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This article will explain in detail how to use the Unsafe class in Java. The editor thinks it is very practical, so I share it with you as a reference. I hope you can get something after reading this article.

The Unsafe object provides a very low-level way to manipulate memory and threads, which is equivalent to opening the back door.

The CAS implementation in the atomic class and the underlying layer of the park unpark in LockSupport all call the methods in UnSafe.

UnSafe is not saying that threads are unsafe, but that manipulating memory can cause unsafe problems.

Of course, for developers,

The Unsafe object cannot be called directly, it can only be obtained through reflection

Get the Unsafe object by reflection

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 {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);}}

Running result

Sun.misc.Unsafe@7ea987ac

Using Unsafe to realize CAS Operation

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 {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") / / get the offset of the member variable long idOffset = unsafe.objectFieldOffset (id); long nameOffset = unsafe.objectFieldOffset (name); Student student = new Student (); / / replace the value unsafe.compareAndSwapInt (student, idOffset, 0,20) of the member variable with the cas method; / / return true 0 as the old value and 20 as the new value unsafe.compareAndSwapObject (student, nameOffset, null, "Zhang San") / / return true the old value is null, and the new value is Zhang San System.out.println (student);}} @ Dataclass Student {volatile int id; volatile String name;}

Running result

Sun.misc.Unsafe@7ea987ac

Student (id=20, name= Zhang San)

Directly use the Unsafe class to implement the previously thread-safe atomic integer BankAccount in AtomicIntegerFieldUpdater

Using AtomicIntegerFieldUpdater to realize money Thread-safe Atomic Integer in atomic

Package com.dongguo.unsafe;import java.util.concurrent.TimeUnit;import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;/** * @ author Dongguo * @ date 2021-9-7 0007-14:41 * manipulates certain fields of non-thread-safe objects in a thread-safe manner. * demand: * 1000 people transfer one yuan to an account at the same time, then the cumulative amount should be increased by 1000 yuan. * in addition to synchronized and CAS, you can also use AtomicIntegerFieldUpdater. * / 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 {bankAccount.transferMoney (bankAccount) }, String.valueOf (I) .start ();} / pause millisecond try {TimeUnit.MILLISECONDS.sleep (500);} catch (InterruptedException e) {e.printStackTrace ();} System.out.println (bankAccount.money);}}

Instead, use UnSafe to implement money thread-safe atomic integers

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 () The offset of the try {/ / money property in the BankAccount object, which is used by Unsafe to access the property DATA_OFFSET = unsafe.objectFieldOffset (BankAccount.class.getDeclaredField ("money")) directly;} catch (NoSuchFieldException e) {throw new Error (e);}} public BankAccount (int money) {this.money = money } public void transferMoney (int amount) {int oldValue; while (true) {/ / get the old value of the shared variable. You can add breakpoints on this line and modify data debugging to deepen the understanding of oldValue = money / / cas attempts to modify data to the old value + amount. If the old value is changed by another thread, return 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 {bankAccount.transferMoney (1);}, String.valueOf (I). Start ();} / pause millisecond try {TimeUnit.MILLISECONDS.sleep (500);} catch (InterruptedException e) {e.printStackTrace ();} System.out.println (bankAccount.money) }} run result 1000 / pause millisecond try {TimeUnit.MILLISECONDS.sleep;} catch (InterruptedException e) {e.printStackTrace ();} System.out.println (bankAccount.money);}}

Running result

one thousand

This is the end of this article on "how Java uses Unsafe classes". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, please share it for more people to see.

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