大厂面试真题-说说AtomicInteger 线程安全原理

public class AtomicInteger extends Number implements java.io.Serializable {
//Unsafe类实例
private static final Unsafe unsafe = Unsafe.getUnsafe();
//内部value值，使用volatile保证线程可见性
private volatile int value;
//value属性值的地址偏移量
private static final long valueOffset;
static {
try {
//计算value属性值的地址偏移量
valueOffset = unsafe.objectFieldOffset(172 | Java 高并发核心编程 卷 2（加强版）：多线程、锁、JMM、JUC、高并发设计模式
AtomicInteger.class.getDeclaredField("value"));
} catch (Exception ex) { throw new Error(ex); }
}
//初始化
public AtomicInteger(int initialValue) {
value = initialValue;
}
//获取当前value值
public final int get() {
return value;
}
//方法：返回旧值并赋新值
public final int getAndSet(int newValue) {
for (;;) {
//自旋
int current = get();
//获取旧值
//以CAS方式赋值，直到成功返回
if (compareAndSet(current, newValue)) return current;
}
}
//方法：封装底层的CAS操作，对比expect(期望值)与value，若不同则返回false
//若expect与value相同，则将新值赋给value，并返回true
public final boolean compareAndSet(int expect, int update) {
return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
}
//方法：安全自增i++
public final int getAndIncrement() {
for (;;) {
//自旋
int current = get();
int next = current + 1;
if (compareAndSet(current, next))
return current;
}
}
//方法：自定义增量数
public final int getAndAdd(int delta) {
for (;;) {
//自旋
int current = get();
int next = current + delta;
if (compareAndSet(current, next))
return current;
}
}
//方法：类似++I，返回自增后的值
public final int incrementAndGet() {
for (;;) {
//自旋
int current = get();
int next = current + 1;
if (compareAndSet(current, next))
return next;
}
}
//方法：返回加上delta后的值
public final int addAndGet(int delta) {
for (;;) {
//自旋
int current = get();第 3 章 CAS 原理与 JUC 原子类 | 173
int next = current + delta;
if (compareAndSet(current, next))
return next;
}
}
//省略其他源码
}