单例与线程安全

饿汉式--本身线程安全

      在类加载的时候，已经进行实例化

/**
 * 饿汉式单例
 * 类加载的时候就实例化
 */
public class HungerSingleton {

    private static HungerSingleton ourInstant = new HungerSingleton();

    public static HungerSingleton getInstance(){
        return ourInstant;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                System.out.println(getInstance().toString());
            }).start();
        }
    }
}

懒汉式--非线程安全 

package threadTest;

/**
 * 懒汉式实例
 * 在需要的时候再实例化
 */
public class LazySingleton {
    private static LazySingleton lazyInstance = null;

    private static LazySingleton getInstance(){
//        判断实力是否为空
        if(null==lazyInstance){
            try {
                Thread.sleep(1000l);
            }catch (InterruptedException e){
                e.printStackTrace();
            }

            lazyInstance = new LazySingleton();
        }
        return lazyInstance;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                System.out.println(getInstance().toString());
            }).start();
        }
    }
}

改成线程安全 双重判定

/**
 * 懒汉式实例
 * 在需要的时候再实例化
 */
public class LazySingleton {
    private static LazySingleton lazyInstance = null;

    private static LazySingleton getInstance(){
//        判断实力是否为空
        if(null==lazyInstance){
            try {
                Thread.sleep(1000l);
            }catch (InterruptedException e){
                e.printStackTrace();
            }
            synchronized (LazySingleton.class){
                if(null==lazyInstance){
                    lazyInstance = new LazySingleton();
                }
            }

        }
        return lazyInstance;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                System.out.println(getInstance().toString());
            }).start();
        }
    }
}