# 5.4. Open MPI Java bindings

Open MPI v5.0.7 provides support for Java-based MPI applications.

Warning

The Open MPI Java bindings are provided on a “provisional” basis — i.e., they are not part of the current or proposed MPI standards. Thus, inclusion of Java support is not required by the standard. Continued inclusion of the Java bindings is contingent upon active user interest and continued developer support.

The rest of this document provides step-by-step instructions on building OMPI with Java bindings, and compiling and running Java-based MPI applications. Also, part of the functionality is explained with examples. Further details about the design, implementation and usage of Java bindings in Open MPI can be found in its canonical reference paper [1]. The bindings follow a JNI approach, that is, we do not provide a pure Java implementation of MPI primitives, but a thin layer on top of the C implementation. This is the same approach as in mpiJava [2]; in fact, mpiJava was taken as a starting point for Open MPI Java bindings, but they were later totally rewritten.

## 5.4.1. Building the Java bindings

Java support requires that Open MPI be built at least with shared libraries (i.e., `--enable-shared`). Note that this is the default for Open MPI, so you don’t have to explicitly add the option. The Java bindings will build only if `--enable-mpi-java` is specified, and a JDK is found in a typical system default location.

If the JDK is not in a place where we automatically find it, you can specify the location. For example, this is required on the Mac platform as the JDK headers are located in a non-typical location. Two options are available for this purpose:

1. `--with-jdk-bindir=<foo>`: the location of `javac` and `javah`

2. `--with-jdk-headers=<bar>`: the directory containing `jni.h`

Some example configurations are provided in Open MPI configuration platform files under `contrib/platform/hadoop`. These examples can provide a starting point for your own custom configuration.

In summary, therefore, you can configure the system using the following Java-related options:

    $ ./configure --with-platform=contrib/platform/hadoop/<your-platform> ...

or:

    $ ./configure --enable-mpi-java --with-jdk-bindir=<foo> --with-jdk-headers=<bar> ...

or simply:

    $ ./configure --enable-mpi-java ...

if JDK is in a “standard” place that `configure` can automatically find.

## 5.4.2. Building Java MPI applications

The `mpijavac` wrapper compiler is available for compiling Java-based MPI applications. It ensures that all required Open MPI libraries and classpaths are defined. For example:

    $ mpijavac Hello.java

You can use the `--showme` option to see the full command line of the Java compiler that is invoked:

    $ mpijavac Hello.java --showme
    /usr/bin/javac -cp /opt/openmpi/lib/mpi.jar Hello.java

Note that if you are specifying a `-cp` argument on the command line to pass your application-specific classpaths, Open MPI will extend that argument to include the `mpi.jar`:

    $ mpijavac -cp /path/to/my/app.jar Hello.java --showme
    /usr/bin/javac -cp /path/to/my/app.jar:/opt/openmpi/lib/mpi.jar Hello.java

Similarly, if you have a `CLASSPATH` environment variable defined, `mpijavac` will convert that into a `-cp` argument and extend it to include the `mpi.jar`:

    $ export CLASSPATH=/path/to/my/app.jar
    $ mpijavac Hello.java --showme
    /usr/bin/javac -cp /path/to/my/app.jar:/opt/openmpi/lib/mpi.jar Hello.java

## 5.4.3. Running Java MPI applications

Once your application has been compiled, you can run it with the standard `mpirun` command line:

    $ mpirun <options> java <your-java-options> <my-app>

`mpirun` will detect the `java` token and ensure that the required MPI libraries and class paths are defined to support execution. You therefore do not need to specify the Java library path to the MPI installation, nor the MPI classpath. Any classpath definitions required for your application should be specified either on the command line or via the `CLASSPATH` environment variable. Note that the local directory will be added to the classpath if nothing is specified.

Note

The `java` executable, all required libraries, and your application classes must be available on all nodes.

## 5.4.4. Basic usage of the Java bindings

There is an MPI package that contains all classes of the MPI Java bindings: `Comm`, `Datatype`, `Request`, etc. These classes have a direct correspondence with handle types defined by the MPI standard. MPI primitives are just methods included in these classes. The convention used for naming Java methods and classes is the usual camel-case convention, e.g., the equivalent of `MPI_File_set_info(fh,info)` is `fh.setInfo(info)`, where `fh` is an object of the class `File`.

Apart from classes, the MPI package contains predefined public attributes under a convenience class `MPI`. Examples are the predefined communicator `MPI.COMM_WORLD` and predefined datatypes such as `MPI.DOUBLE`. Also, MPI initialization and finalization are methods of the `MPI` class and must be invoked by all MPI Java applications. The following example illustrates these concepts:

    import mpi.*;

    class ComputePi {

       public static void main(String args[]) throws MPIException {

           MPI.Init(args);

           int rank = MPI.COMM_WORLD.getRank(),
               size = MPI.COMM_WORLD.getSize(),
               nint = 100; // Intervals.
           double h = 1.0/(double)nint, sum = 0.0;

           for (int i=rank+1; i<=nint; i+=size) {
               double x = h * ((double)i - 0.5);
               sum += (4.0 / (1.0 + x * x));
           }

           double sBuf[] = { h * sum },
                  rBuf[] = new double[1];

           MPI.COMM_WORLD.reduce(sBuf, rBuf, 1, MPI.DOUBLE, MPI.SUM, 0);

           if (rank == 0) {
               System.out.println("PI: " + rBuf[0]);
           }
           MPI.Finalize();
       }
    }

