Develop Reference

Jpype import cannot find module in jar

I have received the task, at work, to find a way to use some methods from an existent jar file in a Python project. I have very limited experience in Python, but I have worked with that specific jar file before (it is part of a project we are working on). I am not allowed to modify much of both projects, and they are required to be as independent as possible.
I have researched multiple ways to include the jar methods in Python. So far, I have tried Jython (which I cannot use because the Python project uses PyQt among other libraries, which force the use of CPython, if my understanding is correct), Pyjnius and JPype. JPype seems the most promising, but I cannot get it working either. I have pasted the my code below, slightly censored because I don't know how much I am allowed to share.
from jpype import *
import jpype.imports
try:
jpype.addClassPath("jars/sdk.jar") #the relative path to the jar file
jpype.startJVM(convertStrings=False)
java.lang.System.out.println(jpype.getClassPath()) #printing the classpath to check, the path is correctly pointing to the sdk.jar file
java.lang.System.out.println("Hello world") #just to check if jpype is installed correctly, works
jpype.imports.registerDomain("a")
from a.b.c.d.e.f.g.h import SomeClass #fails here
except OSError as err:
print(err) # ToDo: Remove print when done
pass
The error I am getting is that the module a.b.c.d.e.f.g.h.SomeClass could not be found. I have tried different ways to give the path (absolute path, relative path, place the jar in different places in the project and outside of it), but that doesn't seem to be the problem, as the path printed is the correct absolute path to the jar file.
Also, the jar is used in other (Java) projects and it works. It is created using maven package (using IntelliJ, if it is relevant, and the same Java version as the one used by the JPype JVM). In the Java projects, the import would be:
import a.b.c.d.e.f.g.h.SomeClass;
I have copied this and just transformed the syntax into Python.
I have also tried to create the class with JObject (which I probably didn't do right anyway) and also tried the older syntax (to my understanding) with JPackage. For the JPackage way, I am getting the exception that the package a.b.c.d.e.f.g.h.SomeClass.someMethod is not Callable, which to my understanding is an equivalent exception to the one I'm getting using jpype imports. I have already gone through all the questions I could find here with similar problems, but none of those solutions have helped me.
Can anyone suggest some possible solution? Or can anyone see what I'm doing wrong? Suggestions of other possibilities to replace JPype are also welcomed. If there is any clarification needed, I will edit the question.

The only thing that seems likely if the jar is on the classpath and failed to import would be for there to be some missing dependency. You have two other ways to try loading the class which may provide additional diagnostics.
jpype.JClass("a.b.c.d.e.f.g.h.SomeClass")
and
jpype.JClass("java.lang.Class").forName("a.b.c.d.e.f.g.h.SomeClass")
The first is manually loading a class by full class specification. It is mechanically what is happening under the import. The second is calling for Java to load the class (bypassing all of JPype). It returns a java.lang.Class which can be passed to JClass to make a wrapper.
Common failures include missing a jar or native library, attempting to start JPype from within a module and having the wrong relative path, error in initialization of the class due to missing resource. JPype is just calling JNI calls, so if everything is fine on Java end it should work. Given that you checked the java.class.path System variable, it has to be something to do with class resources.
The JPype user manual has an alternatives section if you would like to try to find another package. Most of the alternatives with the exception of PyJnius appear to be unmaintained.

