无法借助 Java 中的同步方法获得答案

Question

我必须在 Java 中使用多线程从文件 F1 和 F2 读取内容并将其移动到新文件 (f3)。我面临的问题是，当我在方法中使用同步时，我无法得到答案，但是当我不同步方法时，我能够得到答案。

下面是同步方法的代码，只打印一个文件的内容：

package com.company;
import java.io.*;
import java.io.FileReader;

public class FileMerge{
    public static void main(String[] args) throws IOException, InterruptedException {
        WriteToFile pc = new WriteToFile();
        //Create a Thread1 to read the content of file 1
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                   try {
                    pc.file1();
                } catch (InterruptedException | IOException e) {
                    System.out.println(e);

                }
            }
        });

        // Create  Thread 2 to read the content of file 2
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    pc.file2();
                } catch (InterruptedException | IOException e) {
                    System.out.println(e);

                }
            }
        });


        //Start the Thread
        t1.start();
        t2.start();

        t1.join();
        t2.join();
    }

    public static class WriteToFile{
        PrintWriter pw = new PrintWriter("C:/Users/paras.jain/Desktop/des.txt");

        public WriteToFile() throws FileNotFoundException {
        }


        public synchronized void file1() throws InterruptedException, IOException {
            // BufferedReader object for file1.txt
            BufferedReader br = new BufferedReader(new FileReader("C:\Users\paras.jain\Desktop\f.txt"));
            String line = br.readLine();
            //System.out.println("----------" + line);

            // loop to copy each line of
            // file1.txt to file3.txt
            while (line != null) {
                pw.println(line);

                line = br.readLine();
            }
            br.close();
            pw.close();

        }

        public  synchronized void file2() throws InterruptedException, IOException {

            BufferedReader br1 = new BufferedReader(new FileReader("C:/Users/paras.jain/Desktop/f2.txt"));
            String line2 = br1.readLine();
            //System.out.println("----------" + line2);


            // loop to copy each line of
            // file2.txt to file3.txt
            while (line2 != null) {
                pw.println(line2);
                line2 = br1.readLine();
                //System.out.println("----------" + line2);
            }
            br1.close();
            pw.close();

        }
    }
}

以上代码的输出是：

File 1 line 1 
File 1 line 2
File 1 line 3

当我运行代码 没有同步 方法时，我得到以下输出：

File 1 line 1
File 1 line 2
File 2 line 1
File 2 line 2
File 1 line 3
File 2 line 3

为什么输出会有所不同？以及如何借助 Synchronized 方法获得第二个输出？

Answer 1

当 file1() 和 file2() 方法同步时，其中一个必须先于另一个完成运行。

由于您首先启动运行的 file1() 线程，它可能会赢得比赛，因此 file1() 必须在 file2() 可以运行之前完成.

不幸的是，file1() 关闭了 PrintWriter，所以当 file2() 运行时，所有 pw.println(...) 调用 无声地失败.

为什么默默？因为那是他们定义它的方式。请参阅 PrintWriter:

的 javadoc

Methods in this class never throw I/O exceptions, although some of its constructors may. The client may inquire as to whether any errors have occurred by invoking checkError().

无法借助 Java 中的同步方法获得答案

Not able to get the answer with the help of synchronized method in Java

java

multithreading

synchronized