java CAS及各种锁

CAS

CAS

缺点：循环会耗时；一次性只能保持一个共享变量的原子性；ABA问题

package juc.cas;

import java.util.concurrent.atomic.AtomicInteger;

public class CASDemo {

    // CAS  compareAndSet比较并交换 是CPU的并发原语
    public static void main(String[] args) {
        AtomicInteger atomicInteger = new AtomicInteger(2020);
        // 如果是期望的值就更新，否则一直循环
        System.out.println(atomicInteger.compareAndSet(2020, 2021));// true
        System.out.println(atomicInteger.get());
        atomicInteger.getAndIncrement();// 2021

        System.out.println(atomicInteger.compareAndSet(2020, 2021));// false
        System.out.println(atomicInteger.get());// 2022

    }
}

• ABA问题

package juc.cas;

import java.util.concurrent.atomic.AtomicInteger;

public class CASDemo {

    public static void main(String[] args) {
        AtomicInteger atomicInteger = new AtomicInteger(2020);
        // A对资源操作过了，又改回去了
        System.out.println(atomicInteger.compareAndSet(2020, 2021));// true
        System.out.println(atomicInteger.get());
        System.out.println(atomicInteger.compareAndSet(2021, 2020));// true
        System.out.println(atomicInteger.get());

        // B不知情
        System.out.println(atomicInteger.compareAndSet(2020, 2022));// true
        System.out.println(atomicInteger.get());

    }
}

• 原子引用解决ABA问题

package juc.cas;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicStampedReference;

public class CASDemo {

    public static void main(String[] args) {
        // !如果泛型是一个包装类，注意对象的引用问题
        // 带版本号的原子操作
        AtomicStampedReference<Integer> atomicInteger = new AtomicStampedReference<>(1, 1);

        new Thread(()->{
            // 获得版本号
            System.out.println("A1->" + atomicInteger.getStamp());

            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            System.out.println(atomicInteger.compareAndSet(1, 2,
                    atomicInteger.getStamp(), atomicInteger.getStamp() + 1));
            System.out.println("A2->" + atomicInteger.getStamp());

            System.out.println(atomicInteger.compareAndSet(2, 1,
                    atomicInteger.getStamp(), atomicInteger.getStamp() + 1));
            System.out.println("A3->" + atomicInteger.getStamp());

        },"A").start();

        // 乐观锁原理相同
        new Thread(()->{
            int stamp = atomicInteger.getStamp();// 获得版本号
            System.out.println("B1->" + atomicInteger.getStamp());

            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            System.out.println(atomicInteger.compareAndSet(1, 6, stamp, stamp + 1));
            System.out.println("B2->" + atomicInteger.getStamp());

        },"B").start();
    }
}

• 注意！：Integer有对象缓存机制

Unsafe类

@IntrinsicCandidate
public final int getAndAddInt(Object o, long offset, int delta) {
    int v;
    do {
        v = getIntVolatile(o, offset);
    } while (!weakCompareAndSetInt(o, offset, v, v + delta));// 获取内存地址中的值，自旋锁
    return v;
}

public final int getAndIncrement() {
    return U.getAndAddInt(this, VALUE, 1);
}

各种锁的理解

• 公平锁：不能插队，必须先来后到
• 非公平锁：可以插队，默认都是非公平

public ReentrantLock() {
    sync = new NonfairSync();
}

public ReentrantLock(boolean fair) {
    sync = fair ? new FairSync() : new NonfairSync();
}

• 可重入锁：拿到外面的锁，就可以拿到里面的锁，自动获得

package juc.lock;

public class Demo1 {
    public static void main(String[] args) {
        Phone phone = new Phone();
        new Thread(()->{
            phone.sms();
        }, "A").start();

        new Thread(()->{
            phone.sms();
        }, "B").start();

        /**
         * Asms
         * Acall
         * Bsms
         * Bcall
         */
    }
}

class Phone{
    public synchronized void sms(){
        System.out.println(Thread.currentThread().getName() + "sms");
        call();
    }

    public synchronized void call(){
        System.out.println(Thread.currentThread().getName() + "call");
    }
}

package juc.lock;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class Demo2 {
    public static void main(String[] args) {
        Phone2 phone = new Phone2();
        new Thread(()->{
            phone.sms();
        }, "A").start();

        new Thread(()->{
            phone.sms();
        }, "B").start();

        /**
         * Asms
         * Acall
         * Bsms
         * Bcall
         */
    }
}

class Phone2{
    Lock lock = new ReentrantLock();

    public void sms(){

        lock.lock();// 和call的锁不同，是两把锁
        try {
            System.out.println(Thread.currentThread().getName() + "sms");
            call();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void call(){

        lock.lock();
        try {
            System.out.println(Thread.currentThread().getName() + "call");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}

• 自旋锁

package juc.lock;

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

public class MySpinlock {
    AtomicReference<Thread> atomicReference = new AtomicReference<>();
    // 加锁
    public void myLock() {
        Thread thread = Thread.currentThread();
        System.out.println(Thread.currentThread().getName() + " -> mylock");

        // 自旋锁
        while (!atomicReference.compareAndSet(null, thread)){

        }
    }

    // 解锁
    public void myUnLock() {
        Thread thread = Thread.currentThread();
        System.out.println(Thread.currentThread().getName() + " -> myUnlock");
        atomicReference.compareAndSet(thread, null);
    }

    public static void main(String[] args) throws InterruptedException {
        MySpinlock mySpinlock = new MySpinlock();
        new Thread(()->{
            mySpinlock.myLock();
            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                mySpinlock.myUnLock();
            }
        }, "A").start();

        TimeUnit.SECONDS.sleep(1);

        new Thread(()->{
            mySpinlock.myLock();
            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                mySpinlock.myUnLock();
            }
        }, "B").start();

        /**
         * A -> mylock
         * B -> mylock
         * A -> myUnlock
         * B -> myUnlock
         */
    }
}

• 死锁
  • 使用“jps -l”定位进程号
  • 使用“jstack 进程号”寻找死锁问题