How to read a text file from “assets” directory as a string?

Question

I have a file in my assets folder... how do I read it?

Now I\'m trying:

      public static String readFileAsString(String filePath)
        throws j         


        

Answer 1

Short answer, do this:

public static String readFile( String filePath ) throws IOException
{
    Reader reader = new FileReader( filePath );
    StringBuilder sb = new StringBuilder(); 
    char buffer[] = new char[16384];  // read 16k blocks
    int len; // how much content was read? 
    while( ( len = reader.read( buffer ) ) > 0 ){
        sb.append( buffer, 0, len ); 
    }
    reader.close();
    return sb.toString();
}

It's very straight forward, very fast, and works well for unreasonable large textfiles (100+ MB)

---

Long answer:

(Code at the end)

Many times it won't matter, but this method is pretty fast and quite readable. In fact its an order of complexity faster than @Raceimation's answer -- O(n) instead of O(n^2).

I've tested six methods (from slow to fast):

• concat: reading line by line, concat with str += ... *This is alarmingly slow even for smaller files (takes ~70 seconds for a 3MB file) *
• strbuilder guessing length: StringBuilder, initialized with the files size. i'm guessing that its slow because it really tries to find such a huge chunk of linear memory.
• strbuilder with line buffer: StringBuilder, file is read line by line
• strbuffer with char[] buffer: Concat with StringBuffer, read the file in 16k blocks
• strbuilder with char[] buffer: Concat with StringBuilder, read the file in 16k blocks
• preallocate byte[filesize] buffer: Allocate a byte[] buffer the size of the file and let the java api decide how to buffer individual blocks.

Conclusion:

Preallocating the buffer entirely is the fastest on very large files, but the method isn't very versatile because the total filesize must be known ahead of time. Thats why i suggest using strBuilder with char[] buffers, its still simple and if needed easily changable to accept any input stream instead of just files. Yet its certainly fast enough for all reasonable cases.

Test Results + Code

import java.io.*; 

public class Test
{

    static final int N = 5; 

    public final static void main( String args[] ) throws IOException{
        test( "1k.txt", true ); 
        test( "10k.txt", true ); 
        // concat with += would take ages here, so we skip it
        test( "100k.txt", false ); 
        test( "2142k.txt", false ); 
        test( "pruned-names.csv", false ); 
        // ah, what the heck, why not try a binary file
        test( "/Users/hansi/Downloads/xcode46graphicstools6938140a.dmg", false );
    }

    public static void test( String file, boolean includeConcat ) throws IOException{

        System.out.println( "Reading " + file + " (~" + (new File(file).length()/1024) + "Kbytes)" ); 
        strbuilderwithchars( file ); 
        strbuilderwithchars( file ); 
        strbuilderwithchars( file ); 
        tick( "Warm up... " ); 

        if( includeConcat ){
            for( int i = 0; i < N; i++ )
                concat( file ); 
            tick( "> Concat with +=                  " ); 
        }
        else{
            tick( "> Concat with +=   **skipped**    " ); 
        }

        for( int i = 0; i < N; i++ )
            strbuilderguess( file ); 
        tick( "> StringBuilder init with length  " ); 

        for( int i = 0; i < N; i++ )
            strbuilder( file ); 
        tick( "> StringBuilder with line buffer  " );

        for( int i = 0; i < N; i++ )
            strbuilderwithchars( file ); 
        tick( "> StringBuilder with char[] buffer" );

        for( int i = 0; i < N; i++ )
            strbufferwithchars( file ); 
        tick( "> StringBuffer with char[] buffer " );

        for( int i = 0; i < N; i++ )
            singleBuffer( file ); 
        tick( "> Allocate byte[filesize]         " );

        System.out.println(); 
    }

    public static long now = System.currentTimeMillis(); 
    public static void tick( String message ){
        long t = System.currentTimeMillis(); 
        System.out.println( message + ": " + ( t - now )/N + " ms" ); 
        now = t; 
    }


    // StringBuilder with char[] buffer
    // + works if filesize is unknown
    // + pretty fast 
    public static String strbuilderwithchars( String filePath ) throws IOException
    {
        Reader reader = new FileReader( filePath );
        StringBuilder sb = new StringBuilder(); 
        char buffer[] = new char[16384];  // read 16k blocks
        int len; // how much content was read? 
        while( ( len = reader.read( buffer ) ) > 0 ){
            sb.append( buffer, 0, len ); 
        }
        reader.close();
        return sb.toString();
    }

