设计模式-单例模式

1.懒汉式-线程不安全

public class SingletonLazyNoSafe {
    private static SingletonLazyNoSafe singletonLazyNoSafe;

    private SingletonLazyNoSafe() {}

    public static SingletonLazyNoSafe getInstance(){
        if (singletonLazyNoSafe ==null){
            singletonLazyNoSafe = new SingletonLazyNoSafe();
        }
        return singletonLazyNoSafe;
    }
}

2.懒汉式-线程安全

public class SingletonLazySafe {
    private static SingletonLazySafe singletonLazySafe;
    private SingletonLazySafe(){
    }
    public synchronized static SingletonLazySafe getInstance(){
        if (singletonLazySafe == null){
            singletonLazySafe = new SingletonLazySafe();
        }
        return singletonLazySafe;
    }
}

3.饿汉式-线程安全

public class SingletonHungerSafe {
    private static SingletonHungerSafe singletonHungerSafe = new SingletonHungerSafe();
    private SingletonHungerSafe(){}

    public static SingletonHungerSafe getInstance(){
        return singletonHungerSafe;
    }
}

4.饿汉式-静态代码块-线程安全

public class SingletonHungerStaticBlockSafe {
    private static SingletonHungerStaticBlockSafe singletonHungerStaticBlockSafe;
    static {
        singletonHungerStaticBlockSafe = new SingletonHungerStaticBlockSafe();
    }
    private SingletonHungerStaticBlockSafe(){}

    public static SingletonHungerStaticBlockSafe getInstance(){
        return singletonHungerStaticBlockSafe;
    }
}

5.懒汉式-静态内部类-线程安全

public class SingletonStaticInnerClass {
    private SingletonStaticInnerClass(){}
    private static class InstanceSingleton{
        public static final SingletonStaticInnerClass SINGLETON_STATIC_INNER_CLASS = new SingletonStaticInnerClass();
    }
    public static SingletonStaticInnerClass getInstance(){
        return InstanceSingleton.SINGLETON_STATIC_INNER_CLASS;
    }
}

6.懒汉式-双重校验锁-线程安全

public class SingletonDoubleVerifyLock {
    private static SingletonDoubleVerifyLock singletonDoubleVerifyLock;
    private SingletonDoubleVerifyLock(){}

    public static SingletonDoubleVerifyLock getInstance(){
        if (singletonDoubleVerifyLock == null){
            synchronized (SingletonDoubleVerifyLock.class){
                if (singletonDoubleVerifyLock == null){
                    singletonDoubleVerifyLock = new SingletonDoubleVerifyLock();
                }
            }
        }
        return singletonDoubleVerifyLock;
    }
}

7.懒汉式-枚举实现-线程安全(推荐)

public enum SingletonEnum {
    INSTANCE;
    SingletonEnum() {}
}