java - 并行四任务执行

Question

我想使用四个线程独立执行四个任务。我想减少执行时间。但是这个程序不能减少执行时间。在这个程序中，顺序执行时间小于并行执行时间。我想从顺序执行中更快地执行程序。我怎样才能并行执行四个任务并减少执行时间。请删除此代码的问题并给出解决方案。

package TestParallel;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectOutputStream;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.crypto.Cipher;
import javax.crypto.CipherOutputStream;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;

/**
 *
 * @author Sohel Rana
 */
public class Executor {

    public void encrypt(File fname) throws Exception {
        KeyGenerator keyGen = KeyGenerator.getInstance("AES");
        keyGen.init(256);  //using AES-256
        SecretKey key = keyGen.generateKey();  //generating key
        //  System.out.println("Key = " + bytesToHex(key.getEncoded()));
        Cipher aesCipher = Cipher.getInstance("AES");  //getting cipher for AES
        aesCipher.init(Cipher.ENCRYPT_MODE, key);  //initializing cipher for encryption with key

        //creating file output stream to write to file
        try (FileOutputStream fos = new FileOutputStream(fname + ".aes")) {
            //creating object output stream to write objects to file
            ObjectOutputStream oos = new ObjectOutputStream(fos);
            oos.writeObject(key);  //saving key to file for use during decryption

            //creating file input stream to read contents for encryption
            try (FileInputStream fis = new FileInputStream(fname)) {
                //creating cipher output stream to write encrypted contents
                try (CipherOutputStream cos = new CipherOutputStream(fos, aesCipher)) {
                    int read;
                    byte buf[] = new byte[4096];
                    while ((read = fis.read(buf)) != -1) //reading from file
                    {
                        cos.write(buf, 0, read);  //encrypting and writing to file
                    }
                }
            }
            //  fname.delete();
        }

    }

    public static void main(final String[] args) throws InterruptedException {
        final ExecutorService pool = Executors.newFixedThreadPool(4);

        File file1 = new File("C:\\Users\\Sohel Rana\\Desktop\\test\\33 - Overflow Menu.mp4");
        File file2 = new File("C:\\Users\\Sohel Rana\\Desktop\\test\\Java Cryptography Tutorials 1 AES Encryption and Decryption using Java.mp4");
        File file3 = new File("C:\\Users\\Sohel Rana\\Desktop\\test\\30 - Dank Meme Bro.mp4");
        File file4 = new File("C:\\Users\\Sohel Rana\\Desktop\\test\\How to change API level Android Studio.mp4");

        Executor ex = new Executor();
        long startTime = System.currentTimeMillis();
        pool.execute(() -> {
            try {
                ex.encrypt(file1);
                ex.encrypt(file2);
                 ex.encrypt(file3);
                ex.encrypt(file4);
            } catch (Exception ex1) {
                Logger.getLogger(Executor.class.getName()).log(Level.SEVERE, null, ex1);
            }
        });

        pool.shutdown();

        if (!pool.awaitTermination(1, TimeUnit.DAYS)) {
            System.err.println("Pool did not terminate.");
        }
                long endTime = System.currentTimeMillis();

        System.out.println("Paralle took time " + (endTime - startTime)
                + " milliseconds.");

    }
}

Answer 1

这是创建一个 Runnable，它一个接一个地运行各个 encrypt。

    pool.execute(() -> {
        try {
            ex.encrypt(file1);
            ex.encrypt(file2);
            ex.encrypt(file3);
            ex.encrypt(file4);
        } catch (Exception ex1) {
            Logger.getLogger(Executor.class.getName()).log(Level.SEVERE, null, ex1);
        }
    });

你需要做的是类似

for (File f : new File[] { file1, file2, file3, file4}) {
    pool.execute(() -> {
        try {
            ex.encrypt(f);
        } catch (Exception ex1) {
            Logger.getLogger(Executor.class.getName()).log(Level.SEVERE, null, ex1);
        }
    });
}

附带说明一下，您等待线程完成的方式之所以有效，只是因为线程池的大小足以启动所有提交的任务。如果您要扩展此解决方案以添加更多文件，那么这些文件可能永远不会被编码，因为执行程序将它们放在队列中但尚未启动；这意味着它们根本不会启动，因为这是在 ExecutorService 上调用 shutdown 的效果之一。