Authors: [Lars-Ake Fredlund (lfredlund@fi.upm.es)](mailto:Lars-Ake Fredlund (lfredlund@fi.upm.es)). JavaErlang is a library that attempts to facilitate the communication between Java and Erlang nodes, implemented as an additional layer on top of the JInterface Java interface. The library makes heavy use of the Java reflection capability, provided in the package java.lang.reflect.

The primary functionality is that the library provides functions for creating and interacting with Java objects without the need to write any boilerplate code.

An example:

$ erl -sname anyname
> application:ensure_started(java_erlang).
> {ok,NodeId} = java:start_node().
> Int10 = java:new(NodeId,'java.lang.Integer',[10]).
> String10 = java:call(Int10,toString,[]).

The above code starts a Java node with a specified name (note that the name itself does not matter, just that we start erlang with the -sname or -name option), and establishes a connection to it. Then, a Java integer storing the value 10 is created, and finally a Java string representing "10" is returned from the Java integer just created.

To make your Java classes (and Jar files) visible to the library the option add_to_java_classpath should be provided to java:start_node/1. An example:

{ok,NodeId} = java:start_node([{add_to_java_classpath,["classes"]}]).

This adds the directory classes to the classpath of the started Java interpreter

JavaErlang requires rebar3 for building and testing. See here for getting started with rebar3.

To compile, execute the following command:

$ rebar3 compile

After compilation Erlang beam files will be left in the directory _build/default/lib/java_erlang/ebin/.

Should you wish to install the JavaErlang library in the standard Erlang library structure, the following commands can be used:

$ erl -pa _build/default/lib/java_erlang/ebin/

> java_erlang_install:install().

