Develop Reference

Java intellisense not working when imported projects not available

I'm reviewing an isolated Java project in VSCode. By that I mean that I have the project itself, but not any of the projects that it depends on. From what I can tell, the result is that Intellisense has been turned off for the project, which makes navigating around the code a pain.
For example, you can see below that VSCode can't find the definition of a method, even though it's just 3 lines down. On the side, you can see the errors it's encountered trying to load various artifacts.
Is there a way to turn Intellisense back on, at least for the objects that are defined within the project itself? I have the MS Java extension pack installed.
To clarify, these are internal projects that it depends on and I don't have on my machine.

I just noticed there's a feature that might help you get around this. Check out "Lightweight Mode" here: https://code.visualstudio.com/docs/java/java-project
Maybe this will get away from having access to mvn full resources and just get the quick-n-dirty navigation as the docs seem to indicate!
Lightweight Mode
VS Code for Java supports two modes, lightweight and standard. With
lightweight mode, only source files and JDK are resolved by the
language server; with standard mode, imported dependencies are
resolved and the project is built by the language server.
...works best when you need a super quick-to-start and lightweight
environment to work with your source files, for example, reading
source code, navigating among source code and JDK, viewing outline and
Javadoc, and detecting and fixing syntax errors. Also, code completion
is supported within the scope of source files and JDK... Lightweight mode doesn't resolve imported dependencies nor build the project.

Your project throwed project build error: Non resolved parent pom for com.....
When errors are resolved, the intellisense should work as normal.
Please have a look at this question. Try and see if error goes away.
Project build error: Non-resolvable parent POM.

Code doesn't execute in debug mode, asks for source

I am executing a code that does some database operations.
It performs well when I execute it using the Run command in Eclipse.
But when I execute it in debug mode, I get the source not found
The JAR file mysql-connector-java-5.1.39-bin.jar has no source attachment

You're trying to step into code provided by mysql-connector-java, and you haven't linked in an source code for Eclipse to actually step-in-to.
The easiest thing to do would be to step over that line (since you probably don't care about it) rather than trying to debug the connector code. You may also want to tweak your debugger settings to prevent exceptions from pausing your execution.
As #nitind points out you can also filter types, packages, and patterns so the debugger avoids stepping through them.
If that doesn't work or you do need to step into the connector code you'll need to download a source jar (from Maven) and attach it in your project's build configuration.

Debugging in eclipse program works with the class actually loaded.
The problem you posted looks like the class you are using was not found in the project, but is present without debug info in a distribution jar.
This can be solved most likely by altering the build path of the project to stop using this jar and have the JVM using the project.
or you can try mvn clean eclipse:eclipse

This is happening because the debugger is running into some class file...
To view this, either download the source code and attach it or install some class decompiler plugin (I am using JAD Eclipse Plugin). The first solution is better.

Command works in terminal but not with java [duplicate]

What are the possible causes of a "java.lang.Error: Unresolved compilation problem"?
Additional information:
I have seen this after copying a set of updated JAR files from a build on top of the existing JARs and restarting the application. The JARs are built using a Maven build process.
I would expect to see LinkageErrors or ClassNotFound errors if interfaces changed. The above error hints at some lower level problem.
A clean rebuild and redeployment fixed the problem. Could this error indicate a corrupted JAR?

(rewritten 2015-07-28)
Summary: Eclipse had compiled some or all of the classes, and its compiler is more tolerant of errors.
Long explanation:
The default behavior of Eclipse when compiling code with errors in it, is to generate byte code throwing the exception you see, allowing the program to be run. This is possible as Eclipse uses its own built-in compiler, instead of javac from the JDK which Apache Maven uses, and which fails the compilation completely for errors. If you use Eclipse on a Maven project which you are also working with using the command line mvn command, this may happen.
The cure is to fix the errors and recompile, before running again.
The setting is marked with a red box in this screendump:

try to clean the eclipse project

you just try to clean maven by command
mvn clean
and after that following command
mvn eclipse:clean eclipse:eclipse
and rebuild your project....

Your compiled classes may need to be recompiled from the source with the new jars.
Try running "mvn clean" and then rebuild

The major part is correctly answered by Thorbjørn Ravn Andersen.
This answer tries to shed light on the remaining question: how could the class file with errors end up in the jar?
Each build (Maven & javac or Eclipse) signals in its specific way when it hits a compile error, and will refuse to create a Jar file from it (or at least prominently alert you). The most likely cause for silently getting class files with errors into a jar is by concurrent operation of Maven and Eclipse.
If you have Eclipse open while running a mvn build, you should disable Project > Build Automatically until mvn completes.
EDIT:
Let's try to split the riddle into three parts:
(1) What is the meaning of "java.lang.Error: Unresolved compilation
problem"
This has been explained by Thorbjørn Ravn Andersen. There is no doubt that Eclipse found an error at compile time.
(2) How can an eclipse-compiled class file end up in jar file created
by maven (assuming maven is not configured to used ecj for
compilation)?
This could happen either by invoking Maven with no or incomplete cleaning. Or, an automatic Eclipse build could react to changes in the filesystem (done by Maven) and re-compile a class, before Maven proceeds to collect class files into the jar (this is what I meant by "concurrent operation" in my original answer).
(3) How come there is a compile error, but mvn clean succeeds?
Again several possibilities: (a) compilers don't agree whether or not the source code is legal, or (b) Eclipse compiles with broken settings like incomplete classpath, wrong Java compliance etc. Either way a sequence of refresh and clean build in Eclipse should surface the problem.

