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How to add referenced library JAR files using relative path for eclipse Java project in vscode

I checked this question and many other posts to help me resolve my issue, but nothing worked.
I trying to open an existing Java Project from Eclipse in vscode. The project was opened successfully in Eclipse 2021-06 and I fixed the referenced libraries by selecting the JAR libraries under lib/ folder and right-click - configure build batch - add select add to build path. Then the file .classpath was updated automatically. Then I had to download JDK 11 (needed for vscode) and JDK 1.8 needed for the Java project. I managed to work with eclipse to set the JDK correctly and all worked fine. I was able to compile the project, export the result JAR and deploy and all worked well.
I am having trouble to open this project in vscode using Open Java Project option. I am facing issues adding referenced libraries this Java project as it is a managed project (since it was created in Eclipse). When I try to add the libraries using vscode UI, I am getting mixed results and sometimes it is not working and the build fails. I was surprised as in some instances, things just work, and then they don't.
I tried to update .classpath file in vscode and still same issues. When I try to add the libraries from Referenced Libraries (Read Only) under JAVA PROJECTS tree in vscode by clicking +, it has no effect. This step will modify the file settings.json under .vscode folder by adding the relevant values to the property "java.project.referencedLibraries".
I am confused about how to configure the referenced libraries for Java projects in vscode. Following is a summary of questions:
How to add a library using a relative path to a Java Project based on the project root folder?
What is the difference between adding a library using the .classpath file and by modifying settings.json file? Which one will win?
How to add a folder with all JAR files to be as the referenced library to the Java Project? This is to avoid adding one file at a time.
Why the .classpath and .project files are not showing in vscode explorer view? It will only show when you open the file in vscode from the Windows File Explorer.
I found command Configure Classpath but it is read-only, which I think it is the same as the .classpath file. Is there a way to change it from vscode UI?
When I remove .classpath file or when I removed the classpathentry lines from the .classpath file, the node 'Referenced Libraries (Read Only)' under JAVA PROJECTS in vscode view was removed. Why?
See the snapshots below for more details.
I appreciate your help.

Basically speaking, java extension looks for jars from Referenced Libraries.
Here're my answers to your questions:
Right click the jar and choose copy relative path then add it to settings, click the refresh button then the added jar should be displayed under the option Referenced Libraries:
The CLASSPATH variable is one way to tell applications, including the JDK tools, where to look for user classes. When it comes to add jars, settings java.project.referencedLibraries wins.
No way to select
jar folders but you can use keyboard shortcuts to select all jars then add them.
There's a setting called "java.configuration.checkProjectSettingsExclusions", and it's true by default, so .project and .classpath won't be shown in VS Code.
The command Java: Configure Classpath works on my machine, which can customize current project.
My guess is when you open a new project, vscode popped up a window and ask you if trust it, and you chose Not, then project is read-only. Trusting the workspace then try the command Java: Configure Classpath again, it should be writeable.

The answer by #Molly Want-MSFT helped me a lot. Following is what I did to resolve the problem for good. I applied the steps below many time to verify they will work every time.
Steps to open a Java Project in both Eclipse and vscode:
Download the JDK needed for vscode, Eclipse and your Java Project.
Import the project in Eclipse and setup the JDK for the workspace and the project. Also, make sure to setup the compile level to match the JDK.
Set up the Project JDK to match the default of the workspace.
Build the project (Project-Cleanup) and Export the JAR to make sure all is OK.
Now open the project in vscode.
Setup java.home in Settings, in user and workspace sections. This must be JDK-11 or higher to allow vscode to function properly for Java Projects.
Add references to the installed JDKs in User's settings.json under "java.configuration.runtimes" section.
Restart vscode and take the option Configure Java Runtime from JAVA PROJECTS view. Make sure that the JDK of the Java Project is detected and working correctly.
You may have to open one of the Java Source Code Files. Wait a bit until it will settle down. Check the Java Build Status progress by clicking the spinning icon in the bottom right. This icon is for Language Server and it will turn into an icon that looks like thumbs-up when build/compile is done.
Close and open Configure Java Runtime to verify that the JDK was detected by vscode.
Ensure that both JRE System Libraries and Referenced Libraries under JAVA PROJECTS view are visible without any errors. Check the Problems view and try to resolve all errors.
The Referenced Libraries should be Read Only because this project was setup in Eclipse. You can delete the .classpath and .project files and open the project again in vscode, and try to fix the problems by adding libraries using the + button. When such files are deleted, the project will become Unmanaged. Later, you can restore such files.
When the project is unmanaged, you use Configure Java Runtime from JAVA PROJECTS view and you can check the project type. You can change the JDK to one of the installed ones as per the section "java.configuration.runtimes" in the user's setting.json.
If the the Referenced Libraries is read-only, it has no effect even if you can add libraries into settings.json, but the .classpath file will win.
From JAVA PROJECTS view, you can use the option Build Workspace and Clean Workspace to troubleshoot and try to resolve errors.
Finally, you can Export JAR from the option on JAVA PROJECTS view. This option is not clear and it looks like and arrow pointing to the right |-->.
I hope this helps, and if you have any question, please post a comment and I will try to answer back when possible.

