You first have to select either Modulepath or Classpath.

The Modulepath is a very new thing for Java 9 (here you can watch a short video how it can be used). It exists since the Eclipse Oxygen.1a (4.7.1a), October 11, 2017. In case of doubt, I recommend selecting Classpath.

Answer from howlger on Stack Overflow
Discussions

Java Eclipse cant add external jars - Stack Overflow
I'm watching a YouTube tutorial and the instructor adds external Jar files into the JRE System Library, but the button "Add External JARs" is not clickable for me. How can I fix this? More on stackoverflow.com
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Click buttons like Add jar and Add External Jar disabled in Eclipse - Stack Overflow
Please let me know how to Enable this two buttons in Eclipse oxygen. Thanks in advance. I think I have disabled it mistakenly. More on stackoverflow.com
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November 21, 2017
Add external jars into eclipse plugin project - Jazz Forum
How to add external jars into eclipse plugin project in RTC? (I know there is one option available create plugin from existing jar archives. and somewhere i read that adding external jars through build path doesn't work for plugin project in RTC ) I want to know a method other than this T... More on jazz.net
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June 23, 2015
java - Eclipse - cant add jar to build path. jar is inside the lib folder of project, but not visible - Stack Overflow
I manually copied the my sql connector jar to lib folder of my sql jdbc project. I had some old jars already in that folder and which were also easily added to build path before. Now, I forgot how... More on stackoverflow.com
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Coderanch
coderanch.com › t › 608159 › ide › add-external-jar-file-java
Not able to add external jar file into java project in eclipse through buildpath (IDEs and Version Control forum at Coderanch)
Mani Raju wrote:Yes, It is access related problem of .classpath file in my project and i did checkout for this file. Now it is allowing to add external jar files into my project through build path in eclipse. Thanks. Do you mean this .classpath file is a registered SCM source file?
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Controlyourmindset
controlyourmindset.com › posts › 07f32923f50fa8635d209a91b6950c10
Why I cant add external JARs is disabled in eclipse? – Control Your Mindset
In case of doubt, I recommend selecting Classpath. Show activity on this post. When this window is open first click on the ‘Modulepath’ > After that ‘Add JARs’ option get enabled. ... From the ToolBar to go Project> Properties>Java Build Path > Add External Jars .
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Eclipse
bugs.eclipse.org › bugs › show_bug.cgi
385848 – Cannot add external Jar File; Cannot click Browse
October 10, 2012 - Bugzilla – Bug 385848 Cannot add external Jar File; Cannot click Browse Last modified: 2012-10-10 07:57:33 EDT ... This site is maintained for archival purposes only. Eclipse projects have transitioned to GitHub and Eclipse GitLab.
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Eclipse
bugs.eclipse.org › bugs › show_bug.cgi
457294 – [Mac] Cannot Add External Jars to Project the Second Time
January 14, 2015 - Bugzilla – Bug 457294 [Mac] Cannot Add External Jars to Project the Second Time Last modified: 2019-12-15 04:53:09 EST ... This site is maintained for archival purposes only. Eclipse projects have transitioned to GitHub and Eclipse GitLab.
Find elsewhere
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Controlyourmindset
controlyourmindset.com › posts › 9c9c05cd09430056cb8f3f5e93e2d066
Why can’t I add external JARs in Eclipse? – Control Your Mindset
If your project builds with Java 9+ make sure you’ve selected Classpath (as shown here). You should also be able to add the JAR with right-click on the project > Build Path > Add External Archives…. If that doesn’t help, would you tell use the version and package of Eclipse IDE you’re using?
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Eclipse
bugs.eclipse.org › bugs › show_bug.cgi
552710 – Add External JARs selection ignored when creating a new project
Bugzilla – Bug 552710 Add External JARs selection ignored when creating a new project Last modified: 2025-01-25 19:47:21 EST ... This site is maintained for archival purposes only. Eclipse projects have transitioned to GitHub and Eclipse GitLab.
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Blogger
javarevisited.blogspot.com › 2016 › 07 › eclipse-how-to-add-external-jar-into-classpath.html
