单例模式场景模拟和问题解决

饿汉式单例

private static Student student = new Student();

不存在线程安全问题

懒汉式单例

线程安全问题

package org.example.Singleton;import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;public class SingletonTest {private static Student student = new Student();private static Car car = null;private static AtomicInteger count = new AtomicInteger(0);private static ExecutorService threadPoolExecutor = Executors.newCachedThreadPool();private static final CountDownLatch latch = new CountDownLatch(15);SingletonTest() throws InterruptedException {threadNonSafeLoad();}public static void main(String[] args) throws InterruptedException {loadInstance();latch.await();System.out.println(count.get());}private static void threadSafeLoad() {}private void threadNonSafeLoad() {//        System.out.println(this.car);if (this.car == null) {count.addAndGet(1);this.car = new Car();}latch.countDown();}private static void loadInstance() {for (int i = 0; i < 15; i++) {
//            Thread.sleep(50);Thread thread = new Thread(() -> {try {new SingletonTest();} catch (InterruptedException e) {throw new RuntimeException(e);}});threadPoolExecutor.execute(thread);}}
}class Student {private String name;
}class Car {private String name;
}

运行结果：

会多次创建`Car`对象
1~15

解决方法-双重判断

    private void threadSafeLoad() {if (this.car == null) {// 避免每次都加锁进行判断synchronized (SingletonTest.class) {if (this.car == null) {count.addAndGet(1);this.car = new Car();}}}latch.countDown();}

    SingletonTest() throws InterruptedException {
//        threadNonSafeLoad();threadSafeLoad();}

运行结果：

1