NoSuchMethodError while generating JasperReport's report

I'm generating PDF report using JasperReports library. I this report I have a QRCode which makes problem. I had to add many libraries:
jasperreports, itext, itext-pdfa, itext-pdf, commons-beanutils, commons-collections, commons-digester, commons-logging, groovy-all, barbecue, barcode, barcode4j, batik-anim, batik-bridge, core, w3c.css.sac, w3c,xmlgraphics-commons, batik-all
I receive an exception:
java.lang.ClassCastException: org.apache.batik.anim.dom.SVGOMDocument cannot be cast to org.apache.batik.dom.svg.SVGOMDocument
at org.apache.batik.bridge.BridgeContext.setDocument(Unknown Source)
at org.apache.batik.bridge.GVTBuilder.build(Unknown Source)
at net.sf.jasperreports.renderers.AbstractSvgDataToGraphics2DRenderer.ensureSvg(AbstractSvgDataToGraphics2DRenderer.java:166)
at net.sf.jasperreports.renderers.AbstractSvgDataToGraphics2DRenderer.getDimension(AbstractSvgDataToGraphics2DRenderer.java:111)
at net.sf.jasperreports.engine.export.JRPdfExporter$InternalImageProcessor.processGraphics2D(JRPdfExporter.java:1793)
at net.sf.jasperreports.engine.export.JRPdfExporter$InternalImageProcessor.process(JRPdfExporter.java:1582)
at net.sf.jasperreports.engine.export.JRPdfExporter$InternalImageProcessor.access$300(JRPdfExporter.java:1535)
at net.sf.jasperreports.engine.export.JRPdfExporter.exportImage(JRPdfExporter.java:1475)
at net.sf.jasperreports.engine.export.JRPdfExporter.exportElements(JRPdfExporter.java:1093)
at net.sf.jasperreports.engine.export.JRPdfExporter.exportPage(JRPdfExporter.java:1056)
at net.sf.jasperreports.engine.export.JRPdfExporter.exportReportToStream(JRPdfExporter.java:920)
at net.sf.jasperreports.engine.export.JRPdfExporter.exportReport(JRPdfExporter.java:537)
at reports.JasperReport.generatePdf(JasperReport.java:178)
What is wrong?

If this is your own code (you wrote it), it is just because of you have imported the wrong SVGOMDocument class (which belongs to different Java package).
You should be able to figured it out quickly with decent Java IDE (Eclipse, IntelliJ, NetBeans).
Also, as much as possible try to avoid implicit imports such as:
import org.apache.batik.dom.svg.*;
import org.apache.batik.anim.dom.*;
That might reduce your code stability, e.g. lines added above might alter your reference of existing working code.
Use explicit imports (import org.apache.batik.dom.svg.SVGOMDocument).
Find out what's the return type of the method you invoke, and import from the correct Java package. In Eclipse IDE, you can just remove the import that contains the package/class above, and choose Source > Organize Imports (Ctrl-Shift-O).
If you have both classes in single source code file, you might need to use full class name for one of the classes, e.g.:
org.apache.batik.dom.svg.SVGOMDocument document = SomeLibrary.getSomething();

Java disutil CoordinateConversions import problems

I'm writing a Java class that will be used to send PDUs across a network- to do this, I am following the tutorial at: Tutorial
In the example, the line:
double lla[] = CoordinateConversions.xyzToLatLonDegrees(c);
appears towards the end of the class, and I see that CoordinateConversions has been imported with the line:
import edu.nps.moves.disutil.CoordinateConversions;
I have tried using the xyzToLatLonDegrees(); method in the class that I am writing- calling it in the same way as is done in the example. However, for some reason, I get a compile error that says:
CoordinateConversions cannot be resolved
on the line where I'm trying to use it, and
The import edu.nps.moves.disutil.CoordinateConversions cannot be resolved
on the line where I am importing it.
Does anyone know why this is, and how I can fix the import, so that I can use the xyzToLatLonDegrees() method?

You need to have the CoordinateConversions class on your classpath. Either by obtaining the source and dropping it into your project (possibly adjusting package names, and only if the license allows), or by finding a JAR containing that class and adding it to your build path in your IDE.
You probably need to download the Java files from here.

How do I locate a clashing class in the IntelliJ classpath?

