1、饿汉。线程安全，类装载时就实例化。

public class SingleInstance {

    private SingleInstance(){}

    private static SingleInstance mSingleInstance = new SingleInstance();

    public static SingleInstance getInstance(){
        return mSingleInstance;
    }

}

2、懒汉。懒加载，效率较低。

public class SingleInstance {

    private SingleInstance(){}

    private static SingleInstance mSingleInstance;

    public static synchronized SingleInstance getInstance(){
        if (mSingleInstance == null) {
            mSingleInstance = new SingleInstance();
        }
        return mSingleInstance;
    }
}

3、双重校验锁。使用volatile变量，轻量级的同步机制。

public class SingleInstance {

    private SingleInstance(){}

    private volatile static SingleInstance mSingleInstance;

    public static SingleInstance getInstance(){
         
        if (mSingleInstance == null) {
            synchronized (SingleInstance.class) {
                if (mSingleInstance == null) {
                    mSingleInstance = new SingleInstance();
                }
            }
        }
        return mSingleInstance;
    }
 }

4、静态内部类

public class SingleInstance {

    private SingleInstance(){}

    public static SingleInstance getInstance(){
        return SingleHolder.instance;
    }

    private static class SingleHolder {
        private static SingleInstance instance = new SingleInstance();
    }

}

5、枚举

public enum SingleInstance {
    INSTANCE;
}