Apache HttpComponents詳解

一、簡介

Apache HttpComponents是一個開源的Java HTTP庫，可以用於創建和發送HTTP, HTTPS請求，以及處理HTTP響應。該庫最初由Jakarta項目提供，現已成為Apache項目的一部分。Apache HttpComponents包含兩個模塊：HttpClient和HttpCore。

二、HttpClient模塊

HttpClient模塊是使用最廣泛的HttpComponents模塊之一。HttpClient提供了許多功能，包括但不限於：

1、創建和發送HTTP, HTTPS請求。

2、支持連接池和狀態管理，以便重用HTTP連接並優化性能。

3、支持異步請求以及回調函數。

三、HttpCore模塊

HttpCore模塊是HttpClient模塊的底層模塊。HttpCore提供了HTTP協議的核心功能，包括但不限於：

1、HTTP請求和響應消息的解析和序列化。

2、支持處理HTTP頭和HTTP實體。

3、支持HTTP連接的管理和線程安全。

四、使用HttpComponents發送HTTP請求

以下代碼示例演示如何使用HttpComponents發送HTTP GET請求：

import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;

public class HttpComponentsDemo {
    public static void main(String[] args) throws Exception {
        CloseableHttpClient httpclient = HttpClients.createDefault();
        HttpGet httpget = new HttpGet("http://www.example.com/");
        CloseableHttpResponse response = httpclient.execute(httpget);
        try {
            HttpEntity entity = response.getEntity();
            if (entity != null) {
                System.out.println(EntityUtils.toString(entity));
            }
        } finally {
            response.close();
        }
    }
}

上述代碼中創建了一個默認的CloseableHttpClient對象。使用HttpGet對象來表示HTTP GET請求，並使用execute方法發送HTTP請求。最後，從HTTP響應中獲取實體，並將其轉換為字符串以打印響應內容。

五、處理HTTP響應

以下代碼示例演示如何處理HTTP響應：

import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;
import org.apache.http.HttpResponse;

public class HttpComponentsDemo {
    public static void main(String[] args) throws Exception {
        CloseableHttpClient httpclient = HttpClients.createDefault();
        HttpGet httpget = new HttpGet("http://www.example.com/");

        HttpResponse response = httpclient.execute(httpget);
        int statusCode = response.getStatusLine().getStatusCode();
        System.out.println("statusCode=" + statusCode);

        HttpEntity entity = response.getEntity();
        if (entity != null) {
            String content = EntityUtils.toString(entity);
            System.out.println("content=" + content);
        }
    }
}

上述代碼中，從HttpResponse對象中獲取響應狀態碼，並從HttpEntity對象中獲取響應實體內容。

六、使用連接池和線程安全

以下代碼示例演示如何使用連接池和線程安全的CloseableHttpClient：

import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClientBuilder;

public class HttpComponentsDemo {
    public static void main(String[] args) throws Exception {
        CloseableHttpClient httpClient = HttpClientBuilder.create().build();

        HttpGet httpget = new HttpGet("http://www.example.com/");

        CloseableHttpResponse response = httpClient.execute(httpget);
        try {
            HttpEntity entity = response.getEntity();
            if (entity != null) {
                System.out.println(EntityUtils.toString(entity));
            }
        } finally {
            response.close();
        }

        httpClient.close();
    }
}

HttpClientBuilder.create().build()會創建一個線程安全、支持連接池的CloseableHttpClient實例。使用該實例發送HTTP請求，由於該實例支持連接池，所以可以重用HTTP連接以及提高性能。最後，關閉HttpClient實例釋放資源。

七、使用IO處理和上傳文件

以下代碼示例演示如何使用HttpComponents上傳文件：

import java.io.File;
import java.io.FileInputStream;

import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.ContentType;
import org.apache.http.entity.mime.MultipartEntityBuilder;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;

public class HttpComponentsDemo {
    public static void main(String[] args) throws Exception {
        CloseableHttpClient httpclient = HttpClients.createDefault();

        HttpPost httppost = new HttpPost("http://www.example.com/upload");

        File file = new File("test.txt");
        FileInputStream fis = new FileInputStream(file);

        MultipartEntityBuilder builder = MultipartEntityBuilder.create();
        builder.addBinaryBody("file", fis, ContentType.APPLICATION_OCTET_STREAM, file.getName());

        httppost.setEntity(builder.build());

        CloseableHttpResponse response = httpclient.execute(httppost);
        try {
            HttpEntity entity = response.getEntity();
            if (entity != null) {
                System.out.println(EntityUtils.toString(entity));
            }
        } finally {
            response.close();
        }

        httpclient.close();
    }
}

上述代碼中，使用HttpPost對象來表示HTTP POST請求，並將要上傳的文件包裝到MultipartEntityBuilder對象中。最後，使用CloseableHttpClient發送HTTP請求，並將HTTP響應實體轉換為字符串以打印響應內容。