Apache HttpClient 4.3官方教程：基础与请求执行

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httpClient官方文档是Apache软件基金会提供的一份详尽的教程，主要针对HttpComponents HttpClient库的4.3.x版本进行讲解。这份文档由Oleg Kalnichevski、Jonathan Moore和Jilles van Gurp共同编写，旨在帮助开发者理解和使用HttpClient进行HTTP通信。文档分为多个章节，包括预览、HttpClient的范围以及其功能和限制。在"HttpClient Tutorial"部分，首先介绍了HttpClient的适用范围，它专注于客户端HTTP请求处理，适合在网络应用中发送HTTP请求并处理响应。文档强调了HttpClient并非全方位的HTTP工具集，它专注于HTTP请求执行的基本操作。 "Fundamentals"章节深入解析了HttpClient的核心功能： 1. **Request Execution**：这部分涵盖了如何创建和发送HTTP请求，包括HTTP请求的构建、参数设置以及如何启动请求执行流程。 2. **HTTP Request**：详细解释了如何构造HTTP请求，包括GET、POST、PUT、DELETE等方法，以及如何设置请求头（如User-Agent、Content-Type等）和请求体。 3. **HTTP Response**：介绍了如何处理接收到的HTTP响应，包括响应状态码、响应头信息和响应体内容的解析。 4. **Working with Message Headers**：着重讲解了HTTP消息头的使用，如设置和获取请求头，以及它们在HTTP通信中的作用。 5. **HTTP "..."**: 这里可能提到了更具体的技术细节或示例，如重定向、连接管理、线程池配置等。 "13-12-4"表明这是文档的一个更新日期，可能是指某个版本的发布日期。文档遵循Apache License 2.0，这意味着用户可以在遵守许可协议的前提下使用该库，且软件分发是"AS IS"，不附带任何保证或条件，开发者需自行检查许可证条款了解具体使用限制。这份HttpClient官方文档为开发者提供了强大的工具指导，涵盖了HttpClient的基础架构、API使用方法以及关键概念，对于需要在Java项目中实现高效HTTP通信的开发人员来说，是一份不可或缺的参考资料。

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System.out.println(myEntity.getContentLength());

System.out.println(EntityUtils.toString(myEntity));

System.out.println(EntityUtils.toByteArray(myEntity).length);

stdout >

Content-Type: text/plain; charset=utf-8

important message

1.1.5. Ensuring release of low level resources

In order to ensure proper release of system resources one must close either the

content stream associated with the entity or the response itself

CloseableHttpClient httpclient = HttpClients.createDefault();

HttpGet httpget = new HttpGet("http://localhost/");

CloseableHttpResponse response = httpclient.execute(httpget);

try {

HttpEntity entity = response.getEntity();

if (entity != null) {

InputStream instream = entity.getContent();

try {

// do something useful

} finally {

instream.close();

}

} finally {

response.close();

}

The difference between closing the content stream and closing the response is that the

former will attempt to keep the underlying connection alive by consuming the entity

content while the latter immediately shuts down and discards the connection.

Please note that the HttpEntity#writeTo(OutputStream) method is also required to ensure

proper release of system resources once the entity has been fully written out. If this

method obtains an instance of java.io.InputStream by calling HttpEntity#getContent(), it is also

expected to close the stream in a finally clause.

When working with streaming entities, one can use the EntityUtils#consume(HttpEntity)

method to ensure that the entity content has been fully consumed and the underlying

stream has been closed.

There can be situations, however, when only a small portion of the entire response

content needs to be retrieved and the performance penalty for consuming the

remaining content and making the connection reusable is too high, in which case one

can terminate the content stream by closing the response.

CloseableHttpClient httpclient = HttpClients.createDefault();

HttpGet httpget = new HttpGet("http://localhost/");

CloseableHttpResponse response = httpclient.execute(httpget);

try {

HttpEntity entity = response.getEntity();

if (entity != null) {

InputStream instream = entity.getContent();

int byteOne = instream.read();

int byteTwo = instream.read();

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// Do not need the rest

}

} finally {

response.close();

}

The connection will not be reused, but all level resources held by it will be correctly

deallocated.

