1、餓漢式

public class SingletonHungry {
    private SingletonHungry () {
    }
    private static SingletonHungry singletonHungry = new SingletonHungry();
    public static SingletonHungry getInstance(){
        return singletonHungry;
    }
}

2、懶漢式

public class SingletonLazy {
    private static SingletonLazy singletonLazy;
    private
 
 SingletonLazy () {
        /**
         * 防止被反射強行建立例項
         */
        if(singletonLazy != null) {
            throw new UnsupportedOperationException("SingletonLazy was created!");
        }
    }
    public static SingletonLazy getInstance() {
        if(singletonLazy == null) {
            try {
                singletonLazy = new
 
 SingletonLazy();
                return singletonLazy;
            } catch (Exception e) {
                e.printStackTrace();
            }

        }
        return singletonLazy;
    }
}

3、雙鎖

public class SingletonLock {
    private static SingletonLock singletonLock;
    private SingletonLock
 
() {
    }
    public static SingletonLock getInstance() {
        if(singletonLock == null) {
            synchronized (SingletonLock.class) {
                if(singletonLock == null) {
                    singletonLock = new SingletonLock();
                    return singletonLock;
                }
            }
        }
        return singletonLock;
    }
}

4、內部類

public class SingletonInner {
    private SingletonInner() {
        if (Holder.INSTANCE != null) {
            throw new UnsupportedOperationException("INSTANCE was created!");
        }
    }
    static class Holder {
        private static SingletonInner INSTANCE = new SingletonInner();
    }
    public static SingletonInner getInstance() {
        return Holder.INSTANCE;
    }
}

5、列舉

public enum SingletonEnum{
    INSTANCE;
}