设计模式-单例模式设计模式qq16498553的博客-

单列模式是什么？

单列模式是创建型模式的一种，同样一个对象(实例)在内存中只会有一份，只并只有一个全局的访问点。单列分为好多种实现，其中有线程安全和线程不安全，大类有懒汉和饿汉；

单列模式可以干嘛？

可以解决一个对象重复的创建和销毁，减少内存的开销，保存只在内存只有一份。就比如:一个学校一般只有一个正校长;

代码地址:https://gitee.com/hong99/design-model/issues/I1IMES

代码实现

懒汉：都是被别人推着才去做的，这样线程不安全(也有安全)
饿汉: 先创建对象，每次直接返回实例化后单例结果。
比如:
我去吃饭一到饭管菜都已经上好了，就直接开吃了，这个是饿汉;
我去吃饭一到那里是等了半天，一边上菜一边催，才好不容易把菜上全了，这样的叫懒汉，要别人催才给;

/**  *  * 功能描述:单例(俄汉)  *  * @param:  * @return:  * @auther: csh  * @date: 2020/5/17 10:49  */ public class HungrySingle { //创建 HungrySingle 的一个对象    、 private static HungrySingle instance = new HungrySingle(); //让构造函数为 private，这样该类就不会被实例化 private HungrySingle() { } //获取唯一可用的对象 public static HungrySingle getInstance() { return instance; } public void showMessage() {         System.out.println("Hello World!"); } } 

/**  *  * 功能描述:懒汉（线程不安全）  *  * @param:  * @return:  * @auther: csh  * @date: 2020/5/17 10:55  */ public class LazySingleton { private static LazySingleton instance; private LazySingleton(){} public static LazySingleton getInstance() { if (instance == null) {             instance = new LazySingleton(); } return instance; } } 

/**  * @Auther: csh  * @Date: 2020/5/17 11:27  * @Description:懒汉 （线程安全） 缺点：性能较低  */ public class LazySingleSafery { private static LazySingleSafery lazySingleSafery; private LazySingleSafery(){ } /**      *      * 功能描述:使用了同步类锁导致性能低下      *      * @param:      * @return:      * @auther: csh      * @date: 2020/5/17 11:32      */ public static synchronized LazySingleSafery getInstance(){ if(lazySingleSafery == null){             lazySingleSafery = new LazySingleSafery(); } return lazySingleSafery; } } 

/**  * @Auther: csh  * @Date: 2020/5/17 11:33  * @Description:双检锁/双重校验锁（DCL，即 double-checked locking）  * 性能较高  */ public class DoubleCheckLazySingleSafery { private static DoubleCheckLazySingleSafery instance; private DoubleCheckLazySingleSafery(){ } public static DoubleCheckLazySingleSafery getInstance(){ if(instance == null){ synchronized (DoubleCheckLazySingleSafery.class){ if(instance == null){                     instance = new DoubleCheckLazySingleSafery(); } } } return instance; } } 

/**  * @Auther: csh  * @Date: 2020/5/17 11:39  * @Description:登记式/静态内部类  线程安全  */ public class StaticInnerSingle { private static class SingleHolder{ private static final StaticInnerSingle INSTANCE = new StaticInnerSingle(); } private StaticInnerSingle(){ } public static final StaticInnerSingle getInstance(){ return SingleHolder.INSTANCE; } } 

/**  * @Auther: csh  * @Date: 2020/5/17 10:43  * @Description:实现  */ public class SingletonPatternDemo { public static void main(String[] args) { //不合法的构造函数 //编译时错误：构造函数 HungrySingle() 是不可见的 //HungrySingle object = new HungrySingle(); //获取唯一可用的对象 饿汉         ExecutorService lazyThreadPool = Executors.newFixedThreadPool(10); for (int i =0;i<10;i++){             lazyThreadPool.execute(new Runnable() { @Override public void run() { try {                         Thread.sleep(10); }catch (Exception e){                         e.printStackTrace(); }                     System.out.println("饿汉（安全）"+ HungrySingle.getInstance()); } }); } //懒汉 线程不安全         ExecutorService hungryThreadPool = Executors.newFixedThreadPool(10); for (int i =0;i<10;i++){             hungryThreadPool.execute(new Runnable() { @Override public void run() { try {                         Thread.sleep(10); }catch (Exception e){                         e.printStackTrace(); }                     System.out.println("懒汉（不安全）"+LazySingleton.getInstance()); } }); } //懒汉 线程安全 性能低         ExecutorService hungrySaferyThreadPool = Executors.newFixedThreadPool(10); for (int i =0;i<10;i++){             hungrySaferyThreadPool.execute(new Runnable() { @Override public void run() { try {                         Thread.sleep(10); }catch (Exception e){                         e.printStackTrace(); }                     System.out.println("懒汉（安全）"+LazySingleSafery.getInstance()); } }); } //懒汉 线程安全 性能高         ExecutorService DoubleCheckThreadPool = Executors.newFixedThreadPool(10); for (int i =0;i<10;i++){             DoubleCheckThreadPool.execute(new Runnable() { @Override public void run() { try {                         Thread.sleep(10); }catch (Exception e){                         e.printStackTrace(); }                     System.out.println("懒汉双重检查（安全）"+DoubleCheckLazySingleSafery.getInstance()); } }); } //懒汉 线程安全 性能高         ExecutorService staticInnerSingleThreadPool = Executors.newFixedThreadPool(10); for (int i =0;i<10;i++){             staticInnerSingleThreadPool.execute(new Runnable() { @Override public void run() { try {                         Thread.sleep(10); }catch (Exception e){                         e.printStackTrace(); }                     System.out.println("登记式/静态内部类（安全）"+StaticInnerSingle.getInstance()); } }); } } } 

