初次接触HttpClient4.0

HttpClient程序包是一个实现了 HTTP 协议的客户端编程工具包，要想熟练的掌握它，必须熟悉 HTTP协议。一个最简单的调用如下：

import java.io.IOException;
 import org.apache.http.HttpResponse;
 import org.apache.http.client.ClientProtocolException;
 import org.apache.http.client.HttpClient;
 import org.apache.http.client.methods.HttpGet;
 import org.apache.http.client.methods.HttpUriRequest;
 import org.apache.http.impl.client.DefaultHttpClient;

 public class Test {
     public static void main(String[] args) {

        // 核心应用类
        HttpClient httpClient = new DefaultHttpClient();

         // HTTP请求
        HttpUriRequest request =
                 new HttpGet("http://localhost/index.html");

         // 打印请求信息
        System.out.println(request.getRequestLine());
         try {
             // 发送请求，返回响应
            HttpResponse response = httpClient.execute(request);

             // 打印响应信息
            System.out.println(response.getStatusLine());
         } catch (ClientProtocolException e) {
             // 协议错误
            e.printStackTrace();
         } catch (IOException e) {
             // 网络异常
            e.printStackTrace();
        }
    }
}

import java.io.IOException;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpUriRequest;
import org.apache.http.impl.client.DefaultHttpClient;

public class Test {
    public static void main(String[] args) {

       // 核心应用类
       HttpClient httpClient = new DefaultHttpClient();

        // HTTP请求
        HttpUriRequest request =
                new HttpGet("http://localhost/index.html");

        // 打印请求信息
        System.out.println(request.getRequestLine());
        try {
            // 发送请求，返回响应
            HttpResponse response = httpClient.execute(request);

            // 打印响应信息
            System.out.println(response.getStatusLine());
        } catch (ClientProtocolException e) {
            // 协议错误
            e.printStackTrace();
        } catch (IOException e) {
            // 网络异常
            e.printStackTrace();
        }
    }
}

import java.io.IOException;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpUriRequest;
import org.apache.http.impl.client.DefaultHttpClient;

public class Test {
    public static void main(String[] args) {

       // 核心应用类
       HttpClient httpClient = new DefaultHttpClient();

        // HTTP请求
        HttpUriRequest request =
                new HttpGet("http://localhost/index.html");

        // 打印请求信息
        System.out.println(request.getRequestLine());
        try {
            // 发送请求，返回响应
            HttpResponse response = httpClient.execute(request);

            // 打印响应信息
            System.out.println(response.getStatusLine());
        } catch (ClientProtocolException e) {
            // 协议错误
            e.printStackTrace();
        } catch (IOException e) {
            // 网络异常
            e.printStackTrace();
        }
    }
}

 

如果HTTP服务器正常并且存在相应的服务，则上例会打印出两行结果：

GET http://localhost/index.html HTTP/1.1
HTTP/1.1 200 OK

 核心对象httpClient的调用非常直观，其execute方法传入一个request对象，返回一个response对象。使用httpClient发出HTTP请求时，系统可能抛出两种异常，分别是ClientProtocolException和IOException。第一种异常的发生通常是协议错误导致，如在构造HttpGet对象时传入的协议不对（例如不小心将"http"写成"htp"），或者服务器端返回的内容不符合HTTP协议要求等；第二种异常一般是由于网络原因引起的异常，如HTTP服务器未启动等。
从实际应用的角度看，HTTP协议由两大部分组成：HTTP请求和HTTP响应。那么HttpClient程序包是如何实现HTTP客户端应用的呢？实现过程中需要注意哪些问题呢？

HTTP请求

HTTP 1.1由以下几种请求组成：GET, HEAD, POST, PUT, DELETE, TRACE and OPTIONS， 程序包中分别用HttpGet, HttpHead, HttpPost, HttpPut, HttpDelete, HttpTrace, and HttpOptions 这几个类创建请求。所有的这些类均实现了HttpUriRequest接口，故可以作为execute的执行参数使用。
所有请求中最常用的是GET与POST两种请求，与创建GET请求的方法相同，可以用如下方法创建一个POST请求：

HttpUriRequest request = new HttpPost(
         "http://localhost/index.html");

