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 Class
public 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)
@SpringBootTest
public 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