## 5.4.5. Exception handling

The Java bindings in Open MPI support exception handling. By default, errors are fatal, but this behavior can be changed. The Java API will throw exceptions if the `MPI.ERRORS_RETURN` error handler is set:

    MPI.COMM_WORLD.setErrhandler(MPI.ERRORS_RETURN);

If you add this statement to your program, it will show the line where it breaks, instead of just crashing in case of an error. Error-handling code can be separated from main application code by means of try-catch blocks, for instance:

    try
    {
        File file = new File(MPI.COMM_SELF, "filename", MPI.MODE_RDONLY);
    }
    catch(MPIException ex)
    {
        System.err.println("Error Message: "+ ex.getMessage());
        System.err.println("  Error Class: "+ ex.getErrorClass());
        ex.printStackTrace();
        System.exit(-1);
    }

## 5.4.6. How to specify buffers

In MPI primitives that require a buffer (either send or receive), the Java API admits a Java array. Since Java arrays can be relocated by the Java runtime environment, the MPI Java bindings need to make a copy of the contents of the array to a temporary buffer, then pass the pointer to this buffer to the underlying C implementation. From the practical point of view, this implies an overhead associated to all buffers that are represented by Java arrays. The overhead is small for small buffers but increases for large arrays.

There is a pool of temporary buffers with a default capacity of 64K. If a temporary buffer of 64K or less is needed, then the buffer will be obtained from the pool. But if the buffer is larger, then it will be necessary to allocate the buffer and free it later.

The default capacity of pool buffers can be modified with an Open MPI MCA parameter:

    $ mpirun --mca ompi_mpi_java_eager SIZE ...

The value of `SIZE` can be:

- `N`: An integer number of bytes

- `Nk`: An integer number (suffixed with `k`) of kilobytes

- `Nm`: An integer number (suffixed with `m`) of megabytes

An alternative is to use “direct buffers” provided by standard classes available in the Java SDK such as `ByteBuffer`. For convenience, Open MPI provides a few static methods `new[Type]Buffer` in the `MPI` class to create direct buffers for a number of basic datatypes. Elements of the direct buffer can be accessed with methods `put()` and `get()`, and the number of elements in the buffer can be obtained with the method `capacity()`. This example illustrates its use:

    int myself = MPI.COMM_WORLD.getRank();
    int tasks  = MPI.COMM_WORLD.getSize();

    IntBuffer in  = MPI.newIntBuffer(MAXLEN * tasks),
              out = MPI.newIntBuffer(MAXLEN);

    for (int i = 0; i < MAXLEN; i++)
        out.put(i, myself);      // fill the buffer with the rank

    Request request = MPI.COMM_WORLD.iAllGather(
                      out, MAXLEN, MPI.INT, in, MAXLEN, MPI.INT);
    request.waitFor();
    request.free();

    for (int i = 0; i < tasks; i++) {
        for (int k = 0; k < MAXLEN; k++) {
            if (in.get(k + i * MAXLEN) != i)
                throw new AssertionError("Unexpected value");
        }
    }

Direct buffers are available for: `BYTE`, `CHAR`, `SHORT`, `INT`, `LONG`, `FLOAT`, and `DOUBLE`.

Note

There is no direct buffer for booleans.

Direct buffers are not a replacement for arrays, because they have higher allocation and deallocation costs than arrays. In some cases arrays will be a better choice. You can easily convert a buffer into an array and vice versa.

Important

All non-blocking methods must use direct buffers. Only blocking methods can choose between arrays and direct buffers.

The above example also illustrates that it is necessary to call the `free()` method on objects whose class implements the `Freeable` interface. Otherwise, a memory leak will occur.

## 5.4.7. Specifying offsets in buffers

In a C program, it is common to specify an offset in a array with `&array[i]` or `array+i` to send data starting from a given position in the array. The equivalent form in the Java bindings is to `slice()` the buffer to start at an offset. Making a `slice()` on a buffer is only necessary, when the offset is not zero. Slices work for both arrays and direct buffers.

    import static mpi.MPI.slice;
    // ...
    int numbers[] = new int[SIZE];
    // ...
    MPI.COMM_WORLD.send(slice(numbers, offset), count, MPI.INT, 1, 0);

## 5.4.8. Supported APIs

Complete MPI-3.1 coverage is provided in the Open MPI Java bindings, with a few exceptions:

- The bindings for the `MPI_Neighbor_alltoallw` and `MPI_Ineighbor_alltoallw` functions are not implemented.

- Also excluded are functions that incorporate the concepts of explicit virtual memory addressing, such as `MPI_Win_shared_query`.

## 5.4.9. Known issues

There exist issues with the Omnipath (PSM2) interconnect involving Java. The problems definitely exist in PSM2 v10.2; we have not tested previous versions.

As of November 2016, there is not yet a PSM2 release that completely fixes the issue.

The following `mpirun` command options will disable PSM2:

    shell$ mpirun ... --mca mtl ^psm2 java ...your-java-options... your-app-class

## 5.4.10. Questions? Problems?

The Java API documentation is generated at build time in `$prefix/share/doc/openmpi/javadoc`.

Additionally, this Cisco blog post has quite a bit of information about the Open MPI Java bindings.

If you have any problems, or find any bugs, please feel free to report them to Open MPI user’s mailing list.

Footnotes

[1]

O. Vega-Gisbert, J. E. Roman, and J. M. Squyres. “Design and implementation of Java bindings in Open MPI”. Parallel Comput. 59: 1-20 (2016).

[2]

M. Baker et al. “mpiJava: An object-oriented Java interface to MPI”. In Parallel and Distributed Processing, LNCS vol. 1586, pp. 748-762, Springer (1999).