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Allegro CL version 8.2
Unrevised from 8.1 to 8.2. Significant update since 8.2 release.
8.1 version

jLinker - A Dynamic Link between Lisp and Java

This document contains the following sections:

1.0 Release and update information
2.0 Introduction
3.0 Creating a Dynamically Linked Lisp/Java Application
   3.1 Calling Java
      3.1.1 Two Calling Models
      3.1.2 Data Types and Conversions
      3.1.3 Two Implementations
   3.2 Calling Style from Lisp to Java
      3.2.1 The Funcall Model
      3.2.2 The Class Model
   3.3 Dynamic Linkage Java Reference
   3.4 Initialization Functions and Variables
   3.5 Event Handling
      3.5.1 Lightweight Callback to Lisp Methods
      3.5.2 Lisp Functions to Dispatch Java Events
      3.5.3 Implemented Sub-Classes of AWT Event Handlers
   3.6 I18N Issues
   3.7 Java Applets
   3.8 Re-entrancy
   3.9 Calling Methods of Inner Classes
   3.10 Portability Issues
4.0 Installation
   4.1 Files Involved in Installing jLinker
   4.2 Dedicated Java Server
   4.3 Peer-to-Peer Interaction
   4.4 One Lisp and Several Java Client/Server connections
   4.5 Native Java Issues
      4.5.1 LD_LIBRARY_PATH
      4.5.2 Dumplisp
      4.5.3 Event Polling
      4.5.4 Miscellaneous Caveats
5.0 A Complete Code Example
6.0 Packaging Lisp applications as Java beans and servlets
   6.1 The jLinker Java Bean API
   6.2 The jLinker Servlet API
7.0 Index of classes, operators, and variables


1.0 Release and update information

The jLinker patches released in April, 2011, have been withdrawn. Assuming you have done a recent downloaded the now removed patches, jLinker is as it was before the patches. The patches which are withdrawn contained some backward-incompatible changes and it turned out that retrofiting deployed applications was not reasonably possible. The changes will be part of the 9.0 release.



2.0 Introduction

The symbols in the jLinker module are exported from the package :net.jlinker. The package also has nickname :javatools.jlinker for back-compatibility with earlier versions. This nickname is deprecated and will be removed in a future release.

The purpose of this tool is to automate the interfacing of Lisp programs to Java class libraries. Jlinker supports a socket interface that allows the Lisp and Java parts of an application to run in separate process, and even in separate hosts. Jlinker also supports a native interface (new for 8.0) that allows the Lisp and Java parts of an application to share the address space of a single process. Both interfaces are accessed through the same API and in many cases, the choice of interface does not affect the application program style.

jLinker allows dynamic, unpremeditated access to the public methods, constructors, and members of Java classes from the Lisp runtime environment.

The end result is that the Lisp application may call Java methods as if they were Lisp functions. The documentation of the Java class is all that the Lisp programmer needs to know to use the Java library effectively. For example, the Java statements

	java.awt.Canvas canvas = new java.awt.Canvas();
	canvas.setSize( new java.awt.Dimension(12, 17) );

have the Lisp equivalent

	(setf canvas (jnew "java.awt.Canvas"))
	(jcall "setSize" canvas (jnew "java.awt.Dimension" 12 17))

Remote objects are retained as long as a reference exists in the calling environment. When a Lisp reference to a remote Java object is discarded and garbage collected, a reference to the Java object is eventually eliminated. The retention of the Java object is then controlled by Java rules.

To improve the efficiency of the interface, we allow values returned by methods to be ignored or copied. This prevents the construction of a remote object on the Java side. Ignored values are applicable to values that are already known or irrelevant to the Lisp application. Copied objects are applicable when the Java object is used only for the values of its fields and not for any method invocations.

Calls from Java to Lisp are implemented by the LispCall class in Java. To facilitate some commonly used callbacks from Java to Lisp we provide some adapter and listener classes that send AWT event data to a dispatcher on the Lisp side. This framework has been suitable for all the callbacks used in the java.awt library.

We have tested the concept with several Lisp applications that use the java.awt library for the gui component of the application. The performance of the gui is comparable to a native Java application in most cases. We have demonstrated portability by running the same application on Microsoft Windows NT and Sun Solaris.

