Java 程序因打印行而变慢

Question

您好，我有一个脚本，可以从网上下载文件，同时打印出进度。问题是打印进度的那一行使程序变慢了很多，有什么办法可以阻止它吗？

import java.io.FileOutputStream;
import java.io.InputStream;
import java.net.URL;


public class download {
public static void main(String[] args) {
    try{
        URL u = new URL("http://upload.wikimedia.org/wikipedia/commons/1/16/Appearance_of_sky_for_weather_forecast,_Dhaka,_Bangladesh.JPG");
        FileOutputStream fos = new FileOutputStream("C://Users/xxx/Desktop/test.jpg");
        InputStream is = u.openStream();

        long size = u.openConnection().getContentLengthLong();
        int data;
        long done = 0;
        while((data = is.read())!=-1){
            double progress = (double) (done)/(double)(size)*100;
            System.out.println(progress); // if we dont do this then then it completes fast
            fos.write(data);
            done++;
        }
        fos.close();
    }catch(Exception e){
       e.printStackTrace();
    }
}
}

Answer 1

首先，每一次I/O操作都会有很高的开销。现在，您正在为读取的每个字节打印一条消息!(在 InputStream#read 中注明)。

如果您想要/需要打印进度，请为读取的一堆 KB 执行此操作，通常每 4 KB 执行一次。为此，您可以使用 byte[] 缓冲区 从流中读取和写入数据。

BufferedInputStream input = null;
BufferedOutStream output = null;
final int DEFAULT_BUFFER_SIZE = 4 * 1024;
try {
    input = new BufferedInputStream(is, DEFAULT_BUFFER_SIZE);
    output = new BufferedOutputStream(fos, DEFAULT_BUFFER_SIZE);

    byte[] buffer = new byte[DEFAULT_BUFFER_SIZE];
    int length;
    while ((length = input.read(buffer)) > 0) {
        output.write(buffer, 0, length);
        done += length;
        double progress = (double) (done)/(double)(size)*100
        System.out.println(progress);
    }
} catch (IOException e) {
    //log your exceptions...
} finally {
    closeResource(output);
    closeResource(input);
}

还有这个closeResource方法:

public void closeResource(Closeable resource) {
    if (resource != null) {
        try {
            resource.close();
        } catch (IOException e) {
            logger.error("Error while closing the resource.", e);
        }
    }
}