I just change the path strings of the jars in the .classpath file and it works for me.

Attach the Source in Eclipse of a jar

I added a Student.jar into my Build Path in my eclipse like this-
Right click on the project->BuildPath->Configure Build
Path->Libraries->Add External Jars
There is one class named StudentTest in Student.jar file. When I was debugging my code in eclipse, I stepped into that StudentTest class in the Student.jar.
And after that eclipse shows me like this-
The JAR file S:\some_location\Student.jar has no source attachment.
You can attach the source by clicking Attach Source below
Now I am not sure how should I attach the source in my eclipse. And from where? Can anyone provide me step by step what I need to do.
Update:-
I tried unzipping the Student.jar and I got Student folder. And after that I tried pointing the source to Student folder But still I am not able to see the class properly so that I can debug it properly, it shows the same above behavior.

Use Java Source Attacher !
It does what eclipse should do - a right click context menu that says "Attach Java Source.
It automatically downloads the source for you and attaches it. I've only hit a couple libraries it doesn't know about and when that happens it lets you contribute the url back to the community so no one else will have a problem with that library.

Eclipse is showing no source found because there is no source available . Your jar only has the compiled classes.
You need to import the project from jar and add the Project as dependency .
Other option is to go to the
Go to Properties (for the Project) -> Java Build Path -> Libraries , select your jar file and click on the source , there will be option to attach the source and Javadocs.

A .jar file usually only contains the .class files, not the .java files they were compiled from. That's why eclipse is telling you it doesn't know the source code of that class.
"Attaching" the source to a JAR means telling eclipse where the source code can be found. Of course, if you don't know yourself, that feature is of little help. Of course, you could try googling for the source code (or check wherever you got the JAR file from).
That said, you don't necessarily need the source to debug.

This worked for me for Eclipse-Luna:
Right Click on the *.jar in the Referenced Libraries folder under your project, then click on Properties
Use the Java Source Attachment page to point to the Workspace location or the External location to the source code of that jar.

Go back in to where you added the jar. I believe its the libraries tab, I don't have Eclipse open but that sounds right. to the left of the jar file you added there should be an arrow pointing right, click that and 3 or 4 options expand, one of them being the source file of the library. Click on that and click edit(I think you can also double click it) then locate the file or folder on your hard disk, you probably have to click apply or okay and you're good to go, same with javadoc and i think the last one is native libraries. I don't pay much attention when I'm in there anymore if you couldn't tell. That's what you were asking, right?

I Know it is pretty late but it will be helpful for the other user, as we can do Job using three ways... as below 1)1. Atttach your source code using i.e, Right click on the project then properties --> Java build path--> attach your source in the source tab or you can remove jar file and attach the source in the libraries tab
2. Using eclipse source Analyzer In the eclipse market you can download the plugin java source analyzer which is used to attach the open source jar file's source code. we can achieve it after installing the plugin, by right click on the open source jar and select the attach source option.
3. Using Jadclipse in eclipse you can do it last not the least, you can achieve the decompile your code using this plugin. it is similar way you can download the plugin from the eclipse market place and install in your eclipse.
in jadclipse view you can see your .class file to decomplile source format note here you cannot see the comment and hidden things I think in your scenario you can use the option one and option three, I prefer option three only if i want to the source code not for the debug the code. else i ll code the option 1, as i have the source already available with.