I had this error when I used a launch configuration that had an invalid classpath. In my case, I had a project that initially used Maven and thus a launch configuration had a Maven classpath element in it. I had later changed the project to use Gradle and removed the Maven classpath from the project's classpath, but the launch configuration still used it. I got this error trying to run it. Cleaning and rebuilding the project did not resolve this error. Instead, edit the launch configuration, remove the project classpath element, then add the project back to the User Entries in the classpath.

I got this error multiple times and struggled to work out. Finally, I removed the run configuration and re-added the default entries. It worked beautifully.

Just try to include package name in eclipse in case if you forgot it
Import all packages before using it, EX: import java.util.Scanner before using Scanner class.
These improvements might work and it will not give Java: Unresolved compilation problem anymore.
Also make sure to check compiler compliance level and selected jdk version is same

As a weird case, I encountered such an exception where the exception message (unresolved compilation bla bla) was hardcoded inside of generated class' itself. Decompiling the class revealed this.

I had the same issue using the visual studio Code. The root cause was backup java file was left in the same directory.
Removed the backup java file
When the build failed, selected the Fix it, it cleaned up the cache and restarted the workSpace.

Eclipse Java Remote Debug: How do I ensure that the code I've got in my project is the same as the code running in the JVM?

I've got some existing source code, and jars for a webapp. I am attempting to run the webabb using tomcat and then remote debugging the tomcat server using eclipse to figure out what is happening with the webapp.
For some reason however, eclipse refuses to trap any breakpoints. I've compared the md5sums of the jars on the server with the ones I've got in my debug project, and they match.
Is there some way I can ask eclipse to reconcile the java classes in the remote JVM with the java classes in the local jars and make sure they correspond. Or, perhaps, more specifically, whether the code associated with breakpoints I've set have actually be loaded into the remote JVM?

Check that on the Java Compiler properties page for the java projects in question that the class file options are all checked (you only need the first three but having all five checked is a good thing for debugging)

Headless Eclipse Build returns ERRORLEVEL 13

Everything was going well. Nightly builds ran for more than a month with no problems. However, suddenly when invoking the feature builder from Eclipse the execution ends right away with the message.
ERRORLEVEL 13
As far as I know I haven't changed anything, as this computer is normally not touched. (It is only used for the nightly builds).
I've googled around to see what this error means but to no avail. I found a link from IBM that shows a custom bat file that catches this error, but the explanation given is not very useful.
My guess is that this is related to the workspace, somehow, or that the whole eclipse installation is corrupted.
Do you have any idea what this error means and how can it be corrected?

After reading what the error code means thanks to the answer from VonC I understood where to look. The problem was a lot more obscure that it seems.
I looked into the configuration folder for Eclipse (logs are either written there or in the .metadata folder when something goes wrong), and I found a huge log file. Inside the file I found the following error:
application org.eclipse.sdk not found
and the following exception, followed by a plugin name, several times:
java.util.zip.ZipException: Too many open files
Several plugins could not be loaded, causing a cascade of missing dependencies that prevented Eclipse from launching. I searched the web for this exception and found the following bug description from SUN, which says that Java 1.5.0 can not open more than 2,100 zip files.
The problem started a while after I installed the BABEL project translations for Eclipse into the build computer. These are more than 900 fragments, containing translations for many plugins, one for each language. As I installed it on top of an existing eclipse installation, it seemed that it was not a problem to open them.... until I cleared the workspace for the builds. Then Eclipse wouldn't launch anymore. I went over the limit. It didn't help that the first thing I tried to fix the build was, again.... clearing everything.
Because I only use this computer for headless builds, I didn't realize that the problem was in Eclipse itself and I was looking inside the build process. I only realized when I looked into the log file.
After installing Java 1.6.0_11 I was able to launch Eclipse and go on with my build.

It should mean "ant build failed", meaning the headless ant script fails at some point.
You should check if you can catch the log/output generated by this script to analyze this ant session and see at what point the ant script fails.
If it fails right away, it usually is because of:
a change in rights (writing access), or
in environment variable modifications, or
in resource access (path non accessible).
You also have to check if the computer is still in its original windows domain, and if the rights (admin ?) associated with the account running the ant script are still the same.