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Manual Reference Pages  -  JAVA (3)

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Java - Perl extension for accessing a JVM remotely or locally



  use Java;
  $java = new Java;
  $frame = $java->create_object("java.awt.Frame","Frames Title");
  $array = $java->create_array("java.lang.String",5);
  // Set array element 3 to "Java is lame"
  $array->[3] = "Java is lame";
  $element_value = $array->[3]->get_value();

  $button = $java->create_object("java.awt.Button","Push Me");
  // Listen for Action events from $button object

  // Loop & wait mode
       my $continue = $java->go;
       last if (!defined $continue);

  // Got an event!
  sub event_handler
        my($object_that_caused_event,$event_object) = @_;
        if ($object_that_caused_event->same($button))
                // From $button!
                print "You pushed my button!!\n";


This module allows you to talk to a JVM on a local or remote machine. You can create objects, call functions, access fields, deal with arrays, get events & all the nonsense you can do in Java - from Perl!

Starting a JVM server

First you must run ’JavaServer’ on the machine to which you will make connections. Simply do a ’java JavaServer’ to start the server. By default it will start listening on port 2000. Make sure the ’JavaServer.jar’ is in your classpath - also make sure the Swing stuff (JFC if you prefer) is in your classpath as well if you want to use Swing stuff (note this does not apply to JVM 1.2+).

Creating the root Java object

You connect to a remote (or local) JVM when you create a new Java instance. The new call accepts a hash with the following keys:

        host => hostname of remote machine to connect to
                        default is localhost
        port => port the JVM is listening on (JavaServer)
                        default is 2000
        event_port => port that the remote JVM will send events to
                        default is 2001.  If you specify -1 for this
                        value then the event service will be turned off -
                        if youre not doing any GUI work this might be
                        a good idea as the second event port will NOT
                        get used/opened saving some system resources.
        authfile => The path to a file whose first line is used as a
                        shared secret which will be passed to
                        JavaServer.  To use this feature you must start
                        JavaServer with the --authfile=<filename>
                        command-line option.
                        If the secret words match access will be granted
                        to this client.  By default there is no shared
                        secret.  See the Authorization section below.
        use_old_style_arrays => tell to use old-style arrays
                        which you should NOT be using unless you need
                        backwards compatibility with 3.x &
                        earlier.  By default all arrays returned by
                        JavaServer are tied to the JavaArray class for
                        easier perl-like manipulation.  See the Arrays
                        section futher down for more info.

For example:

        $java = new Java(host => "", event_port => 4032);

        # No events!
        $java2 = new Java(port => 8032, event_port => -1);

You can have any number of java ’environments’ in a Perl program.

Also if you ’use strict’ you must do a ’no struct ’subs’’ ’cuz all Java method calls are AUTOLOAD’ed - sorry.

Creating java primitives

The module will treat all integers encountered in parameter lists as integer and strings as java Strings. All other primitive types must be suffixed with an identifier so knows what primitive Java type to convert it to - for instance boolean types are tagged like:
        true:b or false:b

Here’s a complete list of supported Java primitives:

        Perl String Value  -> (converted to) -> Java Primitive
        -----------------                       --------------
        2344                                    int
        "23:short"                              short
        "23:byte"                               byte
        "a:char"                                char
        "23445:long"                            long
        "3.42:float"                            float
        "3.14159:double"                        double
        "true:b" or "false:b"                   boolean
        "Anything else"                         String
        "Anything else:string"                  String

So... if you need to use an integer as a String say 343:string.

Localization and String encoding

Quick note on String encodings, you can specify that your strings are encoded in a specific format using the :string_<ENCODING> syntax like:

        my $label = $java->create_object("java.awt.Label","Label:string_UTF8");

This specifies that this String uses Unicode encoding. See for the complete list of valid Java String encodings.

Creating java objects

Once you’ve connected to a JVM via the ’new Java’ call you can start creating Java objects. This is accomplished via the ’create_object’ function. The first argument must be the ’fully-qualified’/’full path’ of the Java object you want to create - like ’java.lang.String’ or ’java.awt.Frame’. The remaining arguments are passed to that object’s constructor.

For example:

        my $frame = $java->create_object("java.awt.Frame","Frame Title");
        my $dialog = $java->create_object("java.awt.Dialog",$frame,
                        "Dialog Title","true:b");

Note the use of true:b in the constructor to tell that that value should be a ’true’ Java boolean value.

In these cases a ’java.awt.Frame’ takes a String as the lone parameter, whereas a ’java.awt.Dialog’ takes a Frame, a String, and a boolean value in its constructor.

Calling java methods

You can make both static and instantiated method calls on java objects. The parameter lists work exactly like constructor parameter lists - if you want to pass a java primitive anything other than integers or Strings need to be tagged accordingly. All function calls that return something return a java object - so even if the java function returns an ’int’ it is returned to perl as a ’java.lang.Integer’. To get the value of that Integer you must use the ’get_value’ function. The syntax is exactly what you’d expect (I hope!).

For example:

        $frame->setSize(200, 500);
        $frame->show();  # (or $frame->show)

Note functions that don’t take any parameters don’t need the parentheses! Alternatively you can use the ’call’ function to make method calls:

        $frame->call(setSize, 500, 500);

But that’s no fun!

To call static functions the syntax is slightly different.

For example:

To call the static method ’forName’ in the object ’java.lang.Class’ it looks like this:

        my $class = $java->java_lang_Class("forName", "Test");

Note you use the ’$java’ object returned from the call to ’new Java’ to access static methods - the static object must be fully-qualified separated by ’_’s instead of ’.’s WHEN USED AS A FUNCTION NAME (as opposed to the method below when it’s used as a string - in the below case DO NOT replace ’.’s with ’_’s)! And finally the first parameter is the name of the static function followed by any parameters to it.

If your static class is NOT in a package you MUST use the ’static_call’ function like:

        my $return_value = $java->static_call("MyStaticClass","<function_name>",@params);

Even if your class is in a package you can use the ’static_call’ function (Note when using the ’static_call’ function your fully qualified class name is separated by ’.’ NOT ’_’s as in the example above):

    my $class = $java->static_call("java.lang.Class", "forName", "Test");
        my $class = $java->java_lang_Class("forName", "Test");

Note the use of ’.’s in the first case and ’_’s in the second case. Also the returned value ’$class’ in an OBJECT NOT a string. To ’stringify’ it use the ’get_value’ function as described below. Here’s a sneak peek:

    print "This java.lang.Class objects name is ", $class->get_value, "\n";

Getting and Setting java object fields

You can get and set individual fields in java objects (static or instantiated) using the ’get_field’ and ’set_field’ methods. All ’get_field’ calls return java objects just like calling java functions. You must use the ’get_value’ function to ’unwrap’ primitive types to their actual values.

For example:

Get a static field

        my $win_act = $java->get_field("java.awt.event.WindowEvent",

Note the first parameter must be the fully qualified java object name and the second parameter is the static field.

Get an instantiated field

        my $obj = $java->create_object("");
        my $field = $obj->get_field("my_field");

Similarly to set a field another parameter is added to the ’set_field’ call with the object that the specified field is to be set to:

Set a static field


Set an instantiated field


Getting values

To ’unwrap’ java primitives (including Strings) you need to call the ’get_value’ function. This will stringify any object given to it - typcially this is only useful for ’unwrapping’ java primitives and Strings. Note for all other object the ’toString()’ method is called.

For example:

        my $string1 = $java->create_object("java.lang.String","Mark");
        my $string2 = $java->create_object("java.lang.String","Jim");

        if ($string1 eq $string2)
                # WRONG!!! 
                # $string1 & $string2 are objects!

        if ($string1->get_value eq $string2->get_value)
                # RIGHT!!!
                # now youre comparing actual strings...

Arrays - new style!

Arrays are created with the ’create_array’ function call. It needs a fully-qualified java object or primitive name and a dimension.

        If you specified use_tied_arrays in your constructor to
        (& I think you should unless you have to perserve backwards
        compatibility...) all Java array references will be tied to the
        JavaArray class allowing a more intuitive interface to your array.

        All array references will be _references_ to these objects. 
        Heres how it looks (compare with old style below):

        # This will create a String array with 100 elements
    #       (this is the same)
        my $array  = $java->create_array("java.lang.String",100);

    # Now it gets interesting!
        # Dont forget on primitive arrays to use the : notation!
        $array->[22] = "Mark rules the free world";

        # Get element #99
        my $element_99 = $array->[99];

To get the length or size of an array do what you’d expect (I hope!)

For example:

        my $length = scalar (@$array);
        my $size = $#{@array};

        (remember you get an arrayref there sonny...)

To pass as a function parameter just pass it in as normal:

        my $list = $java->java_util_Arrays("asList",$array);

Arrays - old style

Arrays are created with the ’create_array’ function call. It needs a fully-qualified java object or primitive name and a dimension.

