单例模式

1.饿汉模式：线程安全

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

    private HungryManSingleton() {
    }

    public HungryManSingleton getIntance() {
        return instance;
    }
}

2.懒汉模式：线程不安全

public class LazyManSingleton {

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

3.懒汉模式（线程锁）：线程安全

public class LazyManSingleton {

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

------------------------------------------------------------------------------

4.饿汉模式（强力推荐：并不是在；类加载的时候初始化对象，而是当调用的是时候才会初始化对象。节约资源）：线程安全

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

    private HungerySingleton() {
    }

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