Java 实现浏览器下载文件及文件预览

下面是 Java 实现浏览器下载文件及文件预览的完整攻略。

1. 下载文件

1.1 从网络上下载文件

Java 中可以使用 URLConnection 和 HttpURLConnection 类实现从网络上下载文件，具体方法如下：

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;

public class DownloadFile {
    public static void download(String url, String savePath) throws IOException {
        URL downloadUrl = new URL(url);
        HttpURLConnection connection = (HttpURLConnection) downloadUrl.openConnection();
        connection.setRequestMethod("GET");
        InputStream inputStream = connection.getInputStream();
        BufferedInputStream bufferedInputStream = new BufferedInputStream(inputStream);
        File file = new File(savePath);
        FileOutputStream fileOutputStream = new FileOutputStream(file);
        BufferedOutputStream bufferedOutputStream = new BufferedOutputStream(fileOutputStream);
        byte[] buffer = new byte[1024];
        int bytesRead;
        while ((bytesRead = bufferedInputStream.read(buffer, 0, buffer.length)) != -1) {
            bufferedOutputStream.write(buffer, 0, bytesRead);
            bufferedOutputStream.flush();
        }
        bufferedOutputStream.close();
        fileOutputStream.close();
        bufferedInputStream.close();
        inputStream.close();
    }

    public static void main(String[] args) throws IOException {
        download("https://www.example.com/test.pdf", "test.pdf");
    }
}

1.2 从本地下载文件

使用 Files.copy() 方法可以在 Java 中实现从本地下载文件，具体方法如下：

import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;

public class DownloadFile {
    public static void download(String filePath, String savePath) throws IOException {
        File file = new File(filePath);
        Path path = file.toPath();
        Files.copy(path, new File(savePath).toPath());
    }

    public static void main(String[] args) throws IOException {
        download("test.pdf", "test_download.pdf");
    }
}

2. 文件预览

2.1 预览图片文件

在 Java 中，可以使用 Desktop 类打开本地的图片文件进行预览，具体方法如下：

import java.awt.*;
import java.io.File;
import java.io.IOException;

public class PreviewImage {
    public static void preview(String filePath) throws IOException {
        File imageFile = new File(filePath);
        Desktop.getDesktop().open(imageFile);
    }

    public static void main(String[] args) throws IOException {
        preview("test.jpg");
    }
}

2.2 预览 PDF 文件

在 Java 中，可以使用 PDFRenderer 类预览 PDF 文件，具体方法如下：

import com.sun.pdfview.PDFFile;
import com.sun.pdfview.PDFPage;
import com.sun.pdfview.PDFRenderer;

import javax.swing.*;
import java.awt.*;
import java.awt.geom.Rectangle2D;
import java.io.File;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

public class PreviewPdf {
    public static void preview(String filePath) throws IOException {
        File file = new File(filePath);
        RandomAccessFile raf = new RandomAccessFile(file, "r");
        FileChannel channel = raf.getChannel();
        ByteBuffer buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());
        PDFFile pdfFile = new PDFFile(buffer);

        PDFPage page = pdfFile.getPage(0);
        Rectangle2D rectangle2D = page.getBBox();
        int width = (int) rectangle2D.getWidth();
        int height = (int) rectangle2D.getHeight();
        BufferedImage bufferedImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
        Graphics2D graphics2D = bufferedImage.createGraphics();
        graphics2D.setClip(0, 0, width, height);
        graphics2D.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        graphics2D.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
        PDFRenderer pdfRenderer = new PDFRenderer(page, graphics2D, rectangle2D, null, Color.WHITE);

        pdfRenderer.run();
        ImageIcon image = new ImageIcon(bufferedImage);
        JFrame frame = new JFrame();
        frame.setSize(width, height);
        JLabel label = new JLabel();
        label.setIcon(image);
        frame.add(label);
        frame.setVisible(true);

        graphics2D.dispose();
        channel.close();
        raf.close();
    }

    public static void main(String[] args) throws IOException {
        preview("test.pdf");
    }
}

以上就是 Java 实现浏览器下载文件及文件预览的攻略，希望能对你有所帮助。