path: root/user/jkoenig/java.mdwn

# Java for Hurd (and vice versa)

Contact information:

  * Full name: Jérémie Koenig
  * Email: jk@jk.fr.eu.org
  * IRC: jkoenig on Freenode and OFTC

## Introductions

I am a first year M.Sc. student
in Computer Science at University of Strasbourg (France).
My interests include capability-based security,
programming languages and formal methods
(in particular, object-capability languages and proof-carrying code).

### Proposal summary

This project would consist in improving Java support on Hurd.
The first part would consist in
fixing bugs and porting Java-related packages.
The second part would consist in
creating low-level Java bindings for the Hurd interfaces,
as well as libraries to make translator development easier.

### Previous involvement

I started contributing to Hurd last summer,
during which I participated to Google Summer of Code
as a student for the Debian project.
I worked on porting Debian-Installer to Hurd.
This project was mostly a success,
although we still have to use a special mirror for installation
with a few modified packages
and tweaked priorities
to work around some uninstallable packages
with Priority: standard.

Shortly afterwards,
I rewrote the procfs translator
to fix some issues with memory leaks,
make it more reliable,
and improve compatibility with Linux-based tools
such as `procps` or `htop`.

Although I have not had as much time
as I would have liked to dedicate to the Hurd
since that time,
I have continued to maintain the mirror in question,
and I have started to work
on implementing POSIX threads signal semantics in glibc.

### Project-related skills and interests

I have used Java mostly for university assignments.
This includes non-trivial projects
using threads and distributed programming frameworks
such as Java RMI or CORBA.
I have also used it to experiment with
Google App Engine
(web applications)
and Google Web Toolkit
(a compiler from Java to Javascript which helps with AJAX code),
and I have some limited experience with JNI
(the Java Native Interface, to link Java with C code).

My knowledge of the Hurd and Debian GNU/Hurd is reasonable,
as the Debian-Installer and procfs projects
gave me the opportunity to fiddle with many parts of the system.

Initially,
I started working on this project because I wanted to use
[Joe-E](http://code.google.com/p/joe-e/)
(a subset of Java)
to investigate the potential
[[applications of object-capability languages|objcap]]
in a Hurd context.
I also believe that improving Java support on Hurd
would be an important milestone.

### Organisational matters

I am subscribed to bug-hurd@g.o and
I do have a permanent internet connexion.

I would be able to attend the regular IRC meetings,
and otherwise communicate with my mentor
through any means they would prefer
(though I expect email and IRC would be the most practical).
Since I'm already familiar with the Hurd,
I don't expect I would require too much time from them.

My exams end on May 20 so I would be able to start coding
right at the beginning of the GSoC period.
Next year's term would probably begin around September 15,
so that would not be an issue either.
I expect I would work around 40 hours per week,
and my waking hours would be flexible.

I don't have any other plans for the summer
and would not make any if my project were to be accepted.

Full disclosure:
I also submitted a proposal to the Jikes RVM project
(which is a research-oriented Java Virtual Machine,
itself written in Java)
for implementing a new garbage collector into the MMTk subsystem.

## Improve Java support

### Justification

Java is a popular language and platform used by many desktop and web
applications (mostly on the server side). As a consequence, competitive Java
support is important for any general-purpose operating system.
Better Java support would also be a prerequisite
for the second part of my proposal.

### Current situation

Java is currently supported on Hurd with the GNU Java suite:

   * [GCJ](http://gcc.gnu.org/java/),
     the GNU Compiler for Java, is part of GCC and can compile Java
     source code to Java bytecode, and both source code and bytecode to
     native code;
   * libgcj is the implementation of the Java runtime which GCJ uses.
     It is based on [GNU Classpath](http://www.gnu.org/software/classpath/).
     It includes a bytecode interpreter which enables
     Java applications compiled to native code to dynamically load and execute
     Java bytecode from class files.
   * The gij command is a wrapper around the above-mentioned virtual machine
     functionality of libgcj and can be used as a replacement for the java
     command.

However, GCJ does not work flawlessly on Hurd.r
For instance, some parts of libgcj relies on
the POSIX threads signal semantics, which are not yet implemented.
In particular, this makes ant hang waiting for child processes,
which makes some packages fail to build on Hurd
(“ant” is the “make” of the Java world).

### Tasks

   * **Finish implementing POSIX thread semantics** in glibc (high priority).
     According to POSIX, signal dispositions should be global to a process,
     while signal blocking masks should be thread-specific. Signals sent to the
     process as a whole are to be delivered to any thread which does not block
     them. By contrast, Hurd has per-thread signal dispositions and signals
     sent to a process are delivered to the main thread only. I have been
     working on refactoring the glibc signal code and implementing the POSIX
     semantics as a per-thread option. However, due to lack of time I have not
     yet been able to test and debug my code properly. Finishing this work
     would be my first task.
   * **Fix further problems with GCJ on Hurd** (high priority). While I’m not
     aware of any other problems with GCJ at the moment, I suspect some might
     turn up as I progress with the other tasks. Fixing these problems would
     also be a high-priority task.
   * **Port OpenJDK 6** (medium priority). While GCJ is fine, it is not yet
     100% complete. It is also slower than OpenJDK on architectures where a
     just-in-time compiler is available. Porting OpenJDK would therefore
     improve Java support on Hurd in scope and quality. Besides, it would also
     be a good way to test GCJ, which is used for bootstrapping by the Debian
     OpenJDK packages. Also note that OpenJDK 6 is now the default Java
     Runtime Environment on all released Linux-based Debian architectures;
     bringing Hurd in line with this would probably be a good thing.
   * **Port Eclipse and other Java applications** (low priority). Eclipse is a
     popular, state-of-the-art IDE and tool suite used for Java and other
     languages. It is a dependency of the Joe-E verifier (see part 3 of this
     proposal). Porting Eclipse would be a good opportunity to test GCJ and
     OpenJDK.

### Deliverables

  * The glibc pthreads patch and any other fixes on the Hurd side
    would be submitted upstream
  * Patches against Debian source packages
    required to make them build on Hurd would be submitted
    to the [Debian bug tracking system](http://bugs.debian.org/).


## Create Java bindings for the Hurd interfaces

### Justification

Java is used for many applications and often taught to
introduce object-oriented programming. The fact that Java is a
garbage-collected language makes it easier to use, especially for the less
experienced programmers. Besides, its object-oriented nature is a
natural fit for the capability-based design of Hurd.
The JVM is also used as a target for many other languages,
all of which would benefit from the access provided by these bindings.

Advantages over other garbage-collected, object-oriented languages include
performance, type safety and the possibility to compile a Java translator to
native code and
[link it statically](http://gcc.gnu.org/wiki/Statically_linking_libgcj)
using GCJ, should anyone want to use a
translator written in Java for booting.
Note that Java is
[being](http://www.linuxjournal.com/article/8757)
[used](http://oss.readytalk.com/avian/)
in this manner for embedded development.
Since GCJ can take bytecode as its input,
this expect this possibility would apply to any JVM-based language.

Java bindings would lower the bar for newcomers
to begin experimenting with what makes Hurd unique
without being faced right away with the complexity of
low-level systems programming.

### Tasks summary

  * Implement Java bindings for Mach
  * Implement a libports-like library for Java
  * Modify MIG to output Java code
  * Implement libfoofs-like Java libraries

### Design principles

The principles I would use to guide the design
of these Java bindings would be the following ones:

  * The system should be hooked into at a low level,
    to ensure that Java is a "first class citizen"
    as far as the access to the Hurd's interfaces is concerned.
  * At the same time, the memory safety of Java should be maintained
    and extended to Mach primitives such as port names and
    out-of-line memory regions.
  * Higher-level interfaces should be provided as well
    in order to make translator development
    as easy as possible.
  * A minimum amount of JNI code (ie. C code) should be used.
    Most of the system should be built using Java itself
    on top of a few low-level primitives.
  * Hurd objects would map to Java objects.
  * Using the same interfaces,
    objects corresponding to local ports would be accessed directly,
    and remote objects would be accessed over IPC.

One approach used previously to interface programming languages with the Hurd
has been to create bindings for helper libraries such as libtrivfs. Instead,
for Java I would like to take a lower-level approach by providing access to
Mach primitives and extending MIG to generate Java code from the interface
description files.

This approach would be initially more involved, and would introduces several
issues related to overcoming the "impedance mismatch" between Java and Mach.
However, once an initial implementation is done it would be easier to maintain
in the long run and we would be able to provide Java bindings for a large
percentage of the Hurd’s interfaces.