Symbols naming Lisp operators and variables associated with jLinker are in the javatools.jlinker package. You may want to use this package so symbols need not be qualified with javatools.jlinker. Do this by evaluating (use-package :javatools.jlinker). See use-package.

The jLinker module is loaded by evaluating:

(require :jlinker)

;;;  You may load additional features
;;;  (see Section 6.0 Packaging Lisp applications as Java beans and servlets)
;;;  by evaluating:

(require :jlinkent)

Note that only public methods, constructors, and members of public Java class may be accessed from Lisp. This constraint is implicit throughout this document whenever we mention a Java class, method, constructor, or member.



3.0 Creating a Dynamically Linked Lisp/Java Application


3.1 Calling Java

Once a Jlinker interface has been initialized, Java constructors and methods are called by name.

All the following examples are shown in a :case-sensitive-lower Allegro CL lisp notation. In a standard (:case-insensitive-upper Allegro CL) all Java names would need to be enclosed in string quotes.

The form

   (jconstructor 'java.util.StringTokenizer 
          'java.lang.String 'java.lang.String)

returns a reference to a constructor, and the form

   (jcall (jconstructor 'java.util.StringTokenizer 
                 'java.lang.String 'java.lang.String)
          "ABC DEF GHI " " ")

returns a reference to an instance created by the constructor. These references are ordinary Lisp objects that may be bound to variables and stored in data structures.

   (jcall (jmethod 'java.util.StringTokenizer 'countTokens)
          x)

The operator lookup functions maintain a cache so that only the first mention of a class, constructor, method or field requires a remote call.


3.1.1 Two Calling Models

We provide two calling models that may be used separately or simultaneously at the discretion of the programmer.

In the funcall model, Java classes and methods are referenced by name. This is a somewhat verbose style but is convenient for quick prototyping since all Java classes and methods are immediately available without any additional work on the Java or the Lisp side. For example, the following two statements

(setq x (jnew (jconstructor "java.util.StringTokenizer"
                            "java.lang.String" "java.lang.String")
              "a b c " " "))
(jcall (jmethod "java.util.StringTokenizer" "nextToken") x)

create an instance of the Java java.util.StringTokenizer class and call the nextToken method on the the new instance. In the funcall model, a method or constructor may be specified with an incomplete signature such as

(jconstructor "java.util.StringTokenizer" 2)

This notation specifies a constructor with two arguments. If there is only one constructor with two arguments in the Java class, then we return the constructor. Otherwise, we signal a continuable error where the desired constructor may be selected from a list of choices. A similar short-cut is possible in a call such as

(jcall "nextToken" x)

Here we are calling the (only) nextToken method with zero arguments in the class of the object x. If several methods were available, then a continuable error would again be signalled.

Incomplete signatures are very convenient during development but should be avoided in final applications since searching for methods is a slow process that may require multiple round-trips between Lisp and Java.

In the class model, the user defines Lisp classes that correspond to Java classes and Lisp functions that correspond to Java constructors and Java methods. The Lisp functions that correspond to Java class methods are generic functions specialized on Lisp classes that correspond to Java classes. The Lisp functions that correspond to Java static methods are ordinary functions. To translate the preceding java.util.StringTokenizer example to the class model, we need to make some definitions:

(def-java-class (tokenizer "java.util.StringTokenizer")
                () () () ())
(def-java-constructor tokenizer (tokenizer "java.lang.String"
                                           "java.lang.String"))
(def-java-method (next-token "nextToken") (tokenizer))

When we use these definitions, the code is more compact and Lisp-like:

(setq x (tokenizer "a b c " " "))
(next-token x)

3.1.2 Data Types and Conversions

When Lisp values are passed as arguments in a call to Java, the Lisp values are converted to the Java types specified in the method signature. The value returned by a call to Java is converted to a Lisp value following the rules in the table below.

In Java, values are automatically converted by calling the appropriate method of the LispCall class.

