如何从“assets”目录中读取文本文件作为字符串？

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 java.io.IOException{
            StringBuffer fileData = new StringBuffer(1000);
            BufferedReader reader = new BufferedReader(
                    new FileReader(filePath));
            char[] buf = new char[1024];
            int numRead=0;
            while((numRead=reader.read(buf)) != -1){
                String readData = String.valueOf(buf, 0, numRead);
                fileData.append(readData);
                buf = new char[1024];
            }
            reader.close();
            return fileData.toString();
        }

But it cast a null pointer exception...

the file is called "origin" and it is in folder assets

I tried to cast it with:

readFileAsString("file:///android_asset/origin");

and

readFileAsString("asset/origin");``

but both failed... any advice?

Answer 1

BufferedReader's readLine() method returns a null when the end of the file is reached, so you'll need to watch for it and avoid trying to append it to your string.

The following code should be easy enough:

public static String readFileAsString(String filePath) throws java.io.IOException
{
    BufferedReader reader = new BufferedReader(new FileReader(filePath));
    String line, results = "";
    while( ( line = reader.readLine() ) != null)
    {
        results += line;
    }
    reader.close();
    return results;
}

Simple and to-the-point.

Answer 2

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 :)

Answer 3

You can open an input stream using AssetsManager .

InputStream input = getAssets().open("origin");
Reader reader = new InputStreamReader(input, "UTF-8");

getAssets() is a method of the Context class.

Also note that you shouldn't recreate a buffer of characters ( buf = new char[1024] , the last line of your cycle).

Answer 4

You should try org.appache.commons.io.IOUtils.toString(InputStream is) to get file content as string. There you can pass InputStream object which you will get from

getAssets().open("xml2json.txt")

in your Activity .

To get String use this:

String xml = IOUtils.toString((getAssets().open("xml2json.txt")));

Answer 5

I wrote a function that does the same thing as yours. I wrote it a while back but I believe it still works correctly.

public static final String grabAsSingleString(File fileToUse) 
            throws FileNotFoundException {

        BufferedReader theReader = null;
        String returnString = null;

        try {
            theReader = new BufferedReader(new FileReader(fileToUse));
            char[] charArray = null;

            if(fileToUse.length() > Integer.MAX_VALUE) {
                // TODO implement handling of large files.
                System.out.println("The file is larger than int max = " +
                        Integer.MAX_VALUE);
            } else {
                charArray = new char[(int)fileToUse.length()];

                // Read the information into the buffer.
                theReader.read(charArray, 0, (int)fileToUse.length());
                returnString = new String(charArray);

            }
        } catch (FileNotFoundException ex) {
            throw ex;
        } catch(IOException ex) {
            ex.printStackTrace();
        } finally {
            try {
                theReader.close();
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }

        return returnString;
    }

Now you can use this function if you wish, but when you're passing in the file either through a file object or a string, make sure you either give the full path of the file such as "C:\\Program Files\\test.dat" OR you pass in the relative link from your working directory. Your working directory is commonly the directory you launch the application from (unless you change it). So if the file was in a folder called data, you would pass in "./data/test.dat"

Yes, I know this is working with android so the Windows URI isn't applicable, but you should get my point.