java两个线程对变量进行加1操作实例分析

1--错误的常规写法

public static int i=0;public static void add(){    i=i+1;    action();}public static void action(){    System.out.println("==>"+Thread.currentThread().getName()+":"+i);}public static void main(String[] args) throws InterruptedException {    Thread t1 = new Thread(SysUserServiceImpl::add,"t1");    Thread t2= new Thread(SysUserServiceImpl::add,"t2");    t1.start();    t2.start();}运行结果==>==>t1:1==>t2:2
==>t1:2==>t2:1
==>t1:2==>t2:2

public class ThreadException {    public static volatile int i=0;    public static void add(){        synchronized (ThreadException.class){            i=i+1;            action();        }    }    public static void action(){        System.out.println("==>"+Thread.currentThread().getName()+":"+i);    }    public static void main(String[] args) throws InterruptedException {        Thread t1 = new Thread(ThreadException::add,"t1");        Thread t2= new Thread(ThreadException::add,"t2");        t1.start();        t2.start();
    }}

优点：实现简单

缺点：加锁粒度大，性能低下，分布式环境，多JVM条件，synchronized失效，synchronized 只是本地锁，锁的也只是当前jvm下的对象，在分布式场景下，要用分布式锁

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改进方法2 AtomicInteger

public class ThreadException {    private static AtomicInteger num = new AtomicInteger(0);    public static void add(){        int i = num.getAndIncrement();        action(i);    }    public static void action(int i){        System.out.println("由"+i+"==>"+Thread.currentThread().getName()+":"+num);    }    public static void main(String[] args) throws InterruptedException {        Thread t1 = new Thread(ThreadException::add,"t1");        Thread t2= new Thread(ThreadException::add,"t2");        t1.start();        t2.start();    }}

改进方法3  lock

public class ThreadException {    public static volatile int i=0;    public static void action(){        System.out.println("==>"+Thread.currentThread().getName()+":"+i);    }
    static Lock lock=new ReentrantLock();    public static void inc() {        lock.lock();        try {            Thread.sleep(1);            i=i+1;            action();        } catch (InterruptedException e) {            e.printStackTrace();        } finally {            lock.unlock();        }    }    public static void main(String[] args) throws InterruptedException {        Thread t1 = new Thread(ThreadException::inc,"t1");        Thread t2= new Thread(ThreadException::inc,"t2");        t1.start();        t2.start();    }}