HttpClient教程：基础与连接管理

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"HttpClient教程，由OlegKalnichevski编写，涵盖了HttpClient的基本概念、请求执行、连接管理、异常处理等内容，旨在帮助用户理解和使用HttpClient进行HTTP通信。" HttpClient是Java编程环境中一个强大的库，用于执行HTTP和HTTPS请求。本教程深入介绍了HttpClient的主要功能和用法，帮助开发者构建高效且可靠的网络应用程序。 1. 基本概念 - 请求执行：HttpClient允许创建和发送HTTP请求，并接收响应。请求包括方法（如GET、POST）、URL、头部信息和可选的实体内容。 - HTTP响应：响应包含状态码、头部信息和实体内容。实体内容可以是文本、图像或其他类型的数据。 - 消息头：用于传递额外信息，如内容类型、编码、缓存控制等。 - HTTP实体：请求或响应中的数据部分，可以是任何类型的二进制或文本数据。 - 资源释放：使用后需正确关闭连接和释放资源，防止内存泄漏。 - 内容消费与生产：HttpClient支持读取和写入实体内容。 - 响应处理器：用于自定义响应处理逻辑。 2. 异常处理 - HTTP传输安全性：异常通常与网络问题或服务器错误有关，HttpClient提供安全机制处理这些异常。 - 幂等方法：某些HTTP方法（如GET、HEAD）是幂等的，多次执行结果相同，适合自动重试。 - 自动异常恢复：HttpClient可以配置为在出现异常时尝试恢复。 - 请求重试处理器：控制何时以及如何重试失败的请求。 3. 连接管理 - 连接参数：设置连接超时、读取超时等。 - 连接持久化：HttpClient支持TCP连接的复用，减少建立新连接的开销。 - 路由选择：根据目标URL和代理设置决定最佳连接路径。 - 安全连接：处理SSL/TLS加密的连接，支持自定义SSL上下文和协议。 4. 高级特性 - 协议拦截器：扩展HttpClient的行为，如添加自定义头部或修改请求内容。 - 参数配置：通过参数设置来定制HttpClient的行为，包括全局和请求级别的参数。 - 路由参数：指定特殊路由规则，如代理服务器设置。 - Socket工厂：用于创建自定义的Socket，如支持SSL/TLS的Socket。 HttpClient教程详细解释了这些概念，并提供了实例代码，帮助读者快速上手并熟练使用HttpClient进行HTTP通信。对于需要在网络层进行复杂操作的Java开发者来说，HttpClient是一个不可或缺的工具。

Fundamentals

When creating an entity for a outgoing message, this meta data has to be supplied by the creator of

the entity.

StringEntity myEntity = new StringEntity("important message",

"UTF-8");

System.out.println(myEntity.getContentType());

System.out.println(myEntity.getContentLength());

System.out.println(EntityUtils.getContentCharSet(myEntity));

System.out.println(EntityUtils.toString(myEntity));

System.out.println(EntityUtils.toByteArray(myEntity).length);

stdout >

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

UTF-8

important message

1.1.5. Ensuring release of low level resources

When finished with a response entity, it's important to ensure that all entity content has been

fully consumed, so that the connection could be safely returned to the connection pool and re-

used by the connection manager for subsequent requests. The easiest way to do so is to call the

HttpEntity#consumeContent() method to consume any available content on the stream. HttpClient

will automatically release the underlying connection back to the connection manager as soon as it

detects that the end of the content stream has been reached. The HttpEntity#consumeContent()

method is safe to call more than once.

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, one can simply terminate the request by calling HttpUriRequest#abort() method.

HttpGet httpget = new HttpGet("http://localhost/");

HttpResponse response = httpclient.execute(httpget);

HttpEntity entity = response.getEntity();

if (entity != null) {

InputStream instream = entity.getContent();

int byteOne = instream.read();

int byteTwo = instream.read();

// Do not need the rest

httpget.abort();

}

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 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

Fundamentals

discouraged unless the response entities originate from a trusted HTTP server and are known to be

of limited length.

HttpGet httpget = new HttpGet("http://localhost/");

HttpResponse response = httpclient.execute(httpget);

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

}

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.

HttpGet httpget = new HttpGet("http://localhost/");

HttpResponse response = httpclient.execute(httpget);

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 though 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, "text/plain; charset=\"UTF-8\"");

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

httppost.setEntity(entity);

Please note InputStreamEntity is not repeatable, because it can only read from the underlying data

stream once. Generally it is recommended to implement a custom HttpEntity class which is self-

contained instead of using generic InputStreamEntity. FileEntity can be a good starting point.

1.1.7.1. Dynamic content entities

Often HTTP entities need to be generated dynamically based a particular execution context. HttpClient

provides support for dynamic entities by using EntityTemplate entity class and ContentProducer

interface. Content producers are objects which produce their content on demand, by writing it out to

an output stream. They are expected to be able produce their content every time they are requested to

do so. So entities created with EntityTemplate are generally self-contained and repeatable.

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