Simply import the package of the required source class in your code from jar.
You can find jar's sub packages in
Eclipse -- YourProject --> Referenced libraries --> yourJars --> Packages --> Clases
Like-- I was troubling with the mysql connector jar issue
"the source attachment does not contain the source"
by giving the path of source folder it display this statement
The source attachment does not contain the source for the file StatementImpl.class
Then I just import the package of mysql connector jar which contain the required class:
import com.mysql.jdbc.*;
Then program is working fine.

I faced the same issue and solved using the below steps. Go to Windows->preferences->Editors->File Associations
Here click on Add
then type .class
click on OK
again click on Add
then type .classwithoughtsource
click on OK
Now you will be able to see JadClipse option under Java section in Windows->Preferences
Please provide the path of jad.exe file as shown below.
Path for Decompiler-C:\Users\ahr\Documents\eclipse-jee-galileo-SR2-win32\jad.exe
Directory for temporary Files-C:\Users\ahr.net.sf.jadclipse
click on Apply
Now you should be able to see the classfiles in proper format.

It is quite possible, just go to the jar in the Build Path and choose to attach a source just like follow.

Download JDEclipse from http://java-decompiler.github.io/
Follow the installation steps
If you still didn't find the source, right click on the jar file and select "Attach Library Source" option from the project folder, as you can see below.

I am using project is not Spring or spring boot based application.
I have multiple subprojects and they are nested one within another.
The answers shown here supports on first level of subproject.
If I added another sub project for source code attachement, it is not allowing me saying folder already exists error.
Looks like eclipse is out dated IDE. I am using the latest version of Eclipse version 2015-2019.
It is killing all my time.
My intension is run the application in debug mode navigate through the sub projects which are added as external dependencies (non modifiable).

I have faced same problem and resolved it by using following scenario.
First we have to determine which jar file's source code we want along with version number. For Example "Spring Core" » "4.0.6.RELEASE"
open https://mvnrepository.com/ and search file with name "Spring Core" » "4.0.6.RELEASE".
Now Maven repository will show the the details of that jar file.
In that details there is one option "View All" just click on that.
Then we will navigate to URL "https://repo1.maven.org/maven2/org/springframework/spring-core/4.0.6.RELEASE/".
there so many options so select and download "spring-core-4.0.6.RELEASE-sources.jar" in our our system and attach same jar file as a source attachment in eclipse.

Try removing the breakpoints. :)

giving 'java.library.path' in netbeans for .dll/.so files

How do i give the path for the .dll or .so file in netbeans to load the library using
System.loadLibrary("Foo")
I read this on netbeans page but couldn't help me. as it gives the unsatisfied link error.
These are the two things i tried :
In the following snapshot i created a lib folder inside the netbeans project and placed the dll files inside it.
In the second snapshot i created a lib folder inside the modules folder and placed all the dll files inside lib folder as stated in the link.
But both of them give me unsatisfied link error exception. How do i set java.library.path in netbeans so that i can directly test the application from netbeans,
without having to go and write on the terminal ?

Its working now. Some little bit setting you have to do with your Netbean IDE.Only of Netbean IDE.
Follow the Steps :-
==>Right click on the Project
==>Properties
==>Click on RUN
==>VM Options : -Djava.library.path="C:\Your Directory where Dll is present"
==>Ok
Its working 100%. I have done this in my own project.

