創建型模式：單態

單態模式可能是23種創建模式種最簡單和容易理解的，創建型模式是主要是爲了解決對象的創建的方式，單態則是爲了保證創建的對象的唯一性。

模式場景與說明

有早期編碼經驗的開發者一般在N層應用開發中的業務邏輯層和持久層之間會有數據庫連接的獲取，不管是從緩衝池中獲取還是直接連接JDBC，爲了避免創建多個示例產生資源的浪費，一般會使用單態模式對創建對象進行控制。實際上除此之外，在類似的池化控制中都可以類似的引入，比如線程池等。此類對象創建一般存在：重（類似數據庫連接的獲取）、頻（使用程度較爲頻繁）的特點。

實現方式1

單態模式實現非常簡單，一般只要把握兩個要點，就能快速創建一個單態模式的類：

• 控制構造函數：私有化構造函數保證無法隨意創建對象
• 統一創建對象：在類中提供對象的構建方法並提供公有方法返回創建的實例（一般爲靜態函數）

實現示例

最簡單的示例代碼如下所示

public class Singleton {
    // use private key word to avoid being used directly
    private Singleton() {}
    private static Singleton instance = new Singleton();

    // use public method to get singleton instance
    public static Singleton getInstance(){
        return instance;
    }
}

調用方式：Singleton.getInstance();

這裏稍微修改一下，顯示一下輸出內容，特意在構建函數裏添加sleep，可以在多個終端同時啓動，可以驗證並行時仍然使用同一個實例（hashcode相同）。

package com.liumiao;
import java.time.LocalDateTime;

public class Singleton {

    // use private key word to avoid being used directly
    private Singleton() {
        System.out.printf("%s: Single instance creating begins... \n" ,LocalDateTime.now());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.printf("%s: Single instance creating ends... \n" ,LocalDateTime.now());
    }
    private static Singleton instance = new Singleton();

    // use public method to get singleton instance
    public static Singleton getInstance(){
        System.out.println("get Singleton instance: " + LocalDateTime.now());
        System.out.println(instance.hashCode());
        return instance;
    }
}

測試代碼如下

package com.liumiao;

public class TestSingleton extends Thread {
    public void run() {
        Singleton.getInstance();
    }
    public static void main( String[] args )
    {
        TestSingleton thread1 = new TestSingleton ();
        TestSingleton thread2 = new TestSingleton ();

        thread1.start();
        thread2.start();
    }
}

執行結果如下

2020-06-26T05:37:58.555119: Single instance creating begins... 
2020-06-26T05:37:59.583467: Single instance creating ends... 
get Singleton instance: 2020-06-26T05:37:59.583739
140350290
get Singleton instance: 2020-06-26T05:37:59.583756
140350290

實現方式2

由於實現方式1中對於實例的構建在類裝載的時候就已經完成，雖然解決了線程的同步，但是即使沒有使用到這個實例也會加載，這裏修改一下將new的動作移到getInstance中，通過是否爲空來判斷一下是否需要new進行構建對象，示例代碼如下所示：

package com.liumiao;
import java.time.LocalDateTime;

public class Singleton {

    // use private key word to avoid being used directly
    private Singleton() {
        System.out.printf("%s: Single instance creating begins... \n" ,LocalDateTime.now());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.printf("%s: Single instance creating ends... \n" ,LocalDateTime.now());
    }
    private static Singleton instance = null;

    // use public method to get singleton instance
    public static Singleton getInstance(){
        System.out.println("get Singleton instance: " + LocalDateTime.now());
        if (null == instance) {
            instance = new Singleton();
        }
        System.out.println(instance.hashCode());
        return instance;
    }
}

使用同樣的測試代碼，執行結果如下，所示

get Singleton instance: 2020-06-26T05:39:00.137037
get Singleton instance: 2020-06-26T05:39:00.137051
2020-06-26T05:39:00.137567: Single instance creating begins... 
2020-06-26T05:39:00.137614: Single instance creating begins... 
2020-06-26T05:39:01.162630: Single instance creating ends... 
140350290
2020-06-26T05:39:01.162630: Single instance creating ends... 
1253458049

可以看到，單態出現了問題，產生了不同的實例（hashcode不同）,因爲沒有考慮到線程安全的問題

實現方式3

這裏簡單地使用synchronized來稍作修改, 代碼如下

package com.liumiao;
import java.time.LocalDateTime;

public class Singleton {

    // use private key word to avoid being used directly
    private Singleton() {
        System.out.printf("%s: Single instance creating begins... \n" ,LocalDateTime.now());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.printf("%s: Single instance creating ends... \n" ,LocalDateTime.now());
    }
    private static Singleton instance = null;

    // use public method to get singleton instance
    public static synchronized Singleton getInstance(){
        System.out.println("get Singleton instance: " + LocalDateTime.now());
        if (null == instance) {
            instance = new Singleton();
        }
        System.out.println(instance.hashCode());
        return instance;
    }
}

使用同樣的測試代碼執行結果如下

get Singleton instance: 2020-06-26T05:42:13.732276
2020-06-26T05:42:13.732766: Single instance creating begins... 
2020-06-26T05:42:14.758235: Single instance creating ends... 
346203776
get Singleton instance: 2020-06-26T05:42:14.758679
346203776

所以可以看到已經返回了相同的實例了

其他設計模式

• https://liumiaocn.blog.csdn.net/article/details/106954154