HttpClient4.5.1使用指南：基础与连接管理

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"HttpClient4.5.1教程" HttpClient是一个由Apache基金会开发的Java库，用于执行HTTP请求。HttpClient 4.5.1是该库的一个版本，提供了许多增强的功能和改进，以支持更高效、安全的HTTP通信。本教程旨在帮助开发者理解和使用HttpClient 4.5.1进行网络通信。 1. 基本概念 HttpClient的基础是发送HTTP请求并处理响应。这涉及以下关键组件： - **HTTP请求**：包括方法（GET、POST等）、URL、头信息和可能的实体（请求体）。 - **HTTP响应**：包含状态码、头信息和实体（响应体）。 - **消息头**：用于传递额外信息，如内容类型、编码和缓存控制。 - **HTTP实体**：可以是请求或响应的一部分，用于传输数据，例如JSON或XML。 2. HttpClient接口与线程安全性 - HttpClient接口设计为线程不安全的，这意味着每个请求应对应一个HttpClient实例，以避免并发问题。 - 关于资源释放，使用完毕后，必须确保低级别的资源（如连接）被正确关闭。 - HttpClient实例的生命周期应与应用程序的生命周期匹配，以有效地管理资源。 3. HTTP执行上下文 - HTTP执行上下文存储了在请求执行过程中需要的信息，如认证凭据、重定向策略等。 4. 协议拦截器 - 允许在请求和响应的生命周期中插入自定义逻辑，例如添加头信息或修改实体。 5. 异常处理 - HttpClient处理HTTP传输层的安全性，如网络中断。 - 鉴于HTTP方法的幂等性，某些方法（如GET）即使在网络不稳定时也能成功。 - 自动异常恢复机制可以处理短暂的网络问题。 - RequestRetryHandler接口允许自定义重试策略。 6. 中断请求 - 提供了方法来取消正在进行的请求，这对于用户取消操作或应用程序关闭时清理资源至关重要。 7. 重定向处理 - HttpClient能够自动处理HTTP状态码为3xx的重定向响应，但也可以通过配置禁用或自定义此行为。 8. 连接管理 - 连接持久化可减少建立新连接的开销，提高性能。 - HTTP连接路由考虑了代理服务器、SSL/TLS终止以及根据目标主机和端口选择合适的连接。 - 包括对安全HTTP连接的支持。 9. 连接管理者 - 管理连接和连接池，以有效地复用已建立的连接。 - SimpleConnectionManager适用于基本需求，而更复杂的场景可能需要自定义或高级连接管理器。 HttpClient 4.5.1的高级功能还包括支持多协议、异步执行、Cookie管理、身份验证和更多。理解和熟练使用HttpClient库可以极大地提升Java应用程序的网络功能，同时提供灵活和强大的HTTP通信解决方案。开发者应该根据项目需求选择合适的配置和策略，以实现最佳性能和可靠性。

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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