HttpUriRequest request = new HttpPost(
        "http://localhost/index.html");

HttpUriRequest request = new HttpPost(
        "http://localhost/index.html");

 

HTTP请求格式 告诉我们，有两个位置或者说两种方式可以为request提供参数：request-line方式与request-body方式。

request-line

request-line方式是指在请求行上通过URI直接提供参数。
（1）
我们可以在生成request对象时提供带参数的URI，如：

HttpUriRequest request = new HttpGet(
         "http://localhost/index.html?param1=value1&param2=value2");

HttpUriRequest request = new HttpGet(
        "http://localhost/index.html?param1=value1&param2=value2");

HttpUriRequest request = new HttpGet(
        "http://localhost/index.html?param1=value1&param2=value2");

 

（2）

另外，HttpClient程序包为我们提供了URIUtils工具类，可以通过它生成带参数的URI，如：

URI uri = URIUtils.createURI("http", "localhost", -1, "/index.html",
     "param1=value1&param2=value2", null);
 HttpUriRequest request = new HttpGet(uri);
System.out.println(request.getURI());

URI uri = URIUtils.createURI("http", "localhost", -1, "/index.html",
    "param1=value1&param2=value2", null);
HttpUriRequest request = new HttpGet(uri);
System.out.println(request.getURI());

URI uri = URIUtils.createURI("http", "localhost", -1, "/index.html",
    "param1=value1&param2=value2", null);
HttpUriRequest request = new HttpGet(uri);
System.out.println(request.getURI());

 

上例的打印结果如下：

http://localhost/index.html?param1=value1&param2=value2

 

（3）
需要注意的是，如果参数中含有中文，需将参数进行URLEncoding处理，如：

String param = "param1=" + URLEncoder.encode("中国", "UTF-8") + "&param2=value2";
 URI uri = URIUtils.createURI("http", "localhost", 8080,
 "/sshsky/index.html", param, null);
System.out.println(uri);

String param = "param1=" + URLEncoder.encode("中国", "UTF-8") + "&param2=value2";
URI uri = URIUtils.createURI("http", "localhost", 8080,
"/sshsky/index.html", param, null);
System.out.println(uri);

String param = "param1=" + URLEncoder.encode("中国", "UTF-8") + "&param2=value2";
URI uri = URIUtils.createURI("http", "localhost", 8080,
"/sshsky/index.html", param, null);
System.out.println(uri);

 

上例的打印结果如下：

http://localhost/index.html?param1=%E4%B8%AD%E5%9B%BD&param2=value2

 

（4）
对于参数的URLEncoding处理，HttpClient程序包为我们准备了另一个工具类：URLEncodedUtils。通过它，我们可以直观的（但是比较复杂）生成URI，如：

List<NameValuePair> params = new ArrayList<NameValuePair>();
 params.add(new BasicNameValuePair("param1", "中国"));
 params.add(new BasicNameValuePair("param2", "value2"));
 String param = URLEncodedUtils.format(params, "UTF-8");
 URI uri = URIUtils.createURI("http", "localhost", 8080,
 "/sshsky/index.html", param, null);
System.out.println(uri);

List<NameValuePair> params = new ArrayList<NameValuePair>();
params.add(new BasicNameValuePair("param1", "中国"));
params.add(new BasicNameValuePair("param2", "value2"));
String param = URLEncodedUtils.format(params, "UTF-8");
URI uri = URIUtils.createURI("http", "localhost", 8080,
"/sshsky/index.html", param, null);
System.out.println(uri);

List<NameValuePair> params = new ArrayList<NameValuePair>();
params.add(new BasicNameValuePair("param1", "中国"));
params.add(new BasicNameValuePair("param2", "value2"));
String param = URLEncodedUtils.format(params, "UTF-8");
URI uri = URIUtils.createURI("http", "localhost", 8080,
"/sshsky/index.html", param, null);
System.out.println(uri);

 

上例的打印结果如下：

http://localhost/index.html?param1=%E4%B8%AD%E5%9B%BD&param2=value2

 

request-body

与request-line方式不同，request-body方式是在request-body中提供参数，此方式只能用于POST请求。在HttpClient程序包中有两个类可以完成此项工作，它们分别是UrlEncodedFormEntity类与MultipartEntity类。这两个类均实现了HttpEntity接口。
（1）
使用最多的是UrlEncodedFormEntity类。通过该类创建的对象可以模拟传统的HTML表单传送POST请求中的参数。如下面的表单：

