Created
February 18, 2018 21:32
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How to create a constant for the VM (JIT) that can change
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package fr.umlv.inside.almostconst; | |
import java.util.function.Supplier; | |
public class Main { | |
private static final MostlyConstant<String> FOO = new MostlyConstant<>("HELL"); | |
private static final Supplier<String> FOO_GETTER = FOO.getter(); | |
private static int test(int max, int change) { | |
int sum = 0; | |
for(int i = 0; i < max; i++) { | |
if (i == change) { | |
FOO.set("BAR"); | |
} | |
sum += FOO_GETTER.get().length(); | |
} | |
return sum; | |
} | |
private static int SINK; | |
public static void main(String[] args) { | |
SINK += test(10, 5); | |
SINK += test(10, 5); | |
SINK += test(10, 5); | |
FOO.set("HELL"); | |
System.out.println(test(10_000_000, 5_000_000)); | |
} | |
} |
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package fr.umlv.inside.almostconst; | |
import static java.lang.invoke.MethodHandles.constant; | |
import java.lang.invoke.MethodHandle; | |
import java.lang.invoke.MutableCallSite; | |
import java.lang.reflect.UndeclaredThrowableException; | |
import java.util.function.Supplier; | |
public class MostlyConstant<T> { | |
private final MutableCallSite callSite; | |
private final MethodHandle invoker; | |
public MostlyConstant(T constant) { | |
MutableCallSite callSite = new MutableCallSite(constant(Object.class, constant)); | |
this.callSite = callSite; | |
this.invoker = callSite.dynamicInvoker(); | |
} | |
public void set(T constant) { | |
callSite.setTarget(constant(Object.class, constant)); | |
} | |
public Supplier<T> getter() { | |
MethodHandle invoker = this.invoker; | |
return () -> { | |
try { | |
return (T)invoker.invokeExact(); | |
} catch (Throwable e) { | |
throw rethrow(e); | |
} | |
}; | |
} | |
static UndeclaredThrowableException rethrow(Throwable t) { | |
if (t instanceof RuntimeException) { | |
throw (RuntimeException)t; | |
} | |
if (t instanceof Error) { | |
throw (Error)t; | |
} | |
return new UndeclaredThrowableException(t); | |
} | |
} |
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