Apache HttpClient 4.5 开发全面指南

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"Apache HttpClient开发指导文档4.5版本，涵盖了HttpClient的基本使用、连接管理、异常处理等全面内容，适用于Java开发者进行HTTP客户端编程时参考。" Apache HttpClient是Java平台上广泛使用的HTTP客户端库，用于发送HTTP请求和接收响应。这份4.5版本的开发指导文档深入介绍了HttpClient的核心功能和最佳实践，帮助开发者高效地使用该库。 1. **HttpClient基础** - **请求执行**：HttpClient允许创建并执行HTTP请求，包括GET、POST等方法。 - **HTTP请求**：定义了HTTP请求的组成部分，如方法、URL、请求头和实体。 - **HTTP响应**：包含状态码、响应头和响应实体，表示服务器对请求的回应。 - **消息头处理**：HttpClient支持添加、修改和读取HTTP消息头。 - **HTTP实体**：用于承载请求或响应的数据，可以是文本、二进制数据或流。 - **资源释放**：确保低级资源（如网络连接）在使用后被正确关闭。 - **内容消费与生产**：提供API来读取或写入HTTP实体内容。 - **响应处理器**：允许自定义逻辑来处理响应结果。 2. **HttpClient接口与线程安全** - **HttpClient接口**：理解HttpClient的使用模式和其线程安全特性。 - **线程安全**：讨论HttpClient是否可以在多线程环境中安全使用。 - **资源释放**：介绍如何在HttpClient不再需要时正确关闭和释放资源。 3. **HTTP执行上下文** - **HTTP执行环境**：提供了存储和共享请求执行相关数据的上下文。 4. **拦截器与异常处理** - **协议拦截器**：允许在请求发送前或响应接收后插入自定义逻辑。 - **异常处理**：涵盖如何处理HTTP传输中的错误，区分安全和不安全的操作。 - **幂等方法**：对于幂等请求的特殊考虑。 - **自动异常恢复**：当出现错误时，HttpClient可能尝试恢复连接。 - **请求重试处理**：配置何时以及如何重试失败的请求。 5. **请求取消** - **中止请求**：在请求执行过程中，可以取消请求以停止进一步的处理。 6. **重定向处理** - **重定向策略**：HttpClient处理自动重定向的能力，包括何时跟随重定向和处理重定向链。 7. **连接管理** - **持久连接**：支持TCP连接的复用以减少网络延迟。 - **路由计算**：确定到目标服务器的最佳路径，包括代理和重定向。 - **安全连接**：处理HTTPS连接的建立和验证。 - **连接管理器**：管理HTTP连接的创建、复用和关闭，如SimpleConnectionManager。 8. **其他高级主题** - **多路复用**：HTTP/2支持下的连接多路复用。 - **性能优化**：讨论如何最大化HttpClient的性能和响应时间。 - **身份验证与安全**：介绍认证机制和加密技术在HttpClient中的应用。这份文档不仅适合初学者理解HttpClient的基本用法，也对有经验的开发者提供了解决复杂HTTP场景的深入洞察。通过阅读和实践，开发者可以更好地掌握HttpClient，从而更有效地实现HTTP通信。

Fundamentals

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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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