原子變數比鎖的粒度更細，量級更輕，並且對於在多處理器系統上實現高效能的併發程式碼來說是非常關鍵的。
原子變數類相當於一種泛化的 volatile 變數，能夠支援原子的和有條件的讀-改-寫操作。
原子類在內部使用現代 CPU 支援的 CAS 指令來實現同步。這些指令通常比鎖更快。
原子更新基本型別
- AtomicBoolean - 原子更新布林型別。
- AtomicInteger - 原子更新整型。
- AtomicLong - 原子更新長整型。
示例：
public class AtomicIntegerDemo {
public static void main(String[] args) throws InterruptedException {
ExecutorService executorService = Executors.newFixedThreadPool(5);
AtomicInteger count = new AtomicInteger(0);
for (int i = 0; i < 1000; i++) {
executorService.submit((Runnable) () -> {
System.out.println(Thread.currentThread().getName() + " count=" + count.get());
count.incrementAndGet();
});
}
executorService.shutdown();
executorService.awaitTermination(30, TimeUnit.SECONDS);
System.out.println("Final Count is : " + count.get());
}
}複製程式碼原子更新陣列
- AtomicIntegerArray - 原子更新整型陣列裡的元素。
- AtomicLongArray - 原子更新長整型陣列裡的元素。
- AtomicReferenceArray - 原子更新引用型別陣列的元素。
- AtomicBooleanArray - 原子更新布林型別陣列的元素。
示例：
public class AtomicIntegerArrayDemo {
private static AtomicIntegerArray atomicIntegerArray = new AtomicIntegerArray(10);
public static void main(final String[] arguments) throws InterruptedException {
for (int i = 0; i < atomicIntegerArray.length(); i++) {
atomicIntegerArray.set(i, i);
}
Thread t1 = new Thread(new Increment());
Thread t2 = new Thread(new Compare());
t1.start();
t2.start();
t1.join();
t2.join();
System.out.println("Final Values: ");
for (int i = 0; i < atomicIntegerArray.length(); i++) {
System.out.print(atomicIntegerArray.get(i) + " ");
}
}
static class Increment implements Runnable {
public void run() {
for (int i = 0; i < atomicIntegerArray.length(); i++) {
int add = atomicIntegerArray.incrementAndGet(i);
System.out.println(Thread.currentThread().getName() + ", index " + i + ", value: " + add);
}
}
}
static class Compare implements Runnable {
public void run() {
for (int i = 0; i < atomicIntegerArray.length(); i++) {
boolean swapped = atomicIntegerArray.compareAndSet(i, 2, 3);
if (swapped) {
System.out.println(Thread.currentThread().getName() + ", index " + i + ", value: 3");
}
}
}
}
}複製程式碼原子更新引用型別
- AtomicReference - 原子更新引用型別。
- AtomicReferenceFieldUpdater - 原子更新引用型別裡的欄位。
- AtomicMarkableReference - 原子更新帶有標記位的引用型別。可以原子更新一個布林型別的標記位和應用型別。
public class AtomicReferenceDemo {
private static String message;
private static Person person;
private static AtomicReference<String> aRmessage;
private static AtomicReference<Person> aRperson;
public static void main(String[] args) throws InterruptedException {
Thread t1 = new Thread(new MyRun1());
Thread t2 = new Thread(new MyRun2());
message = "hello";
person = new Person("Phillip", 23);
aRmessage = new AtomicReference<String>(message);
aRperson = new AtomicReference<Person>(person);
System.out.println("Message is: " + message
+ "\nPerson is " + person.toString());
System.out.println("Atomic Reference of Message is: " + aRmessage.get()
+ "\nAtomic Reference of Person is " + aRperson.get().toString());
t1.start();
t2.start();
t1.join();
t2.join();
System.out.println("\nNow Message is: " + message
+ "\nPerson is " + person.toString());
System.out.println("Atomic Reference of Message is: " + aRmessage.get()
+ "\nAtomic Reference of Person is " + aRperson.get().toString());
}
static class MyRun1 implements Runnable {
public void run() {
aRmessage.compareAndSet(message, "Thread 1");
message = message.concat("-Thread 1!");
person.setAge(person.getAge() + 1);
person.setName("Thread 1");
aRperson.getAndSet(new Person("Thread 1", 1));
System.out.println("\n" + Thread.currentThread().getName() + " Values "
+ message + " - " + person.toString());
System.out.println("\n" + Thread.currentThread().getName() + " Atomic References "
+ message + " - " + person.toString());
}
}
static class MyRun2 implements Runnable {
public void run() {
message = message.concat("-Thread 2");
person.setAge(person.getAge() + 2);
person.setName("Thread 2");
aRmessage.lazySet("Thread 2");
aRperson.set(new Person("Thread 2", 2));
System.out.println("\n" + Thread.currentThread().getName() + " Values: "
+ message + " - " + person.toString());
System.out.println("\n" + Thread.currentThread().getName() + " Atomic References: "
+ aRmessage.get() + " - " + aRperson.get().toString());
}
}
static class Person {
private String name;
private int age;
Person(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
int getAge() {
return age;
}
void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "[name " + this.name + ", age " + this.age + "]";
}
}
}複製程式碼原子更新欄位類
- AtomicIntegerFieldUpdater - 原子更新整型的欄位的更新器。
- AtomicLongFieldUpdater - 原子更新長整型欄位的更新器。
- AtomicStampedReference - 原子更新帶有版本號的引用型別。該類將整型數值與引用關聯起來，可用於原子的更新資料和資料的版本號，可以解決使用 CAS 進行原子更新時可能出現的 ABA 問題。
public class AtomicStampedReferenceDemo {
private final static String INIT_REF = "abc";
public static void main(String[] args) throws InterruptedException {
AtomicStampedReference<String> asr = new AtomicStampedReference<>(INIT_REF, 0);
System.out.println("初始物件為：" + asr.getReference());
final int stamp = asr.getStamp();
ExecutorService executorService = Executors.newFixedThreadPool(100);
for (int i = 0; i < 100; i++) {
executorService.submit(() -> {
try {
Thread.sleep(Math.abs((int) (Math.random() * 100)));
} catch (InterruptedException e) {
e.printStackTrace();
}
if (asr.compareAndSet(INIT_REF, Thread.currentThread().getName(), stamp, stamp + 1)) {
System.out.println(Thread.currentThread().getName() + " 修改了物件！");
System.out.println("新的物件為：" + asr.getReference());
}
});
}
executorService.shutdown();
executorService.awaitTermination(60, TimeUnit.SECONDS);
}
}複製程式碼免費Java資料需要自己領取，涵蓋了Java、Redis、MongoDB、MySQL、Zookeeper、Spring Cloud、Dubbo高併發分散式等教程，一共30G。
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