HttpClient 4.0.2 教程：基础与连接管理

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"HttpClient是Apache HttpClient库的最新教程，专注于httpclient-4.0.2版本。这个教程旨在帮助用户快速理解和掌握HttpClient的使用方法，它是一个经典的参考资料，涵盖了HttpClient的各种核心概念和高级特性。" HttpClient是Java编程语言中用于执行HTTP请求的一个库，广泛应用于网络通信和Web服务调用。在本教程中，作者Oleg Kalnichevski详细介绍了HttpClient的各个方面。 1. 基本概念 - 请求执行：HttpClient允许你创建和发送HTTP请求，以及接收和处理响应。HTTP请求包括方法（如GET、POST等）、URL、头部和实体（请求体）。 - HTTP响应：包含状态码、头部和响应实体。HttpClient提供工具来解析这些信息。 - 消息头：用于传递元信息，如内容类型、缓存控制等。 - HTTP实体：请求或响应中的数据部分，可以是文本、二进制数据或其他形式。 - 资源释放：使用完连接后，必须正确关闭，释放网络资源。 - 内容消费和生产：HttpClient支持读取和写入请求或响应实体内容。 - 响应处理器：可自定义处理器来处理特定类型的响应。 2. HTTP执行上下文 - HTTP执行环境包含影响请求处理的上下文信息，如认证、重定向策略等。 3. 异常处理 - HttpClient考虑了网络异常的处理，如重试机制、安全性和幂等性方法。 - 安全性：HTTP传输安全涉及SSL/TLS，确保数据传输的安全。 - 自动异常恢复和请求重试：当请求失败时，可以设置策略进行重试。 - 请求中断：有时需要在请求执行过程中中断，HttpClient提供了相应的机制。 4. 连接管理 - 连接参数：控制连接的创建和配置，如超时设置。 - 连接持久化：保持连接打开以提高性能，避免每次请求都创建新连接。 - 路由计算：确定到达目标服务器的最佳路径，包括处理代理和安全连接。 - Socket工厂：用于创建套接字，包括SSL/TLS的实现和自定义加密层。 5. 其他特性 - 参数层次结构：允许对不同级别的请求和连接设置参数。 - HTTP参数bean：提供方便的接口来管理和设置HttpClient的配置。 - 请求执行参数：调整请求执行的特定方面。通过本教程，读者将能深入理解HttpClient的工作原理，学习如何有效地利用其特性来实现可靠的HTTP通信。无论你是初学者还是经验丰富的开发者，这个教程都能提供宝贵的指导。

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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HttpClient 4.0.2 教程：基础与连接管理

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