单例模式

1.单例模式

1.简单的单例模式

package Slogan;

public class LazySingletonTest {
    public static void main(String[] args) {

            LazySingleton instance1 = LazySingleton.getInstance();
            System.out.println("instance1 = " + instance1);
            LazySingleton instance = LazySingleton.getInstance();
            System.out.println("instance2 = " + instance2);
    }
}

class LazySingleton {
    private static LazySingleton lazySingleton = null;
    LazySingleton() {

    }
    public  static  LazySingleton getInstance() {
        if (lazySingleton == null) {
            try {
                Thread.sleep(200);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            lazySingleton = new LazySingleton();
        }
        return lazySingleton;
    }

}

此时屏幕打印出的两个对象是一致的，但是在多线程的情况下，会产生一些问题

2.多线程下的单例模式

package Slogan;

public class LazySingletonTest {
    public static void main(String[] args) {

        new Thread(() -> {
            LazySingleton instance = LazySingleton.getInstance();
            System.out.println("instance = " + instance);
        }).start();

        new Thread(() -> {
            LazySingleton instance = LazySingleton.getInstance();
            System.out.println("instance = " + instance);
        }).start();

    }
}

class LazySingleton {
    private static LazySingleton lazySingleton = null;
    LazySingleton() {

    }

    public synchronized static LazySingleton getInstance() {
        if (lazySingleton == null) {
            try {
                Thread.sleep(200);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            lazySingleton = new LazySingleton();
        }
        return lazySingleton;
    }

}

需要在方法的前面加锁，也就是synchronized

但是仍是有优化空间

3.优化多线程单例模式

package Slogan;

public class LazySingletonTest {
    public static void main(String[] args) {

        new Thread(() -> {
            LazySingleton instance = LazySingleton.getInstance();
            System.out.println("instance = " + instance);
        }).start();

        new Thread(() -> {
            LazySingleton instance = LazySingleton.getInstance();
            System.out.println("instance = " + instance);
        }).start();

    }
}

class LazySingleton {
    private static LazySingleton lazySingleton = null;

    LazySingleton() {

    }

    public static LazySingleton getInstance() {
        if (lazySingleton == null) {
             //第一次进行加锁
            synchronized (LazySingleton.class) {
                //为了防止并发情况下多次new对象
                if (lazySingleton == null) {
                    lazySingleton = new LazySingleton();
                }
            }
        }
        return lazySingleton;
    }

}