Comments on Venish Joe: Java String Concatenation and Performance

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2011-10-11T22:52:31.626-04:00

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2011-10-02T04:54:00.686-04:00

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2011-09-29T16:49:11.700-04:00

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2011-09-29T02:08:49.766-04:00

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2011-09-25T22:45:09.049-04:00

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2011-08-28T14:01:56.441-04:00

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2011-08-28T14:01:01.067-04:00

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2011-08-25T07:37:15.750-04:00

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There are certainly a lot of details like that to ...

2011-08-25T07:30:12.064-04:00

There are certainly a lot of details like that to take into consideration. That’s a great point to bring up.

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2011-08-11T11:11:17.642-04:00

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2011-06-06T03:56:46.440-04:00

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Thanks for this post. I wasn't aware the diffe...

2011-05-17T07:15:53.557-04:00

Thanks for this post. I wasn't aware the difference between (+), concat() and using StringBuffer/StringBuilder was so big. I have seen a dramatic improvement in performance in my app (outputing HTML). I thought it was I/O related, but it had everything to do with naively concatening Strings with (+) in loops.

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2011-03-14T08:52:09.302-04:00

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I don't think this is a fair analysis of the +...

2009-12-18T11:13:43.899-05:00

I don't think this is a fair analysis of the + operator. Performing this inside a loop will obviously result in allocating a new String on every iteration. As some of the previous commentors indicated, this is not seen often in practice. What I do most often is generate log reports using a variety of known data as follows:

String rep = "Name: " + A.getName() +
+ "\nDescription: " + A.getDesc() +
+ "\nSize: " + A.getSize() +
+ "\nStatus: " + A.generateStatus();

Thus, I basically have the case of;

String rep = A + B + C + D + E + F + G + H;

Now, there are a number of ways that the compiler can behave. The way I've always thought it behaved was as follows:
Allocate String for A+B
Allocate String for (A+B)+C
Allocate String for (A+B+C)+D
...
Allocate String for (A+B+C+D+E+F+G)+H

Thus, there are 7 Strings allocated for the purposes of concatenating 8 strings.

There are a few obvious optimizations that the compiler could make:

Automatically concatenate adjacent constant Strings. This would be known at compile time. For example,

String res = "A" + "B" + C.toString();

could be optimized by the compiler to

String res = "AB" + C.toString();

I would be surprised if the compiler doesn't already do this.

Another optimization it could do is get the total length of all the Strings in the concat list and allocate a single String to hold them all. I don't know if the compiler can do this, but if it did, then I would say that for most uses, the + operator is optimal.

You should always specify either -client or -serve...

2009-11-12T01:44:12.882-05:00

You should always specify either -client or -server as parameter or at least check with -version what will be the default for your setup. Otherwise you'll never know what compiler optimizations will be used. Basically all Java benchmarks are useless without knowing what -client/-server flag was used. With -server depending on the test one can basically see some code to be "removed" or it can make the difference between and endless loop and ending loop (try threading with volatile)...

For the sake of just looking good. Compare the te...

2009-11-11T18:53:36.884-05:00

For the sake of just looking good. Compare the test above with just a print("*"); and at the end print("\n"); and see if it's just plain faster. I mean the code above looks cool, however, is it faster.
I mean the object above can do a lot more, but, I just want a fast printout. And any memory leaks?

Rob, we revamped the Jmol code a couple of years a...

2009-11-11T08:02:28.409-05:00

Rob, we revamped the Jmol code a couple of years ago and saw significant gains in processing speed, though we didn't benchmark it. This is obviously a worst-case scenario, but it's real. The worst case appears to be:

[loop]
x+= xxxx;
x+= xxxx;
x+= xxxx;

[/loop]

A prelude to the upcoming blog entry - string conc...

2009-11-11T07:42:54.862-05:00

A prelude to the upcoming blog entry - string concatenation meters.

http://blog.jinspired.com/?p=832

At the end of the day this whole benchmark is poin...

