Java 23种设计模式 之单例模式 7种实现方式

1、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 只容许在内部进行实例的建立
    private LazySingleton() {
    }


    private static LazySingleton instance = null;

    //建立实例
    public static LazySingleton getInstance() {

        if (instance == null) {
            instance = new LazySingleton();
        }

        return instance;
    }
}

2、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 只容许在内部进行实例的建立
    private SynchronizedLazySingleton() {
    }


    private static SynchronizedLazySingleton instance = null;

    //建立实例  线程安全
    public static synchronized SynchronizedLazySingleton getInstance() {

        if (instance == null) {
            instance = new SynchronizedLazySingleton();
        }

        return instance;
    }

}

3、饿汉式（线程安全）

package com.java.singleton;

//饿汉式
public class HungrySingleton {

    //私有构造方法 只容许在内部进行实例的建立
    private HungrySingleton() {
    }

    //静态初始化 由JVM保证线程安全
    private static HungrySingleton instance = new HungrySingleton();

    //建立实例
    public static HungrySingleton getInstance() {

        return instance;
    }

}

4、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 只容许在内部进行实例的建立
    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";


    //建立实例
    public static CacheSingleton getInstance() {

        //先从缓存中取 没有就建立并放入缓存  有就返回
        CacheSingleton instance = (CacheSingleton) map.get(DEFAULT_KEY);

        if (instance == null) {

            instance = new CacheSingleton();
            map.put(DEFAULT_KEY, instance);
        }

        return instance;
    }

}

5、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 只容许在内部进行实例的建立
    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做用于共享内存 相似于互斥锁
    private volatile static DoubleCheckedLockingSingleton instance = null;

    //建立实例
    public static DoubleCheckedLockingSingleton getInstance() {

        if (instance == null) {

            synchronized (DoubleCheckedLockingSingleton.class) {

                if (instance == null) {
                    instance = new DoubleCheckedLockingSingleton();
                }
            }

        }

        return instance;
    }


}

6、 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 只容许在内部进行实例的建立
    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();
    }


    //建立实例
    public static LazyInitializationHolderClassSingleton getInstance() {

        return LazyInitializationHolder.instance;
    }


}

7、枚举实现单例

package com.java.singleton;

public enum EnumSingleton {

    //枚举类的每一个元素 都表明一个单例
    uniqueEnumSingleton;


    public  void method() {

        System.out.println("EnumSingleton"+uniqueEnumSingleton.hashCode());
    }

}

8、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");
    }


}

9、测试结果