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Eclipse doesn't compile java files into classes

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.

Eclipse Hell . . . Failed to read the project description file (.project)

I think Eclipse is trying to make me miserable. A couple of hours ago, my project was working and compiling well. Suddenly that all changed. Eclipse somehow wipes out all changes I have made to my files(activity, manifest etc.) I make sure to save often but when I go to run the project, I get the error that I have a build error. I checked and there was none, so I go to close Eclipse, so I can reopen and see if the errors will go away. Instead what happens is Eclipse wipes clean all my files and I end up with a project on disk with lots of blank code files. I try to run anyway, and I get the error message below.
Failed to read the project description file (.project) for 'com.example.android.nfc.simulator.FakeTagsActivity.FakeTagsActivity'. The file has been changed on disk, and it now contains invalid information. The project will not function properly until the description file is restored to a valid state.
Anyone have an idea what in the world this is about and how I can rectify this?

I would recommend to wipe out all the eclipse related configuration files(make sure you take the backup if you have manually made some changes in those) and import the project again, by following
File -> Import... -> Existing Project into Workspace
Make sure you take the backup of whole project before doing this.

When Eclipse gets stuck in some obscure status, I usually find useful to recreate the .metadata folder in workspace directory. It is a drastic solution, but it usually works for me. After that you can reimport all your projects.

This happened to me because I had a conflict in my .project file. I opened it up in a text editor and fixed the conflict (in my case, removed from ====... to >>>branch..., as well as <<<HEAD) and then I was able to open the project in eclipse.

I was just encountering this same issue (using Zend Studio 12, which is built on Eclipse). My problem was that I was creating a project from a Remote Server, and I was just downloading everything, which also included the .project file from the old project. It wasn't showing this error until I'd closed the program and tried to open it again (which usually wasn't until the next day).
I seem to have resolved it by making sure not to download the .project when initializing the project.

If the .project file is a text file, so if you have it in version control you might want to copy it over from there.
Before you trying the following make a backup of your current project state.
Assuming you don't have an old copy, you can open it in a text editor and try to see if there is something obvious wrong.
If this fails, copy the source files of your project to a fresh location (without the eclipse configuration files) and import it as a new project into eclipse.

if you still have all of your other source (.java files, AndroidManifest.xml, resources), you might be able to get your project back by simply going to the folder containing the project, removing the .project file that you say is now empty, and then using the new project wizard to recreate the project (and .project file) for you from your existing tree of source.
from the menus, select File -> New... -> Project , and then, in the first dialog, choose Android Project from Existing Code .
if you had done anything special to the .project (reliance on specific .jar files or changes to classpath), you may have to re-do these steps after you re-create your project; but at least it should pick up the code you already have.

If all the files are empty, and you have no working Backup, I can't help. I would recommend to use proper version control in the future.
Use git or mercurial, they have nice UI integrations (see tortoisegit/tortoisehg).
To decouple the build process and library management from eclipse, take a look at maven or gradle, this might help you in future projects.

If you use git, complete the merge e.g. del the head in your androidmanifest.

I also got the Same Issue in Eclipse but the real problem is i removed the actual Source project folder from source location (from which eclipse is trying to take the project) to some other place hence my eclipse is unable to get web.xml file since it is not available. Now i changed my source project folder location to the path where eclipse is trying to search for. Issue Resolved for me.

Just delete the 'servers' folder in your workspace and try again.

eclipse doesn't compile the project

I had running project opened in eclipse. After an accidental restart of windows, now when I open the eclipse I see my project is marked with a little red cross. Now when I run the main method I get a java.lang.NoClassDefFoundError.
I have tried restarting eclipse, Project -> Clean but it doesn't solve the problem.
When I checked the project directory, inside 'target' folder there are no compiled .class files. I tried building the project but I can't get the compiled class files, which is the reason for the error.
How do I solve this?

I tried all the solutions here, but here's what worked for me.
First, remove the project from eclipse.
Then, in your project folder, delete the 2 files that eclipse creates. They're hidden, but they're called .project and .classpath.
Finally, add the project back into eclipse and eclipse will recreate those 2 files and a new configuration for your project.
My specific issue was that it a project that was missing a Java Builder because it thought it was a scala project. The only way for me to reset this was by removing the project, deleting the 2 files, and adding it back in.

Maybe Eclipse's workspace files have become corrupted. Restart Eclipse and choose a new workspace folder (or choose Switch workspace from the menu). Then import the project files into a new project.

Try deleting the run configuration and remaking.
If that doesn't work, open the class, ctrl+f11 to run. That will force Eclipse to update its classpaths & rebuild its main indexes.
To delete the configuration:
- Menu -> Run -> Run Configurations.
- Select your configuration
- Press delete or use the delete icon

If you are using maven, this will likely fix it: Right click your project -> Maven -> Update project.. (or alt + F5), select your project, make sure "Refresh workspace resources from local filesystem" is checked and click ok.

My eclipse project has similar issue though the error appeared slightly different. I imported a third party java project into Eclipse, with Project > Build Automatically checked, do a Project > Clean..., no classes were generated in the output bin folder. Tried different options, no luck.
I noticed a red exclamation mark at the top left corner of the project name.
Right click on project name, Properties > Java Build path, I saw "Build path entry is missing: cci/src". Manually created src folder, the red exclamation mark gone. Do a Project > clean..., project compilation successful
P.S. The eclipse project is not properly organized, there are no code under src folder. I suspected src folder went missing while checked in by original author since there are no code there.

