常用的一些单例模式：

饿汉式单例

package com.example.demo.singleton;


/**
 * @author wangjian
 * @Describe 饿汉式单例
 */
public class HungrySingleton {

    private HungrySingleton(){};


    private final static HungrySingleton hungrySingleton = new HungrySingleton();

    public  static HungrySingleton getInstance(){
        return  hungrySingleton ;
    }

}

懒汉式单例：

package com.example.demo.singleton;

/**
 *  懒汉式单例
 */
public class LazySingleton {

    private LazySingleton(){}

    private static final LazySingleton lazySingleton = null ;

    public static synchronized LazySingleton getInstance(){
        return new LazySingleton() ;
    }
}

双重检查锁式单例

package com.example.demo.singleton;

/**
 *  双重检查锁单例
 * @author wangjian
 */
public class DoubleCheckSingleton {
    private DoubleCheckSingleton(){}

    private static final DoubleCheckSingleton doubleCheckSingleton = null;

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

内部类式单例

package com.example.demo.singleton;

/**
 * 内部类实现单例
 * @author wangjian
 */
public class InnerClassSingleton {
    private InnerClassSingleton(){}

    private static class LazyHolder{
        private static final InnerClassSingleton INNER_CLASS_SINGLETON = new InnerClassSingleton();
    }

    public static InnerClassSingleton getInstance(){
        return LazyHolder.INNER_CLASS_SINGLETON;
    }
}

枚举式单例:

package com.example.demo.singleton;

/**
 * 枚举式单例
 * @author wangjian
 */
public enum EnumSingleton {

    INSTANCE ;
    private Object data ;

    public Object getData() {
        return data;
    }

    public void setData(Object data) {
        this.data = data;
    }

    public static EnumSingleton getInstance(){
        return INSTANCE ;
    }
}

注册式单例：

package com.example.demo.singleton.container;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

/**
 * spring 内部实现的注册式单例
 * @author wangjian
 */
public class RegisterSingleton {
    /**
     * 容器 存放已经存在的实例
     */
    public static final Map<String,Object> ioc = new ConcurrentHashMap<>();

    private RegisterSingleton(){}

    public static Object getBean(String className){
        synchronized (ioc){
            if(!ioc.containsKey(className)){
                try {
                    Object o = Class.forName(className).newInstance();
                    ioc.put(className,o);
                    return o ;
                }catch (Exception e){
                    e.printStackTrace();
                }
            }
        }

        return ioc.get(className);
    }






}

ThreadLocal单例

package com.example.demo.singleton.container;

/**
 * 线程内的单例模式
 * @author wangjian
 *
 */
public class ThreadLocalSingleton {
    private ThreadLocalSingleton(){}

    private static final ThreadLocal<ThreadLocalSingleton> threadLocalSingleton = new ThreadLocal<ThreadLocalSingleton>(){
        @Override
        protected ThreadLocalSingleton initialValue() {
            return new ThreadLocalSingleton() ;
        }
    };

    public static ThreadLocalSingleton getInstance(){
        return threadLocalSingleton.get();
    }
}