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Author image Patrick LeBoutillier


Inline::Java - Write Perl classes in Java.


   my $alu = new alu() ;
   print "9 + 16 = ", $alu->add(9, 16), "\n";
   print "9 - 16 = ", $alu->subtract(9, 16), "\n";

   use Inline Java => <<'END_OF_JAVA_CODE';
      class alu {
         public alu(){

         public int add(int i, int j){
            return i + j ;

         public int subtract(int i, int j){
            return i - j ;


THIS IS ALPHA SOFTWARE. It is incomplete and possibly unreliable. It is also possible that some elements of the interface (API) will change in future releases.


The Inline::Java module allows you to put Java source code directly "inline" in a Perl script or module. A Java compiler is launched and the Java code is compiled. Then Perl asks the Java classes what public methods have been defined. These classes and methods are available to the Perl program as if they had been written in Perl.

The process of interrogating the Java classes for public methods occurs the first time you run your Java code. The namespace is cached, and subsequent calls use the cached version.


Inline::Java is driven by fundamentally the same idea as other Inline language modules, like Inline::C or Inline::CPP. Because Java is both compiled and interpreted, the method of getting your code is different, but overall, using Inline::Java is very similar to any other Inline language module.

This section will explain the different ways to use Inline::Java. For more details on Inline, see 'perldoc Inline'.

Basic Usage

The most basic form for using Inline::Java is:

   use Inline Java => 'Java source code' ;

Of course, you can use Perl's "here document" style of quoting to make the code slightly easier to read:

   use Inline Java => <<'END';

      Java source code goes here.


The source code can also be specified as a filename, a subroutine reference (sub routine should return source code), or an array reference (array contains lines of source code). This information is detailed in 'perldoc Inline'.

In order for Inline::Java to function properly, it needs to know where to find the Java compiler (javac) and the Java Runtime (java) on your machine. This is done using one of the following techniques:

   - set the BIN configuration option to the correct directory
   - set the PERL_INLINE_JAVA_BIN environment variable to the correct directory
   - put the correct directory in your PATH environment variable


There are a number of configuration options that dictate the behavior of Inline::Java:

      Specifies the path to your Java binaries. 
          Ex: BIN => 'my/java/bin/path'

      Specifies the starting port number for the server. If many 
      C<Inline::Java> blocks are declared, the port number is 
      incremented each time.    
      Default is 7890.
      Ex: PORT => 4567

      Specifies the maximum number of seconds that the Perl script
      will try to connect to the Java server. In other this is the
      delay that Perl gives to the Java server to start.
      Default is 15 seconds.
      Ex: STARTUP_DELAY => 20

      Enables debugging info
      Ex: DEBUG => 1

      Adds the specified CLASSPATH to the environment CLASSPATH.
      Ex: CLASSPATH => 'my/other/java/classses'

      Throws a warning when C<Inline::Java> has to 'choose' between 
      different method signatures. The warning states the possible 
      choices and the sugnature chosen.

      Toggles the execution mode. The default is to use the client/server
      mode. To use the JNI extension (you must have built it at install 
      time though. See README and README.JNI for more information), set 
      JNI to 1. 
      Note: This can also be set globally by setting the PERL_INLINE_JAVA_JNI
      environment variable to 1.
      Ex: JNI => 1


Because Java is object oriented, any interface between Perl and Java needs to support Java classes adequately.


   use Inline Java => <<'END';
      class Foo {
         String data = "data" ;
         static String sdata = "static data" ;

         public Foo() {
            System.out.println("new Foo object being created") ;

         public String get_data(){
            return data ;

         public static get_static_data(){
            return sdata ;

         public void set_data(String d){
            data = d ;

   my $obj = new Foo ;
   print $obj->get_data() . "\n" ;
   $obj->set_data("new data") ;
   print $obj->get_data() . "\n" ;

The output from this program is:

   new Foo object being created
   new data

Inline::Java created a new namespace called main::Foo and created the following functions:

   sub main::Foo::new { ... }
   sub main::Foo::Foo { ... }
   sub main::Foo::get_data { ... }
   sub main::Foo::get_sdata { ... }
   sub main::Foo::set_data { ... }
   sub main::Foo::DESTROY { ... }

Note that only the public methods are exported to Perl. Note also that the class itself is not public. With Inline::Java you cannot create public classes because Java requires that they be defined in a .java file of the same name (Inline::Java can't work this way).

