Project is Missing Required Source Folder: "src_ex_common" - classpath

Question

I try to open the sample project from google opensource as provided in the following link:
https://android.googlesource.com/platform/packages/apps/Calendar.git
I have imported the 5 git repositories as required in the program but there is still a red exclamation mark near the project and the "Problems" suggested the project is missing required source folder "src_ex_common". I right click the project name => Properties => Java Build Path => Source, i saw
"MainProjectActivity/src_ex_common (missing)".
I right clicked the project name => new=>source folder and attempt to name a new folder called "src_ex_common" but eclipse said The folder is already a source folder. Then i tried to name a new folder called "src_ex_common" in Windows. The problem above is solved but there are many errors then appear in various java files under src.
I googled that some said this might be due to the problem in ".classpath"
The detail of .classpath is listed below:
<?xml version="1.0" encoding="UTF-8"?>
<classpath>
<classpathentry kind="src" path="src_ex_common"/>
<classpathentry kind="con" path="com.android.ide.eclipse.adt.ANDROID_FRAMEWORK"/>
<classpathentry kind="con" path="com.android.ide.eclipse.adt.LIBRARIES"/>
<classpathentry kind="src" path="src"/>
<classpathentry kind="src" path="gen"/>
<classpathentry kind="output" path="bin/classes"/>
</classpath>
Did anyone encounter this problem before? Great if u would share your approach to tackle it. Many thanks!

Answer 1

If you open the mkprojectfile file in the root folder, you will see that the src_ex_common folder is linked to the commons library:
<link>
<name>src_ex_common</name>
<type>2</type>
<location>SRC_ROOT_DIR/frameworks/ex/common/java</location>
</link>
You can import this project from the same ex repository that contains the chips dependency.