【HttpClient4.5中文教程】【第一章：基础】1.3 HttpClient执行上下文

1.3.Http执行上下文(context)

最初，HTTP是被设计成无状态的，面向请求-响应的协议。然而，现实世界中的应用程序经常需要通过一些逻辑相关的请求-响应交换来保持状态信息。为了使应用程序能够维持一个过程状态， HttpClient允许HTTP请求在一个特定的执行上下文中来执行–称为HTTP上下文。如果相同的上下文在连续请求之间重用，那么多种逻辑相关的请求可以参与到一个逻辑会话中。HTTP上下文功能和java.util.Map<String,Object>很相似。它仅仅是任意命名参数值的集合。应用程序可以在请求之前填充上下文属性，也可以在执行完成之后来检查上下文属性。

HttpContext能够包含任意的对象，因此在两个不同的线程中共享上下文是不安全的，建议每个线程都一个它自己执行的上下文。

在HTTP请求执行的这一过程中， HttpClient添加了下列属性到执行上下文中：

`HttpConnection实例代表连接到目标服务器的当前连接。
`HttpHost实例代表连接到目标服务器的当前连接。
`HttpRoute实例代表了完整的连接路由。
`HttpRequest实例代表了当前的HTTP请求。最终的HttpRequest对象在执行上下文中总是准确代表了状态信息，因为它已经发送给了服务器。每个默认的HTTP/1.0 和 HTTP/1.1使用相对的请求URI，可是，请求以non-tunneling模式通过代理发送时，URI会是绝对的。
`HttpResponse实例代表当前的HTTP响应。
`java.lang.Boolean对象是一个标识，它标志着当前请求是否完整地传输给连接目标。
`RequestConfig代表当前请求配置。
`java.util.List<URI>对象代表一个含有执行请求过程中所有的重定向地址。

你可以使用HttpClientContext适配器类来简化上下文状态的活动。

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print ?

HttpContext context = <…>
HttpClientContext clientContext = HttpClientContext.adapt(context);
HttpHost target = clientContext.getTargetHost();
HttpRequest request = clientContext.getRequest();
HttpResponse response = clientContext.getResponse();
RequestConfig config = clientContext.getRequestConfig();

HttpContext context = <...>
HttpClientContext clientContext = HttpClientContext.adapt(context);
HttpHost target = clientContext.getTargetHost();
HttpRequest request = clientContext.getRequest();
HttpResponse response = clientContext.getResponse();
RequestConfig config = clientContext.getRequestConfig();

代表一个逻辑相关的会话中的多个请求序列应该被同一个HttpContext实例执行，以确保请求之间会话上下文和状态信息的自动传输。

下面的例子中：请求配置在最初被初始化，它将在执行上下文中一直保持。共享与同一个会话中所有连续的请求。

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CloseableHttpClient httpclient = HttpClients.createDefault();
RequestConfig requestConfig = RequestConfig.custom()
.setSocketTimeout(1000)
.setConnectTimeout(1000)
.build();
HttpGet httpget1 = new HttpGet(“http://localhost/1”);
httpget1.setConfig(requestConfig);
CloseableHttpResponse response1 = httpclient.execute(httpget1, context);
try {
HttpEntity entity1 = response1.getEntity();
} finally {
response1.close();
}
HttpGet httpget2 = new HttpGet(“http://localhost/2”);
CloseableHttpResponse response2 = httpclient.execute(httpget2, context);
try {
HttpEntity entity2 = response2.getEntity();
} finally {
response2.close();
}

 CloseableHttpClient httpclient = HttpClients.createDefault();
RequestConfig requestConfig = RequestConfig.custom()
    .setSocketTimeout(1000)
    .setConnectTimeout(1000)
    .build();
HttpGet httpget1 = new HttpGet("http://localhost/1");
httpget1.setConfig(requestConfig);
CloseableHttpResponse response1 = httpclient.execute(httpget1, context);
try {
    HttpEntity entity1 = response1.getEntity();
} finally {
    response1.close();
}
HttpGet httpget2 = new HttpGet("http://localhost/2");
CloseableHttpResponse response2 = httpclient.execute(httpget2, context);
try {
    HttpEntity entity2 = response2.getEntity();
} finally {
    response2.close();
}

=========================总结======================

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print ?

import java.util.List;
import org.apache.http.client.CookieStore;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.protocol.HttpClientContext;
import org.apache.http.cookie.Cookie;
import org.apache.http.impl.client.BasicCookieStore;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;
/**
* This example demonstrates the use of a local HTTP context populated with
* custom attributes.
*/
public class ClientCustomContext {
public final static void main(String[] args) throws Exception {
CloseableHttpClient httpclient = HttpClients.createDefault();
try {
// Create a local instance of cookie store
CookieStore cookieStore = new BasicCookieStore();
// Create local HTTP context
HttpClientContext localContext = HttpClientContext.create();
// Bind custom cookie store to the local context
localContext.setCookieStore(cookieStore);
HttpGet httpget = new HttpGet(“http://localhost/”);
System.out.println(”Executing request ” + httpget.getRequestLine());
// Pass local context as a parameter
CloseableHttpResponse response = httpclient.execute(httpget, localContext);
try {
System.out.println(”—————————————-“);
System.out.println(response.getStatusLine());
List<Cookie> cookies = cookieStore.getCookies();
for (int i = 0; i < cookies.size(); i++) {
System.out.println(”Local cookie: ” + cookies.get(i));
}
EntityUtils.consume(response.getEntity());
} finally {
response.close();
}
} finally {
httpclient.close();
}
}
}

import java.util.List;

import org.apache.http.client.CookieStore;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.protocol.HttpClientContext;
import org.apache.http.cookie.Cookie;
import org.apache.http.impl.client.BasicCookieStore;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;

/**
 * This example demonstrates the use of a local HTTP context populated with
 * custom attributes.
 */
public class ClientCustomContext {

    public final static void main(String[] args) throws Exception {
        CloseableHttpClient httpclient = HttpClients.createDefault();
        try {
            // Create a local instance of cookie store
            CookieStore cookieStore = new BasicCookieStore();

            // Create local HTTP context
            HttpClientContext localContext = HttpClientContext.create();
            // Bind custom cookie store to the local context
            localContext.setCookieStore(cookieStore);

            HttpGet httpget = new HttpGet("http://localhost/");
            System.out.println("Executing request " + httpget.getRequestLine());

            // Pass local context as a parameter
            CloseableHttpResponse response = httpclient.execute(httpget, localContext);
            try {
                System.out.println("----------------------------------------");
                System.out.println(response.getStatusLine());
                List<Cookie> cookies = cookieStore.getCookies();
                for (int i = 0; i < cookies.size(); i++) {
                    System.out.println("Local cookie: " + cookies.get(i));
                }
                EntityUtils.consume(response.getEntity());
            } finally {
                response.close();
            }
        } finally {
            httpclient.close();
        }
    }

}