Inner classes are also supported, you simply need to supply a reference to an outer class object as the first parameter of the constructor:

   use Inline Java => <<'END';
      class Foo {
         public Foo() {

         public class Bar {
            public Bar() {

   my $obj = new Foo() ;
   my $obj2 = new Bar($obj) ;


In the previous example we have seen how to call a method. You can also call static methods in the following manner:

   print Foo->get_sdata() . "\n" ;
   # or
   my $obj = new Foo() ;
   print $obj->get_sdata() . "\n" ;

both of these will print:

   static data   

You can pass any kind of Perl scalar or any Java object to a method. It will be automatically converted to the correct type:

   use Inline Java => <<'END';
      class Foo2 {
         public Foo2(int i, String j, Foo k) {

   my $obj = new Foo() ;
   my $obj2 = new Foo2(5, "toto", $obj) ;

will work fine. These objects can be of any type, even if these types are not known to Inline::Java. This is also true for return types:

   use Inline Java => <<'END';
      import java.util.* ;

      class Foo3 {
         public Foo3() {

         public HashMap get_hash(){
            return new HashMap() ;

         public void do_stuff_to_hash(HashMap h){

   my $obj = new Foo3() ;
   my $h = $obj->gethash() ;
   $obj->do_stuff_to_hash($h) ;

Objects of types unknown to Perl can exist in the Perl space, you just can't call any of their methods.


You can also access all public member variables (static or not) from Perl. As with method arguments, the types of these variables does not need to be known to Perl:

   use Inline Java => <<'END';
      import java.util.* ;

      class Foo4 {
         public int i ;
         public static HashMap hm ;

         public Foo4() {

   my $obj = new Foo4() ;
   $obj->{i} = 2 ;
   my $hm1 = $obj->{hm} ; # instance way
   my $hm2 = Foo4::hm ;   # static way   

Note: Watch out for typos when accessing members in the static fashion, 'use strict' will not catch them since they have a package name...


You can also send and receive arrays. This is done by using Perl lists:

   use Inline Java => <<'END';
      import java.util.* ;

      class Foo5 {
         public int i[] = {5, 6, 7} ;

         public Foo5() {

         public String [] f(String a[]){
            return a ;

         public String [][] f(String a[][]){
            return a ;

   my $obj = new Foo5() ;
   my $i_2 = $obj->{i}->[2] ; # 7
   my $a1 = $obj->f(["a", "b", "c"]) ; # String []

   my $a2 = $obj->f([
      ["00", "01"],
      ["10", "11"],
   ]) ; # String [][]
   print $a2->[1]->[0] ; # "10"


Sometimes when a class as many signatures for the same method, Inline::Java will have to select one of the signatures based on the arguments that are passed:

   use Inline Java => <<'END';
      class Foo6 {
         public Foo6() {

         public void f(int i){

         public void f(char c){

   my $obj = new Foo6() ;
   $obj->f('5') ;

In this case, Inline::Java will call f(int i), because '5' is an integer. But '5' is a valid char as well. So to force the call of f(char c), do the following:

   use Inline::Java qw(cast) ;
   $obj->f(cast('char', '5')) ;
   # or
   $obj->f(Inline::Java::cast('char', '5')) ;

The cast function forces the selection of the matching signature.

Another case where type casting is need is when one wants to pass an array as a java.lang.Object:

   use Inline Java => <<'END';
      Object o ;
      int a[] = {1, 2, 3} ;

      class Foo7 {
         public Foo7() {

   my $obj = new Foo7() ;
   $obj->{o} = [1, 2, 3] ;      # No!

