HttpClient 是 Apache Jakarta Common 下的子项目，能够用来提供高效的、最新的、功能丰富的支持 HTTP 协议的客户端编程工具包，而且它支持 HTTP 协议最新的版本和建议。HttpClient支持的功能以下：
java
支持Http0.九、Http1.0和Http1.1协议。编程
实现了Http所有的方法（GET,POST,PUT,HEAD 等）。安全
支持HTTPS协议。服务器
支持代理服务器。restful
提供安全认证方案。cookie
提供链接池以便重用链接。
多线程
链接管理器支持多线程应用。支持设置最大链接数，同时支持设置每一个主机的最大链接数，发现并关闭过时的链接。app
在http1.0和http1.1中利用KeepAlive保持长链接。框架
本文简单谈下HttpClient源码，而HttpClient源码包有700K，因此这里只会挑重点介绍，详细源码你们能够下载HttpClient的最新的代码来研究。另外：本文所列举的都是 HttpClient4.5 的源码。less
HttpClient常常会用到链接池，以便重用tcp链接，特别是在使用长链接时能节约很多性能（避免了三次握手和四次解握）。HttpClient链接池的逻辑是：先根据域名route 来找是否有空闲的链接，若是有就取出来用，若是没有则会建立一个新的链接，并绑定到route 上，这里使用route 来获取链接是为了重用链接，假设一个长链接在同一个域名下屡次使用就不用屡次握手了。固然，其中还会校验链接池的大小，等待的时间等。源码以下：
private E getPoolEntryBlocking(
final T route, final Object state,
final long timeout, final TimeUnit tunit,
final PoolEntryFuture<E> future)
throws IOException, InterruptedException, TimeoutException {
Date deadline = null;
if (timeout > 0) {
deadline = new Date
(System.currentTimeMillis() + tunit.toMillis(timeout));
}
this.lock.lock();
try {
final RouteSpecificPool<T, C, E> pool = getPool(route);
E entry = null;
while (entry == null) {
Asserts.check(!this.isShutDown, "Connection pool shut down");
for (;;) {
entry = pool.getFree(state);
if (entry == null) {
break;
}
if (entry.isExpired(System.currentTimeMillis())) {
entry.close();
} else if (this.validateAfterInactivity > 0) {
if (entry.getUpdated() + this.validateAfterInactivity <= System.currentTimeMillis()) {
if (!validate(entry)) {
entry.close();
}
}
}
if (entry.isClosed()) {
this.available.remove(entry);
pool.free(entry, false);
} else {
break;
}
}
if (entry != null) {
this.available.remove(entry);
this.leased.add(entry);
onReuse(entry);
return entry;
}
// New connection is needed
final int maxPerRoute = getMax(route);
// Shrink the pool prior to allocating a new connection
final int excess = Math.max(0, pool.getAllocatedCount() + 1 - maxPerRoute);
if (excess > 0) {
for (int i = 0; i < excess; i++) {
final E lastUsed = pool.getLastUsed();
if (lastUsed == null) {
break;
}
lastUsed.close();
this.available.remove(lastUsed);
pool.remove(lastUsed);
}
}
if (pool.getAllocatedCount() < maxPerRoute) {
final int totalUsed = this.leased.size();
final int freeCapacity = Math.max(this.maxTotal - totalUsed, 0);
if (freeCapacity > 0) {
final int totalAvailable = this.available.size();
if (totalAvailable > freeCapacity - 1) {
if (!this.available.isEmpty()) {
final E lastUsed = this.available.removeLast();
lastUsed.close();
final RouteSpecificPool<T, C, E> otherpool = getPool(lastUsed.getRoute());
otherpool.remove(lastUsed);
}
}
final C conn = this.connFactory.create(route);
entry = pool.add(conn);
this.leased.add(entry);
return entry;
}
}
boolean success = false;
try {
pool.queue(future);
this.pending.add(future);
success = future.await(deadline);
} finally {
// In case of 'success', we were woken up by the
// connection pool and should now have a connection
// waiting for us, or else we're shutting down.
