Go to the Package Explorer view with the file open in the current editor. Click the "Link with Editor" toggle (the two arrows). This should jump you to the class file you are viewing, which should be in the jar you are using. If the jar is in a library, you should see the path to the jar in the package explorer. If not you can see the path to it in the preferences. From there you can also use Chin Huang's answer to find where the given source file is.

When you are coding in Eclipse, press CTRL and click on any core Java class name in your source. Eclipse will now show a screen saying you don't have the sources installed. However, in this screen there is a link saying "Attach source...". Click that link and import the src.zip file from your JDK installation directory (src.zip). This should do the trick

There is a shortcut Ctrl+Shift+T (on macOS: Cmd+Shift+T; menu: Navigation > Open Type...) to search for all types that is in your classpath, check the details here

Nice thing about it is that you can search with first letters of each word in uppercase, for example to find FileInputStream, you can use FIS. I love doing so =)

You need to attach source of the corresponding library.

Download Junit's source zip ( from their release site, corresponding to the JUNIT you are using) - example link

Then follow these steps in IBM's link on the topic - with steps

Once you do that, Ctrl + click on the class you interested in - while you are in the editor, you will see its source.

If you have the jar added to a project's classpath, you can view its source by:

1. Expanding Referenced Libraries in the Package Explorer view
2. Expand the jar you're looking at
3. Double clicking on a .class file

You need both the .class and .java file in the jar to view source, though.
If you have only the .java file, the class won't appear at all.
If you have only the .class file, the class will appear as bytecode.

Example:

For taking a look into classes which come without source files (e.g. 3rd party JARs), I use a Java decompiler: JadClipse

This one nicely integrated into Eclipse. Code looks strange sometimes, yet good enough for a rough estimation of what happens behind closed doors. Note that vendors might prohibit decompiling their code, IANAL.

In order to see the source codes of JDK in eclipse you should add JDK path instead of JRE to your workspace or for a specific project SDK. I assume you are a developer and you've installed JDK on your system.

To do that:

1. Right click and select Properties (or press Alt+Enter) on your project.
2. From the left tree styled menu select Java Build Path
3. On the right select the Libraries
4. Select the JRE System Library

5. From the right side buttons click on edit, and edit the path from JRE location to points to the JDK location in your file system.

6. Then click on Finish and the OK buttons of all opened dialogs to save the configurations and you set to go.

Hope this would be helpful.

You can attach source code to libraries. yourProject / Properties / Java Build Path / Libraries / yourLibrary / Source attachment, and then enter the folder or zip file containing the source code (which you need to download separately).

If you only have the object code (.class files), then you need to learn to read ByteCode (which Eclipse shows pretty nicely) or use a Java ByteCode disassembler.