For example:

        # This will create a char array with 100 elements
        my $char_array  = $java->create_array("char",100);

        # This will create a String array with 5 elements
        my $string_array = $java->create_array("java.lang.String",5);

Array elements are get and set using the ’get_field’ and ’set_field’ function calls.

For example:

        # Set element #22 to B
        # Dont forget on primitive arrays to use the : notation!

        # Set element #3 to Mark Rox
        $string_array->set_field(3,"Mark Rox");

        # Get element #99
        my $element_99 = $char_array->get_field(99);

        # Get element #4
        my $element_4 = $string_array->get_field(4);

        # Dont forget to get the actual string value you gotta call
        #       get_value!
        my $char_value = $char_element_99->get_value;
        my $string_value = $string_element_4->get_value;

To get the length of an array use the get_length function.

For example:

        my $length = $string_array->get_length;

Note this will return an actual integer! You do not need to call ’get_value’ on ’get_length’s return value!

Passing & receiving the ’null’ value

To pass a ’null’ in a function parameter list or to set a field or array index, used Perl’s ’undef’. So:

        $object->function($param1, undef, $param2);

Will pass ’null’ as the second parameter to that function. Similarly to set a field or array index to null:

        $object->set_field("fieldname",undef);  # Set field to null
        $array->[4] = undef;    # Set array value to null

Of course if the field or array type is a primimtive type you will get a NullPointerException - Java doesn’t seem to like that!

If a function returns null or a field or array index is equal to null, you will recieve ’undef’ back. Note this is indistinguishable (sp??) from a function with a ’void’ return value. So:

        my $retval = $object->function($param1,$param2,undef,"Another param");
        print "It returned NULL\n" if (!$retval);


        my $f_value = $object->get_field("someField");
        print "someField is NULL\n" if (!$f_value);

        my $a_value = $array->[38];
        print "Array index 38 is NULL\n" if (!$a_value);

If someone can think of a good reason why the null return value should not be undef or should be different than what a void function returns I’d like to hear about it!


Currently will ’croak’ when an Exception is encountered in JavaServer. So the way to deal with them is to enclose your Java expression that might throw an exception in an ’eval’ block & then check the $@ variable to see if an Exception was indeed thrown. You then need to parse the $@ variable to see exactly what Exception was thrown. Currently the format of the $@ string is:

        ERROR: java.lang.Exception: <more info> at $0 line XX

Note the ’<more info>’ part is the result of the getMessage() function of that Exception. Everything after that is the stuff put in there by croak; the filename & line number of your Perl program.


The actual Exception object that was thrown is available via the ’get_exception’ function call.


There is also a convenience function ’get_stack_trace’ which will return the Stack Trace as an array of lines from the most recent Exception thrown. To see how this is done ’Read The Code Luke’ in - basically it just gets the most recent Exception & creates an appropriate PrintWriter into which it has Java dump the Stack Trace & then it just returns the String-ifized version of it - something you can easily (albiet messily) do yourself.

So here’s what an Exception handler can look like:

        my $I;
                $I = $java->java_lang_Integer("parseInt","$some_string:string");
        if ($@)
                # An exception was thrown!!
                $@ =~ s/^ERROR: //;     # Gets rid of ERROR: 
                $@ =~ s/at $0.*$//;     # Gets rid of croak generated stuff

                # Print just the Java stuff
                print "$@\n";

                # This is the actual NumberFormatException object
                my $exception_object = $java->get_exception;

                # Theres also this new convenience routines to give
                #       the Stack Trace as an array of lines
                # This returns the Stack Trace from the most recent
                #       Exception thrown
                my @stack_trace = $java->get_stack_trace;

                local($") = "\n";
                print "Stack Trace:\n@st\n";


So in this example if the scalar $some_string did NOT contain a parsable integer - say ’dd’ - the printed error message would be:

        java.lang.Exception: java.lang.NumberFormatException: dd

        Stack Trace:
        java.lang.Exception: java.lang.NumberFormatException: dd
                at Dealer.callFunction(
                at Dealer.parse(

You can most likely ignore all of the ’Dealer’ stack frames as that is internal to JavaServer. Of course dumping Stack Traces should only be used while you’re debugging anyways!

Comparing Java objects

The ’==’ operator is now overloaded to provide this functionality! Woohoo! So you can now say stuff like:

        if ($object1 == $object2)
                #Theyre the same!

