Ubuntu14.04/16.04 不能使用PPA更新并提示缺失pubkey的方法

在Ubuntu16.04无法直接安装openjdk1.7，但是可以用PPA实现安装，但是进行更新时提示提示缺失pubkey。

//具体错误类似提示如下：
W: GPG error: http://ppa.launchpad.net trusty InRelease:
The following signatures couldn't be verified because 
the public key is not available: NO_PUBKEY 8BAF9A6F

--------------------------------------------
可以通过以下方法解决：

1.    To install it, first add the webupd8 repository for this program:

    sudo add-apt-repository ppa:webupd8team/y-ppa-manager

2.    Update your software list and install Y-PPA-Manager:

    sudo apt-get update

当然如果执行了update之后，提示在apt-get update时遇到出错：由于没有公钥，无法验证下列签名： NO_PUBKEY xxxxxxxxxxxxxxx
解决方法：

$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys XXXXXXXXXXXXXXX

    sudo apt-get install y-ppa-manager

3.    Run y-ppa-manager (i.e. type y-ppa-manager then press enter key).

4.    When the main y-ppa-manager window appears, click on "Advanced."

5.    From the list of advanced tasks, select "Try to import all missing GPG keys" and click OK.

    You're done!

As the warning dialog says when you start the operation, it may take quite a while (about 2 minutes for me) depending on how many PPA's you have and the speed of your connection.

参考链接如下：

https://launchpad.net/~openjdk-r

非常重要，在“Technical details about this PPA”中要选择好对应到Ubuntu版本：

https://launchpad.net/~webupd8team/+archive/ubuntu/y-ppa-manager