设计模式 | 单例模式（singleton）

package designpattern.singleton;

public class HungrySingleton {
    private static HungrySingleton instance = new HungrySingleton();// 静态初始化

    private HungrySingleton() {// 私有化构造方法
    }

    public static HungrySingleton GetInstance() {// 获取实例，static的
        return instance;
    }
}

package designpattern.singleton;

public class LazySingleton {
    private static LazySingleton instance;

    private LazySingleton() {// 私有化构造方法

    }

    public static LazySingleton getInstance() {// 获取实例，static的
        if (instance == null) {
            instance = new LazySingleton();// 方法中创造实例
        }
        return instance;
    }
}

package designpattern.singleton;

public class LazySingleton2 {
    private static LazySingleton2 instance;

    private LazySingleton2() {

    }

    public static synchronized LazySingleton2 getInstance() {// synchronized 修饰
        if (instance == null) {
            instance = new LazySingleton2();
        }
        return instance;
    }
}

package designpattern.singleton;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class LazySingleton3 {
    private static LazySingleton3 instance;
    private static Lock lock = new ReentrantLock();

    private LazySingleton3() {

    }

    public static synchronized LazySingleton3 getInstance() {
        try {
            if (instance == null) {
                lock.lock();
                if (instance == null) {// 有必要再次判断，不然还是存在线程安全问题
                    instance = new LazySingleton3();
                }
                lock.unlock();
            }
        } finally {// 保证锁被释放
            lock.unlock();
        }
        return instance;
    }
}