JavaUnsafe类

Unsafe 对象提供了非常底层的，操作内存、线程的方法，Unsafe 对象不能直接调用，只能通过反射获得。


public class UnsafeAccessor {
    
    static Unsafe unsafe;
    
    static {
        try {
            Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
            theUnsafe.setAccessible(true);
            unsafe = (Unsafe) theUnsafe.get(null);
        } catch (NoSuchFieldException | IllegalAccessException e) {
            throw new Error(e);
        }
    }
    
    static Unsafe getUnsafe() {
        return unsafe;
    }
    
}

CAS操作


Unsafe unsafe = UnsafeAccessor.getUnsafe();
 
Field id = Student.class.getDeclaredField("id");
Field name = Student.class.getDeclaredField("name");
 
// 获得成员变量的偏移量
long idOffset = UnsafeAccessor.unsafe.objectFieldOffset(id);
long nameOffset = UnsafeAccessor.unsafe.objectFieldOffset(name);
 
Student student = new Student();
 
// 使用 cas 方法替换成员变量的值
UnsafeAccessor.unsafe.compareAndSwapInt(student, idOffset, 0, 20); // 返回 true
UnsafeAccessor.unsafe.compareAndSwapObject(student, nameOffset, null, "张三"); // 返回 true
 
System.out.println(student);

使用自定义的 AtomicData 实现之前线程安全的原子整数 Account 实现：


class AtomicData {
 
    private volatile int data;
    static final Unsafe unsafe;
    static final long DATA_OFFSET;
 
    static {
        unsafe = UnsafeAccessor.getUnsafe();
        
        try {
            // data 属性在 DataContainer 对象中的偏移量，用于 Unsafe 直接访问该属性
            DATA_OFFSET = unsafe.objectFieldOffset(AtomicData.class.getDeclaredField("data"));
        } catch (NoSuchFieldException e) {
            throw new Error(e);
        }
    }
 
    public AtomicData(int data) {
        this.data = data;
    }
 
    public void decrease(int amount) {
        int oldValue;
        while(true) {
            // 获取共享变量旧值，可以在这一行加入断点，修改 data 调试来加深理解
            oldValue = data;
            // cas 尝试修改 data 为 旧值 + amount，如果期间旧值被别的线程改了，返回 false
            if (unsafe.compareAndSwapInt(this, DATA_OFFSET, oldValue, oldValue - amount)) {
                return;
            }
        }
    }
 
    public int getData() {
        return data;
    }
 
}