完美的单例模式

在多线程下懒加载保证只实例化一次。

第一种双重判断：

public class SingletonTest {
    private static SingletonTest INSTANCE;

    private SingletonTest() {
    }

    public static SingletonTest getInstance() {
        if (INSTANCE == null) {
            synchronized (SingletonTest.class) {
                if (INSTANCE == null) {
                    INSTANCE = new SingletonTest();
                }
            }
        }
        return INSTANCE;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(() -> {
                System.out.println(SingletonTest.getInstance().hashCode());
            }).start();
        }
    }
}

第二种内部类，JVM会保证内部类只加载一次：

public class SingletonTest {

    private SingletonTest() {
    }

    private static class SingletonTestInnerClass {
        private static final SingletonTest INSTANCE = new SingletonTest();
    }

    public static SingletonTest getInstance() {
        return SingletonTestInnerClass.INSTANCE;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(() -> {
                System.out.println(SingletonTest.getInstance().hashCode());
            }).start();
        }
    }
}

第三种使用枚举：

public enum SingletonTestEnum {
    INSTANCE;

    public void test() {
    }

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(() -> {
                System.out.println(SingletonTestEnum.INSTANCE.hashCode());
            }).start();
        }
    }
}

 

注：这些是自己学习的笔记，如有不准确，欢迎指出！