1.1.6. Consuming entity content

The recommended way to consume the content of an entity is by using its

HttpEntity#getContent() or HttpEntity#writeTo(OutputStream) methods. HttpClient also comes

with the EntityUtils class, which exposes several static methods to more easily read the

content or information from an entity. Instead of reading the java.io.InputStream directly,

one can retrieve the whole content body in a string / byte array by using the methods

from this class. However, the use of EntityUtils is strongly discouraged unless the

response entities originate from a trusted HTTP server and are known to be of limited

length.

CloseableHttpClient httpclient = HttpClients.createDefault();

HttpGet httpget = new HttpGet("http://localhost/");

CloseableHttpResponse response = httpclient.execute(httpget);

try {

HttpEntity entity = response.getEntity();

if (entity != null) {

long len = entity.getContentLength();

if (len != -1 && len < 2048) {

System.out.println(EntityUtils.toString(entity));

} else {

// Stream content out

}

} finally {

response.close();

}

In some situations it may be necessary to be able to read entity content more than

once. In this case entity content must be buffered in some way, either in memory or

on disk. The simplest way to accomplish that is by wrapping the original entity with the

BufferedHttpEntity class. This will cause the content of the original entity to be read into a

in-memory buffer. In all other ways the entity wrapper will be have the original one.

CloseableHttpResponse response = <...>

HttpEntity entity = response.getEntity();

if (entity != null) {

entity = new BufferedHttpEntity(entity);

}

1.1.7. Producing entity content

HttpClient provides several classes that can be used to efficiently stream out content

though HTTP connections. Instances of those classes can be associated with entity

enclosing requests such as POST and PUT in order to enclose entity content into outgoing

HTTP requests. HttpClient provides several classes for most common data containers

such as string, byte array, input stream, and file: StringEntity, ByteArrayEntity,

InputStreamEntity, and FileEntity.

File file = new File("somefile.txt");

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FileEntity entity = new FileEntity(file, ContentType.create("text/plain", "UTF-8"));

HttpPost httppost = new HttpPost("http://localhost/action.do");

httppost.setEntity(entity);

Please note InputStreamEntity is not repeatable, because it can only read from the

underlying data stream once. Generally it is recommended to implement a custom

HttpEntity class which is self-contained instead of using the generic InputStreamEntity.

FileEntity can be a good starting point.

1.1.7.1. HTML forms

Many applications need to simulate the process of submitting an HTML form, for

instance, in order to log in to a web application or submit input data. HttpClient

provides the entity class UrlEncodedFormEntity to facilitate the process.

List<NameValuePair> formparams = new ArrayList<NameValuePair>();

formparams.add(new BasicNameValuePair("param1", "value1"));

formparams.add(new BasicNameValuePair("param2", "value2"));

UrlEncodedFormEntity entity = new UrlEncodedFormEntity(formparams, Consts.UTF_8);

HttpPost httppost = new HttpPost("http://localhost/handler.do");

httppost.setEntity(entity);

The UrlEncodedFormEntity instance will use the so called URL encoding to encode

parameters and produce the following content:

param1=value1&param2=value2

1.1.7.2. Content chunking

Generally it is recommended to let HttpClient choose the most appropriate transfer

encoding based on the properties of the HTTP message being transferred. It is

possible, however, to inform HttpClient that chunk coding is preferred by setting

HttpEntity#setChunked() to true. Please note that HttpClient will use this flag as a hint only.

This value will be ignored when using HTTP protocol versions that do not support

chunk coding, such as HTTP/1.0.

StringEntity entity = new StringEntity("important message",

ContentType.create("plain/text", Consts.UTF_8));

entity.setChunked(true);

HttpPost httppost = new HttpPost("http://localhost/acrtion.do");

httppost.setEntity(entity);

1.1.8. Response handlers

The simplest and the most convenient way to handle responses is by using the

ResponseHandler interface, which includes the handleResponse(HttpResponse response) method. This

method completely relieves the user from having to worry about connection

management. When using a ResponseHandler, HttpClient will automatically take care of

ensuring release of the connection back to the connection manager regardless whether

the request execution succeeds or causes an exception.

CloseableHttpClient httpclient = HttpClients.createDefault();

HttpGet httpget = new HttpGet("http://localhost/json");

ResponseHandler<MyJsonObject> rh = new ResponseHandler<MyJsonObject>() {

@Override

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