Package java.lang.invoke

Class VarHandle

java.lang.Object

java.lang.invoke.VarHandle

public class VarHandle
extends java.lang.Object

A stub for the VarHandle class.
This stub is used to allow Java 8 release compilation to work as expected. The sole limit of this stub is that since java.lang.invoke is a privileged package it cannot be used at runtime (e.g. loaded by a classloader).

For example, if SomeClass is loaded at runtime:

     class SomeClass {
          private static final VarHandle VH = // ... obtained somehow;

          public static void storeStoreFence() {
              if (VH == null) {
                  return;
              }
              VH.storeStoreFence();
          }
     }
 

this is not going to work on Java 8.

To fix it is possible to use an holder class (which won't be loaded at runtime):

     class SomeClass {
          static class Holder {
              private static final VarHandle VH = // ... obtained somehow;
          }

          public static void storeStoreFence() {
              VarHandle vh = Holder.VH;
              if (vh == null) {
                  return;
              }
              vh.storeStoreFence();
          }
     }
 

Or:

     class SomeClass {
          private static final Object VH = // ... obtained somehow;

          public static void storeStoreFence() {
              if (VH == null) {
                  return;
              }
              ((VarHandle)VH).storeStoreFence();
          }
     }
 

The reason why the methods on the stub are declared as native is to allow MethodHandle.PolymorphicSignature to work as expected, see JLS 15.12.3:

   A method is signature polymorphic if all of the following are true:
   - It is declared in the java.lang.invoke.MethodHandle class or the java.lang.invoke.VarHandle class.
   - It has a single variable arity parameter (§8.4.1) whose declared type is Object[].
   - It is native.
 

This seems counter-intuitive since this stub is not going to be used at runtime, but it is required to allow Java 8 compilation to produce VarHandle's method invocations with parameters and result types with the types of the formal ones of the compile-time declaration.

Constructor Summary

Constructors

Constructor	Description
VarHandle()

Method Summary

Modifier and Type	Method	Description
boolean	compareAndSet(java.lang.Object... args)
java.lang.Object	get(java.lang.Object... args)
java.lang.Object	getAcquire(java.lang.Object... args)
java.lang.Object	getAndAdd(java.lang.Object... args)
void	set(java.lang.Object... args)
void	setRelease(java.lang.Object... args)
static void	storeStoreFence()

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

VarHandle

public VarHandle()

Method Detail

get

public java.lang.Object get(java.lang.Object... args)

getAcquire

public java.lang.Object getAcquire(java.lang.Object... args)

set

public void set(java.lang.Object... args)

setRelease

public void setRelease(java.lang.Object... args)

getAndAdd

public java.lang.Object getAndAdd(java.lang.Object... args)

compareAndSet

public boolean compareAndSet(java.lang.Object... args)

storeStoreFence

public static void storeStoreFence()