StringBuilder OOMs earlier with JRE 11 compared to JRE 8

[David Holmes]

Hi Simeon,

Redirecting this to core-libs-dev as it is not a serviceability issue. 
(bcc serviceabillity-dev)

Thanks,
David

On 27/08/2021 8:53 pm, S A wrote:
> Hi all,
> 
> while working on https://bugs.eclipse.org/bugs/show_bug.cgi?id=575641 
> <https://bugs.eclipse.org/bugs/show_bug.cgi?id=575641>, I noticed that 
> StringBuilder throws an OOM "half as early" in OpenJDK 11 (and 17 early 
> access), when compared to OpenJDK 8.
> 
> In particular, I ran the following snippet to see where the limit of 
> appending to a StringBuilder is:
> 
> StringBuilder sb = new StringBuilder();
> long n = 8L * 1024L * 1024L;
> for (long i = 0; i < n; ++i) {
> int m = 1024;
> for (int j = 0; j < m; ++j) {
> int length = sb.length();
> try {
> sb.append((char) j);
> } catch (Error e) {
> System.out.println("Error occurred at length=" + length + " [i=" + i + 
> ", j=" + j + "]");
> throw e;
> }
> 
> }
> }
> 
> JRE 8:
> 
> Error occurred at length=2147483645 [i=2097151, j=1021]
> Exception in thread "main" java.lang.OutOfMemoryError: Requested array 
> size exceeds VM limit
> at java.util.Arrays.copyOf(Arrays.java:3332)
> at 
> java.lang.AbstractStringBuilder.ensureCapacityInternal(AbstractStringBuilder.java:124)
> at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:649)
> at java.lang.StringBuilder.append(StringBuilder.java:202)
> at 
> test.stringbuilder.TestStringbuilderOOM.main(TestStringbuilderOOM.java:13)
> 
> JRE 11:
> 
> Error occurred at length=1073741822 [i=1048575, j=1022]
> Exception in thread "main" java.lang.OutOfMemoryError: Requested array 
> size exceeds VM limit
> at java.base/java.util.Arrays.copyOf(Arrays.java:3745)
> at 
> java.base/java.lang.AbstractStringBuilder.ensureCapacityInternal(AbstractStringBuilder.java:172)
> at 
> java.base/java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:748)
> at java.base/java.lang.StringBuilder.append(StringBuilder.java:241)
> at 
> TestJava11/test.stringbuilder.TestStringbuilderOOM.main(TestStringbuilderOOM.java:13)
> 
> While StringBuilder is not a good choice for holding GBs of text, I 
> wonder that no effort is made to clamp the capacity to its limit when (I 
> assume) it is being doubled upon array expansion.
> 
> Is this something that should be looked into or it can be safely ignored 
> (from JDK users POV)?
> 
> Best regards,
> Simeon