Java 23種類の設計モードの単例モード7種類の実現方式

一、怠け者式(スレッドが安全ではない)

package com.java.singleton;

//         
public class LazySingleton {

    //                    
    private LazySingleton() {
    }


    private static LazySingleton instance = null;

    //    
    public static LazySingleton getInstance() {

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

        return instance;
    }
}

二、怠け者式(スレッド安全)

package com.java.singleton;

//        
public class SynchronizedLazySingleton {

    //                    
    private SynchronizedLazySingleton() {
    }


    private static SynchronizedLazySingleton instance = null;

    //          
    public static synchronized SynchronizedLazySingleton getInstance() {

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

        return instance;
    }

}

三、餓漢式(スレッド安全)

package com.java.singleton;

//   
public class HungrySingleton {

    //                    
    private HungrySingleton() {
    }

    //       JVM      
    private static HungrySingleton instance = new HungrySingleton();

    //    
    public static HungrySingleton getInstance() {

        return instance;
    }

}

四、キャッシュ実装(スレッドが安全でない)

package com.java.singleton;

import java.util.HashMap;
import java.util.Map;

//          
public class CacheSingleton {

    //                    
    private CacheSingleton() {
    }

    //      
    private static Map map = new HashMap<>();

    //       key
    private static final String DEFAULT_KEY = "Cache";


    //    
    public static CacheSingleton getInstance() {

        //                       
        CacheSingleton instance = (CacheSingleton) map.get(DEFAULT_KEY);

        if (instance == null) {

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

        return instance;
    }

}

五、二重検査ロック(怠け者スレッドセキュリティのさらなる最適化)

package com.java.singleton;


//                synchronized      
public class DoubleCheckedLockingSingleton {

    //                    
    private DoubleCheckedLockingSingleton() {
    }

    //volatile                                       
    private volatile static DoubleCheckedLockingSingleton instance = null;

    //    
    public static DoubleCheckedLockingSingleton getInstance() {

        if (instance == null) {

            synchronized (DoubleCheckedLockingSingleton.class) {

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

        }

        return instance;
    }


}

六、Lazy Initialization Holder Class実現単例

package com.java.singleton;


//      Lazy Initialization Holder Class
public class LazyInitializationHolderClassSingleton {

    //                    
    private LazyInitializationHolderClassSingleton() {
    }

    /*
     *                               
     *      LazyInitializationHolder   instance             
     *         
     */
    private static class LazyInitializationHolder {

        /*
         *         JVM      
         *
         */
        private static LazyInitializationHolderClassSingleton instance = new LazyInitializationHolderClassSingleton();
    }


    //    
    public static LazyInitializationHolderClassSingleton getInstance() {

        return LazyInitializationHolder.instance;
    }


}

七、列挙実現単例

package com.java.singleton;

public enum EnumSingleton {

    //                
    uniqueEnumSingleton;


    public  void method() {

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

}

八、テストクラス

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("-------------------------------------------------------------------
");
    }

    @Test
    public void testDoubleCheckedLockingSingleton() {

        for (int i = 0; i < 3; i++) {

            System.out.println("DoubleCheckedLockingSingleton\t" + DoubleCheckedLockingSingleton.getInstance());
        }
        System.out.println("-------------------------------------------------------------------
");
    }

    @Test
    public void testEnumSingleton() {

        for (int i = 0; i < 3; i++) {

            EnumSingleton.uniqueEnumSingleton.method();
        }
        System.out.println("-------------------------------------------------------------------
");
    }


    @Test
    public void testHungrySingleton() {

        for (int i = 0; i < 3; i++) {

            System.out.println("HungrySingleton\t" + HungrySingleton.getInstance());
        }
        System.out.println("-------------------------------------------------------------------
");
    }


    @Test
    public void testLazyInitializationHolderClassSingleton() {

        for (int i = 0; i < 3; i++) {

            System.out.println("LazyInitializationHolderClassSingleton\t" + LazyInitializationHolderClassSingleton.getInstance());
        }
        System.out.println("-------------------------------------------------------------------
");
    }

    @Test
    public void testLazySingleton() {

        for (int i = 0; i < 3; i++) {

            System.out.println("LazySingleton\t" + LazySingleton.getInstance());
        }
        System.out.println("-------------------------------------------------------------------
");
    }


    @Test
    public void testSynchronizedLazySingleton() {

        for (int i = 0; i < 3; i++) {

            System.out.println("SynchronizedLazySingleton\t" + SynchronizedLazySingleton.getInstance());
        }
        System.out.println("-------------------------------------------------------------------
");
    }


}

九、テスト結果