I'm using Mac OS X Yosemite and Netbeans 8.02, I got the same error and the simple solution I have found is like above, this is useful when you need to include native library in the project. So do the next for Netbeans:
1.- Right click on the Project
2.- Properties
3.- Click on RUN
4.- VM Options: java -Djava.library.path="your_path"
5.- for example in my case: java -Djava.library.path=</Users/Lexynux/NetBeansProjects/NAO/libs>
6.- Ok
I hope it could be useful for someone.
The link where I found the solution is here:
java.library.path – What is it and how to use

To me JDeveloper's suggestion didn't work. However, the method that I came across this early afternoon works handy.
It is for netbeans 8.02, and Visual Studio 2013 generated dll.
create a package(eg, "dll") under [Source Packages] node of netbeans
project
put/copy a dll file(eg, simpleDLL.dll) into the package(eg,
"dll")
on the [Libraries] node of the netbeans project, right-click
and choose "Add Jar/Folder...",
navigate to the folder("src") that contains the folder("dll") for the package that you created in step 1 above (it could be "...\src\dll" in my example), select package folder("dll") and click [Open] button to select it.
-- now you should have added a folder("dll") (which contains the dll file{simpleDLL.dll}) to the Libraries node of the project.
That's it. It should work!
Good luck.

IF you are working on "NetBeans Modules"
DLLs or SOs can be placed in the folder release/modules/lib/ in a module project's sources (look in the Files tab). This will make them appear in the final NBM or application in a lib subdirectory beneath where the module's JAR resides. Then just use System.loadLibrary as usual.

How to Build "Thinking in Java" Example Files to Run in Eclipse IDE?

I wanted to learn Java, as recommended by most of the people I downloaded "Thinking in Java" eBook and Source Code Example Lesson files from mindviewinc.com it's download location is this http://www.mindviewinc.com/TIJ4/CodeInstructions.html
These files don't work with Eclipse IDE for some reasons. So we have to build the files using the Build.xml file included in it's source code (Honestly, I do not have any idea what does "build" means here)
I tried building myself but of no use.. It did started building but after reaching some folder it stopped and gave error to install jboss-osgi-installer-1.0.0.jar .. After downloading it, I had no idea what to do with it, so I double clicked it and it asked me the location to install and I installed it in the default location, which was C:\Users\Username/jboss-osgi-1.0.0 and when I tried C:\Program Files\Java** the installation gave error that "this directory cannot be written, please choose another directory!" but anyway I installed it in the default location and it installed..
and again when I tried to build it, it gave me the same error and asked to Install Jboss....... I'm stuck.. This is happening again and again, wasted many days and didn't even crossed 100 pages of it's pdf because of this reason. It's a good eBook though.
I'm stuck in this from months... Please some one build it and make it like a project which could be easily and send me the project file, it would be really appreciated as I can not the solution to this anywhere on search engines..

Ok here's what I did to succesfully import the sources found in the "accesss" folder of you code samples:
Download the javassist jar as per Bruce's instructions, from here:
http://repo2.maven.org/maven2/javassist/javassist/3.9.0.GA/javassist-3.9.0.GA.jar
You must create an Eclipse project for the sources that are used as dependencies for most of the examples, that's the sources in the "net" folder:
in eclipse make new java project.
select the "net" directory, copy it, then in eclipse select the "src" entry in the project you just made, right click on it and select paste.
after eclipse finishes and auto-builds, it will complain about the missing javaassist jar. Right click on the project ->properties->java build path (on the left)->libraries (on the top)-> add external jars... (on the right), browse and select the javassist jar from where you down load it.
now the "net" project should be ok in eclipse
Now let's create the project for the sources in the "access" folder:
create a new java project in eclipse
Now, because Bruce structured his classes like crap, we'll have to make the proper structure in eclipse for them. This means:
a.create a new package called "access"
go to where your the access folder is on your drive, open it, select ONLY the folders (cookie2, desser, mypackage), copy them, then return to eclipse, right click on the "access" package u just made and click paste
now go back to the access dir on your drive, select ONLY the .java files (all of them, not the dirs), copy them, the back to eclipse, click on the src entry in the access project and click paste.
The access project in elcipse will still complain about missing classes form the "net" resouce. Right click on the project in eclipse->properties->java build path->projects (on the top)-> add(on the right)-> check the net project you created earlier-> click ok twice.
Now it should be ok
I hope you got the idea on how dependencies work in eclipse with this example.
For the other parts of you code samples, create separate java projects, add the classes to the proper package (create the packages if Bruce forgot to), and if you are missing external jars, go to http://search.maven.org search and download the jars. Good luck