I'm trying IntelliJ IDEA after many years as an Eclipse user. At the same time, I'm working on a project that I've inherited with many dependencies.
One class will not compile, because IDEA claims that a method in another class does not exist. I can see the method in its source. Control-clicking on the class name in the IDEA editor takes me to the source that looks OK.
My hypothesis is that the compiler isn't using the class compiled from the source within the project, but a class with the same name, somewhere among my dozens of library jars.
How can I find out where IDEA's compiler is finding the clashing class?

CTRL-N and entering the class name should show you all of the matching classes from across the classpath, and which directory/JAR they're in. If there's a clash, you should have duplicates in that list.
Another possibility is that the source you have for the referenced class doesn't match the compiled version of that class.

Java "NoSuchMethodError"

I'm getting:
NoSuchMethodError: com.foo.SomeService.doSmth()Z
Am I understanding correctly that this 'Z' means that return type of doSmth() method is boolean? If true, then that kind of method really does not exist because this method returns some Collection. But on the other hand if I call this method, I'm not assigning its return value to any variable. I just call this method like this:
service.doSmth();
Any ideas why this error occurs? All necessary JAR files exist and all other methods from this class seems to exist.

Looks like method exists in classpath during compilation, but not during running of your application.
I don't think return type is a problem. If it was, it wouldn't compile. Compiler throws error when method call is ambiguous, and it is when two methods differ only by return type.

Normally, this error is caught by the compiler; this error can only occur at run time if the definition of a class has incompatibly changed.
In short - a class/jar file at runtime is not the same that you used at compile time.

This is probably a difference between your compile-time classpath and you run-time classpath.
Here is what seems to be going on:
The code is compiled with a class path that defines the doSmth() method returning a boolean. The byte-code refers to the doSmth()Z method.
At runtime, the doSmth()Z method isn't found. A method returning a Collection is found instead.
To correct this problem, check your (compile time) classpath.

The current reply just tell you why is failing. Usually is even nicer to know how to fix problems. As it is mentioned, the problem usually is that you built your program but when running or exporting it, the library is not included. So the solution is...
If you are running, check the the run configuration
Select Run tab -> Run configurations -> Select the configuration you are running -> Check the Classpath tab -> Ensure the libraries you need are there
If you are exporting (for example a war file), follow this
Select project -> Select properties -> Select Deployment Assembly -> Press Add -> Select Java Build Path Entries -> Select the libraries you want to be included in your exported file (for example a war file)
In both cases, ensure the library which you are referencing in included.
Other frequent problems for this error are not the right type of parameters or visibility but then, the compiler will detect the error before running. In this case, just check the documentation to match the function and package visibility, and ensure that the library is found in Java Build Path in your project properties.

Maybe still can help somebody, but this exception can happen also when you have on the classpath two classes in different jar files that have the same exact signature but they haven't the same public methods.
For example:
On file mylibrary1.jar you have class com.mypackage.mysubpackage.MyClass with method doSmth()
On file mylibrary2.jar you have class com.mypackage.mysubpackage.MyClass without method doSmth()
When searching the class, the classloader may find first mylibrary2.jar depending on the path precedence but can't find the method on that class.
Be sure you don't have the same package + class on two different files.

I noticed this problem occurring while testing some experimental changes in multiple linked projects, after updating them from SVN in Eclipse.
Specifically, I updated all projects from SVN, and reverted the .classpath file rather than edit it manually to keep things simple.
Then I re-added the linked projects to the path, but forgot to remove the related jars. This was how the problem occurred for me.
So apparently the run time used the jar file while the compiler used the project files.

Another way this can happen and is difficult to find:
If a signature of a method in an external jar changes in a way that there is no error found in the IDE because it's still compatible with how you call it the class might not be re-compiled.
If your build checks the files for changes and only then recompiles them, the class might not be recompiled during the build process.
So when you run it this might lead to that problem. Although you have the new jar, your own code expects still the old one but does never complain.
To make it harder it depends on the jvm if it can handle such cases. So in the worst case it runs on the test server but not on the live machine.