HttpClient是Apache旗下的项目，是一个负责创建和维护HTTP和相关协议的工具集。
以下分析使用版本为：
httpclient-4.5.3.jar
httpcore-4.4.6.jar
jdk1.8.0_131
所有示例代码均经过运行测试
发送请求
httpclient最重要的功能就是发送http请求，下面介绍如何执行一个get请求：
CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet get = new HttpGet("http://www.jianshu.com/u/8a3115bb299c");
try(CloseableHttpResponse response = httpclient.execute(get)) {
HttpEntity entity = response.getEntity();
System.out.println(EntityUtils.toString(entity, "UTF-8"));
} catch (Exception e) {
e.printStackTrace();
}
httpclient支持Http/1.1规范中定义的所有方法：GET，HEAD，POST，PUT，DELETE，TRACE，OPTIONS，对应的类是：HttpGet，HttpHead，HttpPost，HttpPut，HttpDelete，HttpTrace，HttpOptions。
请求URI
URI（统一资源标识符），用于标识应用请求的资源，通常由协议版本，主机名，端口（可选），资源路径，参数名（可选），参数值（可选）组成。
请求时，可以通过拼接字符串的形式访问：
HttpGet get = new HttpGet("http://www.jianshu.com/p/7021031d6e49?utm_medium=index-banner&utm_source=desktop");
httpclient也提供了URIBuilder这个类简化请求URI的创建和修改。
URI uri = new URIBuilder()
.setScheme("http")
.setHost("www.jianshu.com")
.setPath("/p/7021031d6e49")
.setParameter("utm_medium", "index-banner")
.setParameter("utm_source", "desktop")
.build();
HttpGet httpget = new HttpGet(uri);
System.out.println(httpget.getURI());
输出
http://www.jianshu.com/p/7021031d6e49?utm_medium=index-banner&utm_source=desktop
http请求是客户端发给服务端的一个消息，消息的第一行包括了请求方法，URI以及使用的协议版本。
HttpGet get = new HttpGet("http://www.jianshu.com/u/8a3115bb299c");
RequestLine requestLine = get.getRequestLine();
System.out.println(requestLine.getMethod());
System.out.println(requestLine.getUri());
System.out.println(requestLine.getProtocolVersion());
System.out.println(requestLine);
输出
GET
http://www.jianshu.com/u/8a3115bb299c
HTTP/1.1
GET http://www.jianshu.com/u/8a3115bb299c HTTP/1.1
处理响应
服务端接收并处理请求后，会返回消息给到客户端，该消息的第一行包括协议版本，状态码以及描述文字。
CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet get = new HttpGet("http://www.jianshu.com/u/8a3115bb299c");
try(CloseableHttpResponse response = httpclient.execute(get)) {
System.out.println(response.getStatusLine());
System.out.println(response.getProtocolVersion());
System.out.println(response.getStatusLine().getStatusCode());
System.out.println(response.getStatusLine().getReasonPhrase());
System.out.println(response.getStatusLine().toString());
} catch (Exception e) {
e.printStackTrace();
}
输出
HTTP/1.1 200 OK
HTTP/1.1
200
OK
HTTP/1.1 200 OK
服务端返回的响应被封装在HttpEntity，通过HttpEntity可以获取请求响应的一些元信息，比如Content-Type，Content-Length以及Content-Encoding，元信息需由服务端提供，否则将是空值。
CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet get = new HttpGet("http://www.jianshu.com/u/8a3115bb299c");
try(CloseableHttpResponse response = httpclient.execute(get)) {
HttpEntity entity = response.getEntity();
System.out.println(entity.getContentType());
System.out.println(entity.getContentLength());
System.out.println(entity.getContentEncoding());
} catch (Exception e) {
e.printStackTrace();
}
输出
Content-Type: text/html; charset=utf-8
-1
null
HttpEntity提供了多种方法来读取正文，官方文档推荐使用以下方法进行读取：HttpEntity.getContent() 与 HttpEntity.writeTo(outputStream)
HttpEntity.getContent()方式
CloseableHttpClient httpclient = null;
CloseableHttpResponse response = null;
BufferedReader reader = null;
try {
httpclient = HttpClients.createDefault();
HttpGet get = new HttpGet("http://www.jianshu.com/u/8a3115bb299c");
response = httpclient.execute(get);
HttpEntity entity = response.getEntity();
reader = new BufferedReader(new InputStreamReader(entity.getContent(), "UTF-8"));
String str = "";
StringBuilder sb = new StringBuilder();
while ((str = reader.readLine()) != null) {
sb.append(str).append("\n");
}
System.out.println(sb);
} catch (Exception e) {
e.printStackTrace();
} finally {
CommonUtils.closeReader(reader);
CommonUtils.closeResponse(response);
CommonUtils.closeHttpClient(httpclient);
}
CommonUtils
public void closeReader(BufferedReader reader) {
try {
if (reader != null) {
reader.close();
}
} catch (Exception e) {
e.printStackTrace();
}
}
public void closeResponse(CloseableHttpResponse response) {
try {
if (response != null) {
response.close();
}
} catch (Exception e) {
e.printStackTrace();
}
}