"This code is designed to work outside of IDEs. Because packages are not introduced until later chapters, and some of the fancier IDEs like Eclipse require all code to be in packages, if you want to use the code inside those IDEs you will have to make some adjustments" - yep, bruce eckel would say that, cause that's the kind of guy he is. He also said there's ONLY checked exceptions in java, and asked for unchecked exeptions to be added as a bonus.
Now, if your sample code has no package, aka it's in the default package, aka when you look in the .java file, right at the top there's no line saying "package what.ever.bla" then all you have to do is:
-in Eclipse create a new Java se project
- open a windows explorer, go to your .java files, select them and copy them
- then in eclipse, right click the src folder in your project and chose "paste"
The files should be added to a "default" package in your eclipse project, without any error

Here is the simplest steps that I find to create a clean Eclipse project for the TIJ4 source code:
Download the source code zip from: http://www.mindviewinc.com/TIJ4/CodeInstructions.html, and unzip it to a folder, say 'TIJ4-code';
Download the additional libraries as in the previous URL, copy tools.jar from your current JDK's lib directory, and put them into TIJ4-code/lib;
Go into the 'TIJ4-code' folder, and run 'python Eclipse.py' (you need python installed on your system). This script adds the missing "package xxx" line to those source java files which don't yet have them, so as to make Eclipse happy. It also generates the Eclipse project files, which doesn't work well for some reason (beyond my limited knowledge on Eclipse), but they aren't needed here;
Open Eclipse and create a new empty java project, say with name 'TIJ4', choose JRE version 1.5, and "create separate folders for sources and class files";
Copy TIJ4-code/lib directory to your new Eclipse project folder;
Right click on the new project and select "Build Path" >> "Configure Build Path", then go to tab "Libraries", "add jars", then select all the jars under your project's lib folder;
Go to "File >> import", choose "General >> File System", select the unzipped folder 'TIJ4-code' as your "From directory". Check the root 'TIJ4-code' in the left panel of the import window, Click on "filter types" and select the .java, so that unnecessary files of other extensions, like *.class, *.py, *.xml, etc can be ignored during the import;
The zipped folder is no longer needed after the new Eclipse project is built. I hope you find this instruction helpful, and have fun learning TIJ4.

If you want to use this code in IDE rather than Eclipse and so on, for example in Intelij Idea or any other, the best way is to compile necessary classes from the TIJ4 with Ant, but it is not easy for the beginners, although it will be a good practice for you. You should make some changes in build.xml files which exists in every directory of TIJ4. Why?
to change version of Java(now we all use Java version higher than 1.5)
to include javaassist.jar library in the "classpath"
to put all compiled files in 'bin' directory(by default without adjustments TIJ4 build.xml puts .class files in the same directory with .java files. That is not a good deal.
For example you need to import and use within IDE(I use Intelij Idea) the class named net.mindview.util.Print ( see Chapter 3):
download TIJ4 and unzip in any directory you want(for example it would be c:\eckel)
download and install AntApache (you can find detailed instructions how to do it in Google) The result is - type in your console ant -version and you should see the answer. So Ant is working.
download javaassist.jar from http://jboss-javassist.github.io/javassist/ . Unzip to any directory, take out javassist.jar and past it to c:\eckel\net
4.There is build.xml file in c:\eckel\net. Now you should make adjustments. Open it with NotePad++ or other editor.
If you don't have Java 1.5 installed on your PC, so change all 'vesion1.5' to 'vesrion1.8' or other version that you have installed.
after tag description put property name = "classpath" value="./;./javassist.jar"/>
<property name = "src_util" value="./mindview/util"/>
<property name = "src_simple" value="./mindview/simple"/>
<property name = "src_atunit" value="./mindview/atunit"/>
<property name = "destdir" value="bin"/>
Then we make targets that will complile util simple or atunit automatically
Change tag available <available
classname="javassist.bytecode.ClassFile"
classpath="${classpath}"
property="javassist"
/>
Now classpath of available = property named 'classpath'. Classpath is the path where java compiler looks for .class files because some of your .java files want them by having import statement. There won't be error message "You must install the Javassist library " any more
Create targets for compiling util simple and so on See the following link with code
Code with targets
You can modify exists targets not to write all the code.
at the top of build.xml file change default="simple"
open cmd, then cd c:\eckel\net , then ant -simple or just ant (simple will run as default)
Please write if you will have some problems. Possibly i forgot mention about something.
Go to Intellij Idea, press Ctrl+Alt+Shift+S
Press +,
Library or directories
choose your c:\eckel\net\bin
5.now you can use import statement for all classes that exist in net.mindview
6.you can make the same with other dirs.

