单例&多线程

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单例最常见的模式为：懒汉式和饿汉式，懒汉式在调用方法的时候才去实例化对象，饿汉式则是直接实例化对象。在多线程模式中，考虑到性能和线程安全，我们一般选择static inner class或者double check instance的单例模式。在性能提高的同时保证了线程安全

static inner class（静态内部类的形式）

public class Singletion {private static class InnerSingletion {private static Singletion single = new Singletion();}public static Singletion getInstance() {return InnerSingletion.single;}}

懒加载（懒汉模式）（double check instance）

public class DubbleSingleton {private static DubbleSingleton ds;public  static DubbleSingleton getDs(){if(ds == null){try {//模拟初始化对象的准备时间...Thread.sleep(3000);} catch (InterruptedException e) {e.printStackTrace();}synchronized (DubbleSingleton.class) {if(ds == null){ds = new DubbleSingleton();}}}return ds;}public static void main(String[] args) {Thread t1 = new Thread(new Runnable() {@Overridepublic void run() {System.out.println(DubbleSingleton.getDs().hashCode());}},"t1");Thread t2 = new Thread(new Runnable() {@Overridepublic void run() {System.out.println(DubbleSingleton.getDs().hashCode());}},"t2");Thread t3 = new Thread(new Runnable() {@Overridepublic void run() {System.out.println(DubbleSingleton.getDs().hashCode());}},"t3");t1.start();t2.start();t3.start();}}

打印结果：

128627259012862725901286272590

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