HttpClient：Java高效HTTP客户端框架详解与核心功能

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HttpClient是Java编程语言中的一个重要组件，它位于`java.net`包之下，旨在提供一个强大且灵活的HTTP客户端库，弥补了`java.net`包在处理高级HTTP请求和连接管理方面的不足。HttpClient的设计目标是支持最新的HTTP标准和最佳实践，适用于各种应用场景，包括Web浏览器、Web服务客户端以及依赖于扩展HTTP协议的分布式通信系统。 HttpClient的核心功能包括： 1. **请求执行**：HttpClient允许开发者创建和执行HTTP请求，包括发送GET、POST、PUT等方法，并处理HTTP响应。这涉及构建HTTP请求（包括URL、头部信息和实体内容）、接收和解析响应，以及管理消息头和实体数据。 - **HTTP请求**：HttpClient提供了API来构建和配置HTTP请求，包括设置请求方法、URI、头部信息等。 - **HTTP响应**：用户可以获取响应状态码、头部信息和响应体内容。 - **消息头管理**：对请求和响应的头部进行操作，确保正确处理Cookie和其他元数据。 - **实体内容**：支持读取和写入HTTP实体，比如文件上传或下载。 2. **HTTP执行上下文**：提供了一个执行环境，用于管理和跟踪HTTP请求的生命周期，包括异常处理、重试策略和请求取消。 - **异常处理**：HttpClient设计了异常处理机制，确保在遇到网络问题或错误时能自动恢复或重试请求，保证请求的安全性。 - **请求重试**：内置的重试策略可以应对短暂的网络中断或服务器问题。 - **请求取消**：允许用户在请求过程中取消请求，释放资源。 3. **连接管理**：HttpClient负责有效的连接管理，包括参数设置、持久连接、路由选择以及定制Socket工厂。 - **连接参数**：控制连接的超时、最大连接数等。 - **连接持久化**：保持连接以提高性能，但也能根据需要关闭。 - **路由计算**：智能地决定连接到哪个服务器，支持安全连接（SSL/TLS）。 - **Socket工厂**：用于创建和配置Socket，允许自定义SSL/TLS安全层和加密选项。 4. **HTTP参数和请求执行参数**：支持配置请求的参数，可以使用参数树或专门的参数Bean，同时也提供参数解析和验证功能。 HttpClient是Java开发人员处理HTTP通信的强大工具，它提供了丰富的功能集，使开发者能够高效地执行HTTP请求，同时关注连接管理、安全性以及异常处理等关键方面。无论是在构建Web应用、构建RESTful API客户端，还是开发分布式系统，HttpClient都是一个不可或缺的组件。

Fundamentals

// Do not need the rest

httpget.abort();

}

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.

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

HttpResponse response = httpclient.execute(httpget);

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

}

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.

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

HttpResponse response = httpclient.execute(httpget);

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");

FileEntity entity = new FileEntity(file, "text/plain; charset=\"UTF-8\"");

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

httppost.setEntity(entity);

Fundamentals

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. Dynamic content entities

Often HTTP entities need to be generated dynamically based a particular execution context. HttpClient

provides support for dynamic entities by using the EntityTemplate entity class and ContentProducer

interface. Content producers are objects which produce their content on demand, by writing it out to

an output stream. They are expected to be able produce their content every time they are requested to

do so. So entities created with EntityTemplate are generally self-contained and repeatable.

ContentProducer cp = new ContentProducer() {

public void writeTo(OutputStream outstream) throws IOException {

Writer writer = new OutputStreamWriter(outstream, "UTF-8");

writer.write("<response>");

writer.write(" <content>");

writer.write(" important stuff");

writer.write(" </content>");

writer.write("</response>");

writer.flush();

}

};

HttpEntity entity = new EntityTemplate(cp);

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

httppost.setEntity(entity);

1.1.7.2. 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, "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

1.1.7.3. 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",

"text/plain; charset=\"UTF-8\"");

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HttpClient：Java高效HTTP客户端框架详解与核心功能

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