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并发编程-原子类 ​

AtomicInteger ​

java
private volatile int value;

public AtomicInteger(int initialValue) {
    value = initialValue;
}

public AtomicInteger() {
}
private volatile int value;

public AtomicInteger(int initialValue) {
    value = initialValue;
}

public AtomicInteger() {
}

构造方法

java
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long valueOffset;

static {
    try {
        valueOffset = unsafe.objectFieldOffset
            (AtomicInteger.class.getDeclaredField("value"));
    } catch (Exception ex) { throw new Error(ex); }
}
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long valueOffset;

static {
    try {
        valueOffset = unsafe.objectFieldOffset
            (AtomicInteger.class.getDeclaredField("value"));
    } catch (Exception ex) { throw new Error(ex); }
}

可以看到最上面是和AQS采用了类似的机制，因为要使用CAS算法更新value的值，所以得先计算出value字段在对象中的偏移地址，CAS直接修改对应位置的内存即可（可见Unsafe类的作用巨大，很多的底层操作都要靠它来完成）

自增操作是怎么在运行的：

java
public final int getAndIncrement() {
    return unsafe.getAndAddInt(this, valueOffset, 1);
}
public final int getAndIncrement() {
    return unsafe.getAndAddInt(this, valueOffset, 1);
}

调用了unsafe.getAndAddInt()，我们接着看看Unsafe里面写了什么：

java
public final int getAndAddInt(Object o, long offset, int delta) {  //delta就是变化的值，++操作就是自增1
    int v;
    do {
      	//volatile版本的getInt()
      	//能够保证可见性
        v = getIntVolatile(o, offset);
    } while (!compareAndSwapInt(o, offset, v, v + delta));  //这里是开始cas替换int的值，每次都去拿最新的值去进行替换，如果成功则离开循环，不成功说明这个时候其他线程先修改了值，就进下一次循环再获取最新的值然后再cas一次，直到成功为止
    return v;
}
public final int getAndAddInt(Object o, long offset, int delta) {  //delta就是变化的值，++操作就是自增1
    int v;
    do {
      	//volatile版本的getInt()
      	//能够保证可见性
        v = getIntVolatile(o, offset);
    } while (!compareAndSwapInt(o, offset, v, v + delta));  //这里是开始cas替换int的值，每次都去拿最新的值去进行替换，如果成功则离开循环，不成功说明这个时候其他线程先修改了值，就进下一次循环再获取最新的值然后再cas一次，直到成功为止
    return v;
}

这是一个do-while循环，采用自旋形式，来不断进行CAS操作，直到成功。

image-20220308131536403

可见，原子类底层也是采用了CAS算法来保证的原子性，包括getAndSet、getAndAdd等方法都是这样。原子类也直接提供了CAS操作方法，我们可以直接使用：

java
public static void main(String[] args) throws InterruptedException {
    AtomicInteger integer = new AtomicInteger(10);
    System.out.println(integer.compareAndSet(30, 20));
    System.out.println(integer.compareAndSet(10, 20));
    System.out.println(integer);
}
public static void main(String[] args) throws InterruptedException {
    AtomicInteger integer = new AtomicInteger(10);
    System.out.println(integer.compareAndSet(30, 20));
    System.out.println(integer.compareAndSet(10, 20));
    System.out.println(integer);
}

如果想以普通变量的方式来设定值，那么可以使用lazySet()方法，这样就不采用volatile的立即可见机制了

java
AtomicInteger integer = new AtomicInteger(1);
integer.lazySet(2);
AtomicInteger integer = new AtomicInteger(1);
integer.lazySet(2);

AtomicIntegerArray ​

java
public static void main(String[] args) throws InterruptedException {
    AtomicIntegerArray array = new AtomicIntegerArray(new int[]{0, 4, 1, 3, 5});
    Runnable r = () -> {
        for (int i = 0; i < 100000; i++)
            array.getAndAdd(0, 1);
    };
    new Thread(r).start();
    new Thread(r).start();
    TimeUnit.SECONDS.sleep(1);
    System.out.println(array.get(0));
}
public static void main(String[] args) throws InterruptedException {
    AtomicIntegerArray array = new AtomicIntegerArray(new int[]{0, 4, 1, 3, 5});
    Runnable r = () -> {
        for (int i = 0; i < 100000; i++)
            array.getAndAdd(0, 1);
    };
    new Thread(r).start();
    new Thread(r).start();
    TimeUnit.SECONDS.sleep(1);
    System.out.println(array.get(0));
}

