HttpClient学习笔记

HttpClient相关的实体类官方文档地址：http://hc.apache.org/httpcomponents-client-ga/httpclient/apidocs/

使用HttpClient发送请求、接收响应很简单，一般需要如下几步即可：

1. 创建HttpClient对象，HttpClient httpClient=new DefaultHttpClient()。

2. 创建请求方法的实例，并指定请求URL。如果需要发送GET请求，创建HttpGet对象；如果需要发送POST请求，创建HttpPost对象。

GET：

HttpGet HttpGet=new HttpGet(“http://www.baidu.com”);
httpClient.execute(httpGet);

3. 如果需要发送请求参数，可调用HttpGet、HttpPost共同的setParams(HetpParams params)方法来添加请求参数；对于HttpPost对象而言，也可调用setEntity(HttpEntity entity)方法来设置请求参数。通过一个NameValuePair集合来存放待提交的参数,并将这个参数集合传入到一个UrlEncodedFormEntity中,然后调用HttpPost的setEntity()方法将构建好的UrlEncodedFormEntity传入。

POST：

HttpPost httpPost=new HttpPost(“http://www.baidu.com”);
List<NameValuePair>params=newArrayList<NameValuePair>();
Params.add(new BasicNameValuePair(“username”,”admin”));
Params.add(new BasicNameValuePair(“password”,”123456”));
UrlEncodedFormEntity entity=newUrlEncodedFormEntity(params,”utf-8”);
httpPost.setEntity(entity);
HttpClient.execute(HttpPost); 

4. 调用HttpClient对象的execute(HttpUriRequest request)发送请求，该方法返回一个HttpResponse。

5. 调用HttpResponse的getAllHeaders()、getHeaders(String name)等方法可获取服务器的响应头；调用HttpResponse的getEntity()方法可获取HttpEntity对象，该对象包装了服务器的响应内容。程序可通过该对象获取服务器的响应内容。

先取出服务器返回的状态码,如果等于200就说明请求和响应都成功了:

If(httpResponse.getStatusLine().getStatusCode()==200){
     //请求和响应都成功了
     HttpEntityentity=HttpResponse.getEntity();//调用getEntity()方法获取到一个HttpEntity实例
     //用EntityUtils.toString()这个静态方法将HttpEntity转换成字符串,防止服务器返回的数据带有中文,所以在转换的时候将字符集指定成utf-8就可以了
     Stringresponse=EntityUtils.toString(entity,”utf-8”);
}

6. 释放连接。无论执行方法是否成功，都必须释放连接。

GET请求示例代码：

	public static String HttpGet(String url) {
		// 关闭HttpClient系统日志;
		System.setProperty("org.apache.commons.logging.Log","org.apache.commons.logging.impl.SimpleLog");
		System.setProperty("org.apache.commons.logging.simplelog.showdatetime","true");
		System.setProperty("org.apache.commons.logging.simplelog.log.org.apache.commons.httpclient","stdout");

		HttpClient httpClient = new DefaultHttpClient();
		HttpGet get = new HttpGet(url);
		HttpResponse response = null;
		HttpEntity httpEntity = null;
		String content = null;
		try {
			response = httpClient.execute(get);
			if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
				//判断gzip,解压缩
				if(!ObjectUtil.isEmpty(response.getLastHeader("Content-Encoding")) && (response.getLastHeader("Content-Encoding").toString()).indexOf("gzip")>=0){
					response.setEntity(new GzipDecompressingEntity(response.getEntity()));
				}
				httpEntity = response.getEntity();
				content = EntityUtils.toString(httpEntity);
			}
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			if (null != httpEntity) {
				try {
					httpEntity.consumeContent();//已经弃用
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
		}
		return content;
	}

POST请求示例代码：

	/**
	 * @param url
	 * @param data (以流的方式发送请求参数)
	 * @return
	 */
	public static String HttpPost(String url, String data) {
		// 关闭HttpClient系统日志;
		System.setProperty("org.apache.commons.logging.Log","org.apache.commons.logging.impl.SimpleLog");
		System.setProperty("org.apache.commons.logging.simplelog.showdatetime","true");
		System.setProperty("org.apache.commons.logging.simplelog.log.org.apache.commons.httpclient","stdout");

		HttpClient httpClient = new DefaultHttpClient();
		HttpPost post = new HttpPost(url);
		HttpResponse response = null;
		HttpEntity httpEntity = null;
		String content = null;
		try {
			StringEntity entity = new StringEntity(data,"UTF-8");
			post.setEntity(entity);
			response = httpClient.execute(post);
			if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
				httpEntity = response.getEntity();
				content = EntityUtils.toString(httpEntity,"UTF-8");
			}
		} catch (Exception e) {
			Logger.getRootLogger().error("HttpPost-流的方式：" + e.getMessage());
		} finally {
			if (null != httpEntity) {
				try {
					httpEntity.consumeContent();
				} catch (IOException e) {
					Logger.getRootLogger().error("HttpPost-流的方式：" + e.getMessage());
				}
			}
		}
		return content;
	}

释放连接注意的问题：

上述示例中使用的httpEntity.consumeContent()自从4.1版本就已经废弃，不建议使用了，建议使用InputStream.close() on the input stream returned bygetContent()来完成。官方sample如下：

CloseableHttpClient httpclient = HttpClients.createDefault();
try {
    HttpGet httpget = new HttpGet("http://www.apache.org/");
    // Execute HTTP request
    CloseableHttpResponse response = httpclient.execute(httpget);
    try {
        // Get hold of the response entity
        HttpEntity entity = response.getEntity();
        // If the response does not enclose an entity, there is no need
        // to bother about connection release
        if (entity != null) {
            InputStream instream = entity.getContent();//获取流对象
            try {
                instream.read();
                // do something useful with the response
            } catch (IOException ex) {
                // In case of an IOException the connection will be released
                // back to the connection manager automatically
                throw ex;
            } finally {
                // Closing the input stream will trigger connection release
                instream.close();
            }
        }
    } finally {
        response.close();
    }
} finally {
    httpclient.close();
}

也可以使用EntityUtils.consume(entity); Ensures that the entity content is fully consumed and the content stream, if exists, is closed.

官方建议使用这种方法：

@Deprecated

void consumeContent()

        throws IOException



Deprecated. (4.1) Use EntityUtils.consume(HttpEntity)

This method is deprecated since version 4.1. Please use standard java convention to ensure resource deallocation by calling InputStream.close() on the input stream returned by getContent()



This method is called to indicate that the content of this entity is no longer required. All entity implementations are expected to release all allocated resources as a result of this method invocation. Content streaming entities are also expected to dispose
of the remaining content, if any. Wrapping entities should delegate this call to the wrapped entity.



This method is of particular importance for entities being received from a connection. The entity needs to be consumed completely in order to re-use the connection with keep-alive.



Throws:

IOException - if an I/O error occurs.

See Also:

and #writeTo(OutputStream)