObj)) {
                    ret = NULL;
                    goto cleanupAndReturn;
                }
                ret1 = setInet6Address_ipaddress(env, iaObj, (char *)&(((struct sockaddr_in6*)iterator->ai_addr)->sin6_addr));
                if (ret1 == JNI_FALSE) {
                    ret = NULL;
                    goto cleanupAndReturn;
                }
                scope = ((struct sockaddr_in6 *)iterator->ai_addr)->sin6_scope_id;
                if (scope != 0) { // zero is default value, no need to set
                    setInet6Address_scopeid(env, iaObj, scope);
                }
                setInetAddress_hostName(env, iaObj, host);
                if ((*env)->ExceptionCheck(env))
                    goto cleanupAndReturn;
                (*env)->SetObjectArrayElement(env, ret, (inet6Index | originalIndex), iaObj);
                inet6Index++;
            }
            if (addressPreference == java_net_InetAddress_PREFER_SYSTEM_VALUE) {
                originalIndex++;
                inetIndex = inet6Index = 0;
            }
            iterator = iterator->ai_next;
        }
    }
cleanupAndReturn:
    JNU_ReleaseStringPlatformChars(env, host, hostname);
    while (resNew != NULL) {
        last = resNew;
        resNew = resNew->ai_next;
        free(last);
    }
    if (res != NULL) {
        freeaddrinfo(res);
    }
    return ret;
}