Java 23种设计模式 之单例模式 7种实现方式

一、懒汉式（线程不安全）package com.java.singleton;//懒汉式 线程不安全public class LazySingleton { //私有构造方法 只允许在内部进行实例的创建 private LazySingleton() { } private static LazySingleton instance = null; //创建实例 public static LazySingleton getInstance() { if (instance == null) { instance = new LazySingleton(); } return instance; }}二、懒汉式（线程安全）package com.java.singleton;//懒汉式 线程安全public class SynchronizedLazySingleton { //私有构造方法 只允许在内部进行实例的创建 private SynchronizedLazySingleton() { } private static SynchronizedLazySingleton instance = null; //创建实例 线程安全 public static synchronized SynchronizedLazySingleton getInstance() { if (instance == null) { instance = new SynchronizedLazySingleton(); } return instance; }}三、饿汉式（线程安全）package com.java.singleton;//饿汉式public class HungrySingleton { //私有构造方法 只允许在内部进行实例的创建 private HungrySingleton() { } //静态初始化 由JVM保证线程安全 private static HungrySingleton instance = new HungrySingleton(); //创建实例 public static HungrySingleton getInstance() { return instance; }}四、缓存实现（线程不安全）package com.java.singleton;import java.util.HashMap;import java.util.Map;//线程不安全 缓存实现public class CacheSingleton { //私有构造方法 只允许在内部进行实例的创建 private CacheSingleton() { } //构造缓存容器 private static Map<String, CacheSingleton> map = new HashMap<>(); //构造默认的存放key private static final String DEFAULT_KEY = “Cache”; //创建实例 public static CacheSingleton getInstance() { //先从缓存中取 没有就创建并放入缓存 有就返回 CacheSingleton instance = (CacheSingleton) map.get(DEFAULT_KEY); if (instance == null) { instance = new CacheSingleton(); map.put(DEFAULT_KEY, instance); } return instance; }}五、双重检查加锁（懒汉式 线程安全的进一步优化）package com.java.singleton;//双重检查加锁 懒汉式在方法上加synchronized的进一步优化public class DoubleCheckedLockingSingleton { //私有构造方法 只允许在内部进行实例的创建 private DoubleCheckedLockingSingleton() { } //volatile 修饰的变量不会被本地线程缓存 对该变量的读写直接作用于共享内存 类似于互斥锁 private volatile static DoubleCheckedLockingSingleton instance = null; //创建实例 public static DoubleCheckedLockingSingleton getInstance() { if (instance == null) { synchronized (DoubleCheckedLockingSingleton.class) { if (instance == null) { instance = new DoubleCheckedLockingSingleton(); } } } return instance; }}六、 Lazy Initialization Holder Class实现单例package com.java.singleton;//静态内部类 Lazy Initialization Holder Classpublic class LazyInitializationHolderClassSingleton { //私有构造方法 只允许在内部进行实例的创建 private LazyInitializationHolderClassSingleton() { } /* * 静态成员式内部类 该内部类的实例对象与外部类的实例无绑定关系 * 有且只有在LazyInitializationHolder被调用instance时 在会对对象实例进行装载 * 从而实现延时加载 / private static class LazyInitializationHolder { / * 静态初始化器 由JVM保证线程安全 * */ private static LazyInitializationHolderClassSingleton instance = new LazyInitializationHolderClassSingleton(); } //创建实例 public static LazyInitializationHolderClassSingleton getInstance() { return LazyInitializationHolder.instance; }}七、枚举实现单例package com.java.singleton;public enum EnumSingleton { //枚举类的每个元素 都代表一个单例 uniqueEnumSingleton; public void method() { System.out.println(“EnumSingleton”+uniqueEnumSingleton.hashCode()); }}八、测试类package com.java.singleton;import org.junit.Test;import org.junit.runner.RunWith;import org.springframework.boot.test.context.SpringBootTest;import org.springframework.test.context.junit4.SpringRunner;@RunWith(SpringRunner.class)@SpringBootTestpublic class SingletonApplicationTests { @Test public void testCacheSingleton() { for (int i = 0; i < 3; i++) { System.out.println(“CacheSingleton\t” + CacheSingleton.getInstance()); } System.out.println("——————————————————————-\n"); } @Test public void testDoubleCheckedLockingSingleton() { for (int i = 0; i < 3; i++) { System.out.println(“DoubleCheckedLockingSingleton\t” + DoubleCheckedLockingSingleton.getInstance()); } System.out.println("——————————————————————-\n"); } @Test public void testEnumSingleton() { for (int i = 0; i < 3; i++) { EnumSingleton.uniqueEnumSingleton.method(); } System.out.println("——————————————————————-\n"); } @Test public void testHungrySingleton() { for (int i = 0; i < 3; i++) { System.out.println(“HungrySingleton\t” + HungrySingleton.getInstance()); } System.out.println("——————————————————————-\n"); } @Test public void testLazyInitializationHolderClassSingleton() { for (int i = 0; i < 3; i++) { System.out.println(“LazyInitializationHolderClassSingleton\t” + LazyInitializationHolderClassSingleton.getInstance()); } System.out.println("——————————————————————-\n"); } @Test public void testLazySingleton() { for (int i = 0; i < 3; i++) { System.out.println(“LazySingleton\t” + LazySingleton.getInstance()); } System.out.println("——————————————————————-\n"); } @Test public void testSynchronizedLazySingleton() { for (int i = 0; i < 3; i++) { System.out.println(“SynchronizedLazySingleton\t” + SynchronizedLazySingleton.getInstance()); } System.out.println("——————————————————————-\n"); }}九、测试结果