Question

我知道Java7中的WatchService，但我需要添加到文件中的内容。我怎样才能做到这一点？

例如：我有一个我想要观看的日志文件，并检查添加到文件中的日志条目。

如果程序退出（例如JVM崩溃），还有什么方法可以继续观看吗？存储文件指针或类似文件？

非常感谢你的帮助！诉

Answer 1

以下代码应该能够满足您的要求。虽然它有点冗长，可能需要一些重构，但它应该可以开箱即用。我在评论之间插入了我的评论，如有必要可以进一步解释。

您只需要创建'/tmp/log.txt'文件，执行代码并向文件添加一些内容。保存文件后，内容将显示在Java控制台上。

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileReader;
import java.io.IOException;
import java.io.PrintWriter;

public class FileWatcher {

    private static final String POINTER_FILE_PATH = "/tmp/pointer.txt";

    private static final String LOG_FILE_PATH = "/tmp/log.txt";

    public static void main(String[] args) throws IOException,
            InterruptedException {
        File logFile = new File(LOG_FILE_PATH);

        //read the previous position in the log file
        long pointer = getLastPointer();

        while (true) {
            long fsize = logFile.length();

            // calculate the length of new content from the pointer position
            int length = (int) (fsize - pointer);

            if (length == 0) {
                // save CPU cycles if nothing happened in the log file
                Thread.sleep(100);
                continue;
            }

            if (fsize < pointer) {
                // throw exception if any file content has been deleted
                // (doesn't cater for caracter replacement)
                throw new RuntimeException("Tampered file content");
            }

            // initialise a new buffer for the new content
            byte[] buf = new byte[length];

            int actualLength = readFileContent(pointer, buf, length);

            // increment the position of the pointer in the file
            pointer = pointer + actualLength;

            //save the current pointer
            storePointer(pointer);

            // print out the new content
            System.out.print(new String(buf, 0, actualLength));
        }

    }

    private static int readFileContent(long pointer, byte[] buf, int length) throws IOException {
        // open a new input stream to the log file
        FileInputStream fis = new FileInputStream(LOG_FILE_PATH);

        try {
            // skip to the previous position in the log file
            fis.skip(pointer);

            // read file content into the buffer and retrieve the actual
            // number of bytes
            int actualLength = fis.read(buf, 0, length);
            return actualLength;
        } finally {
            fis.close();
        }
    }

    private static void storePointer(long pointer) throws IOException {
        PrintWriter pw = null;
        try {
            pw = new PrintWriter(POINTER_FILE_PATH);
            pw.write(String.valueOf(pointer));
            pw.flush();
        } finally {
            if (pw != null) {
                pw.close();
            }
        }
    }

    private static long getLastPointer() throws IOException {
        long pointer = 0;
        BufferedReader br = null;
        try {
            br = new BufferedReader(new FileReader(POINTER_FILE_PATH));
            String line = br.readLine();
            if (line != null && line.length() > 0) {
                pointer = Long.valueOf(line.trim());
            }
        } catch(IOException e) {
            pointer = 0;
        } finally {
            if (br != null)
                br.close();
        }

        return pointer;
    }

}

观察文件以进行修改并获取要添加的内容

1 个答案: