Mercury Java Backend
====================

The Mercury compiler has a backend that generates Java source code, that can
then be compiled into Java bytecode suitable for running using the Java SE
runtime system. The backend is mostly complete, but some parts of the Mercury
standard library are not yet implemented.

The Java backend requires Java SE 8 or higher -- older versions of Java are
*not* supported.

Contents
--------

* Prerequisites
* Installing the `java` grade
* Compiling programs with the `java` grade
* Running programs with the `java` grade
* Limitations
* Library support
* Interfacing with Java
* Performance
* Path length restrictions on Windows
* Java compiler memory exhaustion
* Mercury-level debugging
* Java-level debugging
* Building the Mercury compiler in the `java` grade

Prerequisites
-------------

In order to use Mercury's Java backend you will need:

* The [Java SE platform](https://www.oracle.com/java/technologies/downloads/).

* OR another compatible Java implementation, such as
  [OpenJDK](https://openjdk.org) or [Amazon Corretto](https://aws.amazon.com/corretto/).

Installing the `java` grade
---------------------------

The Mercury compiler uses the grade `java` to target Java source code that is
then compiled into Java bytecode by the Java compiler.

Mercury's autoconfiguration script will cause the `java` grade to be installed
if it finds a suitable Java compiler (e.g. `javac`) and Java runtime
(e.g. `java`) in your `PATH`.

You can check if your Mercury installation has been configured to include the
`java` grade by looking if `java` is included in the output of the Mercury
compiler's `--output-stdlib-grades` option.

Compiling programs with the `java` grade
----------------------------------------

Once you have a Mercury installation that includes the `java` grade, you can
build programs such as `hello.m` or `calculator.m` in the [samples](samples)
directory.

```
    mmc --grade java --make hello
```

When building programs with the `java` grade you *must* use `mmc --make`; using
`mmake` to build programs using the `java` grade is _not_ supported.

Running programs with the `java` grade
--------------------------------------

You can run the `hello` program from the previous section by doing:

```
    ./hello
```

Note that `hello` is a simple shell script generated by the Mercury compiler
that invokes the program using the Java interpreter. The actual class files are
packaged up into a Java archive (JAR) named `hello.jar`.

If you are using the Windows command-line interpreter, i.e. `cmd.exe`, then
setting the value of the option `--target-env-type` to "windows" will cause the
Mercury compiler to generate a batch file that invokes the program, instead of
a shell script.

(See the "Using the Mercury compiler" section of the
[The Mercury User's Guide](https://mercurylang.org/documentation/documentation.html)
for further details.)

Limitations
-----------

The following features of the Mercury implementation are not (currently)
supported by the Java backend:

* Mercury-level debugging (however, see further down).
* Mercury-level profiling.
* Trailing.
* Tabling.
* Backjumping.

Library support
---------------

The Mercury standard library has not been fully ported to Java yet.
The use of unimplemented procedures will result in a run-time error,
with a stack trace and a message like:

```
    Sorry, not implemented: foreign code for this function
```

If you find missing functionality, you can interface to Java using Mercury's
foreign language interface.

The following individual Mercury standard library procedures are either not
supported or not fully implemented:

1. `io.read_binary/{3,4}`    
   `io.write_binary/{3,4}`

    The current implementation of `read_binary` does not work with the
    way Mercury file streams are implemented for the Java backend.

2. `benchmarking.report_stats/0`    
   `benchmarking.report_full_memory_stats/0`

    Memory usage statistics are not yet available, and cpu time
    is not the same as in the C backends, as per `time.m`.

3. `io.environment.set_environment_var/{4,5}`

    The Java APIs do not support setting environment variables, hence
    this predicate throws an exception.

4. `store.arg_ref/5`    
   `store.new_arg_ref/5`

    Due to some limits in RTTI support, dynamic type checking is missing for
    these predicates. They should be used with care.

5.  `time.clock/3`    
    `time.clocks_per_sec/0`    
    `time.times/7`    
    `time.clk_tck/0`

     Because the Java APIs do not provide a way of implementing these
     procedures exactly in pure Java, we have approximated them with what is
     available.