public void closeHttpClient(CloseableHttpClient httpclient) {
try {
if (httpclient != null) {
httpclient.close();
}
} catch (Exception e) {
e.printStackTrace();
}
}
HttpEntity.writeTo方式
CloseableHttpClient httpclient = null;
CloseableHttpResponse response = null;
ByteArrayOutputStream outstream = new ByteArrayOutputStream();
try {
httpclient = HttpClients.createDefault();
HttpGet get = new HttpGet("http://www.jianshu.com/u/8a3115bb299c");
response = httpclient.execute(get);
HttpEntity entity = response.getEntity();
entity.writeTo(outstream);
System.out.println(outstream.toString("UTF-8"));
} catch (Exception e) {
e.printStackTrace();
} finally {
CommonUtils.closeOutputStream(outstream);
CommonUtils.closeResponse(response);
CommonUtils.closeHttpClient(httpclient);
}
CommonUtils
public void closeOutputStream(ByteArrayOutputStream outstream) {
try {
if (outstream != null) {
outstream.close();
}
} catch (Exception e) {
e.printStackTrace();
}
}
为了确保资源的正确释放，采用上述两种方式读取正文时，都需对流进行关闭。除此之外，HttpClient还提供了EntityUtils工具类，方便对正文的读取操作，不过官方文档中建议，只有当响应实体来自受信任的HTTP服务器，并且已知其长度有限，才可以采用这种方法。
EntityUtils.toString方式
CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet get = new HttpGet("http://www.jianshu.com/u/8a3115bb299c");
try(CloseableHttpResponse response = httpclient.execute(get)) {
HttpEntity entity = response.getEntity();
System.out.println(EntityUtils.toString(entity, "UTF-8"));
} catch (Exception e) {
e.printStackTrace();
} finally {
CommonUtils.closeHttpClient(httpclient);
}
EntityUtils.toString内部会对HttpEntity中的输入流进行关闭。
以上三种读取方式，当CloseableHttpClient的实例不再使用时，都需调用close方法进行关闭。
通过BufferedHttpEntity实现响应多次读取
对于流类型的HttpEntity而言，是不可重复读取的，若想多次读取，则需要在某个地方将HttpEntity缓存起来，最简单的方式，可以使用HttpClient提供的BufferedHttpEntity类来实现。
HttpEntity.getContent()+BufferedHttpEntity实现多次读取
CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet get = new HttpGet("http://www.jianshu.com/u/8a3115bb299c");
try(CloseableHttpResponse response = httpclient.execute(get)) {
HttpEntity entity = response.getEntity();
BufferedHttpEntity bufferedEntity = new BufferedHttpEntity(entity);
System.out.println(getEntityContent(bufferedEntity));
System.out.println(getEntityContent(bufferedEntity));
} catch (Exception e) {
e.printStackTrace();
} finally {
CommonUtils.closeHttpClient(httpclient);
}
public String getEntityContent(HttpEntity entity) {
try(BufferedReader reader = new BufferedReader(new InputStreamReader(entity.getContent(), "UTF-8"))) {
String str = "";
StringBuilder sb = new StringBuilder();
while ((str = reader.readLine()) != null) {
sb.append(str).append("\n");
}
return sb.toString();
} catch (Exception e) {
e.printStackTrace();
return "";
}
}
HttpEntity.writeTo+BufferedHttpEntity实现多次读取
CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet get = new HttpGet("http://www.jianshu.com/u/8a3115bb299c");
try(CloseableHttpResponse response = httpclient.execute(get)) {
HttpEntity entity = response.getEntity();
BufferedHttpEntity bufferedEntity = new BufferedHttpEntity(entity);
System.out.println(getContentByWriteTo(bufferedEntity));
System.out.println(getContentByWriteTo(bufferedEntity));
} catch (Exception e) {
e.printStackTrace();
} finally {
CommonUtils.closeHttpClient(httpclient);
}
public String getContentByWriteTo(HttpEntity entity) {
try(ByteArrayOutputStream outstream = new ByteArrayOutputStream()) {
entity.writeTo(outstream);
return outstream.toString("UTF-8");
} catch (Exception e) {
e.printStackTrace();
return "";
}
}
EntityUtils.toString+BufferedHttpEntity实现多次读取
CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet get = new HttpGet("http://www.jianshu.com/u/8a3115bb299c");
try(CloseableHttpResponse response = httpclient.execute(get)) {
HttpEntity entity = response.getEntity();
BufferedHttpEntity bufferedEntity = new BufferedHttpEntity(entity);
System.out.println(EntityUtils.toString(bufferedEntity, "UTF-8"));
System.out.println(EntityUtils.toString(bufferedEntity, "UTF-8"));
} catch (Exception e) {
e.printStackTrace();
} finally {
CommonUtils.closeHttpClient(httpclient);
}
至此，通过HttpClient，简单实现了http get请求的发送和响应处理。