结果

"C:Program FilesJavajdk1.8.0_181binjava.exe" "-javaagent:C:Program FilesJetBrainsIntelliJ IDEA 2018.2.3libidea_rt.jar=58612:C:Program FilesJetBrainsIntelliJ IDEA 2018.2.3bin" -Dfile.encoding=UTF-8 -classpath "C:Program FilesJavajdk1.8.0_181jrelibcharsets.jar;C:Program FilesJavajdk1.8.0_181jrelibdeploy.jar;C:Program FilesJavajdk1.8.0_181jrelibextaccess-bridge-64.jar;C:Program FilesJavajdk1.8.0_181jrelibextcldrdata.jar;C:Program FilesJavajdk1.8.0_181jrelibextdnsns.jar;C:Program FilesJavajdk1.8.0_181jrelibextjaccess.jar;C:Program FilesJavajdk1.8.0_181jrelibextjfxrt.jar;C:Program FilesJavajdk1.8.0_181jrelibextlocaledata.jar;C:Program FilesJavajdk1.8.0_181jrelibextnashorn.jar;C:Program FilesJavajdk1.8.0_181jrelibextsunec.jar;C:Program FilesJavajdk1.8.0_181jrelibextsunjce_provider.jar;C:Program FilesJavajdk1.8.0_181jrelibextsunmscapi.jar;C:Program FilesJavajdk1.8.0_181jrelibextsunpkcs11.jar;C:Program FilesJavajdk1.8.0_181jrelibextzipfs.jar;C:Program FilesJavajdk1.8.0_181jrelibjavaws.jar;C:Program FilesJavajdk1.8.0_181jrelibjce.jar;C:Program FilesJavajdk1.8.0_181jrelibjfr.jar;C:Program FilesJavajdk1.8.0_181jrelibjfxswt.jar;C:Program FilesJavajdk1.8.0_181jrelibjsse.jar;C:Program FilesJavajdk1.8.0_181jrelibmanagement-agent.jar;C:Program FilesJavajdk1.8.0_181jrelibplugin.jar;C:Program FilesJavajdk1.8.0_181jrelibresources.jar;C:Program FilesJavajdk1.8.0_181jrelibrt.jar;D:ideaWorkSpacedesign-modeltargetclasses" com.hong.base.singlemodel.SingletonPatternDemo 饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7 饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7 饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7 饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7 饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7 饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7 饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7 饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7 饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7 饿汉（安全）com.hong.base.singlemodel.HungrySingle@2d164da7 懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752 懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752 懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752 懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752 懒汉（不安全）com.hong.base.singlemodel.LazySingleton@710f2fc4 懒汉（不安全）com.hong.base.singlemodel.LazySingleton@1bd08742 懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752 懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752 懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752 懒汉（不安全）com.hong.base.singlemodel.LazySingleton@7c716752 懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412 懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412 懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412 懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412 懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412 懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412 懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412 懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412 懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412 懒汉（安全）com.hong.base.singlemodel.LazySingleSafery@a49a412 懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d 懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d 懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d 懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d 懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d 懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d 懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d 懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d 懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d 懒汉双重检查（安全）com.hong.base.singlemodel.DoubleCheckLazySingleSafery@63db9a8d 登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c 登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c 登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c 登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c 登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c 登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c 登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c 登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c 登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c 登记式/静态内部类（安全）com.hong.base.singlemodel.StaticInnerSingle@6d037c 

最后我想说，判断一个对象是不是单列只需要看对象里面有没有private 类名 (){} 如果有肯定是单例。

代码地址:https://gitee.com/hong99/design-model/issues/I1IMES

参考资料：

《大话设计模式》
《研磨设计模式》
《设计模式之禅》
《Java与模式》