1.餓漢模式：執行緒安全

public class HungryManSingleton {
    
    private static final HungryManSingleton instance = new HungryManSingleton();

    private HungryManSingleton() {
    }

    public HungryManSingleton getIntance() {
        return instance;
    }
}

2.懶漢模式：執行緒不安全

public class LazyManSingleton {

    
 
private static LazyManSingleton instance = null;
    private LazyManSingleton(){}
    public LazyManSingleton getInstance(){
        if(instance == null){
            instance = new LazyManSingleton();
        }
        return instance;
    }
}

3.懶漢模式（執行緒鎖）：執行緒安全

public class LazyManSingleton {

    
 
private volatile static LazyManSingleton instance = null;
    private LazyManSingleton(){}
    public synchronized LazyManSingleton getInstance(){
        if(instance == null){
            instance = new LazyManSingleton();
        }
        return instance;
    }
}

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4.餓漢模式（強力推薦：並不是在；類載入的時候初始化物件，而是當呼叫的是時候才會初始化物件。節約資源）：執行緒安全

public class HungerySingleton {
    private static class innerClass {
        private static final HungerySingleton instance = new HungerySingleton();
    }

    private HungerySingleton() {
    }

    public static HungerySingleton getInstance() {
        return innerClass.instance;
    }
}