HttpClient4.5.1使用指南

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"HttpClient4.5.1手册是官方提供的详细指南，主要涵盖了HttpClient的主要功能、设计原则以及使用方法。此手册适用于希望通过HttpClient进行HTTP请求和响应处理的开发者。" HttpClient是一个广泛使用的Java库，用于执行HTTP请求并处理响应。HttpClient 4.5.1版本在该系列中是一个稳定且功能丰富的版本，提供了许多改进和特性。 1. **HttpClient的范围**： HttpClient主要关注HTTP协议的客户端实现，提供了一套强大的API来创建、发送和解析HTTP请求及响应。它不是浏览器替代品，不包含渲染或解析HTML的能力，而是专注于网络通信的基础部分。 2. **基础概念**： - **请求执行**：HttpClient允许创建HTTP请求，并发送到服务器，等待响应。HTTP请求包括方法（GET、POST等）、URL、头信息和可选的实体内容。 - **响应处理**：收到服务器的HTTP响应，包括状态码、头信息和实体内容。响应头可以用于获取如内容类型、编码等信息，实体内容可以是文本、图片或其他数据。 3. **HttpClient接口**： HttpClient设计为线程安全，可以在多线程环境中使用。然而，资源管理是关键，确保在操作完成后释放连接和其他资源。HttpClient还支持响应处理器，用于定制化响应的处理方式。 4. **HTTP执行上下文**： HttpClient维护了一个执行上下文，可以存储与请求相关的临时或持久性信息，如认证凭证、重定向策略等。 5. **拦截器**：定义了HTTP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，如添加头信息、修改请求内容等。 6. **异常处理**： HttpClient考虑了网络通信中的异常情况，包括对传输安全的处理，识别幂等方法（如GET），自动异常恢复机制，以及请求重试处理。 7. **取消请求**：支持在请求发送过程中取消操作，这对于多任务并行或用户取消操作的情况非常有用。 8. **重定向处理**： HttpClient提供了内置的重定向策略，能够自动处理服务器返回的重定向响应，也可以自定义重定向策略。 9. **连接管理**： - **连接持久化**：HttpClient支持HTTP/1.1的持久连接，减少建立新连接的开销。 - **路由计算**：根据请求的URL和服务器的配置，决定通过哪个连接发送请求，包括对HTTPS的支持。 - **连接管理器**：管理HTTP连接池，控制连接的创建、复用和关闭，如SimpleConnectionManager等。 HttpClient 4.5.1手册的详细内容还包括更多高级特性，如SSL/TLS配置、身份验证、代理设置、Cookie管理等，旨在帮助开发者构建高效、可靠的网络应用程序。通过深入理解和使用HttpClient，开发者可以更好地控制HTTP通信，实现复杂的网络交互需求。

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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