有一个在线文件(如http://www.example.com/information.asp),我需要抓取并保存到一个目录。我知道有几种逐行抓取和读取在线文件(url)的方法,但是否有一种方法可以使用Java下载并保存文件?
当前回答
这可以读取互联网上的文件,并将其写入文件。
import java.net.URL;
import java.io.FileOutputStream;
import java.io.File;
public class Download {
public static void main(String[] args) throws Exception {
URL url = new URL("https://www.google.com/images/branding/googlelogo/1x/googlelogo_color_272x92dp.png"); // Input URL
FileOutputStream out = new FileOutputStream(new File("out.png")); // Output file
out.write(url.openStream().readAllBytes());
out.close();
}
}
其他回答
总结(并以某种方式润色和更新)之前的答案。以下三种方法实际上是等效的。(我添加了明确的超时,因为我认为这是必须的。没有人希望下载在连接丢失时永远冻结。)
public static void saveUrl1(final Path file, final URL url,
int secsConnectTimeout, int secsReadTimeout))
throws MalformedURLException, IOException {
// Files.createDirectories(file.getParent()); // Optional, make sure parent directory exists
try (BufferedInputStream in = new BufferedInputStream(
streamFromUrl(url, secsConnectTimeout,secsReadTimeout));
OutputStream fout = Files.newOutputStream(file)) {
final byte data[] = new byte[8192];
int count;
while((count = in.read(data)) > 0)
fout.write(data, 0, count);
}
}
public static void saveUrl2(final Path file, final URL url,
int secsConnectTimeout, int secsReadTimeout))
throws MalformedURLException, IOException {
// Files.createDirectories(file.getParent()); // Optional, make sure parent directory exists
try (ReadableByteChannel rbc = Channels.newChannel(
streamFromUrl(url, secsConnectTimeout, secsReadTimeout)
);
FileChannel channel = FileChannel.open(file,
StandardOpenOption.CREATE,
StandardOpenOption.TRUNCATE_EXISTING,
StandardOpenOption.WRITE)
) {
channel.transferFrom(rbc, 0, Long.MAX_VALUE);
}
}
public static void saveUrl3(final Path file, final URL url,
int secsConnectTimeout, int secsReadTimeout))
throws MalformedURLException, IOException {
// Files.createDirectories(file.getParent()); // Optional, make sure parent directory exists
try (InputStream in = streamFromUrl(url, secsConnectTimeout,secsReadTimeout) ) {
Files.copy(in, file, StandardCopyOption.REPLACE_EXISTING);
}
}
public static InputStream streamFromUrl(URL url,int secsConnectTimeout,int secsReadTimeout) throws IOException {
URLConnection conn = url.openConnection();
if(secsConnectTimeout>0)
conn.setConnectTimeout(secsConnectTimeout*1000);
if(secsReadTimeout>0)
conn.setReadTimeout(secsReadTimeout*1000);
return conn.getInputStream();
}
我没有发现明显的差异,在我看来都是对的。它们既安全又高效。(速度的差异似乎无关紧要——我从本地服务器写入180 MB到SSD磁盘的时间大约在1.2到1.5秒之间波动)。它们不需要外部库。所有这些都可以使用任意大小和(根据我的经验)HTTP重定向。
此外,如果没有找到资源(通常是404错误),所有抛出FileNotFoundException,如果DNS解析失败则抛出java.net.UnknownHostException;其他IOException对应传输过程中的错误。
下载一个文件需要你阅读它。无论哪种方式,您都必须以某种方式查看该文件。而不是逐行,你可以从流中逐字节读取:
BufferedInputStream in = new BufferedInputStream(new URL("http://www.website.com/information.asp").openStream())
byte data[] = new byte[1024];
int count;
while((count = in.read(data, 0, 1024)) != -1)
{
out.write(data, 0, count);
}
下面是一个简洁的、可读的、仅使用jdk的解决方案,其中包含适当的封闭资源:
static long download(String url, String fileName) throws IOException {
try (InputStream in = URI.create(url).toURL().openStream()) {
return Files.copy(in, Paths.get(fileName));
}
}
两行代码,没有依赖关系。
下面是一个完整的文件下载示例程序,包含输出、错误检查和命令行参数检查:
package so.downloader;
import java.io.IOException;
import java.io.InputStream;
import java.net.URI;
import java.nio.file.Files;
import java.nio.file.Paths;
public class Application {
public static void main(String[] args) throws IOException {
if (2 != args.length) {
System.out.println("USAGE: java -jar so-downloader.jar <source-URL> <target-filename>");
System.exit(1);
}
String sourceUrl = args[0];
String targetFilename = args[1];
long bytesDownloaded = download(sourceUrl, targetFilename);
System.out.println(String.format("Downloaded %d bytes from %s to %s.", bytesDownloaded, sourceUrl, targetFilename));
}
static long download(String url, String fileName) throws IOException {
try (InputStream in = URI.create(url).toURL().openStream()) {
return Files.copy(in, Paths.get(fileName));
}
}
}
正如so-downloader存储库README中所指出的:
运行文件下载程序:
java -jar so-downloader.jar <source-URL> <target-filename>
例如:
java -jar so-downloader.jar https://github.com/JanStureNielsen/so-downloader/archive/main.zip so-downloader-source.zip
public class DownloadManager {
static String urls = "[WEBSITE NAME]";
public static void main(String[] args) throws IOException{
URL url = verify(urls);
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
InputStream in = null;
String filename = url.getFile();
filename = filename.substring(filename.lastIndexOf('/') + 1);
FileOutputStream out = new FileOutputStream("C:\\Java2_programiranje/Network/DownloadTest1/Project/Output" + File.separator + filename);
in = connection.getInputStream();
int read = -1;
byte[] buffer = new byte[4096];
while((read = in.read(buffer)) != -1){
out.write(buffer, 0, read);
System.out.println("[SYSTEM/INFO]: Downloading file...");
}
in.close();
out.close();
System.out.println("[SYSTEM/INFO]: File Downloaded!");
}
private static URL verify(String url){
if(!url.toLowerCase().startsWith("http://")) {
return null;
}
URL verifyUrl = null;
try{
verifyUrl = new URL(url);
}catch(Exception e){
e.printStackTrace();
}
return verifyUrl;
}
}
简单使用有一个问题:
org.apache.commons.io.FileUtils.copyURLToFile(URL, File)
如果你需要下载和保存非常大的文件,或者在一般情况下,如果你需要自动重试以防连接断开。
在这种情况下,我建议使用Apache HttpClient以及org.apache.commons.io.FileUtils。例如:
GetMethod method = new GetMethod(resource_url);
try {
int statusCode = client.executeMethod(method);
if (statusCode != HttpStatus.SC_OK) {
logger.error("Get method failed: " + method.getStatusLine());
}
org.apache.commons.io.FileUtils.copyInputStreamToFile(
method.getResponseBodyAsStream(), new File(resource_file));
} catch (HttpException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
method.releaseConnection();
}
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