Table of argument conversions from Lisp to Java

Declared Java Type Allowed Lisp Type Actual Java type (or value) Conversion Note
boolean null (false)
boolean any non-nil (true)
char character char
byte integer byte truncate to 7 bits + sign
short integer short truncate to 15 bits + sign
int integer int truncate to 31 bits + sign
long integer long truncate to 63 bits + sign
float number float
double number double
java.lang.String string java.lang.String
byte[ ] sequence byte[ ] truncate to 7 bits + sign
short[ ] sequence short[ ] truncate to 15 bits + sign
int[ ] sequence int[ ] truncate to 31 bits + sign
float[ ] sequence float[ ]
double[ ] sequence double[ ]
String[ ] sequence String[ ]
reference type jwrapper Java reference in wrapper
reference type null (null)
reference type any JLWrapper

Table of argument conversions from Java to Lisp

Java Type Lisp Type (or Value)
boolean (t or nil)
byte integer
short integer
int integer
long integer
char character
String string
float double-float
double double-float
byte[ ] (array (signed-byte 8) (*))
short[ ] (array (signed-byte 16) (*))
int[ ] (array (signed-byte 32) (*))
float[ ] (array double-float (*))
double (array double-float (*))
String[ ] (array t)
null null
JLWrapper Lisp type in wrapper
reference type jwrapper

COMPATIBILITY NOTE

The conversions in the above tables are new for Allegro CL 8.0. In earlier versions of Jlinker, the types in the method signature were not used during argument conversion and the function make-immediate-object was required to coerce many Lisp types to the correct Java type. Existing calls to make-immediate-object will still produce the correct result but in all cases these calls are now redundant.


3.1.3 Two Implementations

There are two distinctly different implementations of the Jlinker interface.

In the socket implementation, the Lisp and Java parts of the application run in separate processes and may even run on separate hosts. A significant feature is that the address spaces of both parts are separate and protected. Each side of the application may be stopped and restarted without affecting the other. A disadvantage is that the speed of interactions is limited by the data transfer rate of sockets and may be subjected to indefinite network delays.

In the native implementation, the Lisp and Java parts of the application run in the same process and share the same address space. A significant feature is that calls between Lisp and Java run at the speed of foreign calls. A possible disadvantage may be that the address space may be reduced for both parts of the application. Anoter potential disadvantage is that a serious error in one part may cause the other part to crash as well.


3.2 Calling Style from Lisp to Java

The styles are funcall and class.


3.2.1 The Funcall Model

In this style, Java methods and constructors are referenced by specifying the full name class and signature of the intended method. This style is verbose, but convenient for occasional use of Java with no additional preparation.

We use the following meta notations for certain arguments:

In the cases where a compiler macro exists for a jLinker function, the compiler macro examines class-ref and method-ref arguments. If the arguments are compile-time constants which evaluate to a string or symbols, the compile-time values of these arguments are used to build the pre-load expressions.


3.2.2 The Class Model

In this style, Java methods and constructors are called by Lisp generic functions specialized on Lisp classes associated with Java classes. The Lisp functions and classes must be defined explicitly by the user.

The macro def-java-class can be used to define a Lisp class which corresponds to a Java class.

The macro def-java-constructor allows defining constructor functions to create instances of the classes defined with def-java-class. The macro def-java-method can be used to define methods. def-java-static defines static methods.

The function jclass-name-equal returns true if two argument strings name the same Java class.


3.3 Dynamic Linkage Java Reference

All the following classes and methods are defined in Java package com.franz.jlinker and supplied in the file jlinker.jar.

Starting with Version 6.5, jlinker requires Java Version 1.5 or later.

The recommended Java interface to jLinker is implemented in Java class LispCall. The class LispConnector is retained for back-compatibility but is now deprecated. Classes are documented in JavaDoc files supplied with the documentation (linked to just below). These two classes provide all the API that a Java program needs to call Lisp or to allow Lisp to call Java. The remainder of this section describes additional Java classes and methods that implement similar but deprecated interfaces retained for compatibility with older versions of ACL.

LispCall Java class documentation: the JavaDoc pages are automatically generated and do not have links to this documentation. Click here to open the LispCall documentation in a new window, and here to (usually) open in in this window.

LispConnector Java class documentation: the JavaDoc pages are automatically generated and do not have links to this documentation. Click here to open the LispConnector documentation in a new window, and here to (usually) open in in this window.

