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