Singleton多种实现方式的在多线程情况下的优缺点

// 不能懒加载
public class SingletonObject1 {

    private static final SingletonObject1 instance = new SingletonObject1();

    private SingletonObject1(){

    }

    public static SingletonObject1 getInstance(){
        return instance;
    }
}

public class SingletonObject2 {

    private static SingletonObject2 instance = null;

    private SingletonObject2(){

    }

    public static SingletonObject2 getInstance(){
        // 线程不安全模式
        if (instance == null)
            instance = new SingletonObject2();
        return SingletonObject2.instance;
    }
}

public class SingletonObject3 {

    private static SingletonObject3 instance = null;
    private SingletonObject3(){

    }
    // 串行执行，效率低下
    public static synchronized SingletonObject3 getInstance(){
        if (instance == null)
            instance = new SingletonObject3();
        return SingletonObject3.instance;
    }
}

public class SingletonObject4 {

    private static SingletonObject4 instance = null;

    private SingletonObject4(){

    }

    // 双重检查机制，严格单例，解决了懒加载的问题，但是也带来了问题：空指针异常
    public static SingletonObject4 getInstance(){
        if (instance == null){
            synchronized (SingletonObject4.class){
                if (instance == null)
                    instance = new SingletonObject4();
            }
        }
        return SingletonObject4.instance;
    }
}

public class SingletonObject5 {

    private static volatile SingletonObject5 instance = null;

    private SingletonObject5(){

    }

    // 双重检查机制，严格单例，解决了懒加载的问题，但是也带来了问题：空指针异常
    public static SingletonObject5 getInstance(){
        if (instance == null){
            synchronized (SingletonObject5.class){
                if (instance == null)
                    instance = new SingletonObject5();
            }
        }
        return SingletonObject5.instance;
    }
}

public class SingletonObject6 {

    private SingletonObject6(){

    }
    private static class InstanceHolder{
        private final static SingletonObject6 instance = new SingletonObject6();
    }

    public static SingletonObject6 getInstance(){
        return InstanceHolder.instance;
    }
}