I have no idea why, but from today Eclipse doesn't compile .java files into .class. I press the "Run" button and Eclipse only tries to run the program rather then compiles it first, so i get this error all the time:
Exception in thread "main" java.lang.NoClassDefFoundError
open problems view from window -> show view -> problems, if there's any error, fix it
eclipse will reject to compile if there is any problems in your project
Close Project -> Open Project -> Clean -> Refresh
Hope it helps for someone else
This problem happens when "for some reasons" your project is corrupted and has a red icon near the project's name in Eclipse. The solution is to save the project's folder, delete the project in Eclipse, than create the project again in Eclipse coping all the previous files saved.
I ran into this problem with Eclipse Luna, for some reason the Java builder was missing from the project. Verify that in your .project file you have something like
<buildSpec>
<buildCommand>
<name>org.eclipse.jdt.core.javabuilder</name>
<arguments>
</arguments>
</buildCommand>
</buildSpec>
You can also configure builders from Project Properties -> Builders
When I have had this exact same problem, its cause has always been the same and the solution has worked 100% of the time for me. It's caused by a missing jar file in the project's Java build path and it's indicated by a red exclamation point decoration on the project icon. To fix the problem, go to Project|Properties|Java Build Path, click on the Libraries tab and either remove or fix the path for all missing jar files. If your project is configured to rebuild automatically, it should start a build as soon as you apply your changes.
The eclipse needs to erase the output folder when building classes.
The folder is found on Project Properties -> Java Build Path -> Default Output Folder. It can differ from general output folder name found under Eclipse Preferences. If this output folder cannot be erased for some reason (for instance, on Windows, when a shell is opened on this location), the project is marked as corrupted (as Marco Micheli describes) and the classes then are not built.
You just need to make sure the output folder can be erased.
Yes I was facing this issue. Coz Eclipse cannot build the project for Build Path error. Check that your external jar files is not showing any error. Delete the Jar file which one is missing or which one is showing re color. delete that one. add a fresh one from the correct path or location. if all the error are gone. then in the project bin folder one class file will be created and then could not load main class error will be removed too.
there can be errors in your project, or other reasons like mentioned in above answers.
or you may be a fool like me
who moved the source code to other folder and not updated source path in project build properties. in this case it was not giving any error or class files too.
check that too.
goto
Project Properties -> Java Build Path -> Source
Add or Link new source folder location.
Do clean all projects ! This will erase all old class files, so eclipse is forced to generate them again.
Some times jars in the build path might be missing even though we are not using it. Since error is there, java files wont compile into classes.
Can check errors from problems view.
Either remove jars from build path or place the missing jars in the build path.
I had problem in eclipse for load module not found and no class file was getting created.
Solution: Go to Create New Project > Under JRE section Choose Use project Specific JRE.
Now Class files are created.
Please check if all the jars are there in your build path settings.
If any jar is missing this issue might come.
I had this problem, I tried the clean, the build manually, open, closing the project, deleting, and open, etc., nothing works, and after 4 hours I get a workaround what works for me.
Please, Make a RAR, ZIP, or copy from your entire project folder as a backup if it doesn't work and you mess around, OK, now:
Clean the Project. It cleaned all my class files from the output folder, but doesn't generate the class files after the clean with the building (but we already know that)
Delete the project (warning, make sure the Delete contents is not selected or your project will dissapear). For me it says something about that desynchronysation with code mix, and hibernate, blabla)
Close myEclipse
Restart your computer.
Open myEclipse
Import the project with the option "Projects from folder or Archive", not with the "existing projects into workspace" option.
Myeclipse start to build the class files, I don't know why, but works for me, I hope this helps somebody else.
Also ensure that there are no projects with fatal errors (projects with ! symbol over it) in the build path of the project which is not generating classes under the bin directory
Just restart your machine.
Its weird but it helps we had loads of problems this type in our company for unknown yet reason. Always restarting the machine helped.
It was normally encountered while trying to run Junit tests and it could not find a class even when the Run configuration was pointed to /bin.
I encountered a similar problem: previously compiled and running java files couldn't run anymore. Eclipse showed "couldn't find class xxxx". When I checked bin folder, the previous .class files were gone. That's why Eclipse couldn't run the files, NOT because of errors in files. I don't know why it can't just re-compile the file, and why the .class file gone missing in the first place.
But I found a solution:
Simply create a dummy java file in the same package (without doing anything just create), then run it. Then magically all previous .class files came back. And I can run those files as before.