    // StringBuffer with char[] buffer
    // + works if filesize is unknown
    // + faster than stringbuilder on my computer
    // - should be slower than stringbuilder, which confuses me 
    public static String strbufferwithchars( String filePath ) throws IOException
    {
        Reader reader = new FileReader( filePath );
        StringBuffer sb = new StringBuffer(); 
        char buffer[] = new char[16384];  // read 16k blocks
        int len; // how much content was read? 
        while( ( len = reader.read( buffer ) ) > 0 ){
            sb.append( buffer, 0, len ); 
        }
        reader.close();
        return sb.toString();
    }

    // StringBuilder init with length
    // + works if filesize is unknown
    // - not faster than any of the other methods, but more complicated
    public static String strbuilderguess(String filePath) throws IOException
    {
        File file = new File( filePath ); 
        BufferedReader reader = new BufferedReader(new FileReader(file));
        String line;
        StringBuilder sb = new StringBuilder( (int)file.length() ); 
        while( ( line = reader.readLine() ) != null)
        {
            sb.append( line ); 
        }
        reader.close();
        return sb.toString();
    }

    // StringBuilder with line buffer
    // + works if filesize is unknown
    // + pretty fast 
    // - speed may (!) vary with line length
    public static String strbuilder(String filePath) throws IOException
    {
        BufferedReader reader = new BufferedReader(new FileReader(filePath));
        String line;
        StringBuilder sb = new StringBuilder(); 
        while( ( line = reader.readLine() ) != null)
        {
            sb.append( line ); 
        }
        reader.close();
        return sb.toString();
    }


    // Concat with += 
    // - slow
    // - slow
    // - really slow
    public static String concat(String filePath) throws IOException
    {
        BufferedReader reader = new BufferedReader(new FileReader(filePath));
        String line, results = "";
    int i = 0; 
        while( ( line = reader.readLine() ) != null)
        {
            results += line;
            i++; 
        }
        reader.close();
        return results;
    }

    // Allocate byte[filesize]
    // + seems to be the fastest for large files
    // - only works if filesize is known in advance, so less versatile for a not significant performance gain
    // + shortest code
    public static String singleBuffer(String filePath ) throws IOException{
        FileInputStream in = new FileInputStream( filePath );
        byte buffer[] = new byte[(int) new File( filePath).length()];  // buffer for the entire file
        int len = in.read( buffer ); 
        return new String( buffer, 0, len ); 
    }
}


/**
 *** RESULTS ***

Reading 1k.txt (~31Kbytes)
Warm up... : 0 ms
> Concat with +=                  : 37 ms
> StringBuilder init with length  : 0 ms
> StringBuilder with line buffer  : 0 ms
> StringBuilder with char[] buffer: 0 ms
> StringBuffer with char[] buffer : 0 ms
> Allocate byte[filesize]         : 1 ms

Reading 10k.txt (~313Kbytes)
Warm up... : 0 ms
> Concat with +=                  : 708 ms
> StringBuilder init with length  : 2 ms
> StringBuilder with line buffer  : 2 ms
> StringBuilder with char[] buffer: 1 ms
> StringBuffer with char[] buffer : 1 ms
> Allocate byte[filesize]         : 1 ms

Reading 100k.txt (~3136Kbytes)
Warm up... : 7 ms
> Concat with +=   **skipped**    : 0 ms
> StringBuilder init with length  : 19 ms
> StringBuilder with line buffer  : 21 ms
> StringBuilder with char[] buffer: 9 ms
> StringBuffer with char[] buffer : 9 ms
> Allocate byte[filesize]         : 8 ms

Reading 2142k.txt (~67204Kbytes)
Warm up... : 181 ms
> Concat with +=   **skipped**    : 0 ms
> StringBuilder init with length  : 367 ms
> StringBuilder with line buffer  : 372 ms
> StringBuilder with char[] buffer: 208 ms
> StringBuffer with char[] buffer : 202 ms
> Allocate byte[filesize]         : 199 ms

Reading pruned-names.csv (~11200Kbytes)
Warm up... : 23 ms
> Concat with +=   **skipped**    : 0 ms
> StringBuilder init with length  : 54 ms
> StringBuilder with line buffer  : 57 ms
> StringBuilder with char[] buffer: 32 ms
> StringBuffer with char[] buffer : 31 ms
> Allocate byte[filesize]         : 32 ms

Reading /Users/hansi/Downloads/xcode46graphicstools6938140a.dmg (~123429Kbytes)
Warm up... : 1665 ms
> Concat with +=   **skipped**    : 0 ms
> StringBuilder init with length  : 2899 ms
> StringBuilder with line buffer  : 2978 ms
> StringBuilder with char[] buffer: 2702 ms
> StringBuffer with char[] buffer : 2684 ms
> Allocate byte[filesize]         : 1567 ms


**/

Ps. You might have noticed that StringBuffer is slightly faster than StringBuilder. This is a bit nonsense because the classes are the same, except StringBuilder is not synchronized. If anyone can (or) can't reproduce this... I'm most curious :)