Httpclient4.4之原理（请求执行）

apache Httpclient基于java BIO实现的，也是基于apache HttpCore项目。他最基本的功能是执行HTTP方法。HttpClient的API的主要入口就是HttpClient接口，看看这个示例：

package httpclienttest;

import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;

public class T1 {
    public static void main(String[] args) {
        HttpGet httpget = new HttpGet("http://www.baidu.com");
        try(CloseableHttpClient httpclient = HttpClients.createDefault();
                CloseableHttpResponse response = httpclient.execute(httpget);){
            System.out.printf("内容类型为：%s",response.getEntity().getContentType());
        }catch(Exception e){
            e.printStackTrace();
        }
}

1. HTTP请求

所有的http请求都由：方法名，请求url，HTTP协议组成。HttpClient支持HTTP/1.1支持的所有方法：GET，HEAD，POST，PUT，DELETE，TRACE和OPTIONS，HttpClient中都有一个特定的类与之对应：HttpGet，HttpHead，HttpPost，HttpPut，HttpDelete，HttpTrace和HttpOptions。

HTTP请求URI包括协议，主机名，可选的端口，资源路径，可选的查询条件等。如下例：

HttpGet httpget = new HttpGet("http://www.google.com/search?hl=en"
                    + "&q=httpclient&btnG=Google+Search&aq=f&oq=");

HttpClient提供了URIBuilder通用类来创建或修改请求URI，如例：

URI uri = new URIBuilder()
    .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())

2. HTTP响应

HTTP响应是HTTP请求发送到服务端处理后响应到客户端的消息。响应第一行是协议与协议版本号，接着是数字状态码和一些文本信息，示例演示一下看看执行结果：

package httpclienttest;

import org.apache.http.HttpResponse;
import org.apache.http.HttpStatus;
import org.apache.http.HttpVersion;
import org.apache.http.message.BasicHttpResponse;

public class T2 {
    public static void main(String[] args) {
        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());
    }
}

输出为：

HTTP/1.1
200
OK
HTTP/1.1 200 OK

3. HTTP消息头

HTTP消息头（header）包含多个消息描述的信息，例如：内容长度，内容类型等。HttpClient提供的方法有检索，添加，删除和枚举等操作。 示例：

package httpclienttest;

import org.apache.http.Header;
import org.apache.http.HttpResponse;
import org.apache.http.HttpStatus;
import org.apache.http.HttpVersion;
import org.apache.http.message.BasicHttpResponse;

public class T3 {
    public static void main(String[] args) {
        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

更有效率的方法是通过HeaderIterator接口获得所有的header信息，示例：

package httpclienttest;

import org.apache.http.HeaderIterator;
import org.apache.http.HttpResponse;
import org.apache.http.HttpStatus;
import org.apache.http.HttpVersion;
import org.apache.http.message.BasicHttpResponse;

public class T4 {
    public static void main(String[] args) {
        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());
        }
    }
}

输出结果：

Set-Cookie: c1=a; path=/; domain=localhost
Set-Cookie: c2=b; path="/", c3=c; domain="localhost"

他也提供了更便利的方法来解析HTTP消息并获得header中一个个独立的header元素，示例：

package httpclienttest;

import org.apache.http.HeaderElement;
import org.apache.http.HeaderElementIterator;
import org.apache.http.HttpResponse;
import org.apache.http.HttpStatus;
import org.apache.http.HttpVersion;
import org.apache.http.NameValuePair;
import org.apache.http.message.BasicHeaderElementIterator;
import org.apache.http.message.BasicHttpResponse;

public class T5 {
    public static void main(String[] args) {
        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\"");
        HeaderElementIterator it = new BasicHeaderElementIterator(
                response.headerIterator("Set-Cookie"));
        while (it.hasNext()) {
            HeaderElement elem = it.nextElement(); 
            System.out.println(elem.getName() + " = " + elem.getValue());
            NameValuePair[] params = elem.getParameters();
            for (int i = 0; i < params.length; i++) {
                System.out.println(" " + params[i]);
            }
        }
    }
}

输出信息：

c1 = a
path=/
domain=localhost
c2 = b
path=/
c3 = c
domain=localhost