Java 线程同步

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

public class Main {
    private Lock lock = new ReentrantLock();
    public void func1(){
        lock.lock();
        try {
            for (int i = 0; i < 10; i++) {
                System.out.println(i+" ");
            }
        }finally {
            lock.unlock();
        }
    }
    public static void main(String[] args) {
        Main main = new Main();
        ExecutorService executorService = Executors.newCachedThreadPool();
        executorService.execute(main::func1);
        executorService.execute(main::func1);
    }
}

输出:

0 1 2 3 4 5 6 7 8 9 -
0 1 2 3 4 5 6 7 8 9 -

其实这段代码等价于

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Main {
    public synchronized void func1(){
            for (int i = 0; i < 10; i++) {
                System.out.print(i+" ");
            }
        System.out.println("-");
    }
    public static void main(String[] args) {
        Main main = new Main();
        ExecutorService executorService = Executors.newCachedThreadPool();
        executorService.execute(main::func1);
        executorService.execute(main::func1);
    }
}

此时可以发现ReentrantLock中和java关键字synchronized起到的效果是一样的.只不过synchronized是JVM提供的,ReentrantLock是jdk中的类.
性能上在jdk1.8上可以优先选用JVM关键字synchronized

notify

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class NotifyExample {
    public synchronized void before(){
        System.out.println("before");
        notify();
    }
    public synchronized void after(){

        try {
            wait();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("after");
    }

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newCachedThreadPool();
        NotifyExample notifyExample = new NotifyExample();
        executorService.execute(notifyExample::after);
        executorService.execute(notifyExample::before);
    }
}

输出

before
after

fun after中的wait()就是进入等待状态并释放线程锁.然后before线程执行,6行中输出然后notify,之前的进入wait()状态的线程继续执行.