StringBuilder setLength（0）处出现奇怪的OutOfMemoryError

Question

It seems no one has reported similar situations... I have absolutely no idea what happened...

I have a StringBuilder of size 8Mb to process a large String result . I am trying to reuse the StringBuilder . I thought setting length = 0 will just reset the counter and not allocate new memory?

try {
      //result.length() around 4Mb
        StringBuilder sBuilder = new StringBuilder(result.length());
        result = DoSomethingToResult1(sBuilder, result); //shrink result a bit using replaceAll

        try {

            sssBuilder.setLength(0);
            result = DoSomethingToResult2(sBuilder, result); //shrink result further using replaceAll
        } catch (OutOfMemoryError e) {
             Log.d(TAG, "Out of Memory on 2");
        }

        try {
            sBuilder.setLength(0);  //OutOfMemory thrown here.          
            result = DoSomethingToResult3(sBuilder, result); //shrink result even further using replaceAll
        } catch (OutOfMemoryError e) {
             Log.d(TAG, "Out of Memory on 3");
        }

    } catch (OutOfMemoryError e) {
                Log.d(TAG, "Cannot create sBuilder");
    }

The process usually dies on the second setLength(0) , sometimes dies on the first setLength(0) , but it can always create the sBuilder in the beginning.

In DoSomethingToResult , I split result into chunk of 100Kb size and append to sBuilder one by one, returning sBuilder.toString(). So problem does not come from replaceAll . And since it passed the first process, I don't think the toString() is the problem either.

I tried:

sBuilder.setLength(0);
System.gc();
try{
    Thread.sleep(1000);
}catch(Exception e){};

or

System.gc();
try{
    Thread.sleep(1000);
}catch(Exception e){};
sBuilder.setLength(0);

Both failed.

Log output:

E/dalvikvm-heap(27130): Out of memory on a 8723792-byte allocation.
I/dalvikvm(27130): "AsyncTask #1" prio=5 tid=11 RUNNABLE
I/dalvikvm(27130):   | group="main" sCount=0 dsCount=0 obj=0x42036a18 self=0x51c2eb08
I/dalvikvm(27130):   | sysTid=27151 nice=10 sched=0/0 cgrp=apps/bg_non_interactive handle=1371731208
I/dalvikvm(27130):   | schedstat=( 0 0 0 ) utm=197 stm=17 core=1
I/dalvikvm(27130):   at java.lang.AbstractStringBuilder.setLength(AbstractStringBuilder.java:~567)
I/dalvikvm(27130):   at java.lang.StringBuilder.setLength(StringBuilder.java:44)

Answer 1

Setting length to 0 doesn't automatically invoke the GC to kick in. What you can do is have a loop and call System.gc, then wait for a while until available memory increases. Then break from the loop and continue with your next process.

You can also try allocating a new builder rather than clearing the buffer in each iteration.