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# Apache HttpClient
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Apache HttpClient is a comprehensive Java library for building robust client-side HTTP services. It provides essential functionality for HTTP/1.1 and HTTP/2 protocols, connection pooling, authentication mechanisms, cookie management, SSL/TLS support, proxy configuration, and automatic redirect handling.
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## Package Information
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- **Package Name**: org.apache.httpcomponents:httpclient
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- **Package Type**: maven
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- **Language**: Java
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- **Installation**: Add to Maven POM:
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  ```xml
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  <dependency>
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    <groupId>org.apache.httpcomponents</groupId>
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    <artifactId>httpclient</artifactId>
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    <version>4.5.14</version>
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  </dependency>
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  ```
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## Core Imports
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```java
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import org.apache.http.client.HttpClient;
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import org.apache.http.impl.client.HttpClients;
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import org.apache.http.client.methods.HttpGet;
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import org.apache.http.HttpResponse;
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```
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For closeable HTTP clients (recommended for proper resource management):
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```java
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import org.apache.http.impl.client.CloseableHttpClient;
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import org.apache.http.impl.client.HttpClients;
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import org.apache.http.client.methods.CloseableHttpResponse;
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```
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## Basic Usage
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```java
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import org.apache.http.impl.client.CloseableHttpClient;
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import org.apache.http.impl.client.HttpClients;
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import org.apache.http.client.methods.HttpGet;
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import org.apache.http.client.methods.CloseableHttpResponse;
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import org.apache.http.util.EntityUtils;
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// Create HTTP client
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try (CloseableHttpClient httpClient = HttpClients.createDefault()) {
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    // Create GET request
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    HttpGet httpGet = new HttpGet("https://api.example.com/data");
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    // Execute request
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    try (CloseableHttpResponse response = httpClient.execute(httpGet)) {
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        // Check response status
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        int statusCode = response.getStatusLine().getStatusCode();
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        // Get response content
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        String responseBody = EntityUtils.toString(response.getEntity());
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        System.out.println("Status: " + statusCode);
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        System.out.println("Response: " + responseBody);
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    }
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} catch (Exception e) {
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    e.printStackTrace();
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}
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```
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## Architecture
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Apache HttpClient is built around several key architectural patterns:
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- **HttpClient Interface**: Main interface for HTTP request execution with implementations for different scenarios
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- **Connection Management**: Pooled and basic connection managers for efficient resource utilization  
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- **Request/Response Model**: Typed HTTP method classes (GET, POST, PUT, etc.) with comprehensive header and entity support
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- **Configuration System**: Builder patterns for client configuration with request-specific overrides
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- **Authentication Framework**: Pluggable authentication schemes (Basic, Digest, NTLM, Kerberos) with credential management
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- **Protocol Handlers**: Support for redirects, retries, and proxy handling with customizable strategies
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This layered architecture provides both simple default behavior and extensive customization capabilities, making it suitable for everything from basic HTTP requests to complex enterprise integration scenarios.
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## Capabilities
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### HTTP Client Creation and Configuration
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Core functionality for creating and configuring HTTP client instances with connection management, timeouts, authentication, and SSL settings.
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```java { .api }
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public static CloseableHttpClient createDefault();
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public static CloseableHttpClient createSystem();
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public static CloseableHttpClient createMinimal();
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public static HttpClientBuilder custom();
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```
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[HTTP Client Configuration](./client-configuration.md)
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### HTTP Request Methods
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Implementation classes for all standard HTTP methods with support for headers, entities, and request-specific configuration.
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```java { .api }
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public class HttpGet extends HttpRequestBase;
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public class HttpPost extends HttpEntityEnclosingRequestBase;
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public class HttpPut extends HttpEntityEnclosingRequestBase;
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public class HttpDelete extends HttpRequestBase;
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```
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[HTTP Request Methods](./http-methods.md)
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### Connection Management
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Connection pooling and lifecycle management for efficient resource utilization and performance optimization.
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```java { .api }
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public interface HttpClientConnectionManager;
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public class PoolingHttpClientConnectionManager implements HttpClientConnectionManager;
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public class BasicHttpClientConnectionManager implements HttpClientConnectionManager;
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```
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[Connection Management](./connection-management.md)
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### Authentication and Security
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Comprehensive authentication mechanisms including Basic, Digest, NTLM, and Kerberos with credential management and SSL/TLS support.
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```java { .api }
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public interface CredentialsProvider;
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public class UsernamePasswordCredentials implements Credentials;
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public class NTCredentials implements Credentials;
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public class AuthScope;
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```
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[Authentication and Security](./authentication.md)
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### Cookie Management
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Full HTTP cookie handling with storage, expiration, and policy management for session-based applications.
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```java { .api }
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public interface CookieStore;
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public interface Cookie;
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public class BasicCookieStore implements CookieStore;
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```
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[Cookie Management](./cookie-management.md)
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### Response Handling
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Flexible response processing with handler interfaces, closeable responses for resource management, and utility methods for content extraction.
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```java { .api }
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public interface ResponseHandler<T>;
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public interface CloseableHttpResponse extends HttpResponse, Closeable;
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```
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[Response Handling](./response-handling.md)
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### Utility Classes
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Helper classes for URI building, URL encoding, and resource cleanup operations.
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```java { .api }
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public class URIBuilder;
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public class URLEncodedUtils;
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public class HttpClientUtils;
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```
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**URIBuilder** - Builder for constructing URIs with query parameters
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**URLEncodedUtils** - Utility methods for URL encoding and form data  
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**HttpClientUtils** - Utility methods for safely closing HTTP resources