JAVA實現文件下載功能：快速下載文件的方法！

一、URL類實現文件下載

import java.io.BufferedInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.net.URLConnection;

public class DownloadUtil {
    public static void download(String urlStr, String savePath) {
        try {
            URL url = new URL(urlStr);
            URLConnection connection = url.openConnection();
            InputStream input = new BufferedInputStream(connection.getInputStream());
            FileOutputStream output = new FileOutputStream(savePath);
            byte[] buffer = new byte[1024];
            int bytesRead = 0;
            while ((bytesRead = input.read(buffer, 0, buffer.length)) >= 0) {
                output.write(buffer, 0, bytesRead);
            }
            output.close();
            input.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

URL類是Java自帶的一個類，可以通過它來獲取URL地址的輸入流，進而下載網路資源。

在該示例中，將通過URL類獲取到指定url下載的網路資源的輸入流，然後以位元組流的形式讀取網路資源，最後以文件輸出流的形式將網路資源保存到本地

二、Apache HttpClient實現文件下載

import java.io.FileOutputStream;
import java.io.IOException;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
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 DownloadUtil2 {
    public static void download(String url, String filePath) throws ClientProtocolException, IOException {
        CloseableHttpClient httpClient = HttpClients.createDefault();
        try {
            HttpGet httpGet = new HttpGet(url);
            HttpResponse response = httpClient.execute(httpGet);
            HttpEntity entity = response.getEntity();
            if (entity != null) {
                FileOutputStream outstream = new FileOutputStream(filePath);
                entity.writeTo(outstream);
                outstream.close();
            }
        } finally {
            httpClient.close();
        }
    }
}

Apache HttpClient是Apache的一個開源組件，該組件通過API提供了一套處理HTTP請求的類和介面，致力於提供更好的性能，簡化API的使用和提供對HTTP標準的全面實現。

在該示例中，通過HttpClient發送HTTP請求，獲取需要下載文件的輸入流，用位元組數組讀取，並以文件輸出流的形式將網路資源保存到本地

三、Java NIO實現文件下載

import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.Channels;
import java.nio.channels.ReadableByteChannel;
import java.nio.channels.WritableByteChannel;
import java.net.URL;
import java.net.URLConnection;

public class DownloadUtil3 {
    public static void download(String urlStr, String savePath) throws IOException {
        URL url = new URL(urlStr);
        URLConnection connection = url.openConnection();
        ReadableByteChannel readableByteChannel = Channels.newChannel(connection.getInputStream());
        FileOutputStream fileOutputStream = new FileOutputStream(savePath);
        WritableByteChannel writableByteChannel = Channels.newChannel(fileOutputStream);
        ByteBuffer byteBuffer = ByteBuffer.allocate(1024 * 1024);
        while (readableByteChannel.read(byteBuffer) > 0) {
            byteBuffer.flip();
            writableByteChannel.write(byteBuffer);
            byteBuffer.clear();
        }
    }
}

Java NIO，即Java New IO，是Java SE 1.4中引入的新API，一套完整的IO API，可以替代原有的Java IO API。NIO提供了非阻塞IO，即選擇器技術，該技術允許伺服器端使用單線程來處理多個並發連接，並處理多個請求。

在該示例中，首先通過URL類獲得需要下載文件的輸入流，然後通過Java NIO的FileChannel和ByteBuffer進行讀寫操作，最終保存到本地文件中。