HttpClient4.3使用指南与常见问题解析

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"HttpClient4.3开发文档" HttpClient是Apache Jakarta Common项目下的一个子项目，它是一个强大而功能丰富的客户端HTTP编程工具包，旨在提供高效、最新且全面的HTTP协议支持，包括对最新版本和建议的支持。这篇开发文档不仅介绍了HttpClient的基本概念，还分享了作者在实际工作中遇到的问题及其解决方案。 1. 基本原理 HttpClient的基础是请求执行过程，主要包括以下几个方面： - HTTP请求：这是与服务器交互的起点，通过构建HttpRequest对象来指定请求方法（如GET、POST等）、URL以及可能的请求头。 - HTTP响应：服务器返回HTTPResponse对象，包含状态码、响应头和响应实体。 - 消息头：用于传递额外信息，如缓存控制、内容类型等。 - HTTP实体：包含请求或响应中的数据，可以是文本、二进制或其他形式。 - 资源释放：确保在操作完成后释放连接和流资源。 - 实体内容消费：读取并处理响应实体内容。 - 实体内容生产：在请求中添加或修改实体内容。 - 响应处理器：自定义逻辑以处理特定类型的响应。 2. HttpClient接口 - 线程安全：HttpClient实例通常不是线程安全的，需要为每个请求创建新的实例或使用连接管理器共享。 - 资源释放：在不再需要HttpClient时，必须正确关闭，以释放系统资源。 3. HTTP执行上下文 - HTTP执行上下文存储了执行请求时的相关信息，如认证凭证、重定向策略等。 4. 异常处理 - HTTP传输安全：处理网络错误和异常，如超时、连接失败等。 - 恒等方法：幂等方法（如GET）在失败后可以重试，非幂等方法（如POST）则需要谨慎处理。 - 自动异常恢复：HttpClient可能尝试自动重试失败的请求。 - 请求重试处理：可以通过实现RequestRetryHandler接口来自定义重试逻辑。 - 请求中断：在请求过程中可以调用abort方法强制取消。 5. HTTP协议拦截器 - 用于在请求发送前或响应接收后添加自定义逻辑，例如日志记录、身份验证等。 6. 重定向处理 - HttpClient可以自动处理重定向，但也可以配置为手动处理。 7. 连接管理 - 连接持久性：保持HTTP连接的打开状态以提高性能。 - 路由选择：根据目标URL和代理设置计算出最佳连接路径，支持HTTPS。 - 连接管理器：管理HTTP连接的生命周期，如SingleClientConnManager和PooledConnectionManager。 2.3.1. 管理连接和连接管理器： - 管理连接是为了更高效地复用已建立的TCP连接。 - SimpleConnectionManager适用于简单场景，而PooledConnectionManager支持多线程和连接池。请注意，以上内容仅涵盖了HttpClient4.3开发文档的部分关键点，完整的文档会提供更深入的细节和示例代码，帮助开发者更好地理解和使用HttpClient进行HTTP通信。在实际开发中，理解并熟练运用这些知识能够有效地优化网络请求的效率和可靠性。

Fundamentals

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

important message

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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