JUC并发编程(16.Java线程-Unsafe）

忘是亡心i 2023-09-24 22:11 51阅读 0赞

# 1.Unsafe #

## 概述 ##

`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`  
**Unsafe CAS 操作**

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;
    	}
    }

**Unsafe CAS 操作**

@Data
    class Student {
        
    	volatile int id;
    	volatile String name;
    }

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);

**输出**

Student(id=20, name=张三)

**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;
    	}
    }

**Account 实现**

Account.demo(new Account() {
        
    	AtomicData atomicData = new AtomicData(10000);
    	@Override
    	public Integer getBalance() {
        
    		return atomicData.getData();
    	}
    	@Override
    	public void withdraw(Integer amount) {
        
    		atomicData.decrease(amount);
    	}
    });

# 2.本章小结 #

`CAS` 与 `volatile`  
`API`  
原子整数  
原子引用  
原子数组  
字段更新器  
原子累加器  
`Unsafe`

*  原理方面  
    `LongAdder`源码  
    `伪共享`