The JavaDoc index: click here to see the index of the available JavaDocs in a new browser window and here in this window (usually). Only the LispCall and LispConnector classes have JavaDoc documentation. The index in a different format is here


3.4 Initialization Functions and Variables

The functions described in this section are used to setup and query the interface between Lisp and Java.

The functions and variables are:

On MS Windows, jLinker is able to locate the Java executable and the required jar files by examining the Windows registry. Therefore, in most installations, it is not necessary to configure jLinker.

On Unix, Linux or MacOSX systems, we have not discovered a general method for discovering the location of the Java executable and libraries. Therefore some configuration is necessary. The sample file jlinker/jl-config.cl is a template that can be customized to set the jLinker configuration variables in several different situations.


3.5 Event Handling

Many Java classes customize their behavior by allowing the programmer to extend them with custom implementations of selected methods. The java.awt package makes extensive use of this facility to handle the events associated with the use of a GUI.

In a distributed computing environment, the question arises of where the custom implementations of the methods should be executed. There is a range of answers to this question and some of the possibilities are discussed in the following sections.

If the custom behavior of an extended method does not require any data from the Lisp side of the application, the method can be implemented in a pure Java extension of the class in question. The extended method may be linked to the application from Lisp.

----- Java ----------

public class MyWindowAdapter extends WindowAdapter {

  public void windowClosing(WindowEvent e) {
    e.getWindow().dispose();
  };


----- Lisp ----------

(jcall "addWindowListener" frame (jnew "MyWindowAdapter"))

The Java method may also call back to Lisp with LispCall methods.


3.5.1 Lightweight Callback to Lisp Methods

When callback methods follow a common pattern, it may be possible to implement a general function that passes enough information from Java to Lisp through a common interface.

In the case of java.awt events, this is a very reasonable approach, and we have subclassed many of the event handlers to transmit event information to Lisp in a common form where it is dispatched by Lisp functions.


(jcall (jmethod "com.franz.jlinker.JLWindowAdapter" "addTo") frame)
(jregister-handler frame :windowClosing #'(lambda (data frame &rest x)
                                            (jcall "dispose" frame)))

This approach can be extended or modified to handle a wide range of callback situations.


3.5.2 Lisp Functions to Dispatch Java Events


3.5.3 Implemented Sub-Classes of AWT Event Handlers

ActionListener

   class com.franz.jlinker.JLActionListener implements ActionListener

   Methods:
     addTo(java.awt.Button)
     addTo(java.awt.List)
     addTo(java.awt.MenuItem)
     addTo(java.awt.TextField)

   Handler arguments:
     object is argument to addTo
     event=   :actionPerformed
     longs=   { event.getModifiers() }
     strings= { event.paramString(), event.getActionCommand() }

ComponentAdapter

   class com.franz.jlinker.JLComponentAdapter extends ComponentAdapter

   Methods:
     addTo(java.awt.Component)

   Handler arguments:
     object is argument to addTo
     event=   :componentResized   :componentMoved
              :componentShown     :componentHidden
     longs=   { }
     strings= { event.paramString() }

ItemListener

   class com.franz.jlinker.JLItemListener implements ItemListener

   Methods:
     addTo(java.awt.Checkbox)
     addTo(java.awt.CheckboxMenuItem)
     addTo(java.awt.Choice)
     addTo(java.awt.ItemSelectable)
     addTo(java.awt.List) 

   Handler arguments:
     object is argument to addTo
     event=   :itemStateChanged
     longs=   { (event.getStateChange()==event.SELECTED)?1:0 }
     strings= { event.paramString(), (event.getItem()).toString() }

KeyAdapter

   class com.franz.jlinker.JLKeyAdapter extends KeyAdapter

   Methods:
     addTo(java.awt.Component)

   Handler arguments:
     object is argument to addTo
     event=   :keyTyped   :keyPressed   :keyReleased
     longs=   { event.getModifiers(), (event.isActionKey()?1:0), 
                event.getKeyCode() }
     strings= { event.paramString() }

MouseAdapter

   class com.franz.jlinker.JLMouseAdapter extends MouseAdapter

   Methods:
     addTo(java.awt.Component)

