Am 08.09.2014 um 20:53 schrieb Jonathan Gibbons:
For example, in the first few lines of the patch, I found this:
Do you see any semantics change here?
diff -r dde9f5cfde5f
src/share/classes/sun/tools/jconsole/inspector/XArrayDataViewer.java
--- a/src/share/classes/sun/tools/jconsole/inspector/XArrayDataViewer.java Tue Aug 05 19:29:00
2014 -0700
+++ b/src/share/classes/sun/tools/jconsole/inspector/XArrayDataViewer.java Sun Aug 10 18:02:01
2014 -0300
@@ -79,25 +79,24 @@
String textColor = String.format("%06x",
foreground.getRGB() & 0xFFFFFF);
StringBuilder sb = new StringBuilder();
- sb.append("<html><body text=#"+textColor+"><table
width=\"100%\">");
+ sb.append("<html><body text=#").append(textColor).append("><table
width=\"100%\">");
for (int i = 0; i < arr.length; i++) {
if (i % 2 == 0) {
- sb.append("<tr style=\"background-color: " +
- evenRowColorStr + "\"><td><pre>" +
- (arr[i] == null ?
- arr[i] : htmlize(arr[i].toString())) +
- "</pre></td></tr>");
+ sb.append("<tr style=\"background-color: ")
+ .append(evenRowColorStr).append("\"><td><pre>")
+ .append(arr[i] == null ?
+ arr[i] : htmlize(arr[i].toString()))
+ .append("</pre></td></tr>");
In this special case javac optimizer could first interpret as (as same as we could help it by just
coding like that):
StringBuilder sb = new StringBuilder(
"<html><body text=#"+textColor+"><table width=\"100%\">");
so no need for an additional implicit StringBuilder. Additionally javac could guess a reasonable
capacity first.
Analyzing the flow to determine it is safe to mutate sb in the face of possible exceptions coming
out of methods like htmlize is more than it would be reasonable to do in javac. For example, what
if the for loop were in a try block and the try block referenced sb?
Good example where such optimization would fail. But:
- manual optimization would fail either
- try catch block anyway would have it's own performance cost, maybe more than the additional
StringBuilder.
Maybe it's not such complicated as we guess to find out if the affected StringBuilder becomes
reliably unreachable in case of exception, and therefore securely could be "mutated".
Also, consider the serviceability implications, if a user is stepping through the code with a
debugger.
Good point. But I think, we are already used with this when stepping through simple String
concatenation or wondering on a StringBuilder in exception stack trace, even there is no
StringBuilder in our code.
-Ulf
