Apache HttpClient深入教程：基础与连接管理

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Apache HttpClient是Apache软件基金会下的一个开源库，用于在Java应用程序中执行HTTP客户端请求。该库在设计时注重高效、可扩展性和安全性，广泛应用于网络服务调用、API集成以及数据抓取等场景。本书《HttpClient Tutorial》由Oleg Kalnichevski、Jonathan Moore和Jilles van Gurp共同编写，深入介绍了HttpClient的各个方面。首先，章节1探讨了HttpClient的基本概念和使用范围（scope）。它强调了HttpClient并非所有HTTP操作的解决方案，而是专注于基于HTTP协议的客户端交互。这部分内容包括基础概念，如如何执行HTTP请求（包括发送GET、POST等请求）、处理HTTP响应（关注消息头、实体内容等）、确保低级资源的正确释放，以及使用不同的响应处理器来处理服务器响应。 HttpClient接口的设计考虑到了线程安全性和资源管理，用户可以在多线程环境中安全地使用，并且可以控制连接的分配和回收。HTTP执行上下文（ExecutionContext）允许设置特定的执行策略和配置，如添加HTTP协议拦截器来处理请求和响应过程中的特定逻辑。异常处理方面，HttpClient提供了安全的HTTP传输机制，支持幂等性方法（如PUT和DELETE），以及自动异常恢复和请求重试机制，同时允许用户自定义请求取消和重定向处理策略。章节2主要聚焦于连接管理，包括连接持久性、路由计算以及安全连接设置。HttpClient支持管理连接池，使用简单的ConnectionManager或更高级的定制选项，如ManagedConnections和ConnectionManagers，来优化网络资源的使用。此外，还讨论了如何通过SecureHTTPconnections来保护数据传输的安全性。 Apache HttpClient教程深入浅出地介绍了如何利用这一强大的工具进行高效的HTTP通信，以及如何有效地管理连接、处理异常和优化性能，对于任何需要在Java应用中进行HTTP通信的开发者来说，这是一本不可或缺的参考资料。学习者不仅能掌握基本操作，还能深入了解其背后的实现原理和最佳实践，从而提升应用程序的稳定性和性能。

Fundamentals

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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();

// Do not need the rest

}

} finally {

response.close();

}

Fundamentals

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 throught 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,

ContentType.create("text/plain", "UTF-8"));

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

httppost.setEntity(entity);

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