HttpClient教程：基础与连接管理

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"HttpClient教程，由Oleg Kalnichevski撰写，涵盖了HttpClient的基本概念、异常处理、连接管理等方面的内容。" HttpClient是Java编程语言中的一个库，用于执行HTTP和HTTPS请求。这个库提供了丰富的功能，包括但不限于请求执行、响应处理、连接管理和协议拦截器等。下面我们将深入探讨这些知识点。 1. 请求执行： - HTTP请求：HttpClient允许开发者构建和发送各种类型的HTTP请求，如GET、POST、PUT等。 - HTTP响应：在发送请求后，HttpClient会接收并处理服务器返回的HTTP响应，包括状态码、消息头和实体内容。 - 消息头：HttpClient支持设置和解析HTTP请求和响应中的消息头，用于传递额外的信息，如缓存控制、内容类型等。 - HTTP实体：请求和响应可以携带实体内容，如文本、图片或二进制数据，HttpClient提供了处理这些内容的方法。 1. 异常处理： - HTTP传输安全：HttpClient处理与网络传输相关的异常，确保在出现问题时能够安全地恢复或关闭连接。 - 没有副作用的方法：区分幂等和非幂等方法，幂等方法（如GET）多次调用结果相同，非幂等方法（如POST）则可能改变服务器状态。 - 自动异常恢复：HttpClient有时会尝试自动恢复失败的请求，例如在网络恢复后重试。 - 请求重试处理：通过RequestRetryHandler接口，开发者可以自定义何时以及如何重试失败的请求。 1. 连接管理： - 连接参数：配置连接相关的属性，如超时、最大连接数等。 - 连接持久化：HttpClient支持持久连接，即TCP连接在多个HTTP事务间复用，以减少网络开销。 - HTTP连接路由：根据目标URL和代理设置选择合适的连接路径，包括处理SSL/TLS加密的连接。 - 路由参数：可以设置特定的路由参数来影响连接建立过程。 - Socket工厂：用于创建和管理套接字，包括提供对SSL/TLS的支持。 2. 协议拦截器： - HttpClient允许插入自定义的协议拦截器，这些拦截器可以在请求发送前或响应接收后进行操作，如添加头信息、修改请求体等。 3. HTTP参数： - 参数层次结构：HttpClient支持多级参数，允许在不同级别设置和获取参数值。 - 参数bean：通过参数bean，可以方便地管理和组织一组相关参数。 HttpClient的灵活性和强大功能使其成为Java开发中处理HTTP通信的重要工具。无论是简单的网页抓取，还是复杂的Web服务交互，HttpClient都能提供可靠的解决方案。通过深入理解这些知识点，开发者可以更好地利用HttpClient来构建高效、稳定的网络应用。

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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