<form action="http://localhost/index.html" method="POST">
     <input type="text" name="param1" value="中国"/>
     <input type="text" name="param2" value="value2"/>
     <inupt type="submit" value="submit"/>
 </form>

<form action="http://localhost/index.html" method="POST">
    <input type="text" name="param1" value="中国"/>
    <input type="text" name="param2" value="value2"/>
    <inupt type="submit" value="submit"/>
</form>

<form action="http://localhost/index.html" method="POST">
    <input type="text" name="param1" value="中国"/>
    <input type="text" name="param2" value="value2"/>
    <inupt type="submit" value="submit"/>
</form>

 

我们可以用下面的代码实现：

List<NameValuePair> formParams = new ArrayList<NameValuePair>();
 formParams.add(new BasicNameValuePair("param1", "中国"));
 formParams.add(new BasicNameValuePair("param2", "value2"));
 HttpEntity entity = new UrlEncodedFormEntity(formParams, "UTF-8");

 HttpPost request = new HttpPost(“http://localhost/index.html”);
request.setEntity(entity);

List<NameValuePair> formParams = new ArrayList<NameValuePair>();
formParams.add(new BasicNameValuePair("param1", "中国"));
formParams.add(new BasicNameValuePair("param2", "value2"));
HttpEntity entity = new UrlEncodedFormEntity(formParams, "UTF-8");

HttpPost request = new HttpPost(“http://localhost/index.html”);
request.setEntity(entity);

List<NameValuePair> formParams = new ArrayList<NameValuePair>();
formParams.add(new BasicNameValuePair("param1", "中国"));
formParams.add(new BasicNameValuePair("param2", "value2"));       
HttpEntity entity = new UrlEncodedFormEntity(formParams, "UTF-8");
 
HttpPost request = new HttpPost(“http://localhost/index.html”);
request.setEntity(entity);

 

当然，如果想查看HTTP数据格式，可以通过HttpEntity对象的各种方法取得。如：

List<NameValuePair> formParams = new ArrayList<NameValuePair>();
 formParams.add(new BasicNameValuePair("param1", "中国"));
 formParams.add(new BasicNameValuePair("param2", "value2"));
 UrlEncodedFormEntity entity = new UrlEncodedFormEntity(formParams, "UTF-8");

System.out.println(entity.getContentType());
System.out.println(entity.getContentLength());
System.out.println(EntityUtils.getContentCharSet(entity));
System.out.println(EntityUtils.toString(entity));

List<NameValuePair> formParams = new ArrayList<NameValuePair>();
formParams.add(new BasicNameValuePair("param1", "中国"));
formParams.add(new BasicNameValuePair("param2", "value2"));
UrlEncodedFormEntity entity = new UrlEncodedFormEntity(formParams, "UTF-8");

System.out.println(entity.getContentType());
System.out.println(entity.getContentLength());
System.out.println(EntityUtils.getContentCharSet(entity));
System.out.println(EntityUtils.toString(entity));

List<NameValuePair> formParams = new ArrayList<NameValuePair>();
formParams.add(new BasicNameValuePair("param1", "中国"));
formParams.add(new BasicNameValuePair("param2", "value2"));       
UrlEncodedFormEntity entity = new UrlEncodedFormEntity(formParams, "UTF-8");
 
System.out.println(entity.getContentType());
System.out.println(entity.getContentLength());
System.out.println(EntityUtils.getContentCharSet(entity));
System.out.println(EntityUtils.toString(entity));

 

上例的打印结果如下：

Content-Type: application/x-www-form-urlencoded; charset=UTF-8
39
UTF-8
param1=%E4%B8%AD%E5%9B%BD&param2=value2

 

（2）
除了传统的application/x-www-form-urlencoded表单，我们另一个经常用到的是上传文件用的表单，这种表单的类型为multipart/form-data。在HttpClient程序扩展包（HttpMime）中专门有一个类与之对应，那就是MultipartEntity类。此类同样实现了HttpEntity接口。如下面的表单：

