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1. 懒汉式线程不安全

public class LazyUnsafeSingleton {private static LazyUnsafeSingleton singleton;private LazyUnsafeSingleton(){}public static LazyUnsafeSingleton getInstance() {if (singleton == null) singleton = new LazyUnsafeSingleton();return singleton;}
}

2. 饿汉式（线程安全）

public class HungrySingleton {private static HungrySingleton singleton = new HungrySingleton();private HungrySingleton(){}public static HungrySingleton getInstance() {return singleton;}
}

3. 懒汉式线程安全

volatile
synchronized

public class LazySafeSingleton {private static volatile LazySafeSingleton singleton;private LazySafeSingleton(){}public static synchronized LazySafeSingleton getInstance() {if (singleton == null) singleton = new LazySafeSingleton();return singleton;}
}

4. 双重检查锁

public class DoubleCheckedLockSingleton {private static volatile DoubleCheckedLockSingleton singleton;private DoubleCheckedLockSingleton(){}public static DoubleCheckedLockSingleton getInstance() {if (singleton == null) {synchronized (DoubleCheckedLockSingleton.class) {if (singleton == null) {singleton = new DoubleCheckedLockSingleton();}}}return singleton;}
}

5. 静态内部类

public class StaticInnerClassSingleton {private static StaticInnerClassSingleton singleton;private static class SingletonHolder {private static StaticInnerClassSingleton INSTANCE = new StaticInnerClassSingleton();}public static StaticInnerClassSingleton getInstance() {return SingletonHolder.INSTANCE;}
}

6. 枚举类型

public enum EnumSingleton {SINGLETON;public String doSomeThing() {return "hello world";}
}

测试类

public class SingletonTest {public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException, InterruptedException {// 1. 懒汉式线程非安全LazyUnsafeSingleton lazyUnsafeSingleton = LazyUnsafeSingleton.getInstance();// 2. 饿汉式HungrySingleton hungrySingleton = HungrySingleton.getInstance();// 3. 懒汉式线程安全LazySafeSingleton lazySafeSingleton = LazySafeSingleton.getInstance();// 4. 双重检查锁DoubleCheckedLockSingleton doubleCheckedLockSingleton = DoubleCheckedLockSingleton.getInstance();// 5. 静态内部类StaticInnerClassSingleton staticInnerClassSingleton = StaticInnerClassSingleton.getInstance();// 6. 枚举类型EnumSingleton enumSingleton = EnumSingleton.SINGLETON;enumSingleton.doSomeThing();// 反射无法破坏枚举类型的单例模式Class<EnumSingleton> enumSingletonClass = EnumSingleton.class;
//        Constructor<EnumSingleton> declaredConstructor = enumSingletonClass.getDeclaredConstructor();Constructor<?> declaredConstructor = enumSingletonClass.getDeclaredConstructors()[0];declaredConstructor.setAccessible(true);Object enumSingleton1 = declaredConstructor.newInstance(); // java.lang.IllegalArgumentException// 7. 反射破坏非枚举类型的单例模式// （1）获取 Singleton 类的 Class 对象Class<LazyUnsafeSingleton> lazyUnsafeSingletonClass = LazyUnsafeSingleton.class;// （2）获取 Singleton 类的私有构造函数Constructor<LazyUnsafeSingleton> constructor = lazyUnsafeSingletonClass.getDeclaredConstructor();// （3）设置私有构造函数的可访问性constructor.setAccessible(true);// （4）使用私有构造函数创建新的实例LazyUnsafeSingleton lazyUnsafeSingletonFromReflect = constructor.newInstance();// 打印新创建的实例和原始实例是否相等System.out.println("Original Instance: " + LazyUnsafeSingleton.getInstance());System.out.println("New Instance: " + lazyUnsafeSingletonFromReflect);System.out.println("Are instances equal? " + (LazyUnsafeSingleton.getInstance() == lazyUnsafeSingletonFromReflect));}
}