6.  `math.fma/3`

    This function is not available because it is not supported by Java 8.
    (It will be supported once the minimum version of Java required by
    Mercury increases.)

Interfacing with Java
----------------------

You can call Java code directly from Mercury using the foreign language
interface. For example:

```
    :- pred to_string(T::in, string::out) is det.
    :- pragma foreign_proc("Java",
        to_string(T::in, Str::out),
        [promise_pure, will_not_call_mercury],
    "
        Str = T.toString();
    ").
```

The implementation will include this Java code in the module's .java file.
You can then call the predicate `to_string/2` exactly the same as if it were
implemented using pure Mercury code.

For more information about the foreign language interface, see the
[Mercury Language Reference Manual](https://www.mercurylang.org/documentation/documentation.html).
Additionally, the [samples/java_interface](samples/java_interface) directory in
the Mercury distribution contains examples of how to use the foreign language
interface with Java.

Performance
-----------

Short programs may run much more slowly in the `java` grade than the C grades.
The runtime is probably dominated by Java class loading and running in
interpreted mode. A long running program should perform reasonably well with a
Just-In-Time compiler. It may also be possible to use an Ahead-Of-Time Java
compiler, but we have not tried that yet.

Path length restrictions on Windows
-----------------------------------

When using the `java` grade on Windows, it is sometimes possible for the fully
qualified names of generated files to exceed the maximum path length. If this
occurs the Mercury compiler will abort with a message like:

```
    Uncaught Mercury exception:
    Software Error: parse_tree.module_cmds: predicate \
    `parse_tree.module_cmds.list_class_files_for_jar'/6: \
    Unexpected: io.file_type failed: No such file or directory
```

In this case all that can (currently) be done is to reduce the length of the
build path, for example by shifting the build directory closer to the root of
the file system (e.g. `C:\mercury`).

Java compiler memory exhaustion
-------------------------------

It is possible for the Java compiler to run out of memory when compiling
Java code generated by the Mercury compiler. If that occurs, you can pass
an option to the `javac` program to increase the limit. For example:

```
    $ mmc --make foo --java --java-flag -J-Xmx512m
```

Or pass an option to `javac` using a `Mercury.options` file:

```
    JAVACFLAGS += -J-Xmx512m
```

Mercury-level debugging
-----------------------

The only Mercury-level debugger available for the Java backend is the
_experimental_ source-to-source debugger; see [README.ssdebug.md](README.ssdebug.md)
for details.

Java-level debugging
--------------------

By default, `javac` already generates line number and source file debugging
information. You can include local variable debugging information by specifying
`--target-debug` when invoking the Mercury compiler. For example:

```
    mmc --grade java --target-debug --make <progname>
```

You can then use the `jdb` debugging tool, which comes as part of the Java SDK
distribution, to debug your program. For more information, see the
documentation for `javac` and `jdb`.

Building the Mercury compiler in the `java` grade
--------------------------------------------------

Building the Mercury compiler and other related tools in the Java grade
is NOT generally supported and should be considered experimental.
In particular, a Mercury compiler built in the Java grade may be slower than
normal and some features may not be available.

However, if you want to give it a try, the required steps are:

1. Ensure that you have an existing working Mercury compiler in your `PATH`
   and a clean version of the Mercury source tree.

2. Run `prepare.sh` and `configure` as normal.

3. Add the line:

        GRADE=java

   to a file named `Mmake.params` at the top-level of the source tree.

4. Build the dependencies using the following command:

        mmake --use-mmc-make depend GRADE=java

5. Compile using the following command:

        mmake --use-mmc-make GRADE=java

6. To install the Java version of the compiler, do:

        mmake --use-mmc-make install GRADE=java

The Java version of the compiler MUST be built using `mmake`'s `--use-mmc-make`
option; the build will not work otherwise. Setting the variable `GRADE` in the
invocations of `mmake` is currently required to avoid variable definition
ordering problems in `Mmake.workspace`.

-----------------------------------------------------------------------------