Here’s the old (other) way of doing the exact same thing:

You can see if two references to java objects actually point to the same object by using the ’same’ function like:

        if ($object1->same($object2))
                # Theyre the same!
                # Nope, not the same

You’ll see why this is useful in the next section ’Events’.


Events are passed from the remote JVM to Perl5 via a separate event port. To enable events on an object use the ’do_event’ function. Your callback function will receive the object that caused the event as its first parameter and the event object itself as the second parameter. Here’s where ya wanna use the ’same’ function (or the new overloaded ’==’ operator) to see what object caused this event if you set up multiple objects to call the same event function.

For example:

        my $frame = $java->create_object("java.awt.Frame","Title");
        my $button = $java->create_object("java.awt.Button","Push Me");

To stop listening for events do:


Where: - $frame is the object for which you’d like to receive events - addWindowListener specifies the types of events you want to listen for - \&event_handler is your event callback routing that will handle these events

You will keep receiving events you registered for until you make a remove call or your Java object goes away (out of scope, you destroy it, whatever).

Note the second parameter MUST be of the form:

        "<add | remove><Event Type>Listener"

Default <Event Types> are:


Swing <Event Types> are:


And within most of these <Event Types> there are a number of specific events. Check out the Java event docs if you don’t know what I’m talking about...

Here’s what an event handler looks like:

        sub event_handler
                my($object,$event) = @_;
                if ($object->same($frame))      # Old sytle
                if ($object == $frame)          # New style!
                        # Event caused by our frame object!
                        # This will get this events ID value
                        my $event_id = $event->getID->get_value;

                        # Get value for a WINDOW_CLOSING event
                        my $closing_id = $java->get_field("java.awt.event.WindowEvent","WINDOW_CLOSING")->get_value;

                        if ($event_id == $closing_id)
                                # Close our frame @ user request
                if ($object->same($button))     # old style
                if ($object == $button)         # new style!
                        print "You Pushed My Button!\n";

Note return values from event handlers are ignored by BUT are returned from the Event Loop as you’ll see in a bit.

Note also how I had to call ’get_value’ to get the actualy integer values of the ’getID’ function return value and the field value of WINDOW_CLOSING.

Event Loops

Once you’ve set up your event handlers you must start the event loop to begin getting events - there are two ways to do this.

        1. Have handle the event loop
        2. Roll your own.’s event loop will block until an events happens - typically this is what you want but sometimes you might want more control, so I’ve decided to be nice this _one_ time & let you roll your own too.

Here’s how’s event loop works for ya:

        # Set up a bunch of events...

                my $cont = $java->go;
                last if (!defined $cont);

Note this works similarly to Tk’s event loop. Your program will now just sit & respond to events via your event handlers. Also note that’s event loop only handles ONE event & then returns - the return value is whatever your event handler returned OR undef if there was an error (like you lost yer connexion to the JVM).

Here’s how you can create yer own Event Loop:

You ask for a FileHandle that represents the incoming event stream. You can then select on this FileHandle or do whatever else you want - remember this is a READ ONLY FileHandle so writing to it ain’t going to do anything. Once you get a ’line’ from this FileHandle you can (and probably should) call ’decipher_event’ & the event will be dispatched to your event handler appropriately - the return value being the return value of your event handler. This can look something like this:

        ## Roll my own event loop

        # Get event FileHandle
        my $event_file_handle = $java->get_event_FH;

        # Set up my select loop
        my $READBITS = 0;
        vec($READBITS,$event_file_handle->fileno,1) = 1;

        # Suck in lines forever & dispatch events
                my $nf = select(my $rb = $READBITS,undef,undef,undef);
                if ($nf)
                        my $event_line = <$event_file_handle>;

Note this example is EXACTLY what’s ’go’ function does - if you roll yer own Event Loop you prolly want to do something more interesting than this!

The upshot is you’ll probably just want to use the ’go’ function but if you’ve got some other FileHandles going on & you don’t want to block on just this one you can (and should) use the ’roll your own’ method.


Using the ’authfile’ key when creating the root Java object specifies a file whose first line is taken to be a password to be passed to the remote JavaServer to authenticate the connexion. JavaServer must be started with the ’--authfile=<filename>’ command-line option and the first line of that file must match to be granted access. Note this is a _very_ basic form of authorization - to maximize it you should restrict the file permissions as much as possible (i.e. 0600). Thanks to Achim Settelmeier for the initial implementation!


None by default.


Mark Ethan Trostler,


perl(1). Any sorta Java documentation you can get yer hands on!


Copyright (c) 2000-2003 Mark Ethan Trostler

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


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