我有一些大的制表符分隔数据文件。这些文件的行数比列数多几个数量级。问题是我想转动这些文件,但在这种情况下,“大”被定义为太大而无法在内存中执行此操作。
我希望能以最快的方式找到一些建议。我主要在UNIX上使用Java,虽然如果出现更快的语言特定解决方案(或使用awk等等),我也会对此持开放态度。
目前我们在内存中这样做,但随着时间的推移,文件超出了我们的内存容量。显然,“买一台更大的机器”是一种解决方案,但目前还不在卡片中。
答案 0 :(得分:1)
以下内容可能适合您。此代码首先以BufferedReader
打开源文件,然后读取第一行并将其与\t
分开。
结果数组的长度是目标文件的行数。创建了一个新的FileHolder
对象数组,其中FileHolder
基本上包含文件描述符和ByteBuffer
以用作缓存(以便不写入每个单词)。创建所有持有者后,将写入第一行。
然后再次读取源文件,逐行拆分,直到空,并附加所有文件夹。
完成后,将创建目标文件(最后),并按数组顺序将所有FileHolder实例写入其中,因此按行顺序。
以下是示例代码(LONG,也可用here)。它当然可以得到改善(资源并没有真正关闭在正确的地方等),但它的工作原理。它在大约25秒内转换275 MB文件(四核Q6600,4 GB RAM,x86_64 Linux 3.1.2-rc5),并以“脆弱”默认值64 MB Sun(64位)JDK运行:
package net.sf.jpam;
import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.io.Reader;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.util.regex.Pattern;
public final class Test
{
private static final Pattern TAB = Pattern.compile("\t");
private static class FileHolder
{
private static final byte TABCHAR[] = "\t".getBytes();
// Size of the buffer size
private static final int BUFSZ = 32768;
// Format string for a file
private static final String FORMAT = "/home/fge/t2.txt.%d";
// The ByteBuffer
private final ByteBuffer buf = ByteBuffer.allocate(BUFSZ);
// The File object
private final File fd;
// RandomAccessFile
private final RandomAccessFile file;
FileHolder(final int index)
throws FileNotFoundException
{
final String name = String.format(FORMAT, index);
fd = new File(name);
file = new RandomAccessFile(fd, "rw");
}
public void write(final String s)
throws IOException
{
final byte[] b = s.getBytes();
if (buf.remaining() < b.length + TABCHAR.length)
flush();
buf.put(b).put(TABCHAR);
}
private void flush()
throws IOException
{
file.write(buf.array(), 0, buf.position());
buf.position(0);
}
public void copyTo(final RandomAccessFile dst)
throws IOException
{
flush();
final FileChannel source = file.getChannel();
final FileChannel destination = dst.getChannel();
source.force(false);
final long len = source.size() - TABCHAR.length;
source.transferTo(0, len, destination);
dst.writeBytes("\n");
}
public void tearDown()
throws IOException
{
file.close();
if (!fd.delete())
System.err.println("Failed to remove file " + fd);
}
@Override
public String toString()
{
return fd.toString();
}
}
public static void main(final String... args)
throws IOException
{
long before, after;
before = System.currentTimeMillis();
final Reader r = new FileReader("/home/fge/t.txt");
final BufferedReader reader = new BufferedReader(r);
/*
* Read first line, count the number of elements. All elements are
* separated by a single tab.
*/
String line = reader.readLine();
String[] elements = TAB.split(line);
final int nrLines = elements.length;
final FileHolder[] files = new FileHolder[nrLines];
/*
* Initialize file descriptors
*/
for (int i = 0; i < nrLines; i++)
files[i] = new FileHolder(i);
/*
* Write first lines, then all others
*/
writeOneLine(elements, files);
while ((line = reader.readLine()) != null) {
elements = TAB.split(line);
writeOneLine(elements, files);
}
reader.close();
r.close();
after = System.currentTimeMillis();
System.out.println("Read time: " + (after - before));
before = System.currentTimeMillis();
final RandomAccessFile out = new RandomAccessFile("/home/fge/t2.txt",
"rw");
for (final FileHolder file: files) {
file.copyTo(out);
file.tearDown();
}
out.getChannel().force(false);
out.close();
after = System.currentTimeMillis();
System.out.println("Write time: " + (after - before));
System.exit(0);
}
private static void writeOneLine(final String[] elements,
final FileHolder[] fdArray)
throws IOException
{
final int len = elements.length;
String element;
FileHolder file;
for (int index = 0; index < len; index++) {
element = elements[index];
file = fdArray[index];
file.write(element);
}
}
}
答案 1 :(得分:0)
@fge: 1)最好使用CharBuffer而不是实例化很多字符串。
2)最好像这样使用模式匹配:
initially..
private Matcher matcher;
Pattern regexPattern = Pattern.compile( pattern );
matcher = regexPattern.matcher("");
and then for matching pattern.. you do this..
matcher.reset(charBuffer).find()
因为,当你看到里面时
Pattern.matcher(CharSequence input) {
Matcher m = new Matcher(this, input);
}
始终避免编写导致大量实例化或字符串使用的代码。这会导致大量内存使用,从而导致性能下降。