设计模式-单例模式

单例模式仅创建一个实例

package designpatterns;
/**
 * 双检锁单例模式 懒汉式 线程安全 高性能
 */
public class Singleton {
    /**
     * volatile 关键字保证实例在多线程中的可见性以及防止指令重排序 创建实例的过程可以被分解成3个步骤 1. memery = allocate();
     * // 分配内存空间 2. initialize(memery); // 初始化内存空间 3. instance = memery; //
     * 将内存地址赋值给instance
     */
    private static volatile Singleton instance;

    /**
     * 构造函数私有化防止被调用
     */
    private Singleton() {

    }

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

    public static void main(String[] args) {
        MyThread[] threads = new MyThread[5];
        for (int i = 0; i < 5; i++) {
            threads[i] = new MyThread(String.format("Thread-%d", i + 1));
            threads[i].start();
        }
    }
}

class MyThread extends Thread {
    String name;

    public MyThread(String name) {
        this.name = name;
    }

    @Override
    public void run() {
        Singleton instance = Singleton.getInstance();
        System.out.println(this.name + ":" + instance.hashCode());
    }
}