Java中的文本文件解析

Question

I am reading in a text file using FileInputStream that puts the file contents into a byte array. I then convert the byte array into a String using new String(byte).

Once I have the string I'm using String.split("\\n") to split the file into a String array and then taking that string array and parsing it by doing a String.split(",") and hold the contents in an Arraylist.

I have a 200MB+ file and it is running out of memory when I start the JVM up with a 1GB of memory. I know I must be doing something in correctly somewhere, I'm just not sure if the way I'm parsing is incorrect or the data structure I'm using.

It is also taking me about 12 seconds to parse the file which seems like a lot of time. Can anyone point out what I may be doing that is causing me to run out of memory and what may be causing my program to run slow?

The contents of the file look as shown below:

"12334", "100", "1.233", "TEST", "TEXT", "1234"
"12334", "100", "1.233", "TEST", "TEXT", "1234"
.
.
.
"12334", "100", "1.233", "TEST", "TEXT", "1234"

Thanks

Answer 1

I'm not sure how efficient it is memory-wise, but my first approach would be using a Scanner as it is incredibly easy to use:

File file = new File("/path/to/my/file.txt");
Scanner input = new Scanner(file);

while(input.hasNext()) {
    String nextToken = input.next();
    //or to process line by line
    String nextLine = input.nextLine();
}

input.close();

Check the API for how to alter the delimiter it uses to split tokens.

Answer 2

It sounds like you're doing something wrong to me - a whole lotta object creation going on.

How representative is that "test" file? What are you really doing with that data? If that's typical of what you really have, I'd say there's lots of repetition in that data.

If it's all going to be in Strings anyway, start with a BufferedReader to read each line. Pre-allocate that List to a size that's close to what you need so you don't waste resources adding to it each time. Split each of those lines at the comma; be sure to strip off the double quotes.

You might want to ask yourself: "Why do I need this whole file in memory all at once?" Can you read a little, process a little, and never have the whole thing in memory at once? Only you know your problem well enough to answer.

Maybe you can fire up jvisualvm if you have JDK 6 and see what's going on with memory. That would be a great clue.

Answer 3

Have a look at these pages. They contain many open source CSV parsers. JSaPar is one of them.

• Text file parsing libraries and projects
• Java Open Source libraries

Answer 4

It sounds like you currently have 3 copies of the entire file in memory: the byte array, the string, and the array of the lines.

Instead of reading the bytes into a byte array and then converting to characters using new String() it would be better to use an InputStreamReader, which will convert to characters incrementally, rather than all up-front.

Also, instead of using String.split("\\n") to get the individual lines, you should read one line at a time. You can use the readLine() method in BufferedReader .

Try something like this:

BufferedReader reader = new BufferedReader(new InputStreamReader(fileInputStream, "UTF-8"));
try {
  while (true) {
    String line = reader.readLine();
    if (line == null) break;
    String[] fields = line.split(",");
    // process fields here
  }
} finally {
  reader.close();
}

Answer 5

If you have a 200,000,000 character files and split that every five characters, you have 40,000,000 String objects. Assume they are sharing actual character data with the original 400 MB String ( char is 2 bytes). A String is say 32 bytes, so that is 1,280,000,000 bytes of String objects.

(It's probably worth noting that this is very implementation dependent. split could create entirely strings with entirely new backing char[] or, OTOH, share some common String values. Some Java implementations to not use the slicing of char[] . Some may use a UTF-8-like compact form and give very poor random access times.)

Even assuming longer strings, that's a lot of objects. With that much data, you probably want to work with most of it in compact form like the original (only with indexes). Only convert to objects that which you need. The implementation should be database like (although they traditionally don't handle variable length strings efficiently).

Answer 6

While calling/invoking your programme you can use this command : java [-options] className [args...]
in place of [-options] provide more memory eg -Xmx1024m or more. but this is just a workaround, u have to change ur parsing mechanism.