Java使用Unsafe类的示例详解

package com.dongguo.unsafe;

import sun.misc.Unsafe;

import java.lang.reflect.Field;


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;
    }

    public static void main(String[] args) {
        Unsafe unsafe = getUnsafe();
        System.out.println(unsafe);
    }
}

package com.dongguo.unsafe;

import lombok.Data;
import sun.misc.Unsafe;

import java.lang.reflect.Field;


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;
    }

    public static void main(String[] args) throws NoSuchFieldException {
        Unsafe unsafe = getUnsafe();
        System.out.println(unsafe);

        Field id = Student.class.getDeclaredField("id");
        Field name = Student.class.getDeclaredField("name");
        // 获得成员变量的偏移量
        long idOffset = unsafe.objectFieldOffset(id);
        long nameOffset = unsafe.objectFieldOffset(name);
        Student student = new Student();
        // 使用 cas 方法替换成员变量的值
        unsafe.compareAndSwapint(student, idOffset, 0, 20); // 返回 true   0为旧值 20为新值
        unsafe.compareAndSwapObject(student, nameOffset, null, "张三"); // 返回 true 旧值为null，新值为张三
        System.out.println(student);
    }
}
@Data
class Student {
    volatile int id;
    volatile String name;
}

package com.dongguo.unsafe;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;


class BankAccount {
    private String bankName = "ACBC";
    public volatile int money = 0;
    AtomicIntegerFieldUpdater<BankAccount> fieldUpdater = AtomicIntegerFieldUpdater.newUpdater(BankAccount.class, "money");

    public void transferMoney(BankAccount bankAccount) {
        fieldUpdater.incrementAndGet(bankAccount);
    }
}

public class AtomicIntegerFieldUpdaterDemo {
    public static void main(String[] args) {
        BankAccount bankAccount = new BankAccount();

        for (int i = 1; i <= 1000; i++) {
            new Thread(() -> {
                bankAccount.transferMoney(bankAccount);
            }, String.valueOf(i)).start();
        }
        //暂停毫秒
        try {
            TimeUnit.MILLISECONDS.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(bankAccount.money);
    }

}

package com.dongguo.unsafe;

import sun.misc.Unsafe;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;


class BankAccount {
    private String bankName = "ACBC";
    public volatile int money;
    static final Unsafe unsafe;
    static final long DATA_OFFSET;

    static {
        unsafe = UnsafeAccessor.getUnsafe();
        try {
            // money 属性在 BankAccount 对象中的偏移量，用于 Unsafe 直接访问该属性
            DATA_OFFSET = unsafe.objectFieldOffset(BankAccount.class.getDeclaredField("money"));
        } catch (NoSuchFieldException e) {
            throw new Error(e);
        }
    }

    public BankAccount(int money) {
        this.money = money;
    }

    public void transferMoney(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 = money;
            // 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 class AtomicIntegerFieldUpdaterDemo {
    public static void main(String[] args) {
        BankAccount bankAccount = new BankAccount(0);

        for (int i = 1; i <= 1000; i++) {
            new Thread(() -> {
                bankAccount.transferMoney(1);
            }, String.valueOf(i)).start();
        }
        //暂停毫秒
        try {
            TimeUnit.MILLISECONDS.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(bankAccount.money);
    }
}
运行结果
1000
/暂停毫秒
        try {
            TimeUnit.MILLISECONDS.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(bankAccount.money);
    }
}