The Java Bindings



This section describes jMSG, the Java API to Simgrid. This API mimicks MSG, which is a simple yet somehow realistic interface. The full reference documentation is provided at the end of this page.

Most of the documentation of the MSG API in C applies directly to the Java bindings (any divergence is seen as a bug that we should fix). MSG structures are mapped to Java objects as expected, and the MSG functions are methods in these objects.

Installing the Java bindings

The easiest is to use a precompiled jarfile, but some people may prefer to compile it from the sources.

Using the Java bindings

In most cases, you can use the SimGrid bindings as if it was a Java library:

$ javac -classpath .:path/to/simgrid.jar your/java/Code.java
$ java -classpath .:path/to/simgrid.jar your.java.Code the/parameter/to/your/code

For example:

$ cd examples/java
$ java -classpath ../../simgrid.jar:. .:../../simgrid.jar app.pingpong.Main ../platforms/platform.xml

Any SimGrid simulation (java or not) is usually constituted of several kind of actors or processes (classes extending @c Msg.Process) that are deployed over the hosts of the virtual platform. So, your code should declare these actors, plus a Main class in charge of deploying your actors on the platform. Please refer to the examples for details.

Troubleshooting

Actually, these bindings are not only implemented in Java. They do use the C implementation of SimGrid. This should be transparent as this library is directly included in the simgrid.jar file but things can still go wrong is several ways.

Error: library simgrid not found

This means that the JVM fails to load the native library. If you use a precompiled jarfile, please report this bug.

If you built it yourself, you can try to use an installed version of the library instead of the one included in the jar. For that, add the path to the native library into the LD_LIBRARY_PATH variable (or in the DYLD_LIBRARY_PATH on Mac OSX).

pthread_create failed

You reached the amount of threads that can be run on your system. Try increasing the thread limits of your operating system.

Other errors

When using jMSG, your program can crash for 3 main reasons:

  • Your Java part is not good: you’ll have a good old java exception thrown, and hence you should be able to correct it by yourself.
  • Our java part is not good: you’ll also have a java exception thrown, but we have real doubts this can happen, since the java part is only a JNI binding. The other option is that it crashed because you used incorrectly the MSG API, so this means also you should have an MSGException. It means you should read carefully MSG samples and/or documentation.
  • Something has crashed in the C part. Okay, here comes the tricky thing. It happens mainly for 2 reasons:
    • When something goes wrong in your simulation, sometimes the C part stops because you used SimGrid incorrectly, and JNI bindings are not fond of that. It means that you’ll have something that looks ugly, but you should be able to identify what’s going wrong in your code by carefully reading the whole error message
    • It may happen that the problem comes directly from SimGrid: in this case, the error should be uglier. In that case, you may submit a bug directly to SimGrid.