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 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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