High CPU due to heavy use of String.replaceAll

Environment

• JBoss Enterprise Application Platform (EAP)

Issue

• We see high CPU usage on our JBoss server and CPU/thread dump data shows that the CPU is consumed by several threads in calls to String.replaceAll(), for example:

"ajp-127.0.0.1-8009-1" daemon prio=10 tid=0x00000000476fd000 nid=0xa73 runnable [0x000000004c1ab000]
   java.lang.Thread.State: RUNNABLE
	at java.util.regex.Pattern.atom(Pattern.java:1980)
	at java.util.regex.Pattern.sequence(Pattern.java:1885)
	at java.util.regex.Pattern.expr(Pattern.java:1752)
	at java.util.regex.Pattern.compile(Pattern.java:1460)
	at java.util.regex.Pattern. (Pattern.java:1133)
	at java.util.regex.Pattern.compile(Pattern.java:823)
	at java.lang.String.replaceAll(String.java:2189)
        ...

"ajp-10.133.72.102-8009-2" daemon prio=10 tid=0x0000000046406000 nid=0xa74 runnable [0x000000004c2ac000]
   java.lang.Thread.State: RUNNABLE
	at java.lang.String.length(String.java:651)
	at java.util.regex.Matcher.getTextLength(Matcher.java:1140)
	at java.util.regex.Matcher.reset(Matcher.java:291)
	at java.util.regex.Matcher. (Matcher.java:211)
	at java.util.regex.Pattern.matcher(Pattern.java:888)
	at java.lang.String.replaceAll(String.java:2189)
        ...

Resolution

• If you're using the same regex repeatedly through replaceAll, you'd likely be better off building the Pattern/Matcher objects themselves and reusing them instead, that way they do not have to be reinitialized and compiled repeatedly by String.replaceAll().
• Matchers are not thread safe though, so if you reuse a Matcher across threads, you will likely want to synchronize access to it and call Matcher.reset() when a thread is finished with it.

Root Cause

• String.replaceAll() is very inefficient and not recommended for repeated use for the same pattern over and over. Each time replaceAll is called, a new regex Pattern is created and compiled, adding a lot of overhead each time.

Diagnostic Steps

• Troubleshoot with CPU data and other diagnostic steps as described in Java application high CPU

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