<form action="http://localhost/index.html" method="POST"
         enctype="multipart/form-data">
     <input type="text" name="param1" value="中国"/>
     <input type="text" name="param2" value="value2"/>
     <input type="file" name="param3"/>
     <inupt type="submit" value="submit"/>
 </form>

<form action="http://localhost/index.html" method="POST"
        enctype="multipart/form-data">
    <input type="text" name="param1" value="中国"/>
    <input type="text" name="param2" value="value2"/>
    <input type="file" name="param3"/>
    <inupt type="submit" value="submit"/>
</form>

<form action="http://localhost/index.html" method="POST"
        enctype="multipart/form-data">
    <input type="text" name="param1" value="中国"/>
    <input type="text" name="param2" value="value2"/>
    <input type="file" name="param3"/>
    <inupt type="submit" value="submit"/>
</form>

 

我们可以用下面的代码实现：

MultipartEntity entity = new MultipartEntity();
 entity.addPart("param1", new StringBody("中国", Charset.forName("UTF-8")));
 entity.addPart("param2", new StringBody("value2", Charset.forName("UTF-8")));
 entity.addPart("param3", new FileBody(new File("C:\\1.txt")));

 HttpPost request = new HttpPost(“http://localhost/index.html”);
request.setEntity(entity);

MultipartEntity entity = new MultipartEntity();
entity.addPart("param1", new StringBody("中国", Charset.forName("UTF-8")));
entity.addPart("param2", new StringBody("value2", Charset.forName("UTF-8")));
entity.addPart("param3", new FileBody(new File("C:\\1.txt")));

HttpPost request = new HttpPost(“http://localhost/index.html”);
request.setEntity(entity);

MultipartEntity entity = new MultipartEntity();
entity.addPart("param1", new StringBody("中国", Charset.forName("UTF-8")));
entity.addPart("param2", new StringBody("value2", Charset.forName("UTF-8")));
entity.addPart("param3", new FileBody(new File("C:\\1.txt")));
 
HttpPost request = new HttpPost(“http://localhost/index.html”);
request.setEntity(entity);

 

HTTP响应

HttpClient程序包对于HTTP响应的处理较之HTTP请求来说是简单多了，其过程同样使用了HttpEntity接口。我们可以从HttpEntity对象中取出数据流（InputStream），该数据流就是服务器返回的响应数据。需要注意的是，HttpClient程序包不负责解析数据流中的内容。如：

HttpUriRequest request = ...;
HttpResponse response = httpClient.execute(request);

 // 从response中取出HttpEntity对象
HttpEntity entity = response.getEntity();

 // 查看entity的各种指标
System.out.println(entity.getContentType());
System.out.println(entity.getContentLength());
System.out.println(EntityUtils.getContentCharSet(entity));

 // 取出服务器返回的数据流
InputStream stream = entity.getContent();

 // 以任意方式操作数据流stream
 // 调用方式 略

HttpUriRequest request = ...;
HttpResponse response = httpClient.execute(request);

// 从response中取出HttpEntity对象
HttpEntity entity = response.getEntity();

// 查看entity的各种指标
System.out.println(entity.getContentType());
System.out.println(entity.getContentLength());
System.out.println(EntityUtils.getContentCharSet(entity));

// 取出服务器返回的数据流
InputStream stream = entity.getContent();

// 以任意方式操作数据流stream
// 调用方式 略

HttpUriRequest request = ...;
HttpResponse response = httpClient.execute(request);
 
// 从response中取出HttpEntity对象
HttpEntity entity = response.getEntity();
 
// 查看entity的各种指标
System.out.println(entity.getContentType());
System.out.println(entity.getContentLength());
System.out.println(EntityUtils.getContentCharSet(entity));
 
// 取出服务器返回的数据流
InputStream stream = entity.getContent();
 
// 以任意方式操作数据流stream
// 调用方式 略

 

附注：

本文说明的是HttpClient 4.0.1 ，该程序包（包括依赖的程序包）由以下几个JAR包组成：
 
commons-logging-1.1.1.jar
commons-codec-1.4.jar
httpcore-4.0.1.jar
httpclient-4.0.1.jar
apache-mime4j-0.6.jar
httpmime-4.0.1.jar