单例创建&线程安全问题

/**
 * 线程安全的
 */
public class Example1 {
    private  Example1(){}
    private static Example1 example1 = new Example1();

    public static Example1 getInstance() {
        return example1;
    }

}

import lombok.extern.slf4j.Slf4j;

/**
 * 线程不安全安全的
 * <p>
 * 线程A 执行到  example = new Example2();还未执行完,
 * 线程B执行要进行 if (example == null)判断,最后可能创建多个实例
 */
@Slf4j
public class Example2 {
    private Example2() {
    }

    private static Example2 example;


    public static Example2 getInstance() {
        if (example == null) {
            example = new Example2();
        }
        return example;
    }

}

import lombok.extern.slf4j.Slf4j;

/**
 * 线程不安全安全的,性能低
 * 每次获取实例都要获取锁性能比较低,
 * 线程A创建了实例,但未写入到主内存中,这时候线程B读到的值便是null
 */
@Slf4j
public class Example3 {
    private Example3() {
    }

    private static Example3 example;


    public static synchronized Example3 getInstance() {
        if (example == null) {
            example = new Example3();
        }
        return example;
    }

}

import lombok.extern.slf4j.Slf4j;

/**
 * 线程不安全的,性能高
 * * 在实例化的情况下不需要每次都获取锁,性能比较好,但是同样线程不安全的,
 * * 线程A创建了实例,但未写入到主内存中,这时候线程B读到的值便是null
 */
@Slf4j
public class Example4 {
    private Example4() {
    }

    private static Example4 example;


    public static Example4 getInstance() {

        if (example == null) {
            synchronized (Example4.class) {
                if (example == null) {
                    example = new Example4();
                }
            }
        }
        return example;
    }

}

import lombok.extern.slf4j.Slf4j;

/**
 * 线程安全的,性能高
 * 在实例化的情况下不需要每次都获取锁,性能比较好,但是同样线程不安全的,
 * 线程A创建了实例,由于变量使用volatile修饰,赋值后会立即写入到主内存,这时候线程B在读的时候也会从主内存中读取
 */
@Slf4j
public class Example5 {
    private Example5() {
    }

    private volatile static Example5 example;

    public static Example5 getInstance() {

        if (example == null) {
            synchronized (Example5.class) {
                if (example == null) {
                    example = new Example5();
                }
            }
        }
        return example;
    }

}

import lombok.extern.slf4j.Slf4j;

/**
 * 使用枚举
 */
@Slf4j
public class Example6 {
    private Example6() {
    }


    public final static Example6 getInstance() {
        return Example6Enum.INSTANCE.getExample6();
    }

    enum Example6Enum {

        INSTANCE(new Example6());
        private Example6 example6;

        Example6Enum(Example6 example6) {
            this.example6 = example6;
        }

        public Example6 getExample6() {
            return example6;
        }
    }
}