Eclipse里如果看不到Attach Source按钮应该怎么办

When I am creating unit test for one of my Java project A, I try to have a look at the source code of this annotation:

Once I press Ctrl and click, I see this message below. The “Attach Source” button is not there.

However I tried another project B in my laptop, it works:

This is strange. Then I compare the build path of two projects and find the reason.
For project A, it uses Eclipse embedded JUnit:

While for project B, the JUnit managed by Maven is used.

So the easiest approach for project A is to also switch to the external JUnit instead of Eclipse embedded one, so that developers have full control on both binary code and source code.

In Eclipse installation folder there is a folder plugin, within it there is an about.html,

the binary file and source code could be downloaded from url mentioned.

After that, go to project A,

replace with:

Now the source code for JUnit is also available for project A:

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