To test, execute the following command:
``` $ rebar3 eunit -v ```

If the eunit tests timeout, try re-running the test to see if the timeout repeats.

To generate Markdown docs, execute the following command:

$ env ERL_LIBS=$PWD/_build/default/lib/edown rebar3 edoc

The test directory contains a number of examples. The subdirectory qc contains a simple Quviq QuickCheck test model for the Java HashSet class -- if QuickCheck is installed the class can be verified by invoking the hashset_test:test() function. The demo subdirectory contains a simple example of using the Swing GUI library from Erlang.

The library now provides a facility to construct Java classes using Erlang, see the java_proxy module for details. To disable this (the option is enabled by default) provide {enable_proxies,false} as an option to start_node/2.

Garbage collection is now implemented. To enable this (the option is currently disabled by default, but the default can change without warning) provide {enable_gc,TrueOrfalse} as an option to start_node/2.

The code for generating an Erlang module corresponding to a Java class has been removed in this version of the library. Thus, Java objects can be created and accessed only using the functions exported from the java module.

Java nodes can now be started on remote Erlang nodes -- see the options for the java:start_node/1 function.

The source code of the library is available on GitHub at git://github.com/fredlund/JavaErlang.git.

Precompiled versions of the library are available too.

The Java null value is represented as the Erlang atom null, the Java boolean values true and false are represented as the corresponding atoms true and false, and integer-like types and floating-point-like types are represented as normal Erlang integers and floats. Arrays are constructed using the normal Erlang syntax for lists and strings. Values can be explicitely type cast using the notation {Type,Value}. For example, {short,5}, {char,$a}, {{array,char,1},"Hello World"}. Java one-dimensional arrays can also be constructed using the funcion java:list_to_array/3. Examples:

> {ok,NodeId} = java:start_node().
{ok,9231}
> False = java:new(NodeId,'java.lang.Boolean',[false]).
{object,0,9231}
> HelloWorldString = java:new(NodeId,'java.lang.String',[java:list_to_array(NodeId,"Hello World!",char)]).
{object,2,9231}
> Zero = java:new(NodeId,'java.lang.Integer',[0]).
{object,3,9232}
> java:call(Zero,intValue,[]).
0
> java:string_to_list(java:new(NodeId,'java.lang.String',[{{array,char,1},"Hello World"}])).
"Hello World"
> java:string_to_list(java:new(NodeId,'java.lang.String',["Hello World"])).
"Hello World"

Boxing and unboxing of primitive method and constructor arguments is done by the library. Examples:

> {ok,NodeId} = java:start_node().
{ok,9231}
> Zero = java:new(NodeId,'java.lang.Integer',[0]).
{object,3,9232}
> java:call(Zero,equals,[0]).
true
> java:call(Zero,equals,[2]).
false
> java:call(Zero,equals,[0.0]).
false

Primitive values returned from a method are represented as normal Erlang values, whereas objects are returned as objects.

> {ok,N} = java:start_node().
{ok,923}
> Zero = java:new(N,'java.lang.Integer',[0]).
{object,3,923}
> java:call(Zero,intValue,[]).
0

Since intValue returnes an int, a primitive type, the library lets the corresponding Erlang function return an Erlang integer.

A Java exceptions Exc is manifest as an Erlang exception {java_exception,Exc}. Example:

  {ok,NodeId} = java:start_node(),
  try java:new(NodeId,'hola',[])
  catch {java_exception,Exc} ->
      io:format("Exception is of type ~p~n",[java:getClassName(Exc)]),
      java:print_stacktrace(Exc)
  end.

The option java_exception_as_value (which can be passed as an argument to java:start_node/1) determines whether Java exceptions are indeed returned as exceptions (the default), as shown in code excerpt above, or whether they are returned as Erlang values (when the option is set to true).

An object of class "c" can be created by calling java:new(NodeId,c,Args), where Args is the list of arguments of the constructor. An example:

{ok,NodeId} = java:start_node(),
I2 = java:new(NodeId,'java.lang.Integer',[2]).

A public method "m" of a Java object "o" of class "c" can be called using the function call java:call(o,m,Args) (or java:get_static(c,m,Args) if it is a static, i.e., class method) where Args is the list of arguments of the method. An example:

{ok,NodeId} = java:start_node(),
I2 = java:new(NodeId,'java.lang.Integer',[2]),
I2b = java:new(NodeId,'java.lang.Integer',[2]),
true = java:call(I2,equals,[I2b]).

This code excerpt creates two Java Integers (of value 2), and checks that the method equals returns true.

A public field "f" of a Java object "o" of class "c" can be accessed using the function call java:get(o,f) (or java:get_static(c,f) if it is a class field). Similarly, the call java:set(o,f,v) is used to assign the value v to the field (and java:set_static(o,f,v) is used for class fields). An example:

{ok,NodeId} = java:start_node(),
Err = java:get_static(NodeId,'java.lang.System',err),
java:call(Err,println,[{int,2}]).

This code excerpt retrieves the Java standard error stream from the field err in java.lang.System, and prints the integer 2.

Currently only the public methods and fields of Java classes are accessible. It is likely that in the future we will provide an option to permit calling methods that are protected, i.e., methods callable only from inside a Java package.

A Java node is created by calling the function java:start_node/0 or java:start_node/1. The JavaErlang interface supports communication between one Erlang node and multiple Java nodes, i.e., java:start_node can be called multiple times from the same Erlang process, and the resulting Java nodes are completely separated. Naturally, a Java object residing on one Java node should not be communicated to a different Java node.

A Java node identifier is an Erlang node (global) resource. That is, an Erlang process P1 can pass a Java node identifier to another Erlang process P2, and then P2 can also call Java code on that Java node.

There is a 1-1 mapping between Erlang processes and Java threads. That is, an Erlang process making a call to Java will not block because another Erlang process is making a Java call.

If the Java code fails to respond to a call from Erlang, and a time limit has been set, an exception java_timeout will be thrown. Note that timeouts can be set on a per-process, per-call basis, using the function set_timeout/1.

The source code for the JavaErlang library is generally licensed under the modified 3-clause BSD software license. See individual files for detailed licensing conditions or exceptions.

The library has a number of practical limitations:

• Performing white box testing is currently difficult; only the public methods, constructors and attributes of classes are callable from Erlang. However, the capability to call non-public methods should be easy to implement (either using the new Java 7 approach, or by changing "permissions" using reflection).

• Speed: the scheme used for communicating between Erlang and Java is clearly suboptimal. Many aspects are trivial to improve: use integer tags instead of atom tags, etc.

• Inability to use the calling context of a Java call to determine which Java method to invoke.