Eclipse - How to add/remove external JAR into Java Project's Classpath? Example
6) Alternatively, Click Add External JAR to add JAR from any folder on your machine. It will open the file chooser to choose the JAR file from the file system and add that into CLASSPATH. Once you click Open, the JAR will be shown in your project's build path as shown below : 8) Once you add JAR, you can see them in same Library window. You can remove them by selecting and clicking Remove button. 5) Click Add JAR to add JAR from any other project in the same Eclipse workspace.
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Jazz Community Site
jazz.net › forum › questions › 199375 › add-external-jars-into-eclipse-plugin-project
Add external jars into eclipse plugin project - Jazz Forum
June 23, 2015 - on the project, use context menu>PDE Tools>Update Classpath to correctly add the jars to the eclipse project classpath. This worked for me first time and in the "lib" folder it created Three more folders which are "com", "META-INF", "sqlj" and one file name "T4XAIndbtPkg.SJPeofile().ser" I ...
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Wikihow
wikihow.com › computers and electronics › software › programming › java › how to add jars to project build paths in eclipse (java)
How to Add JARs to Project Build Paths in Eclipse (Java)
August 10, 2022 - Click Add External JARs. It's on the right side of the project properties window. ... Select the JARs you want and click Open. The JARs will now appear in the list of libraries in the build path. ... Click OK to close the properties window. The JARs will now be in Referenced Libraries. ... Include your email address to get a message when this question is answered. Submit ... Whenever you add new files or folders to your projects in Eclipse via anything but Eclipse, you must refresh the affected projects to let Eclipse know that the new files are there.
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Eclipse
bugs.eclipse.org › bugs › show_bug.cgi
457340 – "add External jar" button doesn't browse dialog except first time use for User Library
January 13, 2015 - Bugzilla – Bug 457340 "add External jar" button doesn't browse dialog except first time use for User Library Last modified: 2015-01-13 12:21:49 EST ... This site is maintained for archival purposes only. Eclipse projects have transitioned to GitHub and Eclipse GitLab.
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Stack Overflow
stackoverflow.com › questions › 36617987 › eclipse-add-external-jars-doesnt-work-to-add-them-as-referenced-libraries
java - Eclipse add external jars doesn't work to add them as Referenced libraries - Stack Overflow
I suppose your Eclipse perspective is selected "Resource", not "Java" or "Java EE". After you change the perspective to "Java", you will see the Referenced Libraries. ... I did change the perspective, but the projects look the same. It's obvious that there's an error when I import the jar files since they can't be accessed or used.
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The Eclipse Foundation
eclipse.org › forums › index.php › t › 1106698
Eclipse Community Forums: Java Development Tools (JDT) » [CLOSED]classes in jar files not accessible | The Eclipse Foundation
January 22, 2021 - The Eclipse Foundation - home to a global community, the Eclipse IDE, Jakarta EE and over 350 open source projects, including runtimes, tools and frameworks.
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Java67
java67.com › 2017 › 04 › how-to-add-jar-file-in-eclipse-project.html
How to add JAR files in Eclipse Project's Build path? Example | Java67
This is really useful when you are stepping into the source code of a third-party library or any internal library while debugging. In my earlier article, I have shown you how to attach sources to a JAR file, which ideally should be next to this article, but never mind you can refer to them now. Though you can add any external JAR files in your Java project on Eclipse, I suggest using Maven as a build tool for dependency management, it's a lot easier to manage a central local repository using Maven for keeping those useful JAR files, instead of searching on the internet, downloading it and then manually adding into your Java project's build path.
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Programmersought
programmersought.com › article › 56983343756
Eclipse, "Add External JARs" is grayed solution - Programmer Sought
Want right project in Eclipse projects -> Build Path-> Configure Build Path-> Java Build Path used to add a jar experiment, it was found Library under Add External JARs button is grayed state, shown in Figure 1 .