How to write java singleton pattern

Singleton Pattern is one of the simplest design patterns in Java. This type of design pattern is a creational pattern, which provides an optimal way to create objects.

This pattern involves a single class that is responsible for creating its own objects while ensuring that only a single object is created. This class provides a way to access its unique objects directly, without the need to instantiate an object of the class.

Note:

1. A singleton class can only have one instance. 2. A singleton class must create its own unique instance. 3. The singleton class must provide this instance to all other objects.

java Five ways to write singleton mode:

1. Full-bodied mode (lazy mode)

// 饱汉
// UnThreadSafe
public class Singleton1 {
  private static Singleton1 singleton = null;
  private Singleton1() {
  }
  public static Singleton1 getInstance() {
    if (singleton == null) {
      singleton = new Singleton1();
    }
    return singleton;
  }
}

Advantages: Lazy loading starts quickly, takes up little resources, is instantiated when used, and is lock-free.

Disadvantages: Not thread-safe.

2. Lazy mode (lazy mode) - thread safety

public class Singleton {
    /**
     * 定义一个变量来存储创建好的类实例
     */
    private static Singleton uniqueInstance = null;
    /**
     * 私有化构造方法，好在内部控制创建实例的数目
     */
    private Singleton(){
    }
    /**
     * 定义一个方法来为客户端提供类实例
     * @return 一个Singleton的实例
     */
    public static synchronized Singleton getInstance(){
        //判断存储实例的变量是否有值
        if(uniqueInstance == null){
            //如果没有，就创建一个类实例，并把值赋值给存储类实例的变量
            uniqueInstance = new Singleton();
        }
        //如果有值，那就直接使用
        return uniqueInstance;
    }
    /**
     * 示意方法，单例可以有自己的操作
     */

    public void singletonOperation(){
//功能处理
    }
    /**
     * 示意属性，单例可以有自己的属性
     */
    private String singletonData;
    /**
     * 示意方法，让外部通过这些方法来访问属性的值
     * @return 属性的值
     */
    public String getSingletonData(){
        return singletonData;
    }
}

Advantages: Same as above, but locked.

Disadvantages: synchronized is an exclusive lock and has poor concurrency performance. Even after the creation is successful, obtaining the instance is still a serialized operation.

3. Lazy Mode (Lazy Mode) - Double Check Lock DCL (Double Check Lock)

public class Singleton {
    /**
     * 对保存实例的变量添加volatile的修饰
     */
    private volatile static Singleton instance = null;
    private Singleton(){
    }

    public static Singleton getInstance(){
//先检查实例是否存在，如果不存在才进入下面的同步块
       if(instance == null){
//同步块，线程安全的创建实例
            synchronized(Singleton.class){
//再次检查实例是否存在，如果不存在才真的创建实例
                if(instance == null){
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

Advantages: Lazy loading, thread safety.

Note: The instance must be modified with the volatile keyword to ensure complete initialization.

4. Hungry mode

public class Singleton {
//4：定义一个静态变量来存储创建好的类实例
//直接在这里创建类实例，只会创建一次
    private static Singleton instance = new Singleton();
//1：私有化构造方法，好在内部控制创建实例的数目
    private Singleton(){
    }
//2：定义一个方法来为客户端提供类实例
//3：这个方法需要定义成类方法，也就是要加static
//这个方法里面就不需要控制代码了
   public static Singleton getInstance(){
//5：直接使用已经创建好的实例
       return instance;
    }
}

Advantages: Hungry mode is inherently thread-safe and there is no delay when using it.

Disadvantages: Instances are created at startup, slow startup, and possible waste of resources.

5. Holder mode

public class Singleton {
    /**
     * 类级的内部类，也就是静态的成员式内部类，该内部类的实例与外部类的实例
     * 没有绑定关系，而且只有被调用到才会装载，从而实现了延迟加载
     */
    private static class SingletonHolder{
        /**
         * 静态初始化器，由JVM来保证线程安全
         */
        private static Singleton instance = new Singleton();
    }
    /**
     * 私有化构造方法
     */
    private Singleton(){
    }
    public static  Singleton getInstance(){
        return SingletonHolder.instance;
    }
}

Advantages: A way to perfectly combine lazy loading and thread safety (no locks). (Recommended)

Related learning recommendations: java basic tutorial

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