引用类型原子类
引用类型原子类介绍
基本类型原子类只能更新一个变量，如果需要原子更新多个变量，需要使用引用类型原子类。
- AtomicReference：引用类型原子类
- AtomicStampedReference：原子更新引用类型里的字段原子类
- AtomicMarkableReference ：原子更新带有标记位的引用类型
上面三个类提供的方法几乎相同，所以这里以 AtomicReference 为例子来介绍。
AtomicReference 类使用示例
import java.util.concurrent.atomic.AtomicReference;
public class AtomicReferenceTest {
public static void main(String[] args) {
AtomicReference<Person> ar = new AtomicReference<Person>();
Person person = new Person("SnailClimb", 22);
ar.set(person);
Person updatePerson = new Person("Daisy", 20);
ar.compareAndSet(person, updatePerson);
System.out.println(ar.get().getName());
System.out.println(ar.get().getAge());
}
}
class Person {
private String name;
private int age;
public Person(String name, int age) {
super();
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
}上述代码，首先创建了一个 Person 对象，然后把 Person 对象设置进 AtomicReference 对象中，然后调用 compareAndSet() 方法，该方法就是通过通过 CAS 操作设置 ar。如果 ar 的值为 person 的话，则将其设置为 updatePerson。实现原理与 AtomicInteger 类中的 compareAndSet() 方法相同。运行上面的代码后的输出结果如下：
Daisy
20AtomicStampedReference 类使用示例
import java.util.concurrent.atomic.AtomicStampedReference;
public class AtomicStampedReferenceDemo {
public static void main(String[] args) {
// 实例化、取当前值和 stamp 值
final Integer initialRef = 0, initialStamp = 0;
final AtomicStampedReference<Integer> asr = new AtomicStampedReference<>(initialRef, initialStamp);
System.out.println("currentValue=" + asr.getReference() + ", currentStamp=" + asr.getStamp());
// compare and set
final Integer newReference = 666, newStamp = 999;
final boolean casResult = asr.compareAndSet(initialRef, newReference, initialStamp, newStamp);
System.out.println("currentValue=" + asr.getReference()
+ ", currentStamp=" + asr.getStamp()
+ ", casResult=" + casResult);
// 获取当前的值和当前的 stamp 值
int[] arr = new int[1];
final Integer currentValue = asr.get(arr);
final int currentStamp = arr[0];
System.out.println("currentValue=" + currentValue + ", currentStamp=" + currentStamp);
// 单独设置 stamp 值
final boolean attemptStampResult = asr.attemptStamp(newReference, 88);
System.out.println("currentValue=" + asr.getReference()
+ ", currentStamp=" + asr.getStamp()
+ ", attemptStampResult=" + attemptStampResult);
// 重新设置当前值和 stamp 值
asr.set(initialRef, initialStamp);
System.out.println("currentValue=" + asr.getReference() + ", currentStamp=" + asr.getStamp());
// [不推荐使用，除非搞清楚注释的意思了] weak compare and set
// 困惑！weakCompareAndSet 这个方法最终还是调用 compareAndSet 方法。[版本: jdk-8u191]
// 但是注释上写着 "May fail spuriously and does not provide ordering guarantees,
// so is only rarely an appropriate alternative to compareAndSet."
// todo 感觉有可能是 jvm 通过方法名在 native 方法里面做了转发
final boolean wCasResult = asr.weakCompareAndSet(initialRef, newReference, initialStamp, newStamp);
System.out.println("currentValue=" + asr.getReference()
+ ", currentStamp=" + asr.getStamp()
+ ", wCasResult=" + wCasResult);
}
}输出结果如下：
currentValue=0, currentStamp=0
currentValue=666, currentStamp=999, casResult=true
currentValue=666, currentStamp=999
currentValue=666, currentStamp=88, attemptStampResult=true
currentValue=0, currentStamp=0
currentValue=666, currentStamp=999, wCasResult=true4.4 AtomicMarkableReference 类使用示例
import java.util.concurrent.atomic.AtomicMarkableReference;
public class AtomicMarkableReferenceDemo {
public static void main(String[] args) {
// 实例化、取当前值和 mark 值
final Boolean initialRef = null, initialMark = false;
final AtomicMarkableReference<Boolean> amr = new AtomicMarkableReference<>(initialRef, initialMark);
System.out.println("currentValue=" + amr.getReference() + ", currentMark=" + amr.isMarked());
// compare and set
final Boolean newReference1 = true, newMark1 = true;
final boolean casResult = amr.compareAndSet(initialRef, newReference1, initialMark, newMark1);
System.out.println("currentValue=" + amr.getReference()
+ ", currentMark=" + amr.isMarked()
+ ", casResult=" + casResult);
// 获取当前的值和当前的 mark 值
boolean[] arr = new boolean[1];
final Boolean currentValue = amr.get(arr);
final boolean currentMark = arr[0];
System.out.println("currentValue=" + currentValue + ", currentMark=" + currentMark);
// 单独设置 mark 值
final boolean attemptMarkResult = amr.attemptMark(newReference1, false);
System.out.println("currentValue=" + amr.getReference()
+ ", currentMark=" + amr.isMarked()
+ ", attemptMarkResult=" + attemptMarkResult);
// 重新设置当前值和 mark 值
amr.set(initialRef, initialMark);
System.out.println("currentValue=" + amr.getReference() + ", currentMark=" + amr.isMarked());
// [不推荐使用，除非搞清楚注释的意思了] weak compare and set
// 困惑！weakCompareAndSet 这个方法最终还是调用 compareAndSet 方法。[版本: jdk-8u191]
// 但是注释上写着 "May fail spuriously and does not provide ordering guarantees,
// so is only rarely an appropriate alternative to compareAndSet."
// todo 感觉有可能是 jvm 通过方法名在 native 方法里面做了转发
final boolean wCasResult = amr.weakCompareAndSet(initialRef, newReference1, initialMark, newMark1);
System.out.println("currentValue=" + amr.getReference()
+ ", currentMark=" + amr.isMarked()
+ ", wCasResult=" + wCasResult);
}
}输出结果如下：
currentValue=null, currentMark=false
currentValue=true, currentMark=true, casResult=true
currentValue=true, currentMark=true
currentValue=true, currentMark=false, attemptMarkResult=true
currentValue=null, currentMark=false
currentValue=true, currentMark=true, wCasResult=true本文参与 腾讯云自媒体同步曝光计划，分享自作者个人站点/博客。
原始发表：2019.07.21 ，如有侵权请联系 cloudcommunity@tencent.com 删除
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