技術標籤：設計模式

動機

1. 在軟體系統中，經常有這樣一些特殊的類，必須保證它們在系統中只存在一個例項，才能確保它們的邏輯正確性、以及良好的效率。
2. 如何繞過常規的構造器，提供一種機制來保證一個類只要一個例項？
3. 這應該是類設計者的責任，而不是使用者的責任。

模式定義

保證一個類僅有一個例項，並提供一個該例項的全域性訪問點。

類圖

總結

1. Singleton模式中的例項構造器可以設定為protected已允許子類派生。
2. Singleton模式一般不要支援拷貝建構函式和Clone介面，因為這有可能導致多個物件例項，與Singleton模式的初衷違背。
3. 如何實現多執行緒環境下安全的Singleton？注意對雙檢查鎖的正確實現。

例項

注意點

• 單例模式需要把建構函式設定為虛擬函式
• 需要考慮多執行緒的問題
• 使用雙檢查鎖機制
• 防止指令重排，java中使用volatile關鍵字

1. 單列類

public class HouseSingleton {
    // 新增volatile是為了避免指令重排
    private static volatile HouseSingleton houseSingleton = null;

    private HouseSingleton() {
        System.out.println("Create HouseSingleton");
    }

    public
 
 static HouseSingleton getInstance() {
        // 雙檢查鎖機制，可以保證只有例項每建立的時候鎖一次，其他情況下都不會鎖執行緒，不會導致效能下降。
        // 問題是，可能出現指令重排，先分配記憶體然後直接把記憶體地址賦值給引用變數，導致還沒執行建構函式；
        // 因此部分執行緒不能正常使用例項，因此需要藉助volatile關鍵字避免指令重排。
        if (houseSingleton == null) {
            synchronized (HouseSingleton.class) {
                if
 
 (houseSingleton == null) {
                    houseSingleton = new HouseSingleton();
                    return houseSingleton;
                }
            }
        }

        return houseSingleton;
    }

    public void say() {
        System.out.println("Say hello, name:" + Thread.currentThread().getName());
    }
}

2. 多執行緒呼叫單例類

public class Client {
    public static void main(String[] args) throws InterruptedException {

        for (int i = 0; i < 100; i++) {
            Thread thread = new Thread(() -> {
                HouseSingleton houseSingleton = HouseSingleton.getInstance();
                houseSingleton.say();
            }, "thread" + (i + 1));

            thread.start();
        }

        Thread.sleep(10000);

    }
}
// 執行結果
Create HouseSingleton
Say hello, name:thread1
Say hello, name:thread2
Say hello, name:thread3
Say hello, name:thread4
Say hello, name:thread86
Say hello, name:thread6
Say hello, name:thread7
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