Was having the same problem and finally found the glorious hint here.
In my project there is one *.scala File and Eclipse Neon is incompatible with the Scala IDE, thus there is no builder for scala and the whole project is not built.
After removing the Scala file and the Scala Package from the linked Libraries )Project Properties --> Build Path --> Libraries) and afterward re-adding the Java Builder (no idea why this one went missing, but here's a useful help on how to add the Java builder when missing), the project could be built and run again.

Try refreshing and then cleaning the project.

For me the problem is in missing jars,the jars were added first and the location of those jars where changed after.
So When I correctly added all the required jars again to the build path, it starts to build project correctly.

When does Eclipse copy resources to the output folder?

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.

Run NetBeans project just with ant

I have a sample code that was built with Netbeans.
It has a build.xml file so I downloaded ant and try to run it.
I've got this error message:
...... nbproject\build-impl.xml:76: Platform is not correctly set up
For what I can see, this is fixed by "simply" downloading Netbeans and running the sample from there, but... I don't want to install it to run a 10 files sample.
Is there a workaround to run Netbeans projects with Java? What's the correct .properties file I have to modify?

It is possible to run the NetBeans generated projects straight from Java/ANT, but you may need to manually set some of the properties and/or add paths to jar files.
Unfortunately, NetBeans tends to include taskdef's using their own JAR files and reference properties that are defined only in the /nbproject/private/private.properties files, which usually get set when you first open the NetBeans project or modified as you edit the project in the IDE.
If you inspect the build-impl.xml you should be able to find the property and derive what value needs to be set(OS platform), then either:
create/set the property in the
/nbproject/private.properties
add that property definition in the
parent build.xml
pass in the commandline when invoking your ant
target using -DPlatform=Foo
Personally, I like the structure of the NetBeans generated ANT files and targets, but hate how much custom/proprietary stuff they jam in that makes it hard to run without NetBeans.
For example:
ant -Dplatforms.JDK_1.7.home=/opt/jdk

I've just successfully built NetBeans project with ant. These were the things I had to do:
Copy <NetBeans-folder>/java2/ant to a "Netbeanless" machine
Copy nbproject/project.properties to, say, ant.properties
Replace every ${} expression in ant.properties with it's value
Add platform.<platform-name>.home=<path to platform>
Add libs.CopyLibs.classpath=<path to nb-ant>/extra/org-netbeans-modules-java-j2seproject-copylibtask.jar
Add other needed classpaths into javac.classpath (e.g. path to servlet-api.jar)
ant -propertyfile ant.properties
It works, but doesn't make me happy. I would either like to find the way to reuse project.properties, or to automatically translate it to a "resolved" version (step 3). Build could then be automated.

I just went through this exercise with my NetBeans 7.0 project. What I was able to do was copy my build.properties file from my .netbeans\7.0 directory on my Windows system and make a server.properties file for my build server. This isn't much of a stretch, since every developer's build.properties may vary, so having another file for the server is to be expected. I then put together a simple server-build.xml file that references this file and does only the basics of init, compile, dist, and clean. I committed these two files to my CVS repository at the top level of my project directory, since they don't conflict with other project files and serve as a reminder in case something needs to be updated. Now I can build my project on my CI server with "ant -f server-build.xml" and everything just works.
My whole init section looks like this, giving my server paths priority, but including the necessary information from the NetBeans project properties.
<target name="init">
<property file="server.properties"/>
<property file="nbproject/project.properties"/>
</target>
I also had to do something similar when defining the ant tasks for my nested projects:
<target name="compile">
<ant antfile="${project.MyProj-common}/build.xml" inheritall="false" target="jar">
<property location="${build.dir}" name="dist.ear.dir"/>
<property file="server.properties"/>
<property file="${project.MyProj-common}/nbproject/project.properties"/>
</ant>
...
</target>
I had to copy the j2ee.platform.classpath from project.properties to my server.properties file to ensure the references to j2ee.server.home resolved as I needed. I didn't expect to have to do this, but the classpath was wrong otherwise, causing the build to fail.
Thanks for the information on this question, as it helped guide me to this solution.

I just faced the same problem. I hope I could get rid of netbeans to go to eclipse and maven, just for that.
But here is a good link to export a ant built project from netbeans into a continuous integration server (or could be into any other IDE too).
Technique is going pretty well. Here is a summary:
install netbeans on the CI server (there is an option to do it without gui, use -silent on the netbeans installer)
include nbproject in SVN
add to ignore list the private folder
create your own private folder on CI server
make it point to a folder of your CI server (mimicking a private user folder in the account used for CI)
copy a real folder from a user to this folder and change every path (replace strings) to point to your netbeans install on your CI server.
And it should work.

Just to add to Mads' answer... usually, you need to install and open Netbeans at least once on the target machine. The ANT projects also rely on a few settings from the USERDIR/.netbeans/... directory. This may have changed with 6.5+.
This will get some of the base settings configured and define the classpath's to netbeans jars. If your dependencies (i.e. libraries) or project is being run from a different directory since the last time you opened the project in Netbeans, you will need to tweak a few settings in the private.properties file as Mads' described.

I'm using Netbeans 6.8 and java projects that were created with Netbeans can be run from the Netbeans auto generated build files with just ant on the cli. All the regular targets like ant compile,ant run,ant clean, etc "just work". (I'm using Fedora 13 if that matters)

You can use this repo just for that https://github.com/albfan/ant-netbeans
It overcomes all the oddities of netbeans wrapped ant config so you just need:
To compile:
$ ant.sh compile
To run:
$ ant.sh run
To whatever (autocompletion):
$ ant.sh <tab><tab>