Just Remove Your all Jars (Libraries) from the Build Path and refresh and add all jars and clean and build the project
This is probably a rare event but I declared a servlet with annotations and left off the "/" before the servlet mapping name. I spent over an hour chasing my tail, installing new servers, thinking I might even need to re-install my IDE and it was this simple error! Every time I tried to load any page from my Dynamic Web Project I got a 404 error. I could not even load a plain vanilla html page. There really needs to be better error logging with the IDE and Server interaction as there was zero indication of the error and my servlet was never called at the start of my web app. Hope this helps save someone else some frustration. Always name your servlet with a / slash like /myServlet never just myServlet.
I tried many different potential solutions: cleaning, rebuilding, removing, re importing, Update Maven Project and nothing. I still couldn't run my app from Eclipse. In my case the reason was that I was running Eclipse using openjdk-11 while my project was in Java 8. After I've reconfigured Eclipse to be ran from Java 8 everything started to work (in eclipse.ini).
-vm
/usr/lib/jvm/jdk1.8.0_202/bin
I'll explain my solution to this problem: "eclipse not compiling java files into class files". I'm using Eclipse version '2020-03 (4.15.0)'. I have a project that is dependent on a java project. For the dependent project, I was not able to export a jar file containing the compiles classes and resources. To be more precise, the exported jar file did not contain any class files. The output bin folder for the project is empty after doing a project build. In examining the console output when I tried to build all projects indicated that the build failed because the dependent project was missing. In looking at one of the log outputs, I saw something indicating that the build failed because some eclipse class was missing.
One of the things I tried was to do an eclipse update. Eclipse offered up the list of changes and I tried to apply them, but the update failed. I tried to update several times but they all failed. Looking more carefully at the offered up list of updates I noticed that one of the suggested update involved "deleting" Java Development Tools. That line had an warning indicating that the entire feature could not be deleted due to other dependencies and that only the non dependent features would be deleted. Doesn't that sound suspicious. Why was the update trying to delete Java Development Tools? My speculation is that something got deleted from the Java Development tool that was causing the build process to fail. The missing class I saw earlier also suspiciously sounds like it might be part of the Java Development Tools feature.
So what I did to solve the problem was to reinstall eclipse. I used the eclipse loader application to do the reinstall. (Note, I did not uninstall it first). After the reinstall, the dependent project was able to produce classes, and things once again worked as they should.
I certainly don't know how the eclipse release got into this compromised state, but in my situation the re-install of eclipse was the only way I could find to resolve the problem.
There would be the case : if you have opened project explorer in eclipse then it may not show you classes generated under /**/build/classes, in such case open Navigator view of that project and check build directory under the same project.
I've got a Java Web Project which I'm deploying to a Tomcat embedded in Eclipse's Servers Plugin, alas, when I navigate to the location of that webapp after publishing/starting the server, the .class files are all missing. I can navigate as far as ProjectName\WEB-INF\classes\path\to\packages\ -> All packages are empty.
Any Idea how to solve this problem?
Edit
Build Automatically is checked and it compiles just fine for another web project I've just included, just not for the one specified
Edit 2
Reinstalled Eclipse (because why not) - nothing's changed, works for Project #2, doesn't work for Project #1. Not even the path to the packages is existant now. The content of the src folder (that's supposed to be moved to WEB-INF/classes) isn't there at all - but the resources (I've got a resources folder with the same deployment assembly command) are.
A few steps to perform to restore sanity...
Refresh your project. It is possible that something was modified on the file system external to Eclipse.
Kick of Project -> Clean.
Now look at your project (not the deployed location) on the file system. Do you see the expected files in the /bin directory (or whatever you may have changed class output directory to)?
Let's now try exporting a war using the wizard. File -> Export -> Web -> WAR or something like that. Check the WAR contents. Are you .class files there?
If everything is good so far, there is nothing wrong with your project. The Tomcat server adapter just got confused.
In the Servers view... Make sure Tomcat is running. Next right click on Tomcat and select add/remove projects. Remove your project. Confirm that your web app is no longer accessible.
Now re-add your project via add/remove projects or invoke one of the run as actions.
The above should reset Tomcat server adapter and hopefully things should work as expected.
Check if you had deleted or moved your external JARs associated with the project on your IDE. I had moved it and ran into the same issue.
I need to run Maven from the command line while Eclipse is open. Since this can confuse Eclipse, I configured the two to use different output folders (Maven writes to target, Eclipse m2e uses target-eclipse). This works very well.
But in one of my unit tests, I need to load classes which are part of a project but not on the classpath. Basically, I build my own URL classloader. Now, I have a problem: Which class folder to add?
If the project is open, target-eclipse/classes is the correct choice, otherwise I should use target/classes.
Assume that I know the Eclipse folder and the workspace folder. How can I tell from Java code running in a child process of Eclipse (not an Eclipse plugin) which projects are open and which aren't?
The existence of the folder is not a good criteria since the folder isn't deleted when I close a project.
[EDIT] Basically my question is: Eclipse must store that information somewhere in $workspace/.metadata or in $ECLIPSE_HOME/configuration. In which file do I need to look?
