Java - CAS用法及说明

CAS即compare and swap，表示比较并交换，在java中依赖Unsafe类来实现，常见的CAS实现有AtomicInteger、AtomicLong、AtomicReference等，这些都是使用乐观锁的形式来实现多线程线程编程。
下面以AtomicInteger为例介绍:

1. AtomicInteger的使用

public static void main(String[] args) {
    AtomicInteger atomicInteger = new AtomicInteger();
    int i = atomicInteger.incrementAndGet();
    System.out.println(i);
}

2. 源码分析

构造AtomicInteger的时候默认创建一个Unsafe对象，他是实现CAS的关键。
valueOffset代表字段在对象中的偏移量，为的是能通过偏移量直接索引到字段值。

public class AtomicInteger extends Number implements java.io.Serializable {
    private static final long serialVersionUID = 6214790243416807050L;

    // setup to use Unsafe.compareAndSwapInt for updates
    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;
    ...
}

• incrementAndGet方法

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

• getAndAddInt

在做incrementAndGet的时候，会一直do-while循环，var1、var2找到对象字段的位置，持续更新var5获取当前值，var5+var4作为更新值，循环往复进行更新，知道更新成功。

public final int getAndAddInt(Object var1, long var2, int var4) {
    int var5;
    do {
        var5 = this.getIntVolatile(var1, var2);
    } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));

    return var5;
}

• 最后的native方法compareAndSwapInt

public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);

3. 参考AtomicInteger自己操作Unsafe类

import sun.misc.Unsafe;
import java.lang.reflect.Field;
public class UnsafeTest {

    public static void main(String[] args) throws Exception {
        Unsafe unsafe = getUnsafe();
        long offset = unsafe.objectFieldOffset(Entry.class.getDeclaredField("id"));
        System.out.println("offset: " + offset);
        Entry entry = new Entry();
        entry.setId(1);
        boolean b = unsafe.compareAndSwapInt(entry, offset, 1, 2);
        System.out.println(b);
        System.out.println(entry.getId());
    }

    /**
     * 通过反射获取Unsafe实例
     */
    public static Unsafe getUnsafe() throws IllegalAccessException, NoSuchFieldException {
        Field field = Unsafe.class.getDeclaredField("theUnsafe");
        field.setAccessible(true);
        return (Unsafe)field.get(null);
    }

    public static class Entry{

        private int id;

        public int getId() {
            return id;
        }

        public void setId(int id) {
            this.id = id;
        }
    }
}