java并发J.U.C AtomicReference VS Volatile
2024-08-27 18:39:19
SpinLock实现,摘自并发编程网
package com.juc.simple;
import java.util.concurrent.atomic.AtomicReference;
/**
* a implement of spinlock based on cas
*
*/
public class SpinLock {
private AtomicReference<Thread> sign = new AtomicReference<>();
/**
* 让当前线程不停地的在循环体内执
* 当循环的条件被其他线程改变时 才能进入临界区
*/
public void lock() {
Thread current = Thread.currentThread();
//自旋,如果sign为null才能设置为current or memcachedb: checkandset
while (!sign.compareAndSet(null, current)) {
}
}
public void unlock() {
Thread current = Thread.currentThread();
sign.compareAndSet(current, null);
}
}
这里是通过 AtomicReference实现的，而AtomicReference是通过volatile与Unsafe的CAS实现
volatile保证可见性，所有的线程看到的都是最新（主内存？）的值,但是volatile的操作不具有原子性，这样像（a++)这种操作就不能保证安全了，实际是读--修改--写操作的过程
AtomicReference的CAS操作更新值具有原子性
先来一段AtomicReference与volatile的比较
package com.juc.simple;
import java.util.concurrent.atomic.AtomicReference;
public class VolatileVsAtomicReference {
private static volatile Integer volatileVar = 0;
//初始值value为0
private static AtomicReference<Integer> atomicReference=new AtomicReference<Integer>(volatileVar);
public static void main(String[] args) {
try {
testAtomicReference();
testVolatile();
} catch (InterruptedException e) {
System.out.println(e.getMessage());
}
}
public static void testAtomicReference() throws InterruptedException{
for (int i = 0; i < 1000; i++) {
new Thread(new Runnable(){
@Override
public void run() {
for (int i = 0; i < 1000; i++)
while(true){
Integer temp=atomicReference.get();
if(atomicReference.compareAndSet(temp, temp+1)){
break;
}
}
}
}).start();
}
Thread.sleep(1000);
System.out.println("atomicReference "+atomicReference.get()); //1000000
}
public static void testVolatile() throws InterruptedException{
for (int i = 0; i < 1000; i++) {
new Thread(new Runnable(){
@Override
public void run() {
for (int i = 0; i < 1000; i++) {
volatileVar=volatileVar++;
}
}
}).start();
}
Thread.sleep(1000);
System.out.println("volatileVar "+volatileVar); //may 8545
}
}
结果：试过好几次volatileVar值都是不一样的
atomicReference 1000000
volatileVar 8545
这里可以看出如果用了CAS的话AtomicReference是可以保证原子性的，但如果只是简单的get()和set()方法是不行的，其实就等同于volatile，因为这货本身就是volatile实现的
参考
Java volatile reference vs. AtomicReference
get has the memory effects of reading a volatile variable.
set has the memory effects of writing (assigning) a volatile variable.
AtomicReference的原代码实现
package java.util.concurrent.atomic;
import sun.misc.Unsafe;
/**
* An object reference that may be updated atomically. See the {@link
* java.util.concurrent.atomic} package specification for description
* of the properties of atomic variables.
* @since 1.5
* @author Doug Lea
* @param <V> The type of object referred to by this reference
*/
public class AtomicReference<V> implements java.io.Serializable {
private static final long serialVersionUID = -1848883965231344442L;
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long valueOffset;
static {
try {
valueOffset = unsafe.objectFieldOffset
(AtomicReference.class.getDeclaredField("value"));
} catch (Exception ex) { throw new Error(ex); }
}
//volatile 变量
private volatile V value;
/**
* Creates a new AtomicReference with the given initial value.
*
* @param initialValue the initial value
*/
public AtomicReference(V initialValue) {
value = initialValue;
}
/**
* Creates a new AtomicReference with null initial value.
*/
public AtomicReference() {
}
/**
* Gets the current value.
*
* @return the current value
*/
public final V get() {
return value;
}
/**
* Sets to the given value.
*
* @param newValue the new value
*/
//volatile变量赋值
public final void set(V newValue) {
value = newValue;
}
/**
* Eventually sets to the given value.
*
* @param newValue the new value
* @since 1.6
*/
//设置为新的值
public final void lazySet(V newValue) {
unsafe.putOrderedObject(this, valueOffset, newValue);
}
/**
* Atomically sets the value to the given updated value
* if the current value {@code ==} the expected value.
* @param expect the expected value
* @param update the new value
* @return true if successful. False return indicates that
* the actual value was not equal to the expected value.
*/
//如果当前值与期望值一致，则设置为新的值
public final boolean compareAndSet(V expect, V update) {
return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
}
/**
* Atomically sets the value to the given updated value
* if the current value {@code ==} the expected value.
*
* <p>May <a href="package-summary.html#Spurious">fail spuriously</a>
* and does not provide ordering guarantees, so is only rarely an
* appropriate alternative to {@code compareAndSet}.
*
* @param expect the expected value
* @param update the new value
* @return true if successful.
*/
public final boolean weakCompareAndSet(V expect, V update) {
return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
}
/**
* Atomically sets to the given value and returns the old value.
*
* @param newValue the new value
* @return the previous value
*/
//以原子方式设置为新的值，并且返回旧值
public final V getAndSet(V newValue) {
while (true) {
V x = get();
if (compareAndSet(x, newValue))
return x;
}
}
/**
* Returns the String representation of the current value.
* @return the String representation of the current value.
*/
public String toString() {
return String.valueOf(get());
}
}
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