Java使用Unsafe类的示例详解 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

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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 true0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); }}@Dataclass 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); }}

【Java使用Unsafe类的示例详解】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 = https://www.it610.com/article/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 falseif (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