1、HttpClient的最常用功能是execute()方法。执行一次execute()会包括了一次或多次request请求 - response响应事件。HttpClient会将request请求发送给目标服务器以取得response响应对象，也可有可能在执行失败后抛出一个异常。
一个简单的代码如下：
CloseableHttpClienthttpclient = HttpClients.createDefault();
HttpGet httpget = newHttpGet("http://localhost/");
CloseableHttpResponse response = httpclient.execute(httpget);
try {
<...>
} finally {
response.close();
}
2、所有的Http request请求都包括了一个方法名、一个请求地址和一个HTTP协议版本号。HttpClient支持HTTP/1.1协议下所有的HTTP方法，包括：Get、HEAD、POST、PUT、DELETE、TRACE和OPTIONS。每一种方法都通过特定的类来实现，包括HttpGet、HttpHead、HttpPost、HttpPut、HttpDelete、HttpTrace和HttpOptions。
请求地址由一个URI（统一资源定位符）来描述。Http URI包括了协议、主机名、端口、地址、请求参数等。
例如：
HttpGet httpget = newHttpGet(
"http://www.google.com/search?hl=en&q=httpclient&btnG=Google+Search&aq=f&oq=");
HttpClient 提供了URIBuilder类来简化请求地址的创建和修改，例如：
URI uri = newURIBuilder()
.setScheme("http")
.setHost("www.google.com")
.setPath("/search")
.setParameter("q","httpclient")
.setParameter("btnG","Google Search")
.setParameter("aq","f")
.setParameter("oq","")
.build();
HttpGet httpget = new HttpGet(uri);
System.out.println(httpget.getURI());
输出：
http://www.google.com/search?q=httpclient&btnG=Google+Search&aq=f&oq=
3、Httpresponse是服务器返回给客户端的一段报文。报文的第一行包括了协议版本、数字状态码和文字描述。例如：
HttpResponse response= new BasicHttpResponse(HttpVersion.HTTP_1_1,
HttpStatus.SC_OK, "OK");
System.out.println(response.getProtocolVersion());
System.out.println(response.getStatusLine().getStatusCode());
System.out.println(response.getStatusLine().getReasonPhrase());
System.out.println(response.getStatusLine().toString());
4、一段Http报文包括了多个header描述属性，包括内容长度、内容类型等等，HttpClient提供了查询、删除、处理header的方法，例如：
HttpResponse response= new BasicHttpResponse(HttpVersion.HTTP_1_1,
HttpStatus.SC_OK,"OK");
response.addHeader("Set-Cookie",
"c1=a; path=/;domain=localhost");
response.addHeader("Set-Cookie",
"c2=b; path=\"/\",c3=c; domain=\"localhost\"");
Header h1 = response.getFirstHeader("Set-Cookie");
System.out.println(h1);
Header h2 =response.getLastHeader("Set-Cookie");
System.out.println(h2);
Header[] hs =response.getHeaders("Set-Cookie");
System.out.println(hs.length);
输出：
Set-Cookie: c1=a;path=/; domain=localhost
Set-Cookie: c2=b; path="/", c3=c; domain="localhost"
2
最有效率的做法是用HeaderIterator来获得特定类型的所有header。
例如：
HttpResponse response= new BasicHttpResponse(HttpVersion.HTTP_1_1,
HttpStatus.SC_OK,"OK");
response.addHeader("Set-Cookie",
"c1=a; path=/;domain=localhost");
response.addHeader("Set-Cookie",
"c2=b; path=\"/\",c3=c; domain=\"localhost\"");
HeaderIterator it =response.headerIterator("Set-Cookie");
while (it.hasNext()){
System.out.println(it.next());
}
输出：
Set-Cookie: c1=a;path=/; domain=localhost
Set-Cookie: c2=b; path="/", c3=c; domain="localhost"
5、HttpEntity 是Http request-reponse交互的报文内容。HttpClient区分了三种报文内容：
（1）流形式的报文内容，报文内容不可重复。
（2）独立的报文内容，报文内容是可重复的。
（3）从其他报文内容中获取的报文内容。
通过HttpEntity的getContent()可以获得Java.io.Inputstream的输入流，也可以通过HttpEntity的writeTo(OutputStream)写入内容。
HttpEntity的getContentType()和getContentLength()方法可以获取Content-Type和Content-Length的元数据，getContentEncoding()方法可以获取编码格式。
例如：
StringEntity myEntity= new StringEntity("important message",
ContentType.create("text/plain", "UTF-8"));
System.out.println(myEntity.getContentType());
System.out.println(myEntity.getContentLength());
System.out.println(EntityUtils.toString(myEntity));
System.out.println(EntityUtils.toByteArray(myEntity).length);
输出：
Content-Type:text/plain; charset=utf-8
17
important message
17
读取HttpEntity，例如：
CloseableHttpClienthttpclient = HttpClients.createDefault();
HttpGet httpget = newHttpGet("http://localhost/");
CloseableHttpResponse response = httpclient.execute(httpget);
try {
HttpEntity entity =response.getEntity();
if (entity != null) {
long len =entity.getContentLength();
if (len != -1 && len <2048) {
System.out.println(EntityUtils.toString(entity));
