yes, puffinman . that works.

but i have trouble there if i do that way.

i have writen a java code to read the file, but sometimes it is producing result and sometimes it is not producing result.

my java code follows the following logic

1) It locks the log file wth FileLock.
2)it reads the file.
3)prints the file.
4)unlocks the file.

but when i run this java code sometimes i get the file output sometimes i dont get the output.

so, at some point of time i may not get the output from my java code and for that reason my whole code would be erroneous .

the following observation i have noticed when i run your code.

i saw the log fle size is changing form 0 to 5.8 MB very rapidly (because of overwrite ?).

may be when my java code is reading the log file the file size is 0 and as a result java code is not outputting anything.

but somehow, i have to recover this problem. i need the CPU data correctly whenever i need it.

So your original question wasn't really "I need to write the output of top to a file", it was rather "I need to get the output of top into my java program." In fact, writing it to a file is not the optimal solution (for reasons you are describing). Opening a child top process from Java and piping its output directly to your running program is more along the lines of what you want. I am sure it is possible in Java but I could not tell you how.

In general you should ask what you are really trying to do rather than only asking about what you think the answer is.

And by the way, a 5.8 MB top listing?? How many processes are running on your computer??

Excellent advice.
Read the data directly from /proc - at whatever frequency you decide upon; just like top probably does. This is a pseudo filesystem - for simplicity, you can assume the data only exists when you request it.

Puffman, thank you for your answer. It's really useful.
Perl rocks.