// Just continue in the loop, both cases are checked.
pool.unqueue(future);
this.pending.remove(future);
}
// check for spurious wakeup vs. timeout
if (!success && (deadline != null) &&
(deadline.getTime() <= System.currentTimeMillis())) {
break;
}
}
throw new TimeoutException("Timeout waiting for connection");
} finally {
this.lock.unlock();
}
}
在发送数据前HttpClient 确定会和后台的服务（好比restful服务）进行socket链接，创建socket时的参数好比connectTimeout、soTimeout等均可配置。另外，此时会开启套接字的输出流和输入流：输出流用于向后台的restful服务输出Request的参数、header、cookie和body等，该部分源码以下：
@Override
public void connect(
final ManagedHttpClientConnection conn,
final HttpHost host,
final InetSocketAddress localAddress,
final int connectTimeout,
final SocketConfig socketConfig,
final HttpContext context) throws IOException {
final Lookup<ConnectionSocketFactory> registry = getSocketFactoryRegistry(context);
final ConnectionSocketFactory sf = registry.lookup(host.getSchemeName());
if (sf == null) {
throw new UnsupportedSchemeException(host.getSchemeName() +
" protocol is not supported");
}
final InetAddress[] addresses = host.getAddress() != null ?
new InetAddress[] { host.getAddress() } : this.dnsResolver.resolve(host.getHostName());
final int port = this.schemePortResolver.resolve(host);
for (int i = 0; i < addresses.length; i++) {
final InetAddress address = addresses[i];
final boolean last = i == addresses.length - 1;
Socket sock = sf.createSocket(context);
sock.setSoTimeout(socketConfig.getSoTimeout());
sock.setReuseAddress(socketConfig.isSoReuseAddress());
sock.setTcpNoDelay(socketConfig.isTcpNoDelay());
sock.setKeepAlive(socketConfig.isSoKeepAlive());
final int linger = socketConfig.getSoLinger();
if (linger >= 0) {
sock.setSoLinger(true, linger);
}
conn.bind(sock);
final InetSocketAddress remoteAddress = new InetSocketAddress(address, port);
if (this.log.isDebugEnabled()) {
this.log.debug("Connecting to " + remoteAddress);
}
try {
sock = sf.connectSocket(
connectTimeout, sock, host, remoteAddress, localAddress, context);
conn.bind(sock);
if (this.log.isDebugEnabled()) {
this.log.debug("Connection established " + conn);
}
return;
} catch (final SocketTimeoutException ex) {
if (last) {
throw new ConnectTimeoutException(ex, host, addresses);
}
} catch (final ConnectException ex) {
if (last) {
final String msg = ex.getMessage();
if ("Connection timed out".equals(msg)) {
throw new ConnectTimeoutException(ex, host, addresses);
} else {
throw new HttpHostConnectException(ex, host, addresses);
}
}
} catch (final NoRouteToHostException ex) {
if (last) {
throw ex;
}
}
if (this.log.isDebugEnabled()) {
this.log.debug("Connect to " + remoteAddress + " timed out. " +
"Connection will be retried using another IP address");
}
}
}
@Override
public Socket connectSocket(
final int connectTimeout,
final Socket socket,
final HttpHost host,
final InetSocketAddress remoteAddress,
final InetSocketAddress localAddress,
final HttpContext context) throws IOException {
final Socket sock = socket != null ? socket : createSocket(context);
if (localAddress != null) {
sock.bind(localAddress);
}
try {
sock.connect(remoteAddress, connectTimeout);
} catch (final IOException ex) {
try {
sock.close();
} catch (final IOException ignore) {
}
throw ex;
}
return sock;
}
创建链接成功后就能够发送数据了，像前面所说的那样：httpclient是使用输出流向后台的restful服务输出Request的参数、header、cookie和body等。源码以下：
public HttpResponse execute(
final HttpRequest request,
final HttpClientConnection conn,
final HttpContext context) throws IOException, HttpException {
Args.notNull(request, "HTTP request");
Args.notNull(conn, "Client connection");
Args.notNull(context, "HTTP context");
try {
HttpResponse response = doSendRequest(request, conn, context);
if (response == null) {
