HttpClient使用详解与最佳实践

Httpclient

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"HttpClient使用指南最新版" HttpClient是一个广泛应用于Java开发中的组件，用于处理HTTP请求和响应。本指南旨在提供HttpClient的基本使用方法和高级特性，帮助开发者更有效地使用该工具。 1. 请求执行基础 - HTTP请求：HttpClient允许开发者构建和发送各种类型的HTTP请求，如GET、POST、PUT等，可以通过设置请求头和实体内容来定制请求。 - HTTP响应：接收到服务器的响应后，HttpClient能解析响应的状态码、头信息以及响应体内容。 - 消息头：可以设置和处理请求头与响应头，包括内容类型、编码、授权等信息。 - HTTP实体：用于处理请求和响应中的数据，可以是文本、二进制数据或其他形式。 - 资源释放：确保在完成操作后释放底层资源，如TCP连接。 - 实体内容：处理请求的输入和响应的输出内容，支持流式处理和缓冲区处理。 - 响应处理器：自定义逻辑来处理特定的响应情况。 2. HttpClient接口 - 线程安全：HttpClient设计为线程安全，可以在多线程环境中使用。 - 资源释放：注意HttpClient对象的生命周期管理，避免内存泄漏。 3. HTTP执行上下文 - 执行上下文提供了配置请求执行环境的机制，可以设置全局或特定请求的参数。 4. HTTP协议拦截器 - 允许在请求发送前和响应接收后插入自定义逻辑，例如日志记录、性能监控或动态修改请求。 5. 异常处理 - HTTP传输安全性：处理网络中断、超时等问题。 -幂等方法：对于幂等的HTTP方法（如GET、HEAD），即使多次重试也不会改变结果。 - 自动异常恢复：HttpClient可配置以自动处理某些异常并尝试重新发送请求。 - 请求重试处理：控制何时和如何重试失败的请求。 6. 中断请求 - 在请求执行过程中，可以通过调用相关方法中断请求，释放资源。 7. 重定向处理 - HttpClient支持自动处理HTTP状态码3xx的重定向。 8. 连接管理 - 连接持久化：减少建立新连接的开销，提高性能。 - 路由计算：根据目标URL选择合适的连接，包括通过代理或SSL加密的连接。 - 连接管理器：管理和复用HTTP连接，如SimpleConnectionManager。 9. 高级特性 - 定制连接管理策略：针对不同的应用需求，如连接池大小、超时时间等。 - 请求与响应拦截器：进一步扩展功能，如认证、压缩等。 HttpClient提供了强大的HTTP客户端功能，涵盖基本的请求响应处理到复杂的连接管理和错误处理，使得开发者能够灵活、高效地处理HTTP通信任务。正确理解和使用HttpClient的各种特性和配置，将有助于提升应用程序的网络性能和稳定性。

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Fundamentals

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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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