package st.gravel.support.jvm.runtime;
import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.lang.invoke.MutableCallSite;
import java.lang.invoke.SwitchPoint;
public class AlmostFinalValue {
private final AlmostFinalCallSite callsite =
new AlmostFinalCallSite(this);
protected Object initialValue() {
return null;
}
public MethodHandle createGetter() {
return callsite.dynamicInvoker();
}
public Object setValue(Object value) {
callsite.setValue(value);
return value;
}
private static class AlmostFinalCallSite extends MutableCallSite {
private Object value;
private SwitchPoint switchPoint;
private final AlmostFinalValue volatileFinalValue;
private final MethodHandle fallback;
private final Object lock;
private static final Object NONE = new Object();
private static final MethodHandle FALLBACK;
static {
try {
FALLBACK = MethodHandles.lookup().findVirtual(AlmostFinalCallSite.class, "fallback",
MethodType.methodType(Object.class));
} catch (NoSuchMethodException|IllegalAccessException e) {
throw new AssertionError(e.getMessage(), e);
}
}
AlmostFinalCallSite(AlmostFinalValue volatileFinalValue) {
super(MethodType.methodType(Object.class));
Object lock = new Object();
MethodHandle fallback = FALLBACK.bindTo(this);
synchronized(lock) {
value = NONE;
switchPoint = new SwitchPoint();
setTarget(fallback);
}
this.volatileFinalValue = volatileFinalValue;
this.lock = lock;
this.fallback = fallback;
}
Object fallback() {
synchronized(lock) {
Object value = this.value;
if (value == NONE) {
value = volatileFinalValue.initialValue();
}
MethodHandle target = switchPoint.guardWithTest(MethodHandles.constant(Object.class, value), fallback);
setTarget(target);
return value;
}
}
void setValue(Object value) {
synchronized(lock) {
SwitchPoint switchPoint = this.switchPoint;
this.value = value;
this.switchPoint = new SwitchPoint();
SwitchPoint.invalidateAll(new SwitchPoint[] {switchPoint});
}
}
}
}