HttpClient入门指南：实现高效HTTP客户端

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"HttpClient教程(Apache)" HttpClient是Apache组织提供的一款强大的HTTP客户端库，它为Java开发者提供了高效、现代化且功能丰富的HTTP标准与建议的实现，弥补了java.net包在处理HTTP请求时灵活性和功能上的不足。HttpClient设计时考虑了可扩展性，并提供了对基本HTTP协议的稳健支持，适用于构建各种HTTP意识的客户端应用，如网页浏览器、Web服务客户端或分布式通信系统。 HttpClient教程主要分为以下几个部分： 1. **HttpClient的范围** 这部分介绍HttpClient的基本定位，强调它不仅适用于用户驱动的网页浏览器，也适用于需要HTTP支持的Web服务和网络计算应用。 2. **HttpClient不是什么** 这里澄清HttpClient并不是一个完整的HTTP服务器实现，而是专注于客户端的功能。 3. **基础概念** - **请求执行**：涵盖HTTP请求的构造、发送以及响应的接收。 - **HTTP请求**：包括如何创建和设置请求头、方法（GET、POST等）。 - **HTTP响应**：解析和处理响应头、状态码和实体内容。 - **消息头工作**：管理请求和响应头，如设置、获取和删除。 - **HTTP实体**：处理请求和响应中的数据内容。 - **资源释放**：确保低级别的套接字和其他资源在使用后被正确关闭。 - **消费实体内容**：如何读取和处理响应实体的数据。 - **生产实体内容**：如何在请求中添加自定义数据。 - **响应处理器**：用于处理特定类型的响应。 4. **HTTP执行上下文** 这部分讨论了执行请求时的上下文环境，如线程局部变量和执行策略。 5. **异常处理** - **HTTP传输安全性**：处理网络异常和错误。 - **幂等方法**：处理幂等HTTP方法（如GET）的重试。 - **自动异常恢复**：当出现异常时，HttpClient可能尝试恢复连接。 - **请求重试处理器**：定义何时和如何重试失败的请求。 6. **终止请求** 学习如何在执行过程中取消或中断请求。 7. **HTTP协议拦截器** 使用拦截器来插入自定义逻辑，如日志记录、性能测量或身份验证。 8. **HTTP参数** - **参数层次结构**：理解参数如何组织和继承。 - **HTTP参数bean**：使用参数对象进行配置。 9. **HTTP请求执行参数** 设置影响请求执行的各种参数。 10. **连接管理** - **连接参数**：配置连接的超时、缓冲等。 - **连接持久化**：管理持久连接以提高效率。 - **HTTP连接路由**：包括计算最佳路由，处理代理和重定向。 - **安全HTTP连接**：使用SSL/TLS加密连接。 - **HTTP路由参数**：控制路由决策的细节。 - **Socket工厂**：自定义套接字创建，包括SSL/TLS层的定制。通过这个HttpClient教程，开发者可以深入理解如何利用HttpClient库构建复杂和高效的HTTP客户端应用，同时掌握HTTP协议的各个方面，包括连接管理、异常处理和扩展性机制。

Fundamentals

When creating an entity for a outgoing message, this meta data has to be supplied by the creator of

the entity.

StringEntity myEntity = new StringEntity("important message",

"UTF-8");

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

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

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

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

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

stdout >

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

UTF-8

important message

1.1.5. Ensuring release of low level resources

When finished with a response entity, it's important to ensure that all entity content has been

fully consumed, so that the connection could be safely returned to the connection pool and re-

used by the connection manager for subsequent requests. The easiest way to do so is to call the

HttpEntity#consumeContent() method to consume any available content on the stream. HttpClient

will automatically release the underlying connection back to the connection manager as soon as it

detects that the end of the content stream has been reached. The HttpEntity#consumeContent()

method is safe to call more than once.

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, one can simply terminate the request by calling HttpUriRequest#abort() method.

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

HttpResponse response = httpclient.execute(httpget);

HttpEntity entity = response.getEntity();

if (entity != null) {

InputStream instream = entity.getContent();

int byteOne = instream.read();

int byteTwo = instream.read();

// 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 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

Fundamentals

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);

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 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 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.

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