古怪的需求

在實習的公司碰到一個古怪的需求：在一臺伺服器上寫日誌檔案，每當日誌檔案寫到一定大小時，比如是1G，會將這個日誌檔案改名成另一個名字，並新建一個與原檔名相同的日誌檔案，再往這個新建的日誌檔案裡寫資料；要求寫一個程式能實時地讀取日誌檔案中的內容，並且不能影響寫操作與重新命名操作。

RandomAccessFile類中seek方法可以從指定位置讀取檔案，可以用來實現檔案實時讀取。JDK文件對RandomAccessFile的介紹

Instances of this class support both reading and writing to a random access file. A random access file behaves like a large array of bytes stored in the file system. There is a kind of cursor, or index into the implied array, called the file pointer; input operations read bytes starting at the file pointer and advance the file pointer past the bytes read. If the random access file is created in read/write mode, then output operations are also available; output operations write bytes starting at the file pointer and advance the file pointer past the bytes written. Output operations that write past the current end of the implied array cause the array to be extended.

在每一次讀取後，close一下就不會影響重新命名操作了。

編碼實現

寫日誌檔案，每秒寫200條記錄，並且記錄寫的時間

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.text.SimpleDateFormat;
import java.util.Date;

public class LogReader implements Runnable {
    private File logFile = null;
    private long lastTimeFileSize = 0; // 上次檔案大小
    private static SimpleDateFormat dateFormat = new SimpleDateFormat(
            "yyyy-MM-dd HH:mm:ss");

    public LogReader(File logFile) {
        this.logFile = logFile;
        lastTimeFileSize = logFile.length();
    }

    /**
     * 實時輸出日誌資訊
     */
    public void run() {
        while (true) {
            try {
                long len = logFile.length();
                if (len < lastTimeFileSize) {
                    System.out.println("Log file was reset. Restarting logging from start of file.");
                    lastTimeFileSize = len;
                } else if(len > lastTimeFileSize) {
                    RandomAccessFile randomFile = new RandomAccessFile(logFile, "r");
                    randomFile.seek(lastTimeFileSize);
                    String tmp = null;
                    while ((tmp = randomFile.readLine()) != null) {
                        System.out.println(dateFormat.format(new Date()) + "\t"
                                + tmp);
                    }
                    lastTimeFileSize = randomFile.length();
                    randomFile.close();
                }
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            
            try {
                Thread.sleep(50);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

}
 

實時讀取日誌檔案，每隔1秒讀一次

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.text.SimpleDateFormat;
import java.util.Date;

public class LogReader implements Runnable {
    private File logFile = null;
    private long lastTimeFileSize = 0; // 上次檔案大小
    private static SimpleDateFormat dateFormat = new SimpleDateFormat(
            "yyyy-MM-dd HH:mm:ss");

    public LogReader(File logFile) {
        this.logFile = logFile;
        lastTimeFileSize = logFile.length();
    }

    /**
     * 實時輸出日誌資訊
     */
    public void run() {
        while (true) {
            try {
                RandomAccessFile randomFile = new RandomAccessFile(logFile, "r");
                randomFile.seek(lastTimeFileSize);
                String tmp = null;
                while ((tmp = randomFile.readLine()) != null) {
                    System.out.println(dateFormat.format(new Date()) + "\t"
                            + tmp);
                }
                lastTimeFileSize = randomFile.length();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

}
 

開啟寫執行緒、讀執行緒，將實時資訊列印在控制檯。

import java.io.File;

public class RunRun {
    public static void main(String[] args) {
        File logFile = new File("mock.log");
        Thread wthread = new Thread(new LogWrite(logFile));
        wthread.start();
        Thread rthread = new Thread(new LogReader(logFile));
        rthread.start();
    }
}

在讀寫的過程中，我們可以手動將mock.log檔案重新命名，發現依舊可以實時讀。