LongAdder ​

在JDK8之后，新增了DoubleAdder和LongAdder，在高并发情况下，LongAdder的性能比AtomicLong的性能更好，主要体现在自增上，它的大致原理如下：在低并发情况下，和AtomicLong是一样的，对value值进行CAS操作，但是出现高并发的情况时，AtomicLong会进行大量的循环操作来保证同步，而LongAdder会将对value值的CAS操作分散为对数组cells中多个元素的CAS操作（内部维护一个Cell[] as数组，每个Cell里面有一个初始值为0的long型变量，在高并发时会进行分散CAS，就是不同的线程可以对数组中不同的元素进行CAS自增，这样就避免了所有线程都对同一个值进行CAS），只需要最后再将结果加起来即可。

image-20220406100926787

使用如下：

java
public static void main(String[] args) throws InterruptedException {
    LongAdder adder = new LongAdder();
    Runnable r = () -> {
        for (int i = 0; i < 100000; i++)
            adder.add(1);
    };
    for (int i = 0; i < 100; i++)
        new Thread(r).start();   //100个线程
    TimeUnit.SECONDS.sleep(1);
    System.out.println(adder.sum());   //最后求和即可
}
public static void main(String[] args) throws InterruptedException {
    LongAdder adder = new LongAdder();
    Runnable r = () -> {
        for (int i = 0; i < 100000; i++)
            adder.add(1);
    };
    for (int i = 0; i < 100; i++)
        new Thread(r).start();   //100个线程
    TimeUnit.SECONDS.sleep(1);
    System.out.println(adder.sum());   //最后求和即可
}

LongAdder&AtomicLong性能对比:

java
public class Main {
    public static void main(String[] args) throws InterruptedException {
        System.out.println("使用AtomicLong的时间消耗："+test2()+"ms");
        System.out.println("使用LongAdder的时间消耗："+test1()+"ms");
    }

    private static long test1() throws InterruptedException {
        CountDownLatch latch = new CountDownLatch(100);
        LongAdder adder = new LongAdder();
        long timeStart = System.currentTimeMillis();
        Runnable r = () -> {
            for (int i = 0; i < 100000; i++)
                adder.add(1);
            latch.countDown();
        };
        for (int i = 0; i < 100; i++)
            new Thread(r).start();
        latch.await();
        return System.currentTimeMillis() - timeStart;
    }

    private static long test2() throws InterruptedException {
        CountDownLatch latch = new CountDownLatch(100);
        AtomicLong atomicLong = new AtomicLong();
        long timeStart = System.currentTimeMillis();
        Runnable r = () -> {
            for (int i = 0; i < 100000; i++)
                atomicLong.incrementAndGet();
            latch.countDown();
        };
        for (int i = 0; i < 100; i++)
            new Thread(r).start();
        latch.await();
        return System.currentTimeMillis() - timeStart;
    }
}
public class Main {
    public static void main(String[] args) throws InterruptedException {
        System.out.println("使用AtomicLong的时间消耗："+test2()+"ms");
        System.out.println("使用LongAdder的时间消耗："+test1()+"ms");
    }

    private static long test1() throws InterruptedException {
        CountDownLatch latch = new CountDownLatch(100);
        LongAdder adder = new LongAdder();
        long timeStart = System.currentTimeMillis();
        Runnable r = () -> {
            for (int i = 0; i < 100000; i++)
                adder.add(1);
            latch.countDown();
        };
        for (int i = 0; i < 100; i++)
            new Thread(r).start();
        latch.await();
        return System.currentTimeMillis() - timeStart;
    }

    private static long test2() throws InterruptedException {
        CountDownLatch latch = new CountDownLatch(100);
        AtomicLong atomicLong = new AtomicLong();
        long timeStart = System.currentTimeMillis();
        Runnable r = () -> {
            for (int i = 0; i < 100000; i++)
                atomicLong.incrementAndGet();
            latch.countDown();
        };
        for (int i = 0; i < 100; i++)
            new Thread(r).start();
        latch.await();
        return System.currentTimeMillis() - timeStart;
    }
}

AtomicReference ​

java
public static void main(String[] args) throws InterruptedException {
    String a = "Hello";
    String b = "World";
    AtomicReference<String> reference = new AtomicReference<>(a);
    reference.compareAndSet(a, b);
    System.out.println(reference.get());
}
public static void main(String[] args) throws InterruptedException {
    String a = "Hello";
    String b = "World";
    AtomicReference<String> reference = new AtomicReference<>(a);
    reference.compareAndSet(a, b);
    System.out.println(reference.get());
}

AtomicIntegerFieldUpdater ​

JUC还提供了字段原子更新器，可以对类中的某个指定字段进行原子操作（注意字段必须添加volatile关键字）：

java
public class Main {
    public static void main(String[] args) throws InterruptedException {
        Student student = new Student();
        AtomicIntegerFieldUpdater<Student> fieldUpdater =
                AtomicIntegerFieldUpdater.newUpdater(Student.class, "age");
        System.out.println(fieldUpdater.incrementAndGet(student));
    }

    public static class Student{
        volatile int age;
    }
}
public class Main {
    public static void main(String[] args) throws InterruptedException {
        Student student = new Student();
        AtomicIntegerFieldUpdater<Student> fieldUpdater =
                AtomicIntegerFieldUpdater.newUpdater(Student.class, "age");
        System.out.println(fieldUpdater.incrementAndGet(student));
    }

    public static class Student{
        volatile int age;
    }
}