Java使用Unsafe类的示例详解

Unsafe 对象提供了非常底层的，操作内存、线程的方法，相当于开了后门。

在atomic类中CAS实现、LockSupport中park unpark的底层都调用了UnSafe中的方法。

UnSafe并不是说线程不安全，而是说操作内存有可能会造成不安全问题。

当然对于开发人员来说

Unsafe 对象不能直接调用，只能通过反射获得

通过反射获得Unsafe对象

package com.dongguo.unsafe;

import sun.misc.Unsafe;

import java.lang.reflect.Field;

/**
* @author Dongguo
* @date 2021/9/12 0012-21:32
* @description:
*/
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);
  }
}

运行结果

sun.misc.Unsafe@7ea987ac

使用Unsafe实现 CAS 操作

package com.dongguo.unsafe;

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

import java.lang.reflect.Field;

/**
* @author Dongguo
* @date 2021/9/12 0012-21:32
* @description:
*/
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;
}

运行结果

sun.misc.Unsafe@7ea987ac
Student(id=20, name=张三)

直接使用Unsafe类实现之前AtomicIntegerFieldUpdater中线程安全的原子整数 BankAccount

在atomic中使用AtomicIntegerFieldUpdater实现money线程安全的原子整数

package com.dongguo.unsafe;

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

/**
* @author Dongguo
* @date 2021/9/7 0007-14:41
* 以一种线程安全的方式操作非线程安全对象的某些字段。
* 需求：
* 1000个人同时向一个账号转账一元钱，那么累计应该增加1000元，
* 除了synchronized和CAS,还可以使用AtomicIntegerFieldUpdater来实现。
*/
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);
  }

}

改为使用UnSafe实现money线程安全的原子整数

package com.dongguo.unsafe;

import sun.misc.Unsafe;

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

/**
* @author Dongguo
* @date 2021/9/7 0007-14:41
*/
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);
  }
}

运行结果

1000

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原文链接：https://blog.csdn.net/m0_37450089/article/details/120462281