Java将字符串列表写入文件，但是文件为空

Question

I have found this question in other languages, but have yet to find a solution to this issue in a java application.

I have a large .txt file with millions of records. Each record is /n delimited. Basically it is a single column of data from a table. The goal is to read the data from the input file and partition it. Then write the partitioned data to a new file. For example, a file with 2 million records will become 200 files with 10,000 records each (with the last file containing <10,000.)

I am successfully reading and partitioning the data. I am successfully creating the first file and it is being named properly.

The problem is only 1 file is created and it is empty. The code as is compiles and runs without errors or exceptions.

My code is below:

    import java.io.BufferedReader;
    import java.io.BufferedWriter;
    import java.io.FileReader;
    import java.io.FileWriter;
    import java.io.IOException;
    import java.io.StringWriter;
    import java.util.ArrayList;
    import java.util.Collection;
    import java.util.List;
    import java.util.concurrent.atomic.AtomicInteger;
    import java.util.stream.Collectors;

    public class ChunkTextFile {

    private static final String inputFilename = "inputFile.txt";

    public static void main(String[] args) {

        BufferedReader reader = null;

        BufferedWriter fileWriter = null;

        BufferedWriter lineWriter = null;

        StringWriter stringWriter = null;

        // Create an ArrayList object to hold the lines of input file

        List<String> lines = new ArrayList<String>();

        try {
            // Creating BufferedReader object to read the input file

            reader = new BufferedReader(new FileReader("src" + "//" + inputFilename));

            // Reading all the lines of input file one by one and adding them into ArrayList
            String currentLine = reader.readLine();

            while (currentLine != null) {
                lines.add(currentLine);

                currentLine = reader.readLine();

            }
            // End of file read.

           //Partition ArrayList into a collection of smaller Lists<String>
            final AtomicInteger counter = new AtomicInteger(0);
            final int size = 10000;

            Collection<List<String>> partitioned = lines.stream()
                    .collect(Collectors.groupingBy(it -> counter.getAndIncrement() / size)).values();

            //Printing partitions. Each partition will be written to a file.
            //Testing confirms the partitioning works correctly.
            partitioned.forEach(System.out::println);

            //Iterate through the Collections and create a file for List<String> object.
            //Testing confirms that multiple files are created and properly named.
            Integer count = 0;
            for (List<String> chunks : partitioned) {
                // Prepare new incremented file name.
                String outputFile = "batched_items_file_";
                String txt = ".txt";
                count++;


                String filename = outputFile + count + txt;

                // Write file to directory.
                fileWriter = new BufferedWriter(new FileWriter("src" + "//" + outputFile));
                fileWriter = new BufferedWriter(new FileWriter(filename));

                //Iterate through the List of Strings and write each String to the file.
                //Writing is not successful. Only 1 file is created and it is empty.
                for (String chunk : chunks) {
                    stringWriter = new StringWriter();
                    lineWriter = new BufferedWriter(stringWriter);
                    // Prepare list of strings to be written to new file.
                    // Write each item number to file.
                    lineWriter.write(chunk);
                    lineWriter.flush();
                }
                lineWriter.close(); // <- flush the BufferedWriter

                fileWriter.close();
            }

        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            // Closing the resources
            System.out.println("Finished");

            try {
                if (reader != null) {
                    reader.close();
                }

                if (fileWriter != null) {
                    fileWriter.close();
                }

                if (stringWriter != null) {
                    stringWriter.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

Input file example:

230449
235659
295377
329921
348526
359836
361447
384723
396202
571490

Thank you in advance.

Answer 1

You don't need all those extra writers in your for and the writer supposed to write (fileWriter) to the file is not being called. Replace your for by this one:

for (String chunk : chunks) {
    fileWriter.write(chunk);
}

Tip: Just call fileWriter.close() once inside the finally block. The close method will automatically flush the writer for you (there's no need to call fileWriter.flush()).

Answer 2

There are several issues with your code. The files are empty, because you don't close the writers. You are even creating redundant writers as in this sequence

fileWriter = new BufferedWriter(new FileWriter("src" + "//" + outputFile));
fileWriter = new BufferedWriter(new FileWriter(filename));

To handle resources like readers and writers in the optimal way, use the try-with-resources statement .

The missing new lines is only a small problem.

Further, you are unnecessarily reading the entire input file into the heap memory, just to be able to perform a questionable Stream operation on it. While it is possible to stream over a file directly, eg with Files.lines , the grouping with an AtomicInteger is not the intended way of using a Stream anyway. And the end result would still hold the entire input lines in memory while it would be straight-forward to write the lines to the target file immediately.

