单例模式（饿汉式，懒汉式）

------改进后线程安全------

// 懒汉式
class LazySingleton {
    // 私有化构造器
    private LazySingleton() {}
    // 类的内部创建实例
    private static LazySingleton instance = null;
     
    public static LazySingleton getInstance() {
        if (instance == null) {
            // 外面加一层的原因：if判断比synchronized效率高
            synchronized (LazySingleton.class) {
                if (instance == null) {
                    instance = new LazySingleton();
                }
            }
        }
        return instance;
    }
}
// 测试输出的地址是否一致
public class SingletonTest {
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new Thread() {
                @Override
                public void run() {
                    System.out.println(LazySingleton.getInstance());
                }
            }.start();
        }
    }
}

------本身线程安全------

// 饿汉模式
class HungrySingleton {
    // 私有化构造器
    private HungrySingleton() {}
    
    private static HungrySingleton instance = new HungrySingleton();
    
    public static HungrySingleton getInstance() {
        return instance;
    }
}
// 测试输出的地址是否一致
public class SingletonTest {
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new Thread() {
                @Override
                public void run() {
                    System.out.println(HungrySingleton.getInstance());
                }
            }.start();
        }
    }
}

标签：饿汉,class,LazySingleton,懒汉,instance,static,单例,public,HungrySingleton
来源： https://www.cnblogs.com/lxh-daniel/p/16672178.html