How do I read last n characters from a file?

In the following post you will learn how to read last n characters from a file. The JDK 7 introduces a new SeekableByteChannel interface which allows its implementation to change the position and the size of the byte channel. One of its implementation is the FileChannel class (java.nio.channels.FileChannel).

The FileChannel class make it possible to get hold the current position of where we are going to read from or write to a file. The code snippet below shows you how you can read the last 1000 characters from a log file.

package org.kodejava.examples.nio;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;

public class FileReadLastCharacters {
    public static void main(String[] args) {
        //
        // Defines the path to the log file and creates a ByteBuffer.
        //
        Path logPath = 
                Paths.get("D:\Appservers\apache-tomcat-7.0.37\logs\catalina.out");
        ByteBuffer buffer = ByteBuffer.allocate(1024);

        try {
            //
            // Creates FileChannel and open the file channel for read access.
            //
            FileChannel channel = FileChannel.open(logPath, StandardOpenOption.READ);

            //
            // Read a sequence of bytes from the channel into the buffer starting
            // at given file position, which is the channel size - 1000. Because
            // we are going to read the last 1000 characters from the file.
            //
            channel.read(buffer, channel.size() - 1000);
            System.out.println("Characters = " + new String(buffer.array()));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

The steps in the code snippet above are:

  1. Get the path to the log file.
  2. Create a ByteBuffer, a buffer where the read bytes to be transfered.
  3. Using the FileChannel.open() method we open  a file to be read and return a FileChannel object.
  4. The read() method of the FileChannel reads a sequence of bytes from the channel and transfer them to the given buffer starting and the position defined by channel.size() - 1000. This method returns the number of bytes read, possible zero, or -1 if the given position is greater than or equal to the file’s current size.
  5. Print out the buffered string.

How to monitor file or directory changes?

package org.kodejava.example.nio;

import java.io.IOException;
import java.nio.file.*;

import static java.nio.file.StandardWatchEventKinds.*;

public class FileWatchDemo {
    public static void main(String[] args) {
        try {
            // Creates a instance of WatchService.
            WatchService watcher = FileSystems.getDefault().newWatchService();

            // Registers the logDir below with a watch service.
            Path logDir = Paths.get("/Users/kodejava/temp/");
            logDir.register(watcher, ENTRY_CREATE, ENTRY_MODIFY, ENTRY_DELETE);

            // Monitor the logDir at listen for change notification.
            while (true) {
                WatchKey key = watcher.take();
                for (WatchEvent<?> event : key.pollEvents()) {
                    WatchEvent.Kind<?> kind = event.kind();

                    if (ENTRY_CREATE.equals(kind)) {
                        System.out.println("Entry was created on log dir.");
                    } else if (ENTRY_MODIFY.equals(kind)) {
                        System.out.println("Entry was modified on log dir.");
                    } else if (ENTRY_DELETE.equals(kind)) {
                        System.out.println("Entry was deleted from log dir.");
                    }
                }
                key.reset();
            }
        } catch (IOException | InterruptedException e) {
            e.printStackTrace();
        }
    }
}

To get the created, modified or deleted file you can see the following example: How to get the file name when using WatchService?.

How to get the file name when using WatchService?

package org.kodejava.example.nio;

import java.io.IOException;
import java.nio.file.*;

import static java.nio.file.StandardWatchEventKinds.*;

public class WatchServiceGetFilename {
    public static void main(String[] args) {
        try {
            // Create a WatchService and register the logDir path with the
            // WatchService for ENTRY_CREATE.
            WatchService watcher = FileSystems.getDefault().newWatchService();
            Path logDir = Paths.get("/Users/kodejava/temp");
            logDir.register(watcher, ENTRY_CREATE);

            while (true) {
                WatchKey key;
                try {
                    key = watcher.take();
                } catch (InterruptedException e) {
                    return;
                }

                for (WatchEvent<?> event : key.pollEvents()) {
                    if (event.kind() == ENTRY_CREATE) {
                        // Get the name of created file.
                        WatchEvent<Path> ev = cast(event);
                        Path filename = ev.context();

                        System.out.printf("A new file %s was created.%n",
                                filename.getFileName());
                    }
                }
                key.reset();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @SuppressWarnings("unchecked")
    static <T> WatchEvent<T> cast(WatchEvent<?> event) {
        return (WatchEvent<T>)event;
    }
}

How to write file using Files.newBufferedWriter?

To open a file for writing in JDK 7 you can use the Files.newBufferedWriter() method. This method takes three arguments. We need to pass the Path, the Charset and a varargs of OpenOption.

For example, in the snippet below we pass the path of our log file, we use the StandardCharsets.UTF_8 charset and we use the StandardOpenOption.WRITE to open a file for writing. If you want to open a file and append its contents instead of rewriting it you can use the StandardOpenOption.APPEND.

package org.kodejava.example.nio;

import java.io.BufferedWriter;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;

public class FilesNewBufferedWriter {
    public static void main(String[] args) {
        Path logFile = Paths.get("app.log");
        try (BufferedWriter writer = Files.newBufferedWriter(logFile,
                StandardCharsets.UTF_8, StandardOpenOption.WRITE)) {

            for (int i = 0; i < 10; i++) {
                writer.write(String.format("Message %s%n", i));
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Because we use the StandardOpenOption.WRITE we have to make sure that the file to be written is exists. If the file is not available we will get error like java.nio.file.NoSuchFileException.

How to read file using Files.newBufferedReader?

In the snippet below you’ll learn to open file for reading using Files.newBufferedReader() method in JDK 7. This method returns a java.io.BufferedReader which makes a backward compatibility with the old I/O system in Java.

To read a file you’ll need to provide a Path and the Charset to the newBufferedReader() method arguments.

package org.kodejava.example.nio;

import java.io.BufferedReader;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

public class FilesNewBufferedReader {
    public static void main(String[] args) {
        Path logFile = Paths.get("D:\Temp\logs\app.log");
        try (BufferedReader reader = Files.newBufferedReader(logFile,
                StandardCharsets.UTF_8)) {
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}