[java] Most efficient way to check if a file is empty in Java on Windows

I am trying to check if a log file is empty (meaning no errors) or not, in Java, on Windows. I have tried using 2 methods so far.

Method 1 (Failure)

FileInputStream fis = new FileInputStream(new File(sLogFilename));  
int iByteCount = fis.read();  
if (iByteCount == -1)  
    System.out.println("NO ERRORS!");
else
    System.out.println("SOME ERRORS!");

Method 2 (Failure)

File logFile = new File(sLogFilename);
if(logFile.length() == 0)
    System.out.println("NO ERRORS!");
else
    System.out.println("SOME ERRORS!");

Now both these methods fail at times when the log file is empty (has no content), yet the file size is not zero (2 bytes).

What is the most efficient and accurate method to check if the file is empty? I asked for efficiency, as I have to keep checking the file size thousands of times, in a loop.

Note: The file size would hover around a few to 10 KB only!

Method 3 (Failure)

Following @Cygnusx1's suggestion, I had tried using a FileReader too, without success. Here's the snippet, if anyone's interested.

Reader reader = new FileReader(sLogFilename);
int readSize = reader.read();
if (readSize == -1)
    System.out.println("NO ERRORS!");
else
    System.out.println("SOME ERRORS!");

This question is related to java file logging file-io filesystems

The answer is


Why not just use:

File file = new File("test.txt");

if (file.length() == 0) {
    // file empty
} else {
    // not empty
}

Is there something wrong with it?


The idea of your first snippet is right. You probably meant to check iByteCount == -1: whether the file has at least one byte:

if (iByteCount == -1)  
    System.out.println("NO ERRORS!");
else
    System.out.println("SOME ERRORS!");

This is an improvement of Saik0's answer based on Anwar Shaikh's comment that too big files (above available memory) will throw an exception:

Using Apache Commons FileUtils

private void printEmptyFileName(final File file) throws IOException {
    /*Arbitrary big-ish number that definitely is not an empty file*/
    int limit = 4096;
    if(file.length < limit && FileUtils.readFileToString(file).trim().isEmpty()) {
        System.out.println("File is empty: " + file.getName());
    }        
}

I combined the two best solutions to cover all the possibilities:

BufferedReader br = new BufferedReader(new FileReader(fileName));     
File file = new File(fileName);
if (br.readLine() == null && file.length() == 0)
{
    System.out.println("No errors, and file empty");
}                
else
{
    System.out.println("File contains something");
}

Another way to do this is (using Apache Commons FileUtils) -

private void printEmptyFileName(final File file) throws IOException {
    if (FileUtils.readFileToString(file).trim().isEmpty()) {
        System.out.println("File is empty: " + file.getName());
    }        
}

I think the best way is to use file.length == 0.

It is sometimes possible that the first line is empty.


String line = br.readLine();
String[] splitted = line.split("anySplitCharacter");
if(splitted.length == 0)
    //file is empty
else
    //file is not empty

I had the same problem with my text file. Although it was empty, the value being returned by the readLine method was not null. Therefore, I tried to assign its value to the String array which I was using to access the splitted attributes of my data. It did work for me. Try this out and tell me if it works for u as well.


You can choose try the FileReader approach but it may not be time to give up just yet. If is the BOM field destroying for you try this solution posted here at stackoverflow.

Byte order mark screws up file reading in Java


Try FileReader, this reader is meant to read stream of character, while FileInputStream is meant to read raw data.

From the Javadoc:

FileReader is meant for reading streams of characters. For reading streams of raw bytes, consider using a FileInputStream.

Since you wanna read a log file, FileReader is the class to use IMO.


Stolen from http://www.coderanch.com/t/279224/Streams/java/Checking-empty-file

FileInputStream fis = new FileInputStream(new File("file_name"));  
int b = fis.read();  
if (b == -1)  
{  
  System.out.println("!!File " + file_name + " emty!!");  
}  

Updated: My first answer was premature and contained a bug.


Now both these methods fail at times when the log file is empty (has no content), yet the file size is not zero (2 bytes).

Actually, I think you will find that the file is NOT empty. Rather I think that you will find that those two characters are a CR and a NL; i.e. the file consists of one line that is empty.

If you want to test if a file is either empty or has a single empty line then a simple, relatively efficient way is:

try (BufferedReader br = new BufferedReader(FileReader(fileName))) {
    String line = br.readLine();
    if (line == null || 
        (line.length() == 0 && br.readLine() == null)) {
        System.out.println("NO ERRORS!");
    } else {
        System.out.println("SOME ERRORS!");
    }
}

Can we do this more efficiently? Possibly. It depends on how often you have to deal with the three different cases:

  • a completely empty file
  • a file consisting of a single empty line
  • a file with a non-empty line, or multiple lines.

You can probably do better by using Files.length() and / or reading just the first two bytes. However, the problems include:

  • If you both test the file size AND read the first few bytes then you are making 2 syscalls.
  • The actual line termination sequence could be CR, NL or CR NL, depending on the platform. (I know you say this is for Windows, but what happens if you need to port your application? Or if someone sends you a non-Windows file?)
  • It would be nice to avoid setting up stream / reader stack, but the file's character encoding could map CR and NL to something other than the bytes 0x0d and 0x0a. (For example ... UTF-16)
  • Then there's the annoying habit of some Windows utilities have putting BOM markers into UTF-8 encoded files. (This would even mess up the simple version above!)

All of this means that the most efficient possible solution is going to be rather complicated.


Examples related to java

Under what circumstances can I call findViewById with an Options Menu / Action Bar item? How much should a function trust another function How to implement a simple scenario the OO way Two constructors How do I get some variable from another class in Java? this in equals method How to split a string in two and store it in a field How to do perspective fixing? String index out of range: 4 My eclipse won't open, i download the bundle pack it keeps saying error log

Examples related to file

Gradle - Move a folder from ABC to XYZ Difference between opening a file in binary vs text Angular: How to download a file from HttpClient? Python error message io.UnsupportedOperation: not readable java.io.FileNotFoundException: class path resource cannot be opened because it does not exist Writing JSON object to a JSON file with fs.writeFileSync How to read/write files in .Net Core? How to write to a CSV line by line? Writing a dictionary to a text file? What are the pros and cons of parquet format compared to other formats?

Examples related to logging

How to redirect docker container logs to a single file? Console logging for react? Hide strange unwanted Xcode logs Where are logs located? Retrieve last 100 lines logs Spring Boot - How to log all requests and responses with exceptions in single place? How do I get logs from all pods of a Kubernetes replication controller? Where is the Docker daemon log? How to log SQL statements in Spring Boot? How to do logging in React Native?

Examples related to file-io

Python, Pandas : write content of DataFrame into text File Saving response from Requests to file How to while loop until the end of a file in Python without checking for empty line? Getting "java.nio.file.AccessDeniedException" when trying to write to a folder How do I add a resources folder to my Java project in Eclipse Read and write a String from text file Python Pandas: How to read only first n rows of CSV files in? Open files in 'rt' and 'wt' modes How to write to a file without overwriting current contents? Write objects into file with Node.js

Examples related to filesystems

Get an image extension from an uploaded file in Laravel Notepad++ cached files location No space left on device How to create a directory using Ansible best way to get folder and file list in Javascript Exploring Docker container's file system Remove directory which is not empty GIT_DISCOVERY_ACROSS_FILESYSTEM not set Trying to create a file in Android: open failed: EROFS (Read-only file system) Node.js check if path is file or directory