https://issues.apache.org/bugzilla/show_bug.cgi?id=55609
Bug ID: 55609
Summary: c:forEach loop on integer range consumes unnecessary
heap space in proportion to value for "end"
Product: Taglibs
Version: unspecified
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P2
Component: Standard Taglib
Assignee: dev@tomcat.apache.org
Reporter: supp...@aoindustries.com
The implementation of iteration for a forEach tag over an integer range builds
an array of all the Integer objects in the range, and then obtains an iterator
via Arrays.asList(...).iterator(). The unnecessarily consumes a large amount
of heap space when the value of "end" is large.
The method in this comment about performance is basically correct from a *time*
perspective, but completely ignores *space* considerations.
The patch to iterate over an integer range without undue heap space follows.
For JSTL 1.1.1 in
src/org/apache/taglibs/standard/tag/common/core/ForEachSupport.java:
// Begin added by AO Industries, Inc.
import java.util.NoSuchElementException;
// End added by AO Industries, Inc.
private ForEachIterator beginEndForEachIterator() {
// Begin removed by AO Industries, Inc.
// /*
// * To plug into existing support, we need to keep 'begin', 'end',
// * and 'step' as they are. So we'll simply create an Integer[]
// * from 0 to 'end', inclusive, and let the existing implementation
// * handle the subsetting and stepping operations. (Other than
// * localizing the cost of creating this Integer[] to the start
// * of the operation instead of spreading it out over the lifetime
// * of the iteration, this implementation isn't worse than one that
// * created new Integers() as needed from next(). Such an adapter
// * to ForEachIterator could easily be written but, like I said,
// * wouldn't provide much benefit.)
// */
// Integer[] ia = new Integer[end + 1];
// for (int i = 0; i <= end; i++)
// ia[i] = new Integer(i);
// return new SimpleForEachIterator(Arrays.asList(ia).iterator());
// End removed by AO Industries, Inc.
// Begin added by AO Industries, Inc.
// This implementation does not require heap space in proportion to the
number of iterations.
return new SimpleForEachIterator(
new Iterator() {
private final int end = ForEachSupport.this.end;
private int next = 0;
public boolean hasNext() {
return next <= end;
}
public Object next() {
if(next > end) throw new NoSuchElementException();
return Integer.valueOf(next++);
}
public void remove() {
throw new UnsupportedOperationException();
}
}
);
// End added by AO Industries, Inc.
}
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