并发编程--原子类AotmicInteger

分手后的思念是犯贱 2022-06-18 08:10 319阅读 0赞

前几篇博客中我们已经介绍了线程、volatile、synchronized和cas自旋相关的知识，接下来我介绍一下jdk提供的并发编程包java.util.concurrent中相关的实现类知识。

AtomicInteger简单来说就是一个能进行原子操作的Integer，这样在多线程操作下对AtomicInteger的操作是原子操作的，操作后的值对所有线程都是立即可见的。简单来说其实现就是使用的volatile变量 value和sun.misc.Unsafe提供的CAS操作完成的，volatile变量value就表明任意一个线程对value的操作都是对其他线程可见的，并且对于value的修改使用Unsafe提供的方法实现线程不阻塞来完成对value值的修改等操作。

AtomicInteger的变量值：

private volatile int value;

get和set操作就是直接赋值给value

public final int get() {
            return value;
        }
    
     public final void set(int newValue) {
            value = newValue;
        }

加加和减减操作就是使用Unsafe提供的方法完成CAS操作

public final int incrementAndGet() {
            return unsafe.getAndAddInt(this, valueOffset, 1) + 1;
        }
    
        public final int decrementAndGet() {
            return unsafe.getAndAddInt(this, valueOffset, -1) - 1;
        }

简单来说AtomicInteger的实现原理就是通过一个volatile的变量value以及Unsafe类提供的CAS操作来完成的。

AtomicInteger源码：

public class AtomicInteger extends Number implements java.io.Serializable {
        private static final long serialVersionUID = 6214790243416807050L;
    
        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 volatile int value;
    
        public AtomicInteger(int initialValue) {
            value = initialValue;
        }
    	
        public AtomicInteger() {
        }
    
        public final int get() {
            return value;
        }
    
        public final void set(int newValue) {
            value = newValue;
        }
    	
        public final void lazySet(int newValue) {
            unsafe.putOrderedInt(this, valueOffset, newValue);
        }
    
      
        public final int getAndSet(int newValue) {
            return unsafe.getAndSetInt(this, valueOffset, newValue);
        }
    
        public final boolean compareAndSet(int expect, int update) {
            return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
        }
    
        public final boolean weakCompareAndSet(int expect, int update) {
            return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
        }
    
        public final int getAndIncrement() {
            return unsafe.getAndAddInt(this, valueOffset, 1);
        }
    
        public final int getAndDecrement() {
            return unsafe.getAndAddInt(this, valueOffset, -1);
        }
    
        public final int getAndAdd(int delta) {
            return unsafe.getAndAddInt(this, valueOffset, delta);
        }
    
        public final int incrementAndGet() {
            return unsafe.getAndAddInt(this, valueOffset, 1) + 1;
        }
    
        public final int decrementAndGet() {
            return unsafe.getAndAddInt(this, valueOffset, -1) - 1;
        }
    
        public final int addAndGet(int delta) {
            return unsafe.getAndAddInt(this, valueOffset, delta) + delta;
        }
    
        public final int getAndUpdate(IntUnaryOperator updateFunction) {
            int prev, next;
            do {
                prev = get();
                next = updateFunction.applyAsInt(prev);
            } while (!compareAndSet(prev, next));
            return prev;
        }
    
        public final int updateAndGet(IntUnaryOperator updateFunction) {
            int prev, next;
            do {
                prev = get();
                next = updateFunction.applyAsInt(prev);
            } while (!compareAndSet(prev, next));
            return next;
        }
    
        public final int getAndAccumulate(int x,
                                          IntBinaryOperator accumulatorFunction) {
            int prev, next;
            do {
                prev = get();
                next = accumulatorFunction.applyAsInt(prev, x);
            } while (!compareAndSet(prev, next));
            return prev;
        }
    
        public final int accumulateAndGet(int x,
                                          IntBinaryOperator accumulatorFunction) {
            int prev, next;
            do {
                prev = get();
                next = accumulatorFunction.applyAsInt(prev, x);
            } while (!compareAndSet(prev, next));
            return next;
        }
    
        public String toString() {
            return Integer.toString(get());
        }
    
        public int intValue() {
            return get();
        }
    
        public long longValue() {
            return (long)get();
        }
    
        public float floatValue() {
            return (float)get();
        }
    
        public double doubleValue() {
            return (double)get();
        }
    
    }