} else {
// Stream contentout
}
}
} finally {
response.close();
}
也可以生成报文内容
File file = newFile("somefile.txt");
FileEntity entity = new FileEntity(file,
ContentType.create("text/plain", "UTF-8"));
HttpPost httppost =newHttpPost("http://localhost/action.do");
httppost.setEntity(entity);
6、可以通过UrlEncodeFormEntity来提交表单，例如
List<NameValuePair>formparams = new ArrayList<NameValuePair>();
formparams.add(new BasicNameValuePair("param1","value1"));
formparams.add(new BasicNameValuePair("param2","value2"));
UrlEncodedFormEntity entity = new UrlEncodedFormEntity(formparams,Consts.UTF_8);
HttpPost httppost = newHttpPost("http://localhost/handler.do");
httppost.setEntity(entity);
7、可以采用ResponseHandler来简化连接操作，例如
CloseableHttpClienthttpclient = HttpClients.createDefault();
HttpGet httpget = new HttpGet("http://localhost/json");
ResponseHandler<MyJsonObject>rh = new ResponseHandler<MyJsonObject>() {
@Override
public JsonObjecthandleResponse(
final HttpResponse response)throws IOException {
StatusLine statusLine =response.getStatusLine();
HttpEntity entity =response.getEntity();
if (statusLine.getStatusCode()>= 300) {
throw newHttpResponseException(
statusLine.getStatusCode(),
statusLine.getReasonPhrase());
}
if (entity == null) {
throw newClientProtocolException("Response contains no content");
}
Gson gson = newGsonBuilder().create();
ContentType contentType =ContentType.getOrDefault(entity);
Charset charset =contentType.getCharset();
Reader reader = newInputStreamReader(entity.getContent(), charset);
return gson.fromJson(reader,MyJsonObject.class);
}
};
MyJsonObject myjson = client.execute(httpget, rh);
8、 httpClient不是线程安全的，所以如果不再使用的话，可以通过close()方法进行关闭，例如
CloseableHttpClienthttpclient = HttpClients.createDefault();
try {
<...>
} finally {
httpclient.close();
}
9、HttpContext可以作为Http交互的上下文，是以java.util.Map存储的对象。包括了HttpConnection、HttpHost、HttpRoute、HttpRequest等，可以通过HttpclientContext来定义上下文状态。
HttpContext context =<...>
HttpClientContext clientContext =HttpClientContext.adapt(context);
HttpHost target = clientContext.getTargetHost();
HttpRequest request = clientContext.getRequest();
HttpResponse response = clientContext.getResponse();
RequestConfig config = clientContext.getRequestConfig();
10、如果要实现重试机制，可以采用HttpRequestRetryHandler接口来实现。例如：
HttpRequestRetryHandlermyRetryHandler = new HttpRequestRetryHandler() {
public booleanretryRequest(
IOExceptionexception,
intexecutionCount,
HttpContext context){
if (executionCount >= 5){
// Do not retry if over maxretry count
return false;
}
if (exception instanceofInterruptedIOException) {
// Timeout
return false;
}
if (exception instanceofUnknownHostException) {
// Unknown host
return false;
}
if (exception instanceofConnectTimeoutException) {
// Connectionrefused
return false;
}
if (exception instanceofSSLException) {
// SSL handshakeexception
return false;
}
HttpClientContext clientContext =HttpClientContext.adapt(context);
HttpRequest request =clientContext.getRequest();
boolean idempotent = !(requestinstanceof HttpEntityEnclosingRequest);
if (idempotent) {
// Retry if the request isconsidered idempotent
return true;
}
return false;
}
};
CloseableHttpClient httpclient = HttpClients.custom()
.setRetryHandler(myRetryHandler)
.build();
11、重定向处理，例如：
CloseableHttpClienthttpclient = HttpClients.createDefault();
HttpClientContext context = HttpClientContext.create();
HttpGet httpget = newHttpGet("http://localhost:8080/");
CloseableHttpResponse response = httpclient.execute(httpget, context);
try {
HttpHost target =context.getTargetHost();
List<URI> redirectLocations =context.getRedirectLocations();
URI location =URIUtils.resolve(httpget.getURI(), target, redirectLocations);
System.out.println("Final HTTPlocation: " + location.toASCIIString());
// Expected to be an absoluteURI
} finally {
response.close();
}