Java: split text file into multiple List

Question

I'm looking for an efficient way to split a text file into a set of ArrayList. The text file is a Thread dump and I'd like to create a List for every single Thread. Every thread is separated by an empty line. For example, taken the following file:

"management-handler-thread - 66" prio=10 tid=0x00007fe960111000 nid=0x4cea waiting on condition [0x00007fe96c25c000]
   java.lang.Thread.State: TIMED_WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
        - parking to wait for  <0x00000006019cbbd0> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
        at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:226)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2082)
        at java.util.concurrent.LinkedBlockingQueue.poll(LinkedBlockingQueue.java:467)

"management-handler-thread - 65" prio=10 tid=0x00007fe968185800 nid=0x4ce9 waiting on condition [0x00007fe96c35d000]
   java.lang.Thread.State: TIMED_WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
        - parking to wait for  <0x00000006019cbbd0> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
        at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:226)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2082)
        at java.util.concurrent.LinkedBlockingQueue.poll(LinkedBlockingQueue.java:467)

The first List should contain:

"management-handler-thread - 66" prio=10 tid=0x00007fe960111000 nid=0x4cea waiting on condition [0x00007fe96c25c000]
   java.lang.Thread.State: TIMED_WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
. . .

And the second List should contain:

"management-handler-thread - 65" prio=10 tid=0x00007fe968185800 nid=0x4ce9 waiting on condition [0x00007fe96c35d000]
   java.lang.Thread.State: TIMED_WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
. . . .

What would you recommend to do that ? (Iteration, Regular expression ..) Thanks

Iterate over lines, insert depending on read data to proper list — Robert Wadowski
– Robert Wadowski, Commented Jul 4, 2015 at 20:51
How big is your input file? You most likely don't want to load a big log file into memory at once but use streams. — Sebastian S
– Sebastian S, Commented Jul 4, 2015 at 20:52
Why do you want to write the result into ArrayLists? You could also write them into separate files directly. Use a BufferedReader for the input and PrintWriters for the output files and each time you encounter an empty line you write the current entry into the appropriate file. — Tesseract
– Tesseract, Commented Jul 4, 2015 at 21:07

pobrelkey · Accepted Answer · 2015-07-04 21:06:08Z

3

No need for regular expressions, just read the file line by line...

List<List<String>> dumpedThreads = new ArrayList<>();
try (FileReader fr = new FileReader("path/to/thread-dump.txt")) {
    try (BufferedReader br = new BufferedReader(fr)) {
        List<String> thisThread = null;
        for (String line = br.readLine(); line != null; line = br.readLine()) {
            if (line.trim().length() == 0) {
                thisThread = null;
            } else {
                if (thisThread == null) {
                    thisThread = new ArrayList<>();
                    dumpedThreads.add(thisThread);
                }
                thisThread.add(line);
            }
        }
    }
}

answered Jul 4, 2015 at 21:06

pobrelkey

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1 Comment

Tesseract Over a year ago

That will only work if thread-dump.txt is sorted by thread.

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