### Bindings for Mach system calls

In this low-level approach, my intention is to enable Java code to use Mach
system calls (in particular, mach_msg) more or less directly. This would
ensure full access to the system from Java code, but it raises a number of
issues:

  * the Java code must be able to manipulate Mach-level entities, such as port
    rights or page-aligned buffers mapped outside of the garbage-collected
    heap (for out-of-line transfers);
  * putting together IPC messages requires control of the low-level
    representation of data.

In order to address these concerns, classes would be encapsulating these
low-level entities so that they can be referenced through normal, safe objects
from standard Java code. Bindings for Mach system calls can then be provided
in terms of these classes. Their implementation would use C code through the
Java Native Interface (JNI).

More specifically, this functionality would be provided by the `org.gnu.mach`
package, which would contain at least the following classes:

  * `MachPort` would encapsulate a `mach_port_t`. (Some of) its constructors
    would act as an interface for the `mach_port_allocate()` system call.
    `MachPort` objects would also be instantiated from other parts of the JNI
    C code to represent port rights received through IPC. The `deallocate()`
    method would call `mach_port_deallocate()` and replace the encapsulated
    port name with `MACH_PORT_DEAD`. We would recommend that users call it
    when a port is no longer used, but the finalizer would also deallocate the
    port when the `MachPort` object is garbage collected.
  * `Buffer` would represent a page-aligned buffer allocated outside of the
    Java heap, to be transferred (or having been received) as out-of-line
    memory. The JNI code would would provide methods to read and write data at
    an arbitrary offset (but within bounds) and would use `vm_allocate()` and
    `vm_deallocate()` in the same spirit as for `MachPort` objects.
  * `Message` would allow Java code to put together Mach messages. The
    constructor would allocate a `byte[]` member array of a given size.
    Additional methods would be provided to fill in or query the information
    in the message header and additional data items, including `MachPort` and
    `Buffer` objects which would be translated to the corresponding port names
    and out-of-line pointers.
    A global map from port names to the corresponding `MachPort` object
    would probably be needed to ensure that there is a one-to-one
    correspondence.
  * `Syscall` would provide static JNI methods for performing system calls not
    covered by the above classes, such as `mach_msg()` or
    `mach_thread_self()`.  These methods would accept or return `MachPort`,
    `Buffer` and `Message` objects when appropriate. The associated C code
    would access the contents of such objects directly in order to perform the
    required unsafe operations, such as constructing `MachPort` and `Buffer`
    objects directly from port names and C pointers.

Note that careful consideration should be given to the interfaces of these
classes to avoid “safety leaks” which would compromise the safety guarantees
provided by Java. Potential problematic scenarios include the following
examples:

  * It must not be possible to write an integer at some position in a
    `Message` object, and to read it back as a `MachPort` or `Buffer` object,
    since this would allow unsafe access to arbitrary memory addresses and
    mach port names.
  * Providing the `mach_task_self()` system call would also provide access to
    arbitrary addresses and ports by using the `vm_*` family of RPC operations
    with the returned `MachPort` object. This means that the relevant task
    operations should be provided by the `Syscall` class instead.

Finally, access should be provided to the initial ports and file descriptors
in `_hurd_ports` and provided by the `getdport()` function,
for instance through static methods such as
`getCRDir()`, `getCWDir()`, `getProc()`, ... in a dedicated class such as
`org.gnu.hurd.InitPorts`.

A realistic example of code based on such interfaces would be:

    import org.gnu.mach.MsgType;
    import org.gnu.mach.MachPort;
    import org.gnu.mach.Buffer;
    import org.gnu.mach.Message;
    import org.gnu.mach.Syscall;
    import org.gnu.hurd.InitPorts;

    public class Hello
    {   
       public static main(String argv[])
           /* Parent class for all Mach-related exceptions */
           throws org.gnu.mach.MachException
       {   
           /* Allocate a reply port */
           MachPort reply = new MachPort();

           /* Allocate an out-of-line buffer */
           Buffer data = new Buffer(MsgType.CHAR, 13);
           data.writeString(0, "Hello, World!");

           /* Craft an io_write message */
           Message msg = new Message(1024);
           msg.setRemotePort(InitPorts.getdport(1));
           msg.setLocalPort(reply, Message.Type.MAKE_SEND_ONCE);
           msg.setId(21000);
           msg.addBuffer(data);

           /* Make the call, MACH_MSG_SEND | MACH_MSG_RECEIVE */
           Syscall.machMsg(msg, true, true, reply);

           /* Extract the returned value */
           msg.assertId(21100);
           int retCode = msg.readInt(0);
           int amount = msg.readInt(1);
       }
    }

Should this paradigm prove insufficient,
more ideas could be borrowed from the
[`org.vmmagic`](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.151.5253&rep=rep1&type=pdf)
package used by [Jikes RVM](http://jikesrvm.org/),
a research Java virtual machine itself written in Java.