2009-11-10T08:55:46.102-05:00

At the end of the day this whole benchmark is pointless as no one code in an real-world executes like any of the scenarios tested above. The most common case is someone creating a string (or buffer or builder) and performing a few appends or inserts. This is what should be tested if one really has too much "waste" time with oneself. But then you would be much better off benchmarking the creation and copying of char[] arrays which is what is effectively happening underneath the thin service veneer of these classes.

What is much more useful to note in an application is how many times one allocs/inserts/appends/resizes/tostrings builders and buffers in the course of an execution which is the point of my upcoming blog entry.

Switching positions can make a difference, because...

2009-11-10T08:37:35.435-05:00

Switching positions can make a difference, because the JIT-compiler needs some warmup. When JIT kicks in with runtime optimizations, the numbers change dramatically.

"What IS an example of commonly written code ...

2009-11-10T03:17:33.605-05:00

"What IS an example of commonly written code that involves concatenating large numbers of Strings in a loop? I can't think of any time that situation turned up in my 12 years of Java development."

I am actually writing a blog post today about this very issue and you be very surprised how common it is once you filter out a lot of noise in the execution of a web request. Does one need to time it? Absolutely NOT. In fact the customer case that serves as the basis for this blog entry used StringBuilder for the most part. The problem with String(Buffers/Builders) is that they are used wrongly as generic output queues. I will post the link later when the article is finished.

Is there a point to the above comparisons. NO.

There have been so very many articles and blog pos...

2009-11-09T19:12:29.461-05:00

There have been so very many articles and blog posts about the "pitfalls" of string concatenation in Java, and I'm still waiting for the one that starts with "there was a performance hotspot in our code that we started researching...".

It's all entirely academic unless it solves an *actual* performance problem in real-live code, and it results in a lot of bad code by new developers who waste their time rewriting working code to "clean up all that unoptimal string concatenation"... accomplishing nothing but making code less readable and introducing bugs.

What IS an example of commonly written code that involves concatenating large numbers of Strings in a loop? I can't think of any time that situation turned up in my 12 years of Java development.

What I *have* seen is lots of ugly code where people are jumping through hoops to avoid using "+" when, for example, generating one line from 3 fields "city, state zip" -- an utterly useless optimization -- so that after a while I kinda get pissed off seeing this optimization pop up again in another post/article/etc.. Any discussion of an optimization needs to cover when to use it (with real-life examples...), because there are always times they should not be used, often for reasons of readability/maintainability.

@Anonymous I ran the tests more than 5 times with...

2009-11-09T12:55:23.855-05:00

@Anonymous

I ran the tests more than 5 times with default run-time parameters and the difference in the standard deviation was almost negligible. I am not sure if running in a EA JRE 7 made any difference, probably I will try this in JRE6 in a 32bit machine and see if it makes any difference.

@Anonymous

Switching the positions didn't make any difference, I don't see why that would make any difference. Java compiler will optimize the (+) statements as you mentioned as long as they are in the same statement, but in a loop there wouldn't be any big optimization by compiler.

Please change the positions of the + and concat te...

2009-11-09T11:54:38.775-05:00

Please change the positions of the + and concat tests. What do you get?

Using concat instead of + isn't a good idea, as the java compiler will optimize the + statements where possible.

String myVar = "abcdef";
String myVar = "a"+"b"+"c"+"d"+"e"+"f";
Those statement will be exactly the same at bytecode level.

You say that the results are conclusive and expect...

2009-11-09T07:57:42.762-05:00

You say that the results are conclusive and expected. I would say that the difference between "+" and StringBuilder is unexpected and consequently the results are far from conclusive. The std dev. on all runs are way too high which suggest that there are interferences in your benchmarks. Also there is no warmup in the code.

One thing to note, StringBuilder and StringBuffer all delegate their calls to a common abstract super class. The only difference is synchronization. Synchronization behaves very differently on single core and multi-core and is also very sensitive to the version of the VM you are using as well as any command line settings. IOWs, it is very easy to make StringBuffer worse than any of the numbers you've published here.