response = doReceiveResponse(request, conn, context);
}
return response;
} catch (final IOException ex) {
closeConnection(conn);
throw ex;
} catch (final HttpException ex) {
closeConnection(conn);
throw ex;
} catch (final RuntimeException ex) {
closeConnection(conn);
throw ex;
}
}
@Override
public void flush() throws IOException {
flushBuffer();
flushStream();
}
private void flushBuffer() throws IOException {
final int len = this.buffer.length();
if (len > 0) {
streamWrite(this.buffer.buffer(), 0, len);
this.buffer.clear();
this.metrics.incrementBytesTransferred(len);
}
}
private void flushStream() throws IOException {
if (this.outstream != null) {
this.outstream.flush();
}
}
发送完数据后就会接受套接字输入流的数据，上面源码块里面的 response = doReceiveResponse(request, conn, context)就是用于接受套接字数据。源码以下：
protected HttpResponse doReceiveResponse(
final HttpRequest request,
final HttpClientConnection conn,
final HttpContext context) throws HttpException, IOException {
Args.notNull(request, "HTTP request");
Args.notNull(conn, "Client connection");
Args.notNull(context, "HTTP context");
HttpResponse response = null;
int statusCode = 0;
while (response == null || statusCode < HttpStatus.SC_OK) {
response = conn.receiveResponseHeader();
if (canResponseHaveBody(request, response)) {
conn.receiveResponseEntity(response);
}
statusCode = response.getStatusLine().getStatusCode();
} // while intermediate response
return response;
}
其中：conn.receiveResponseHeader() 用于获取Response的header数据，而conn.receiveResponseEntity(response)或获取Response的body数据。固然，在获取body数据前会先判断Response的状态码是否合法，源码以下：
protected boolean canResponseHaveBody(final HttpRequest request, final HttpResponse response) {
if ("HEAD".equalsIgnoreCase(request.getRequestLine().getMethod())) {
return false;
}
final int status = response.getStatusLine().getStatusCode();
return status >= HttpStatus.SC_OK
&& status != HttpStatus.SC_NO_CONTENT
&& status != HttpStatus.SC_NOT_MODIFIED
&& status != HttpStatus.SC_RESET_CONTENT;
}
获取Response数据后会处理这些数据，好比保持链接之类的设置，Httpclient会根据response返回的协议以及header里的Connect等参数来设置是否保持链接，源码以下：
if (reuseStrategy.keepAlive(response, context)) {
// Set the idle duration of this connection
final long duration = keepAliveStrategy.getKeepAliveDuration(response, context);
if (this.log.isDebugEnabled()) {
final String s;
if (duration > 0) {
s = "for " + duration + " " + TimeUnit.MILLISECONDS;
} else {
s = "indefinitely";
}
this.log.debug("Connection can be kept alive " + s);
}
connHolder.setValidFor(duration, TimeUnit.MILLISECONDS);
connHolder.markReusable();
} else {
connHolder.markNonReusable();
}
@Override
public boolean keepAlive(final HttpResponse response,
final HttpContext context) {
Args.notNull(response, "HTTP response");
Args.notNull(context, "HTTP context");
// Check for a self-terminating entity. If the end of the entity will
// be indicated by closing the connection, there is no keep-alive.
final ProtocolVersion ver = response.getStatusLine().getProtocolVersion();
final Header teh = response.getFirstHeader(HTTP.TRANSFER_ENCODING);
if (teh != null) {
if (!HTTP.CHUNK_CODING.equalsIgnoreCase(teh.getValue())) {
return false;
}
} else {
if (canResponseHaveBody(response)) {
final Header[] clhs = response.getHeaders(HTTP.CONTENT_LEN);
// Do not reuse if not properly content-length delimited
if (clhs.length == 1) {
final Header clh = clhs[0];
try {
final int contentLen = Integer.parseInt(clh.getValue());
if (contentLen < 0) {
return false;
}
} catch (final NumberFormatException ex) {
return false;
}
} else {
return false;
}
}
}
// Check for the "Connection" header. If that is absent, check for
// the "Proxy-Connection" header. The latter is an unspecified and
// broken but unfortunately common extension of HTTP.