### Generating Java stubs with MIG

Once the basic machinery is in place to interface with Mach, Java programs
have more or less equal access to the system functionality without resorting
to more JNI code. However, as illustrated above, this access is far from
convenient.

As a solution I would modify MIG to add the option to output Java code. MIG
would emit a Java interface, a client class able to implement the interface
given a Mach port send right, an a server class which would be able to handle
incoming messages. The class diagram below, although it is by no means
complete or exempt of any problem, illustrates the general idea:

[[gsoc2011_classes.png]]

This structure is somewhat reminiscent of
[Java RMI](http://en.wikipedia.org/wiki/Java_remote_method_invocation)
or similar systems,
which aim to provide more or less transparent access to remote objects.
The exact way the Java code would be generated still needs to be determined,
but basically:

  * An interface, corresponding to the header files generated by MIG, would
    enumerate the operations listed in a given .defs files. Method names would
    be transformed to adhere to Java conventions (for instance,
    `some_random_identifier` would become `someRandomIdentifier`).
  * A user class, corresponding to the `*User.c` files,
    would implement this interface by doing RPC over a given MachPort object.
  * A server class, corresponding to `*Server.c`, would be able to handle
    incoming messages using a user-provided implementation of the interface.
    (Possibly, a skeleton class providing methods which would raise
    `NotImplementedException`s would be provided as well.
    Users would derive from this class and override the relevant methods.
    This would allow them not to implement some operations,
    and would avoid pre-existing code from breaking when new operations are
    introduced.)

In order to help with the implementation of servers, some kind of library
would be needed to associate Mach receive rights with server objects and to
handle incoming messages on dedicated threads, in the spirit of libports.
This would probably require support for port sets at the level of the Mach
primitives described in the previous section.

When possible, operations involving the transmission of send rights
of some kind would be expressed in terms of the MIG-generated interfaces
instead of `MachPort` objects.
Upon reception of a send right,
a `FooUser` object would be created
and associated with the corresponding `MachPort` object.
If the received send right corresponds to a local port
to which a server object has been associated,
this object would be used instead.
This way,
subsequent operations on the received send right
would be handled as direct method calls
instead of going through RPC mechanisms.

Some issues will still need to be solved regarding how MIG will convert
interface description files to Java interfaces. For instance:

  * `.defs` files are not explicitly associated with a type. For instance in
    the example above, MIG would have to somehow infer that io_t corresponds
    to `this` in the `Io` interface.
  * More generally, a correspondence between MIG and Java types would have
    to be determined. Ideally this would be automated and not hardcoded
    too much.
  * Initially, reply port parameters would be ignored. However they may be
    needed for some applications.

So the details would need to be flushed out during the community bonding
period and as the implementation progresses. However I’m confident that a
satisfactory solution can be designed.

Using these new features, the example above could be rewritten as:

    import org.gnu.hurd.InitPorts;
    import org.gnu.hurd.Io;
    import org.gnu.hurd.IoUser;

    class Hello {
       static void main(String argv[]) throws ...
       {
           Io stdout = new IoUser(InitPorts.getdport(1));
           String hello = “Hello, World!\n”;

           int amount = stdout.write(hello.getBytes(), -1);

           /* (A retCode corresponding to an error
               would be signalled as an exception.) */
       }
    }

An example of server implementation would be:

    import org.gnu.hurd.Io;
    import java.util.Arrays;

    class HelloIo implements Io {
       final byte[] contents = “Hello, World!\n”.getBytes();

       int write(byte[] data, int offset) {
           return SOME_ERROR_CODE;
       }

       byte[] read(int offset, int amount) {
           return Arrays.copyOfRange(contents, offset,
                                     offset + amount - 1);
       }

       /* ... */
    }

A new server object could then be created with `new IoServer(new HelloIo())`,
and associated with some receive right at the level of the ports management
library.