A file [Workspace_Home]/.metadata/.lock tells You the workspace is open.
The list and settings of projects in the workspace are in [Workspace_Home]/.metadata/.plugins/org.eclipse.core.resources/.projects.
It seems like open Java projects have a folder [Workspace_Home]/.metadata/.plugins/org.eclipse.core.resources/.projects/[Project_name]/.indexes, and closed don't have it. - I double checked it, and it seems I was wrong here.
But I just checked that closed Java projects have a file: [Workspace_Home]/.metadata/.plugins/org.eclipse.core.resources/.projects/[Project_name]/org.eclipse.jdt.core/state.dat, and open projects don't have it.
I would just use some configuration parameter (a system property for example), telling where the folder is. See http://maven.apache.org/plugins/maven-surefire-plugin/examples/system-properties.html for how to pass system properties when running tests with Maven. The first example in this page shows how to pass the build directory as a system property.
I noticed that "sometimes" the resources i put into a source folder in my Eclipse project will not be copied to the output folder ("bin") immediately.
E.g. i change a properties file using the eclipse editor and save... and "bin" still has the old version.
Does anyone know what exactly triggers the copying (and how i can trigger that from a plugin)?
I thought it happened automatically when a resource changes.
-- EDIT --
To clarify what i meant by triggering it from a plugin: I have a plugin that depends on up to date resources in the output folder, but apparently the resources are outdated when my plugin is called. And that's why i need to better understand what happens and how i can force it when i need to.
If you don't add the resource through eclipse (but for instance through the windows explorer), eclipse wont notice some changes until you refresh your project/folder. (right click in the packater explorer for instance).
After it notices the change it will copy it to the output folder when you build (often this is done automatically upon changes)
This is a Bug in almost all Eclipse versions, it happens without any reason. My eclipse 3.5.2 and 3.6.2 both meet this problem in someday. In my case, I need to copy modified .clj clojure source files to classes directory for immediate effection in a web app, now I have to set /src before /classes in CLASSPATH of web appserver startup script, It's solve my problem temporarily.
As #Thirler said, if you don't add the resource through Eclipse, it may not notice the files until you refresh your project. However, if the Refresh/Clean doesn't work (in my case, it didn't), you have to add the files to the folder structure IN ECLIPSE.
Just drag the file from the folder where it is in the file system using Finder/Windows Explorer, drop it into the same folder in Eclipse's Project Explorer (Eclipse will ask for overwriting, just say yes) and Voila!
EDIT:
An easier way is to just navigate into the folder using Project Explorer and then Refresh (F5).
As far as I know it happens when you build the project. You don't need a plug-in, you just need to tell eclipse to do a build of the project.
There is an Eclipse setting Java > Compiler > Building > Output folder > Filtered resources.
For unknown reasons, this was set to *.launch,*.testsuite,*.deploy,*.location,*.execution,*.datapool,*.artifact,*.html,*.svg in my workspace (while the default seems to be just *.launch). This was the reason, why my Eclipse constantly refused to copy a classpath resource index.html.
Late response, but I just stumbled over this question but non of the answers helped me to convince Eclipse to do its job.
This happened to me when I imported an Eclipse project as a whole, including both src and bin directories. The fix is to Clean the project: Project -> Clean... -> select your project and press Clean. This should re-build the project and properly locate the resources needed.
Ok I'm completely googled-out.
I have a few java projects in my eclipse workspace (about 25). Most projects use linked source folders.
When I rename a class in Project1, the references to that class in the other projects are not updated. The references within the project are updated just fine. The net result is compilation errors on the next automatic recompile.
I'm using Eclipse 3.5.1, but the same behavior was shown by 3.4.0 and 3.4.2
Any ideas about how to fix this?
My current plan - after googling for 20 minutes - is to write a script to convert the linked source folders to be OS links (I'm on linux) in the project folders themselves. So then I'm no longer using linked source folders.
Like Philippe Faes said, it works fine if your projects are set as a dependency on each other. Make sure that your project's build path are set up that way.
IMHO linked folders are for external files, not for another projects.
Another problem is that if you are just referencing a jar as a dependency (ie, on the project's class path), eclipse will not be able to know that the jar was build based on another project's source thus will not refactor properly.
Try to ask your colleagues for the eclipse project files (.project and .classpath) and edit the files if you need, then check if your refactor still doesn't work. I am guessing that your project is set up differently than your colleagues.
Cross-project renames work just fine if your projects depend on each other.
What exactly do you mean with linked source folders: do you link to the same source folder more than once?
This is a shot in the dark, but make sure that your project is a Java project. I am not sure if other project types (like the generic Project) might not refactor properly. I have never seen this problem before and it has always worked as expected for me...