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Panama upcall example

This is a small example application, demonstrating that an exception, thrown inside a native upcall using Java Panama, cannot be caught.
The project comes with a small native library called libupcall, consisting of a single function (perform_upcall), that takes a function pointer as its parameter and calls this function pointer.
The Java application uses Project Panama to pass a Java method as parameter to perform_upcall. Inside this method, a RuntimeException is thrown. perform_upcall is called inside a try-block, catching every type of Throwable, so that the exception, thrown inside the upcall should be caught. However, instead of that, the exception's stack trace is printed and the JVM exits with a return code of 1.

Build and run

First, you need to build the native library:

  1. Change directory to src/main/native.
  2. Execute ./build.sh.
  3. You should have the native library built as libupcall.so inside src/main/native. Change directory back into the project's root directory

Now, you can build the Java project:

  1. Make sure, you have a current version of JDK 19 with Project Panama installed on your system. You can always download the latest nightly build of the OpenJDK including Project Panama from here.
  2. Edit gradle.properties inside the project's root directory and let org.gradle.java.installations.paths point to your Panama JDK.
  3. Execute ./gradlew installDist. For that, your default JDK must be any JDK with a version number up to and including 17, since Gradle does not support building with newer versions of Java. However, it can use the JDK pointed to inside your gradle.properties to build this project using Java 19.
  4. You can now execute the application (change JAVA_HOME to your Panama JDK): JAVA_HOME="/path/to/your/panama-jdk" LD_LIBRARY_PATH="$LD_LIBRARY_PATH:src/main/native" ./build/install/panama-upcall/bin/panama-upcall

Output

Upon execution, you should see the following the output:

WARNING: Using incubator modules: jdk.incubator.foreign
Hello from native code!
Hello from Java code!
java.lang.RuntimeException: Throwing an exception inside an upcall
        at de.hhu.bsinfo.panama.upcall.Application.lambda$static$0(Application.java:10)
        at de.hhu.bsinfo.panama.upcall.UpcallHelper.perform_upcall(UpcallHelper.java:15)
        at de.hhu.bsinfo.panama.upcall.Application.main(Application.java:15)
  • Hello from native code! is printed using printf() inside the native library.
  • Hello from Java code! is printed inside the Java upcall.
  • The stacktrace is printed after the exception is thrown. However, since we run the upcall inside a try-block, it should actually be caught and no stack trace be printed.

The expected output, if the exception had been caught, would look like the following:

WARNING: Using incubator modules: jdk.incubator.foreign
Hello from native code!
Hello from Java code!
Exception caught!
Upcall finished!