   Handler arguments:
     object is argument to addTo
     event= :mouseClicked  :mousePressed  :mouseReleased
                           :mouseEntered  :mouseExited
     longs=   { event.getModifiers(), (event.isPopupTrigger()?1:0), 
                event.getClickCount(), event.getX(), event.getY() }
     strings= { event.paramString() }

MouseMotionAdapter

   class com.franz.jlinker.JLMouseMotionAdapter extends MouseMotionAdapter

   Methods:
     addTo(java.awt.Component)

   Handler arguments:
     object is argument to addTo
     event= :mouseDragged   :mouseMoved
     longs=   { event.getModifiers(), (event.isPopupTrigger()?1:0), 
                event.getClickCount(), event.getX(), event.getY() }
     strings= { event.paramString() }

WindowAdapter

   class com.franz.jlinker.JLWindowAdapter extends WindowAdapter

   Methods:
     addTo(java.awt.Window)

   Handler arguments:
     object is argument to addTo
     event=   :windowOpened  :windowClosing    :windowClosed
                             :windowIconified  :windowDeiconified
	                     :windowActivated  :windowDeactivated
     longs=   { }  
     strings= { }

Generic Event Handler

The following code examples show parts of some of the above adapter implementations. The examples illustrate how to add a new event handler that propagates the Java event to the Lisp jregister-handler interface.

When the Java object supplied with the event is also the object registered in Lisp:

package com.franz.jlinker;
import java.awt.*;
import java.awt.event.*;


public class JLKeyAdapter extends KeyAdapter {

  // One addTo method is needed for each argument type.
  public static synchronized void addTo( Component comp ) {
    comp.addKeyListener( (KeyListener)(new JLKeyAdapter()) );
  }



  // One event method is needed for each event defined in the 
  // listener or adapter interface.
  public void keyTyped(KeyEvent e) {
    String s = { e.paramString() };
    int[] l = { e.getModifiers(), (e.isActionKey()?1:0), e.getKeyCode() };
    LispCall.dispatchEvent("keyTyped", (Object)(e.getComponent()), s, l);
  }

}

When the Java object associated with the event is not the object registered in Lisp:

package com.franz.jlinker;
import java.awt.*;
import java.awt.event.*;

public class JLActionListener implements ActionListener {

  private Object handle;

  // One addTo method is needed for each argument type.
  public static synchronized void addTo( Button comp ) {
    JLActionListener l = new JLActionListener();
    l.handle = (Object)comp;
    comp.addActionListener( (ActionListener)l );
  }



  // One event method is needed for each event defined in the 
  // listener or adapter interface.
  public void actionPerformed(ActionEvent e) {

    String[] s = { e.paramString(), e.getActionCommand() };
    int[]    l = { e.getModifiers() };
    
    LispCall.dispatchEvent("actionPerformed", handle, s, l);
  }


}



3.6 I18N Issues

Characters are converted to 16-bit positive integers for transmission and then converted back to characters using the following primitive sequences. This should yield consistent results when client and host are on the same machine.

                  Lisp                     Java

Start String      s                        s
       
Extraction     c<-(char s i)            c<-s.charAt(i)

Conversion     n<-(char-code c)         n<-(int)c

Transmit          16-bit n                 16-bit n
              
Conversion     c<-(code-char n)         c<-(char)n

Construction   s<-(make-string len)     sb<-new StringBuffer(len)
	          (setf (char s i) c)       sb.append(c)
	                                s <-sb.toString()

Result String     s                         s

3.7 Java Applets

When calling Lisp from a Java Applet, the normal mode is to advertise in Lisp and use connect() in Java.

NOTE: The behavior of the plain APPLET tag in Netscape is not reliable. The plug-in style of applet activation seems to work without problems. In Netscape this is invoked with the EMBED html tag; in Internet Explorer, the OBJECThtml tag.

When a jLinker application is running as an applet in a browser, the security measures in the browser prevent the use of run-time method lookup in the Lisp application. All methods and constructors must be named with a complete signature in the Lisp code.


3.8 Re-entrancy

One Java VM can support exactly one socket connection to Lisp (because static variables are used extensively).

If Lisp calls Java then Java may call back to Lisp before returning from the initial call, but a call to Java from the callback will block until the initial call returns. This will typically lead to a deadlock.

