Thursday, May 08, 2008

Bad concurrency advice: interned Strings

I just read Thread Signaling from Jacob Jenkov. It is fine as far as it goes to introduce the reader to Object.wait() and Object.notify().

But it has one fatal flaw: it uses a literal java.lang.String for coordinating between threads. Why is this wrong?

Strings are interned by the compiler. To quote the javadocs: All literal strings and string-valued constant expressions are interned. Using a literal string means that any other code anywhere in the JVM, even in other libraries, which use the same String literal value all share the same object for wait() and notify(). This code:

public void dastardly() {

will wake up a thread waiting on empty string, including one in utterly unrelated code.

Don't do that. Instead, create a fresh Object for coordinating threads. This age-worn advice for lock objects (synchronize(lock)) applies just as much to objects used to coordinate threads.

Wednesday, May 07, 2008

JVM hints from Dautelle

Jean-Marie Dautelle on Realistically real-time, or getting real-time-like behavior from the non-real-time JVM.

Friday, May 02, 2008

Good News - JavaOne 2008

JPMorgan is sending me to JavaOne 2008!

Sorry for no posts since November — work has been inordinately busy. I will post from San Francisco.