If you wish to you skip the entire Ant build process, I have converted the project to a Maven project (over JDK8), which you can simply import into your Eclipse workspace.
https://github.com/gauravojha/tij4-maven

Thanks to #Sergey Oliv's example. In addition, I needed to provide uncompiled javaassist package with all java files in the 'net' folder too. The .jar wouldn't work by itself.
I must say, this was a lot of work to get going.

How to get Java Decompiler / JD / JD-Eclipse running in Eclipse Helios

Java Decompiler (JD) is generally recommended as a good, well, Java Decompiler. JD-Eclipse is the Eclipse plugin for JD.
I had problems on several different machines to get the plugin running. Whenever I tried to open a .class file, the standard "Source not found" editor would show, displaying lowlevel bytecode disassembly, not the Java source output you'd expect from a decompiler.
Installation docs in http://java.decompiler.free.fr/?q=jdeclipse are not bad but quite vague when it comes to troubleshooting.
Opening this question to collect additional information: What problems did you encounter before JD was running in Eclipse Helios? What was the solution?

Here's the stuff I ran into:
1) RTFM and install the "Microsoft Visual C++ 2008 SP1 Redistributable Package" mentioned
at top of the installation docs. I missed this at first because the Helios instructions are at the end.
2) Close all open editor tabs before opening a class file. Otherwise it's easy to get an outdated editor tab from a previous attempt.
3) Open the class file in the "Java Class File Editor" (not "Java Class File Viewer"). Use "Open With" in the context menu to get the right editor. If pleased with results, make it the default editor in the File Association settings, in Window/Preference General/Editors/File Associations select *.class to open with "Java Class File Editor".
4) This guy recommends installing the Equinox SDK from the Helios update site. I did, but I'm not sure if this was really necessary. Anyone know?
5) If the class files you are trying to view are in an Eclipse Java project, they need to be in the project's build path. Otherwise, an exception ("Not in the build path") will show up in the Eclipse error log, and decompile will fail. I added the class files as a library / class file folder to the build path.
6) Drag/dropping a class file from Windows Explorer or opening it with File/Open File... will not work. In my tests, I gives a "Could not open the editor: The Class File Viewer cannot handle the given input ('org.eclipse.ui.ide.FileStoreEditorInput')." error. That is probably the wrong editor anyways, see 3).
7) After getting the plugin basically running, some files would still not decompile for an unknown reason. This disappeared after closing all tabs, restarting Helios, and trying again.

To Make it work in Eclipse Juno - I had to do some additional steps.
In General -> Editors -> File Association
Select "*.class" and mark "Class File Editor" as default
Select "*.class without source" -> Add -> "Class File Editor" -> Make it as default
Restart eclipse

The JD-eclipse plugin 0.1.3 can only decompile .class files that are visible from the classpath/Build Path.
If your class resides in a .jar, you may simply add this jar to the Build Path as another library. From the Package Explorer browse your new library and open the class in the Class File Editor.
If you want to decompile any class on the file system, it has to reside in the appropriate folder hierachy, and the root folder has to be included in the build path. Here is an example:
Class is foo.bar.MyClass in .../someDir/foo/bar/MyClass.class
In your Eclipse project, add a folder with arbitrary name aClassDir, which links to .../someDir.
Add that linked folder to the Build Path of the project.
Use the Navigator View to navigate and open the .class file in the Class File Editor. (Note: Plain .class files on the file system are hidden in the Package Explorer view.)
Note: If someDir is a subfolder of your project, you might be able to skip step 2 (link folder) and add it directly to the Build Path. But that does not work, if it is the compiler output folder of the Eclipse project.
P.S. I wish I could just double click any .class file in any project subfolder without the need to have it in the classpath...