A simple and efficient solution would be

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;

public class ChunkTextFile {

    private static final String inputFilename = "inputFile.txt";

    public static void main(String[] args) {
        final int size = 10000;
        try(BufferedReader reader=Files.newBufferedReader(Paths.get("src", inputFilename))) {
            String line = reader.readLine();
            for(int count = 0; line != null; count++) {
                try(BufferedWriter writer = Files.newBufferedWriter(
                        Paths.get("batched_items_file_" + count + ".txt"))) {
                    for(int i = 0; i < size && line != null; i++) {
                        writer.write(line);
                        writer.newLine();
                        line = reader.readLine();
                    }
                }
            }
        }
        catch(IOException ex) {
            ex.printStackTrace();
        }
    }
}

Answer 3

StringWriter不是用于编写字符串 ，而是用于写入字符串 。

Answer 4

You can use just

Path file = Paths.get(filename);
Files.write(file, chunks, Charset.forName("UTF-8"));

And, you should put count=0 before loop, otherwise it will be always 0.

Overall it will be like this:

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.stream.Collectors;

public class ChunkTextFile {

private static final String inputFilename = "inputFile.txt";

public static void main(String[] args) {

    BufferedReader reader = null;


    // Create an ArrayList object to hold the lines of input file

    List<String> lines = new ArrayList<String>();

    try {
        // Creating BufferedReader object to read the input file

        reader = new BufferedReader(new FileReader(inputFilename));

        // Reading all the lines of input file one by one and adding them into ArrayList
        String currentLine = reader.readLine();

        while (currentLine != null) {
            lines.add(currentLine);

            currentLine = reader.readLine();

        }
        // End of file read.

        //Partition ArrayList into a collection of smaller Lists<String>
        final AtomicInteger counter = new AtomicInteger(0);
        final int size = 10;

        Collection<List<String>> partitioned = lines.stream()
                .collect(Collectors.groupingBy(it -> counter.getAndIncrement() / size)).values();

        //Printing partitions. Each partition will be written to a file.
        //Testing confirms the partitioning works correctly.
        partitioned.forEach(System.out::println);

        //Iterate through the Collections and create a file for List<String> object.
        //Testing confirms the file is created and properly named.
        Integer count = 0;
        for (List<String> chunks : partitioned) {
            // Prepare new incremented file name.
            String outputFile = "batched_items_file_";
            String txt = ".txt";

            count++;

            String filename = outputFile + count + txt;

            Path file = Paths.get(filename);
            Files.write(file, chunks, Charset.forName("UTF-8"));
        }

    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        // Closing the resources
        System.out.println("Finished");

        try {
            if (reader != null) {
                reader.close();
            }

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
 }
 }

Answer 5

I am accepting the above answer as it solved my problem, but I wanted to expand on it for anyone that finds this question and answer. For the created files to be in the same format as the input file (newline delimited) I changed my code using the accepted answer and added System.lineSeparator() .

The final solution looks like this.

fileWriter.write(chunk + System.lineSeparator());

Thank you again for the quick responses.

This is the working version. I recommend commenting out or removing partitioned.forEach(System.out::println); to improve performance.

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.StringWriter;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.stream.Collectors;

public class ChunkTextFile {

private static final String inputFilename = "inputFile.txt";

public static void main(String[] args) {

    BufferedReader reader = null;

    BufferedWriter fileWriter = null;


    // Create an ArrayList object to hold the lines of input file

    List<String> lines = new ArrayList<String>();

    try {
        // Creating BufferedReader object to read the input file

        reader = new BufferedReader(new FileReader("src" + "//" + inputFilename));

        // Reading all the lines of input file one by one and adding them into ArrayList
        String currentLine = reader.readLine();

        while (currentLine != null) {
            lines.add(currentLine);

            currentLine = reader.readLine();

        }
        // End of file read.

        final AtomicInteger counter = new AtomicInteger(0);
        final int size = 10000;

        Collection<List<String>> partitioned = lines.stream()
                .collect(Collectors.groupingBy(it -> counter.getAndIncrement() / size)).values();

        //Printing partitions. Each partition will be written to a file.
        //Testing confirms the partitioning works correctly.
        partitioned.forEach(System.out::println);

        //Iterate through the Collections and create a file for List<String> object.
        //Testing confirms the file is created and properly named.
        Integer count = 0;
        for (List<String> chunks : partitioned) {
            // Prepare new incremented file name.
            String outputFile = "batched_items_file_";
            String txt = ".txt";
             count++;

            String filename = outputFile + count + txt;

            // Write file to directory.
            fileWriter = new BufferedWriter(new FileWriter("src" + "//" + outputFile));
            fileWriter = new BufferedWriter(new FileWriter(filename));

            //Iterate through the List of Strings and write each String to the file.
            //Writing is not successful. Only 1 file is created and it is empty.
            for (String chunk : chunks) {
                // Prepare list of strings to be written to new file.
                // Write each item number to file.
                fileWriter.write(chunk + System.lineSeparator());
            }

        }

    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        // Closing the resources
        System.out.println("Finished");

        try {
            if (reader != null) {
                reader.close();
            }

            if (fileWriter != null) {
                fileWriter.close();
            }

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
  }
}