java单例写法

1、枚举法

enum Singleton {
	INSTANCE;

	public void doSomeThing() {}
}

2、静态内部类

public class Singleton {
	private static class SingletonHolder {
		static final Singleton INSTANCE = new Singleton();
	}
	
	public static Singleton getInstance() {
		return SingletonHolder.INSTANCE:
	}
}

3、双检锁

public class Singleton {
	//volatile必须要加，禁止new操作的指令重排序
	private static volatile Singleton INSTANCE = null;
	private static final Object LOCK = new Object();

    private Singleton() {}
     
    public static Singleton getInstance() {
		if (INSTANCE == null) {
			synchronized(LOCK) {
				if (INSTANCE == null) {
					INSTANCE = new Singleton();
				}
			}
		}
		return INSTANCE;
    }
}

4、懒汉式

class Singleton{
    private static Singleton instance = new Singleton();
    public static Singleton getInstance() {
        return instance;
    }
    private Singleton(){}
}

5、饿汉式（线程不安全，并发问题）

public class Singleton {
	private static Singleton INSTANCE = null;

    private Singleton() {}
     
    public static Singleton getInstance() {
		if (INSTANCE == null) {
			INSTANCE = new Singleton();
		}
		return INSTANCE;
    }
}

6、spring实现的单例

@Component
@Scope("singleton") //默认就是单例，可以不加
public class Singleton {
}

或
@Configuration
public class Configuration {

	@Bean
    @Scope("singleton") //默认就是单例，可以不加
    public Singleton getSingleton() {
       return new Singleton();
    }
}