I am using Eclipse 3.7 Indigo and Windows 7 64-bit:
What I did was to install the Microsoft Visual C++ 2008 SP1 Redistributable Package as suggested by the site and reminded by #Universalspezialist.
Then install the plugin as stated in the site: http://java.decompiler.free.fr/?q=jdeclipse
Go to preference, then find "File Associations"
Click on the *.class, then set the "class File Editor" as default.
Restart Eclipse perhaps? (I did this, but I'm not sure if it's necessary or not)

Simple thing i did to get it working:
Went in eclipse > Window > Preferences
(Optional)typed in the search box "file" to help trim the tree of options.
Went to General > Editors > File associations.
Clicked the ".class" type. Below there were 2 editors present, i clicked on the "Class File Editor" - the one with the icon from JD, clicked the "Default" button on the right.
Done.
Now all ur class are belong to us.

After testing on Juno, Kepler and Luna, I found JD only works for *.class files on build path.
Adding the jar as a lib of an existing project
Go to Preferences->General->Editors->File Associations, set *.class without source to Class File Editor with a cup icon

Its should work for all version of eclipse even in Spring tool suit(STS).
Here is the steps
Go to the URl
Follow The link to download or click the bellow link to direct download
Click Here to download
Download JD-Eclipse.
Download and unzip the JD-Eclipse Update Site,
Launch Eclipse,
Click on "Help > Install New Software...",
Click on button "Add..." to add an new repository,
Enter "JD-Eclipse Update Site" and select the local site directory,
Select extracted folder and give any name. I have given JDA.
and click ok.
Check "Java Decompiler Eclipse Plug-in",
Next, next, next... and restart Eclipse.

I use jadeclipse instead, because it can't work in 3.6/3.7 eclipse
Update site
http://webobjects.mdimension.com/jadclipse/3.6/
Intallation
http://5thcross.wordpress.com/2009/05/20/installing-jadclipse-in-eclipse/

JAD Decomplier plug-in for Eclipse version 3.x and 4.x.
update site: http://feeling.sourceforge.net/update
Steps:
Open Eclipse IDE.
Click Help->Install New software
Paste above URL and give name as JAD.
Select the Eclipse Class Decompiler
Click on Next and accept agreements
Install it.
Restart Eclipse and check now.

if you need to decompile standalone jar try JD-GUI by the same autor (of JD-Eclipse). It is a standalone application (does not need eclipse). It can open both *.class and *.jar files. Interesting enough it needs .Net installed (as do JD-Eclipse indeed), but otherwise works like a charm.
Find it here:
http://jd.benow.ca/
Regards,

I made the steps 1, 2, 3 and the 7. and I put the folder with the class files in the project build path (right click, properties, java build path, libraries, add class folder, create new folder, advanced>>, link to folder in the file system, browse,...) then restart eclipse.

Download the JD-Eclipse Update Site(github.com/java-decompiler/jd-eclipse)
Launch Eclipse,
Click on "Help > Install New Software...",
Click on button "Add..." to add an new repository,
Enter "JD-Eclipse Update Site" and select the local site directory,
Check "Java Decompiler Eclipse Plug-in",
Next, next, next... and restart Eclipse.

Just download the site from the JD page. I was able to install from a local site in the isntalled software section of eclipse.

Steps to add the Java Decompiler in Eclipse :
Open Eclipse IDE.
Click Help->Eclipse Marketplace Wizard
In Search tab find the JD (JD is keyword to get the Eclipse Class Decompiler)
Select the Eclipse Class Decompiler
Click on Install.
Restart Eclipse and check

I used Intellij or Android studio both are working awesome for decompiling purpose.
Internally Intellij following FernFlower Decompiler.