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| Code Block | ||
|---|---|---|
| ||
// Works with acquire/release semantics for volatile
// Broken under JDK 1.4 and earlier
class Foo {
private volatile Helper helper = null;
public Helper getHelper() {
if (helper == null) {
synchronized(this) {
if (helper == null) {
helper = new Helper();
}
}
}
return helper;
}
}
|
Exceptions
EX1: Explicitly synchronized code does not require the use of double-checked locking.
| Wiki Markup |
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*EX2:* "Although the double-checked locking idiom cannot be used for references to objects, it can work for 32-bit primitive values (e.g., int's or float's). Note that it does not work for long's or double's, since unsynchronized reads/writes of 64-bit primitives are not guaranteed to be atomic." \[[Pugh 04|AA. Java References#Pugh 04]\] |
Risk Assessment
Using incorrect forms of the double checked locking idiom can lead to synchronization issues.
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