HttpClient教程：基础与进阶指南

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"HttpClient教程" 本教程是关于Apache HttpClient的详细指南，主要针对使用HttpClient进行HTTP通信的开发者。HttpClient是一个功能强大的Java库，用于执行HTTP请求并处理响应。它旨在提供灵活、可配置且高效的HTTP客户端实现，适用于各种复杂的HTTP用例。 1. 基本概念 - HTTP请求：HttpClient允许创建各种类型的HTTP请求（GET、POST、PUT等），并支持添加请求头和实体内容。 - HTTP响应：在发送请求后，HttpClient接收HTTP响应，包括状态码、响应头和实体内容。 - 消息头：HttpClient提供了处理请求和响应头的方法，以便设置或获取HTTP头信息。 - HTTP实体：可以使用HttpClient发送和接收包含数据的HTTP实体，如文本、图像或文件。 - 资源释放：在操作完成后，HttpClient确保释放底层资源，如网络连接。 - 内容消费与生产：HttpClient支持读取和写入HTTP实体内容，例如下载文件或上传数据。 - 响应处理器：可以通过自定义响应处理器来处理特定类型的响应。 2. HttpClient接口 - 线程安全：HttpClient的设计考虑了多线程环境，但需要注意某些方法可能不是线程安全的。 - 资源释放：在不再需要HttpClient实例时，正确关闭和释放资源至关重要。 3. HTTP执行上下文 - 这部分介绍了如何在执行请求时使用和管理上下文信息，例如认证、缓存策略等。 4. HTTP协议拦截器 - 拦截器允许在请求发送前或响应接收后插入自定义逻辑，如日志记录、修改请求头等。 5. 异常处理 - HttpClient处理网络传输的异常，并提供了一套机制来处理幂等性方法和自动恢复策略。 - 请求重试：当请求失败时，可以配置HttpClient使用请求重试策略。 6. 中断请求 - 如果需要取消正在进行的请求，HttpClient提供了中断请求的功能。 7. 重定向处理 - HttpClient支持自动处理HTTP重定向，但也可以根据需求关闭或自定义重定向行为。 8. 连接管理 - 连接持久化：HttpClient支持TCP连接的复用，减少建立新连接的开销。 - 路由计算：根据目标URL和代理设置计算出合适的路由。 - 安全连接：处理HTTPS连接和SSL/TLS握手。 - 连接管理器：管理和维护HTTP连接池，以提高性能和资源利用。 9. 进一步深入 - HttpClient还提供了更高级的功能，如连接池配置、自定义连接参数、代理服务器支持、以及对HTTP/1.1和HTTP/2的支持。本教程适合对HttpClient有一定了解但希望深入学习的开发者，通过学习，你将能够熟练地使用HttpClient进行高效的HTTP通信。

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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