BufferedWriter not writing everything to its outpu

Question

I have a Java program that reads some text from a file, line by line, and writes new text to an output file. But not all the text I write to my BufferedWriter appears in the output file after the program has finished. Why is that?

The details: the program takes a CSV text document and converts it into SQL commands to insert the data into a table. The text file has more than 10000 lines which look similar to following:

2007,10,9,1,1,1006134,19423882

The program seems to work fine except it just stops in the file randomly half way through creating a new SQL statement having printed it into the SQL file. It looks something like:

insert into nyccrash values (2007, 1, 2, 1, 4, 1033092, 259916);
insert into nyccrash values (2007, 1, 1, 1, 1, 1020246, 197687);
insert into nyccrash values (2007, 10, 9, 1

This happens after about 10000 lines but several hundred lines before the end of the file. Where the break happens is between a 1 and a ,. However, the characters doesn't seem important because if I change the 1 to a 42 the last thing written to the new file is 4, which is cutting off the 2 from that integer. So it seems like the reader or writer must just be dying after writing/reading a certain amount.

My Java code is as follows:

import java.io.*;

public class InsertCrashData
{
    public static void main (String args[])
    {
        try
        {   
            //Open the input file.
            FileReader istream = new FileReader("nyccrash.txt");
            BufferedReader in = new BufferedReader(istream);
            //Open the output file.
            FileWriter ostream = new FileWriter("nyccrash.sql");
            BufferedWriter out = new BufferedWriter(ostream);
            String line, sqlstr;

            sqlstr = "CREATE TABLE nyccrash (crash_year integer, accident_type integer, collision_type integer, weather_condition integer, light_condition integer, x_coordinate integer, y_coordinate integer);\n\n"; 
            out.write(sqlstr);

            while((line = in.readLine())!= null)
            {
                String[] esa = line.split(",");
                sqlstr = "insert into nyccrash values ("+esa[0]+", "+esa[1]+", "+esa[2]+", "+esa[3]+", "+esa[4]+", "+esa[5]+", "+esa[6]+");\n";
                out.write(sqlstr);
            }
        }
        catch(Exception e)
        {
            System.out.println(e);
        }
    }
}

Answer 1

A resource that must be closed when it is no longer needed.

finally {
    out.close();//this would resolve the issue
    }

Some things to consider:

• BufferedWriter.close() flushes the buffer to the underlying stream, so if you forget to flush() and don't close, your file may not have all the text you wrote to it.
• BufferedWriter.close() also closes the wrapped Writer. When that's a FileWriter, this will ultimately close a FileOutputStream and tell the OS that you're done writing to the file.
• The garbage collector will automatically call close(), not on the BufferedWriter or the wrapped FileWriter, but on the FileOuputStream. So the OS will be happy, but you have to wait for the GC.
• However, you always want to release OS resources as soon as you no longer need them. This goes for open files, database connections, print queues ... anything. Trust me on this one.
• BufferedWriter.close() does clear up the internal character buffer, so that memory will be available for garbage collection, even while the BufferedWriter itself remains in scope.

So, Always close your resources (not just files) when you're done with them.

If you really want a peek under the covers, most of the Java API's source is available. BufferedWriter is here.

Answer 2

Since you're using BufferedWriter you can also flush the buffer when appropriate:

out.flush()

This will write the rest of the buffer to the actual file. Close-method also flushes the buffer and closes the file.

out.close()

There might be situations when you want to flush the buffer without closing the file. In these situations you can use the flush-method.

You can also use BuffredWriter's newline-method instead of adding \n to the end of the line. Newline-method uses system specific line separator so your code works on different platforms.

out.newLine()