使用java.util.scanner平台從文件或URI的底部n行（尾部）掃描-不可知論

Question

我正在編寫一個包（出於學習目的），該包采用URI或本地文件路徑，並從HTML或文本文檔中讀取指定數量的行。

選項之一是，如果用戶將傳遞底數的負數作為要讀取的行數，則用戶應該能夠閱讀文檔的底行。

如果我打算在Linux / Unix上執行此操作，則我相信類似這樣的內容（我省去了URL v。File的代碼部分，只關注問題的核心）：

Runtime r = Runtime.getRuntime()
Process p = r.exec("tail -f")
Scanner s = new Scanner(p.getInputStream())
while (s.hasNextLine()) {
    String line = s.nextLine()
    // Do whatever you want with the output.
}

會工作。 但是，這不適用於Windows。

有平台無關的方法嗎？

Answer 1

由於沒有人發布答案，因此可能沒有一個非常直接的解決方案，但這就是我最終得到的結果。

文件1：MainTest.java

/**
 * File:        MainTest.java
 */
package mod8;
//import mod8.CaptureURI.*;
import java.io.*;

/**
 * @author Jason D. Miller, Miller Intelligence LLC
 *
 */
public class MainTest {

    /**
     * Write a program that will display lines of a text you specify. 
     * It should take two arguments: a file or a URL and the number of lines to display. 
     * 
     * If the number of lines given is positive, it should display the first n lines of the text. 
     * If it is 0, it should display all the lines of the text. 
     * If it is a negative number, it should display the last n lines of the text. 
     * 
     * Your program should handle errors appropriately.
     */
    public MainTest() {}

    /**
     * @param args
     */
    public static void main(String[] args) throws IOException {
        //Greet and instruct user
        System.out.println("********************************************");
        System.out.println("* Welcome to Capture URI (aka Module 8)!   *");
        System.out.println("*                                          *");
        System.out.println("*    Please enter a local file location    *");
        System.out.println("*      or web URL, then press ENTER        *");
        System.out.println("*    Next, please input the # of lines to  *");
        System.out.println("*      capture, then press ENTER           *");
        System.out.println("********************************************");

        //Get params
        BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in), 1);
        System.out.println("File or web URI:");
        String uri = stdin.readLine();
        System.out.println("Number of lines as an integer (0 = all, negative implies tail):");
        Integer lines = stdin.read();

        //Pass to backend
        String text = "";
        text = CaptureURI.CURI(uri, lines);

        //Print results
        System.out.println(text);
    }

}

文件2：CaptureURI.java

package mod8;
import mod8.Tail;
import java.net.*;
import java.io.*;
import java.util.*;

public class CaptureURI  {


    public static boolean isLocalFile(String file) {
        try {
            new URL(file);
            return false;
        } catch (MalformedURLException e) {
            return true;
        }
    }


    public static String CURI(String uri, Integer lines) throws IOException {
        String out = "";

        // Get the entire file/URI
        if(lines == 0){     
            if(isLocalFile(uri)){
                try {
                     out = new Scanner(new File(uri)).useDelimiter("\\Z").next();               
                } 
                catch (FileNotFoundException e) {
                    e.printStackTrace();
                }
            }else{
            Scanner scanner = new Scanner(new URL(uri).openStream(), "UTF-8");
            out = scanner.useDelimiter("\\A").next();
            scanner.close();
            }
        }

        //Take "head" lines of file/URI
        if(lines > 0){
            if(isLocalFile(uri)){
                try {
                    File file = new File(uri);                      
                    Scanner s = new Scanner(file);
                    for(int i = 0; i < lines; i++)
                        {
                        out += s.nextLine();
                        }
                    s.close();
                } 
                catch (FileNotFoundException e) {
                    e.printStackTrace();
                }
            } else{
            Scanner scanner = new Scanner(new URL(uri).openStream(), "UTF-8");
            for(int i = 0; i < lines; i++) 
            {
                out += scanner.next();
            }
            scanner.close();
            }

        }

        // Take "tail" lines of file/URI
        if(lines < 0){
            if(isLocalFile(uri)){
                try {
                    File file = new File(uri);            
                    Scanner s = new Scanner(file);
                    out = s.toString();
                    //String[] ary = out.split("");
                    out = Tail.JavaTail(out);
                    s.close();
                } 
                catch (FileNotFoundException e) {
                    e.printStackTrace();
                }
            } else{
            Scanner scanner = new Scanner(new URL(uri).openStream(), "UTF-8");
            for(int i = 0; i < lines; i++) 
            /*{
                out += scanner.next();
            }*/
            out = Tail.JavaTail(out);   
            scanner.close();
            }

        }


        return(out);
    }
}

文件3：Tail.java

package mod8;

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

/**
 * Java implementation of the Unix tail command
 * 
 * @param args[0] File name
 * @param args[1] Update time (seconds). Optional. Default value is 1 second
 * 
 * @author Luigi Viggiano (original author) http://it.newinstance.it/2005/11/19/listening-changes-on-a-text-file-unix-tail-implementation-with-java/
 * @author Alessandro Melandri (modified by)
 * @author Jason Millerm hack-r.com (modified by)
 * */
public class Tail {

  static long sleepTime = 1000;

  public static String JavaTail(String args) throws IOException {
    String out = "";

      BufferedReader input = new BufferedReader(new FileReader(args));
      String currentLine = null;

      while (true) {

    if ((currentLine = input.readLine()) != null) {
      out += currentLine;
      continue;
    }
    input.close();
    return(out);

  /* try {
      Thread.sleep(sleepTime);
    } catch (InterruptedException e) {
      Thread.currentThread().interrupt();
      break;
    } */


      //
      //return(out);

    } 
  }
}