httpclient的使用(二)

一、HTTP context的使用

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HTTP协议是无状态的协议，但是在使用时，我们总希望一些状态信息能够更长的时间,我们称之为上下文。我们可以采用以下方式为我们的会话增加状态信息。

HttpContext httpContext = new BasicHttpContext();
HttpGet httpGet = <...>
HttpResponse response = httpClient.execute(httpGet, httpContext);

HttpClient提供了一下属性可以添加到我们中会话中，这是属性以键值对的方式存在于context中

• HttpConnection
• HttpHost
• HttpRoute
• HttpRequest
• HttpResponse
• java.lang.Boolean
• RequestConfig
• java.util.List<URI>

下面的代码是增加RequestConfig属性

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();
}

context测试样例

package download.test;

import java.io.IOException;

import org.apache.http.HttpEntity;
import org.apache.http.HttpHost;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpUriRequest;
import org.apache.http.client.protocol.HttpClientContext;
import org.apache.http.entity.ContentType;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.protocol.BasicHttpContext;
import org.apache.http.protocol.HttpContext;
import org.apache.http.util.EntityUtils;

public class Download {

    public static void main(String[] args) throws ClientProtocolException,
            IOException {
        /**
         * 获取重定向之后的网址信息
         * 
         * @see HttpClient缺省会自动处理客户端重定向
         * @see 即访问网页A后,假设被重定向到了B网页,那么HttpClient将自动返回B网页的内容
         * @see 若想取得B网页的地址
         *      ,就需要借助HttpContext对象,HttpContext实际上是客户端用来在多次请求响应的交互中,保持状态信息的
         * @see 我们自己也可以利用HttpContext来存放一些我们需要的信息,以便下次请求的时候能够取出这些信息来使用
         */

        CloseableHttpClient httpClient = HttpClients.createDefault();
        HttpContext httpContext = new BasicHttpContext();
        HttpGet httpGet = new HttpGet("http://127.0.0.1:80/");
        try {
            // 将HttpContext对象作为参数传给execute()方法,则HttpClient会把请求响应交互过程中的状态信息存储在HttpContext中
            HttpResponse response = httpClient.execute(httpGet, httpContext);
            // 将httpContext放入到适配器中以方便访问
            HttpClientContext clientContext = HttpClientContext
                    .adapt(httpContext);
            HttpHost targetHost = clientContext.getTargetHost();
            // 获取实际的请求对象的URI
            HttpUriRequest realRequest = (HttpUriRequest) clientContext.getRequest();
            // .getRequest();
            System.out.println("主机地址:" + targetHost);
            System.out.println("URI信息:" + realRequest.getURI());
            HttpEntity entity = response.getEntity();
            if (null != entity) {
                System.out.println("响应内容:"
                        + EntityUtils.toString(entity, ContentType
                                .getOrDefault(entity).getCharset()));
                EntityUtils.consume(entity);
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            httpClient.close();;
        }
    }
}

二、HTTP拦截器的使用

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 HTTP拦截器可以对特定的请求、应答甚至页面进行处理，也能通过HTTP上下文环境进行信息共享，如果多个拦截器协同工作时要考虑执行顺序的问题。

下面的例子是将本地context的信息添加到所有的请求头中

CloseableHttpClient httpclient = HttpClients.custom()
        .addInterceptorLast(new HttpRequestInterceptor() {
            public void process(
                    final HttpRequest request,
                    final HttpContext context) throws HttpException, IOException {
                AtomicInteger count = (AtomicInteger) context.getAttribute("count");
                request.addHeader("Count", Integer.toString(count.getAndIncrement()));
            }
        })
        .build();
AtomicInteger count = new AtomicInteger(1);
HttpClientContext localContext = HttpClientContext.create();
localContext.setAttribute("count", count);
HttpGet httpget = new HttpGet("http://localhost/");
for (int i = 0; i < 10; i++) {
    CloseableHttpResponse response = httpclient.execute(httpget, localContext);
    try {
        HttpEntity entity = response.getEntity();
    } finally {
        response.close();
    }
}

 三、请求retry操作

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在发起http请求时可能因为网络拥塞等原因，使得发送失败，这时我们会尝试重新发送请求。

HttpRequestRetryHandler myRetryHandler = new HttpRequestRetryHandler() {
    public boolean retryRequest(
            IOException exception,
            int executionCount,
            HttpContext context) {
        if (executionCount >= 5) {
            // Do not retry if over max retry count
            return false;
        }
        if (exception instanceof InterruptedIOException) {
            // Timeout
            return false;
        }
        if (exception instanceof UnknownHostException) {
            // Unknown host
            return false;
        }
        if (exception instanceof ConnectTimeoutException) {
            // Connection refused
            return false;
        }
        if (exception instanceof SSLException) {
            // SSL handshake exception
            return false;
        }
        HttpClientContext clientContext =         HttpClientContext.adapt(context);
        HttpRequest request = clientContext.getRequest();
        boolean idempotent = !(request instanceof HttpEntityEnclosingRequest);
        if (idempotent) {
            // Retry if the request is considered idempotent
            return true;
        }
        return false;
    }
};
CloseableHttpClient httpclient = HttpClients.custom()
        .setRetryHandler(myRetryHandler)
        .build();