### Base classes for common types of translators

Once MIG can target Java code, and a libports equivalent is available,
creating new translators in Java would be greatly facilitated. However,
we would probably want to introduce basic implementations of file system
translators in the spirit of libtrivfs or libnetfs. They could take the form
of base classes implementing the relevant MIG-generated interfaces which
would then be derived by users,
or could define a simpler interface
which would then be used by adapter classes
to implement the required ones.

I would draw inspiration from libtrivfs and libnetfs
to design and implement similar solutions for Java.

### Deliverables

  * A hurd-java package would contain the Java code developed
    in the context of this project.
  * The Java code would be documented using javadoc
    and a tutorial for writing translators would be written as well.
  * Modifications to MIG would be submitted upstream,
    or a patched MIG package would be made available.

The Java libraries resulting from this work,
including any MIG support classes
as well as the class files built from the MIG-generated code
for the Mach and Hurd interface definition files,
would be provided as single `hurd-java` package for
Debian GNU/Hurd.
This package would be separate from both Hurd and Mach,
so as not to impose unreasonable build dependencies on them.

I expect I would be able to act as its maintainer in the foreseeable future, 
either as an individual or as a part of the Hurd team.
Hopefully,
my code would be claimed by the Hurd project as their own,
and consequently the modifications to MIG
(which would at least conceptually depend on the Mach Java package)
could be integrated upstream.

Since by design,
the Java code would use only a small number of stable interfaces,
it would not be subject to excessive amounts of bitrot.
Consequently,
maintenance would primarily consist in
fixing bugs as they are reported,
and adding new features as they are requested.
A large number of such requests
would mean the package is useful,
so I expect that the overall amount of work
would be correlated with the willingness of more people
to help with maintenance
should I become overwhelmed or get hit by a bus.


## Timeline

The dates listed are deadlines for the associated tasks.

  * *Community bonding period.*
    Discuss, refine and complete the design of the Java bindings
    (in particular the MIG and "libports" parts)
  * *May 23.*
    Coding starts.
  * *May 30.*
    Finish implementing pthread signal semantics.
  * *June 5.*
    Port OpenJDK
  * *June 12.*
    Fix the remaining problems with GCJ and/or OpenJDK,
    possibly port Eclipse or other big Java packages.
  * *June 19.*
    Create the bindings for Mach.
  * *June 26.*
    Work on some kind of basic Java libports
    to handle receive rights.
  * *July 3.*
    Test, write some documentation and examples.
  * *July 17 (two weeks).*
    Add the Java target to MIG.
  * *July 24.*
    Test, write some documentation and examples.
  * *August 7 (two weeks).*
    Implement a modular libfoofs to help with translator development.
    Try to write a basic but non-trivial translator
    to evaluate the performance and ease of use of the result,
    rectify any rough edges this would uncover.
  * *August 22. (last two weeks)*
    Polish the code and packaging,
    finish writing the documentation.


## Conclusion

This project is arguably ambitious.
However, I have been thinking about it for some time now
and I'm confident I would be able to accomplish most of it.

In the event multiple language bindings projects
would be accepted,
some work could probably be done in common.
In particular,
[ArneBab](http://www.bddebian.com/~hurd-web/community/weblogs/ArneBab/2011-04-06-application-pyhurd/)
seems to favor a low-level approach for his Python bindings as I do for Java,
and I would be happy to discuss API design and coordinate MIG changes with him.
I would also have an extra month after the end of the GSoC period
before I go back to school,
which I would be able to use to finish the project
if there is some remaining work.
(Last year's rewrite of procfs was done during this period.)

As for the project's benefits,
I believe that good support for Java
is a must-have for the Hurd.
Java bindings would also further the Hurd's agenda
of user freedom by extending this freedom to more people:
I expect the set of developers
who would be able to write Java code against a well-written libfoofs
is much larger than
those who master the intricacies of low-level systems C programming.
From a more strategic point of view,
this would also help recruit new contributors
by providing an easier path to learning the inner workings of the Hurd.

Further developments
which would build on the results of this project
include my planned [[experiment with Joe-E|objcap]]
(which I would possibly take on as a university project next year).
Another possibility would be to reimplement some parts
of the Java standard library
directly in terms of the Hurd interfaces
instead of using the POSIX ones through glibc.
This would possibly improve the performance
of some Java applications (though probably not by much),
and would otherwise be a good project
for someone trying to get acquainted with Hurd.

Overall, I believe this project would be fun, interesting and useful.
I hope that you will share this sentiment
and give me the opportunity to spend another summer working on Hurd.