HeaderIterator hit = response.headerIterator(HTTP.CONN_DIRECTIVE);
if (!hit.hasNext()) {
hit = response.headerIterator("Proxy-Connection");
}
if (hit.hasNext()) {
try {
final TokenIterator ti = createTokenIterator(hit);
boolean keepalive = false;
while (ti.hasNext()) {
final String token = ti.nextToken();
if (HTTP.CONN_CLOSE.equalsIgnoreCase(token)) {
return false;
} else if (HTTP.CONN_KEEP_ALIVE.equalsIgnoreCase(token)) {
// continue the loop, there may be a "close" afterwards
keepalive = true;
}
}
if (keepalive){
return true;
// neither "close" nor "keep-alive", use default policy
}
} catch (final ParseException px) {
// invalid connection header means no persistent connection
// we don't have logging in HttpCore, so the exception is lost
return false;
}
}
// default since HTTP/1.1 is persistent, before it was non-persistent
return !ver.lessEquals(HttpVersion.HTTP_1_0);
}
而后若是Response返回302,时httpclient会根据策略来判断并决定是否须要重定向，源码以下：
for (int redirectCount = 0;;) {
final CloseableHttpResponse response = requestExecutor.execute(
currentRoute, currentRequest, context, execAware);
try {
if (config.isRedirectsEnabled() &&
this.redirectStrategy.isRedirected(currentRequest, response, context)) {
if (redirectCount >= maxRedirects) {
throw new RedirectException("Maximum redirects ("+ maxRedirects + ") exceeded");
}
redirectCount++;
final HttpRequest redirect = this.redirectStrategy.getRedirect(
currentRequest, response, context);
if (!redirect.headerIterator().hasNext()) {
final HttpRequest original = request.getOriginal();
redirect.setHeaders(original.getAllHeaders());
}
currentRequest = HttpRequestWrapper.wrap(redirect);
if (currentRequest instanceof HttpEntityEnclosingRequest) {
RequestEntityProxy.enhance((HttpEntityEnclosingRequest) currentRequest);
}
final URI uri = currentRequest.getURI();
final HttpHost newTarget = URIUtils.extractHost(uri);
if (newTarget == null) {
throw new ProtocolException("Redirect URI does not specify a valid host name: " + uri);
}
// Reset virtual host and auth states if redirecting to another host
if (!currentRoute.getTargetHost().equals(newTarget)) {
final AuthState targetAuthState = context.getTargetAuthState();
if (targetAuthState != null) {
this.log.debug("Resetting target auth state");
targetAuthState.reset();
}
final AuthState proxyAuthState = context.getProxyAuthState();
if (proxyAuthState != null) {
final AuthScheme authScheme = proxyAuthState.getAuthScheme();
if (authScheme != null && authScheme.isConnectionBased()) {
this.log.debug("Resetting proxy auth state");
proxyAuthState.reset();
}
}
}
currentRoute = this.routePlanner.determineRoute(newTarget, currentRequest, context);
if (this.log.isDebugEnabled()) {
this.log.debug("Redirecting to '" + uri + "' via " + currentRoute);
}
EntityUtils.consume(response.getEntity());
response.close();
} else {
return response;
}
} catch (final RuntimeException ex) {
response.close();
throw ex;
} catch (final IOException ex) {
response.close();
throw ex;
} catch (final HttpException ex) {
// Protocol exception related to a direct.
// The underlying connection may still be salvaged.
try {
EntityUtils.consume(response.getEntity());
} catch (final IOException ioex) {
this.log.debug("I/O error while releasing connection", ioex);
} finally {
response.close();
}
throw ex;
}
}
能够看到，首先获取Response，而后根据config.isRedirectsEnabled() 和this.redirectStrategy.isRedirected(currentRequest, response, context)的结果来进行重定向，都为true则重定向，不然直接返回Response。固然了，这里的redirectsEnabled和redirectStrategy都是能够自定义的。
HttpClient 是Apache 下又一个十分优秀的开源框架，最经常使用用于封装长链接、调用后台的resetful接口等。然而它还有不少本文没有提到的功能，好比安全认证、使用HTTPS协议等。你们感兴趣的话能够更深刻地了解下。