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

server/2025/1/12 15:36:30/

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GETconnection.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 permissionplainConnect0();}}protected void plainConnect0()  throws IOException {// try to see if request can be served from local cacheif (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 failedhttp = 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 proxyif (!failedOnce) {http = getNewHttpClient(url, null, connectTimeout);http.setReadTimeout(readTimeout);} else {// make sure to construct new connection if first// attempt failedhttp = 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 failedhttp = getNewHttpClient(url, instProxy, connectTimeout, false);http.setReadTimeout(readTimeout);}}ps = (PrintStream)http.getOutputStream();} catch (IOException e) {throw e;}// constructor to HTTP client calls openserverconnected = 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 openedreturn;}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 connectionopenServer(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 connectionsuper.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 ports = 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 literalif (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 notthrow invalidIPv6LiteralException(host, false);}}// Check and try to parse host string as an IP address literalif (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 firstint 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-idthrow 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;}}// Rotateif (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 buffermemset(&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 getifaddrsret = lookupIfLocalhost(env, hostname, JNI_TRUE);if (ret != NULL || (*env)->ExceptionCheck(env)) {goto cleanupAndReturn;}
#endif// report errorNET_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 duplicatesint 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 typesskip = 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 addressesret = (*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 setsetInet6Address_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;
}


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