Android HttpClient类的使用

一、HttpClient类的介绍

HttpClient是Apache下的一个Java开源项目，其作用是提供了一个高效、灵活、可扩展的处理HTTP请求的Java工具包，是用于Android上进行网络通信的标准工具。使用HttpClient可以很方便地在Android应用程序中执行HTTP请求，例如GET、POST等请求，获取服务器发送的响应和状态信息。

二、HttpClient请求流程

HttpClient请求的流程主要分为以下几个步骤：

创建 HttpClient 实例

HttpClient httpClient = new DefaultHttpClient();

创建 HttpGet 或 HttpPost 请求对象

HttpGet httpGet = new HttpGet(url);
HttpPost httpPost = new HttpPost(url);

设置请求参数

List<NameValuePair> paramList = new ArrayList<>();
paramList.add(new BasicNameValuePair("username", "test"));
paramList.add(new BasicNameValuePair("password", "123456"));
UrlEncodedFormEntity entity = new UrlEncodedFormEntity(paramList, "UTF-8");
httpPost.setEntity(entity);

客户端执行请求操作，并接收服务端返回的响应结果
```
HttpResponse httpResponse = httpClient.execute(httpGet);
```

三、HttpClient基本请求

1、GET请求

GET请求通过在URL后添加参数来传递参数信息。下面是一个使用HttpClient发送GET请求的例子。

public static String sendHttpGetRequest(String url) {
    String response = "";
    try {
        HttpClient httpClient = new DefaultHttpClient();
        HttpGet httpGet = new HttpGet(url);
        HttpResponse httpResponse = httpClient.execute(httpGet);
        if (httpResponse.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
            HttpEntity entity = httpResponse.getEntity();
            if (entity != null) {
                response = EntityUtils.toString(entity, "UTF-8");
            }
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return response;
}

2、POST请求

POST请求通过在请求体中添加参数来传递参数信息。下面是一个使用HttpClient发送POST请求的例子。

public static String sendHttpPostRequest(String url, Map<String, String> params) {
    String response = "";
    try {
        HttpClient httpClient = new DefaultHttpClient();
        HttpPost httpPost = new HttpPost(url);
        List<NameValuePair> paramList = new ArrayList<>();
        for (Map.Entry<String, String> entry : params.entrySet()) {
            paramList.add(new BasicNameValuePair(entry.getKey(), entry.getValue()));
        }
        UrlEncodedFormEntity entity = new UrlEncodedFormEntity(paramList, "UTF-8");
        httpPost.setEntity(entity);
        HttpResponse httpResponse = httpClient.execute(httpPost);
        if (httpResponse.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
            HttpEntity httpEntity = httpResponse.getEntity();
            if (httpEntity != null) {
                response = EntityUtils.toString(httpEntity, "UTF-8");
            }
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return response;
}

四、HttpClient高级请求

1、HttpClient连接池

默认情况下，每次HttpClient请求操作都会创建一个新的HttpClient实例，如果在同一个应用程序中进行多次请求，这样就会导致资源的浪费。因此，在实际开发中可以考虑使用HttpClient连接池来维护一组可用的HttpClient实例，减少资源的浪费。

public class HttpClientPool {
    private static PoolingHttpClientConnectionManager connMgr;
    private static RequestConfig requestConfig;
    private static final int MAX_TIMEOUT = 5000;
    static {
        connMgr = new PoolingHttpClientConnectionManager();
        connMgr.setMaxTotal(200);
        connMgr.setDefaultMaxPerRoute(connMgr.getMaxTotal());
        requestConfig = RequestConfig.custom()
                .setSocketTimeout(MAX_TIMEOUT)
                .setConnectTimeout(MAX_TIMEOUT)
                .setConnectionRequestTimeout(MAX_TIMEOUT)
                .build();
    }
    public static CloseableHttpClient getHttpClient() {
        CloseableHttpClient httpClient = HttpClients.custom()
                .setConnectionManager(connMgr)
                .setDefaultRequestConfig(requestConfig)
                .build();
        return httpClient;
    }
}

2、HttpClient拦截器

HttpClient拦截器是一个在请求执行前后对请求进行处理的机制。拦截器可以在请求执行前添加一些公共属性，比如请求头、cookies等。也可以在请求执行成功后对返回的响应结果进行加工处理。

public class HttpHeaderInterceptor implements HttpRequestInterceptor {
    private Map<String, String> header;
    public HttpHeaderInterceptor(Map<String, String> header) {
        this.header = header;
    }
    @Override
    public void process(HttpRequest request, HttpContext context) {
        for (Map.Entry<String, String> entry : header.entrySet()) {
            request.addHeader(entry.getKey(), entry.getValue());
        }
    }
}

3、HttpClient文件上传

HttpClient可以通过MultipartEntity来进行文件上传操作。

public static String uploadFile(String url, Map<String, String> params, File file) {
    String response = "";
    try {
        CloseableHttpClient httpClient = HttpClients.createDefault();
        HttpPost httpPost = new HttpPost(url);
        MultipartEntityBuilder builder = MultipartEntityBuilder.create();
        builder.setMode(HttpMultipartMode.BROWSER_COMPATIBLE);
        for (Map.Entry<String, String> entry : params.entrySet()) {
            builder.addPart(entry.getKey(), new StringBody(entry.getValue(), ContentType.TEXT_PLAIN));
        }
        builder.addPart("filename", new FileBody(file));
        httpPost.setEntity(builder.build());
        CloseableHttpResponse httpResponse = httpClient.execute(httpPost);
        if (httpResponse.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
            response = EntityUtils.toString(httpResponse.getEntity(), "UTF-8");
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return response;
}