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This compliant solution uses a local variable to reduce the number of unsynchronized reads of the helper field to 1. As a result, if the read of helper yields a non-null value, it is cached in a local variable that is inaccessable to other threads, and safely returned.
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public final class Helper {
private final int n;
public Helper(int n) {
this.n = n;
}
// Other fields and methods, all fields are final
}
final class Foo {
private Helper helper = null;
public Helper getHelper() {
Helper h = helper; // only one racyunsynchronized read of helper
if (h == null) {
synchronized (this) {
h = helper; // in synchronized block, so this is safe
if (h == null) {
h = new Helper(42);
helper = h;
}
}
}
return h;
}
}
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