【设计模式】9.创建型模式-单例模式(Singleton)
一、描述:
某个类只能生成一个实例,该类提供了一个全局访问点供外部获取该实例,其拓展是有限多例模式。
二、特点:
1.单例类只有一个实例对象
2.该单例对象必须由单例类自行创建
3.单例类对外提供一个访问该单例的全局访问点。
三、优点:
1.单例模式可以保证内存里只有一个实例,减少了内存的开销。
2.可以避免对自愿的多重占用。
3.单例模式设置全局访问点,可以优化和共享资源的访问。
四、应用场景:
1.需要频繁创建的一些类,使用单例可以降低系统的内存压力,减少GC。
2.某些类创建实例时占用资源较多,或实例化耗时较长,且经常使用。
3.某些类需要频繁实例化,而创建的对象又频繁被销毁的时候,如多线程的线程池,网络连接池等。
4.频繁访问数据库或文件的对象。
5.当对象需要被共享的场合。由于单例模式只允许创建一个对象,共享该对象可以节省内存,并加快对象访问速度。如Web中的配置对象、数据库的连接池等。
五、实现:
1.饿汉模式(类初始化时就创建,线程安全)
public class HungaryTest { private static final HungaryTest hungaryTest = new HungaryTest(); private HungaryTest() { } public static HungaryTest getHungaryTest() { return hungaryTest; } }
2.懒汉模式(需要的时候再创建,加volatile线程安全)
public class SafetyLazy2Test { private static volatile SafetyLazy2Test safetyLazy2Test; private SafetyLazy2Test() { } public static synchronized SafetyLazy2Test getLazyTest() { if (safetyLazy2Test == null) { System.out.println("safetyLazy2Test是空"); safetyLazy2Test = new SafetyLazy2Test(); } System.out.println("safetyLazy2Test不是空"); return safetyLazy2Test; } }
双重检测机制(DCL机制),同样可以实现线程安全
public class SafetyLazyTest { private static volatile SafetyLazyTest safetyLazyTest; private SafetyLazyTest() { } public static SafetyLazyTest getLazyTest() { if (safetyLazyTest == null) { synchronized (SafetyLazyTest.class) { if (safetyLazyTest == null) { System.out.println("lazyTest是空"); safetyLazyTest = new SafetyLazyTest(); } } } System.out.println("lazyTest不是空"); return safetyLazyTest; } }
线程测试:
@Slf4j public class SafetyLazyThread extends Thread { @Override public void run() { super.run(); log.info("safetyLazy:{}", SafetyLazyTest.getLazyTest().hashCode()); log.info("safetyLazy2:{}", SafetyLazy2Test.getLazyTest().hashCode()); } } public class ThreadTest { public static void main(String[] args) { safetyThreadTest(); } private static void safetyThreadTest() { SafetyLazyThread t1 = new SafetyLazyThread(); SafetyLazyThread t2 = new SafetyLazyThread(); SafetyLazyThread t3 = new SafetyLazyThread(); SafetyLazyThread t4 = new SafetyLazyThread(); t1.start(); t2.start(); t3.start(); t4.start(); } }
运行结果:
lazyTest是空 lazyTest不是空 lazyTest不是空 lazyTest不是空 lazyTest不是空 10:55:32.381 [Thread-2] INFO com.my.project.designmode.singletoninstance.SafetyLazyThread - safetyLazy:1149747804 10:55:32.381 [Thread-1] INFO com.my.project.designmode.singletoninstance.SafetyLazyThread - safetyLazy:1149747804 10:55:32.381 [Thread-0] INFO com.my.project.designmode.singletoninstance.SafetyLazyThread - safetyLazy:1149747804 10:55:32.381 [Thread-3] INFO com.my.project.designmode.singletoninstance.SafetyLazyThread - safetyLazy:1149747804 safetyLazy2Test是空 safetyLazy2Test不是空 safetyLazy2Test不是空 safetyLazy2Test不是空 10:55:32.387 [Thread-2] INFO com.my.project.designmode.singletoninstance.SafetyLazyThread - safetyLazy2:1682859985 10:55:32.387 [Thread-0] INFO com.my.project.designmode.singletoninstance.SafetyLazyThread - safetyLazy2:1682859985 10:55:32.387 [Thread-1] INFO com.my.project.designmode.singletoninstance.SafetyLazyThread - safetyLazy2:1682859985 safetyLazy2Test不是空 10:55:32.387 [Thread-3] INFO com.my.project.designmode.singletoninstance.SafetyLazyThread - safetyLazy2:1682859985
3.静态内部类(线程安全,推荐使用。调用时才会创建。)
@Slf4j public class StaticTest { private static class StaticSingletonTest { private static final StaticTest staticTest = new StaticTest(); } private StaticTest() { } public static StaticTest getInstance() { return StaticSingletonTest.staticTest; } }
4.枚举模式
public enum EnumSingleton { INSTANCE; public EnumSingleton getInstance(){ return INSTANCE; } }