The reason why this will not work is simple. When Inline::Java sees an array, it checks the Java type you are trying to match it against to validate the construction of your Perl list. But in this case, it can't validate the array because you're assigning it to an Object. You must use the 3 parameter version of the cast function to do this:

   $obj->{o} = Inline::Java::cast(
     [1, 2, 3],
     "[Ljava.lang.String;") ;

This tells Inline::Java to validate your Perl list as a String [], and then cast it as an Object.

Here is how to construct the array type representations:

  [<type> ->  1 dimensional <type> array
  [[<type> -> 2 dimensional <type> array

  where <type>is one of:
    B byte     S short     I int     J long  
    F float    D double    C char    Z boolean

    L<class>; array of <class> objects

This is described in more detail in most Java books that talk about reflection.

But you only need to do this if you have a Perl list. If you already have a Java array reference obtained from elsewhere, you don't even need to cast:

   $obj->{o} = $obj->{a} ;


Starting in version 0.20, it is possible to use the JNI (Java Native Interface) extension. This enables Inline::Java to load the Java virtual machine as a shared object instead of running it as a stand-alone server. This brings an improvement in performance.

However, the JNI extension is not available on all platforms (see README and README.JNI for more information). For that reason, if you have built the JNI extension, you must enable it explicitely by doing one of the following:

   - set the JNI configuration option to 1
   - set the PERL_INLINE_JAVA_JNI environment variable to 1

Note: Inline::Java only creates one virtual machine instance. Therefore you can't use JNI for some sections and client/server for others. The first section determines the execution mode.


This is an ALPHA release of Inline::Java. Further testing and expanded support for other operating systems and platforms will be a focus for future releases. It has been tested on:

   - Solaris 2.5.1 + Perl 5.6 + Java SDK 1.2.2
   - Solaris 2.8   + Perl 5.6 + Java SDK 1.3.0
   - Windows 2000  + Perl 5.6 + Java SDK 1.3.0
   - Windows 95    + Perl 5.6 + Java SDK 1.2.2 (fix required in Makefile)

It likely will work with many other configurations.


This is how Inline::Java works. Once the user's code is compiled by the javac binary, Inline::Java's own Java code is compiled. This code implements a server (or not if you use the JNI mode) that receives requests from Perl to create objects, call methods, destroy objects, etc. It is also capable of analyzing Java code to extract the public symbols. Once this code is compiled, it is executed to extract the symbols from the Java code.

Once this is done, the user's code information is fetched and is bound to Perl namespaces. Then Inline::Java's code is run to launch the server. The Perl script then connects to the server using a TCP socket (or not if you use the JNI mode). Then each object creation or method invocation on "Java objects" send requests to the server, which processes them and returns object ids to Perl which keeps them the reference te objects in the future.


For information about using Inline, see Inline.

For information about other Inline languages, see Inline-Support.

Inline::Java's mailing list is inline@perl.org

The subscribe, send email to inline-subscribe@perl.org


When reporting a bug, please do the following:

 - Put "use Inline REPORTBUG;" at the top of your code, or 
   use the command line option "perl -MInline=REPORTBUG ...".
 - Run your code.
 - Follow the printed instructions.

Here are some things to watch out for:

  1. You can't use the "package" Java directive when using Inline::Java.

  2. You can't create public classes when using Inline::Java. This is due to the fact that Java requires that public classes be defined in a .java file of the same name (Inline::Java can't work this way).


Patrick LeBoutillier <patl@cpan.org>

Brian Ingerson <INGY@cpan.org> is the author of Inline.


Copyright (c) 2001, Patrick LeBoutillier.

All Rights Reserved. This module is free software. It may be used, redistributed and/or modified under the terms of the Perl Artistic License.

(see http://www.perl.com/perl/misc/Artistic.html)