singleton (单例)

1. 饿汉式

/**
 * 饿汉式
 * 类加载到内存后，就实例化一个单例，JVM保证线程安全
 * 简单实用
 */
public class Singleton01 {

    private static final Singleton01 singleton = new Singleton01();

    private Singleton01() {
    }

    public static Singleton01 getInstance() {
        return singleton;
    }

}    

2. 懒汉式

/**
 * 懒汉式
 * 虽然达到了按需初始化的目的，但是带来了多线程访问不安全的问题
 * 当多个线程同时进到if判断里，会多次被创建
 */
public class Singleton02 {

    private static Singleton02 singleton;

    private Singleton02() {
    }

    public static Singleton02 getInstance() {
        if (singleton == null) {
            singleton = new Singleton02();
        }
        return singleton;
    }
}

3. 懒汉式增强式

/**
 * 懒汉式增强式
 */
public class Singleton04 {

    private static volatile Singleton04 singleton;

    private Singleton04() {
    }

    public synchronized static Singleton04 getInstance() {
      if (singleton == null) {
          synchronized (Singleton04.class) {
              if (singleton == null) {
                  singleton = new Singleton04();
              }
          }
      }
      return singleton;
    }
 } 

4. 静态内部类

/**
 * 静态内部类
 * jvm保证单例
 * 加载外部类不会加载内部类，这样可以实现懒加载
 */
public class Singleton05 {


    private Singleton05() {
    }

    private static class Singleton05Holder {
        private static final Singleton05 singleton = new Singleton05();
    }

    public static Singleton05 getInstance() {
        return Singleton05Holder.singleton;
    }


    /**
     * 测试代码
     *
     * @param args
     */
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                Singleton05 singleton02 = Singleton05.getInstance();
                System.out.println(singleton02.hashCode());
            }).start();
        }
    }
}

5. 完美方式

/**
 * effective java 给出的完美方式
 * 不仅解决线程同步，还可以防止反序列化
 */
public enum Singleton06 {

    INSTANCE;

    /**
     * 测试代码
     *
     * @param args
     */
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                Singleton06 singleton02 = Singleton06.INSTANCE;
                System.out.println(singleton02.hashCode());
            }).start();
        }


    }
}