If Java calls Lisp then Lisp may call back to Java, but a call to Lisp from the callback will block until the initial call to Lisp returns. This will typically lead to a deadlock also.

On the Lisp side, the variable net.jlinker::*transport-timeout* may be set to a positive number. This will trigger an error when a call to Java blocks for more than the specified number of seconds. If the number is larger than most reasonable delays in Java, this should detect most deadlock situations in the Lisp code. There is no corresponding timeout feature in Java.

In the native (JNI) implementation of jlinker, there are no recursion restrictions on calls between Lisp and Java. There is a thread restriction in ACL impementations that do not use OS threads (at this time all Unix ports): Java may call Lisp only in the thread in which Lisp initially started the Java VM. Any Lisp LWP may call Java since from the Java perspective all Lisp LWPs are the same thread.


3.9 Calling Methods of Inner Classes

JLinker uses Java Reflection methods to make all the method calls requested by the Lisp application. When an application attempts to call a method of an inner class as in the example below:

(let* ((al (jnew "java.util.ArrayList"))
       (it (jcall "iterator" al)))
   (jcall "hasNext" it))

Java throws java.lang.IllegalAccessException.

Our experience shows that the accessibility of inner class methods is tested when Java reflection methods are used on them and the default accessibility of all methods is False. If the special variable *jlinker-set-accessible* is set to a non-nil value, then jLinker will automatically re-try the call after changing the accessibility of the method to True.

The application programmer can avoid the overhead of a double call by evaluating a form such as

(jcall "setAccessible" m (make-immediate-object t :boolean))

for any methods known to be implemented in inner classes.

Naturally, if Java security settings prevent access to the accessibility setting of the method, then the method simply cannot be called from Lisp. One workaround in this case is to add a Java class that implements the desired method call from Java:

public class Wrap {
  public static boolean hasNext( java.util.Iterator x ) {
    return x.hasNext();
  }
}

The Lisp code for the previous example is then:

(let* ((al (jnew "java.util.ArrayList"))
       (it (jcall "iterator" al)))
   (jstatic "hasNext" "Wrap" it))

A single wrapper class can be used to define any number of these helper methods.


3.10 Portability Issues

Lisp applications can interface to Java through both jlinker implementations with the same code. The only place where the jlinker implementation is apparent is in the call to jlinker-init. This part of the application can be made more portable with a suitable binding for *jlinker-init*.

Java applications that use only LispCall can interface to Lisp through both jlinker implementations with the same code.

Any part of the Java application that depends on the LispConnector, JavaLinkDist, and TransStruct classes can only be used with the socket implementation.

There is one important difference in the jlinker behavior that depends on the threads implementation of the Lisp image. In a Lisp implementation that uses native OS threads, Java and Lisp threads may call back and forth freely. In a Lisp implementation that does not use native OS threads, a Java application may call Lisp only from the one thread in which Lisp is running. Thus a call from Java to Lisp is possible only when Java is running in call from Lisp.

To overcome this limitation in graphic applications, the jlinker adapter and listener classes may be used to handle AWT events. These adapters and listeners queue Java events on the Java side. A Lisp process periodically polls this queue and dispatches the events to the Lisp handlers.

The mayCall() method in the LispCall class returns an integer that describes the thread callback restrictions:



4.0 Installation


4.1 Files Involved in Installing jLinker


4.2 Dedicated Java Server

When the interface is to a dedicated Java server, the interface is setup and controlled entirely from the Lisp application.

To Prepare the Environment

To Prepare the Application

  1. Start Allegro CL, and make sure that jLinker is loaded.
  2. Compile application files with preload instructions. This will start the jLinker interfaces with a call to jlinker-init and update the preload file.
  3. Optionally, start the application and exercize it to uncover dynamic class and method references.
  4. Create a class and method preload file, with a call to (jlookup :gen-preload).

To Run the Application

  1. Start Allegro CL, and make sure that jLinker is loaded.
  2. Make sure the application is loaded.
  3. Start the jLinker interfaces with a call to jlinker-init.
  4. Start the application.

4.3 Peer-to-Peer Interaction

If the jLinker interface is to an independently running Java application, the steps needed to establish the interface depend on which side initiates the interaction.

Lisp advertises, THEN, Java connects

     Advertise in a (default) file:

     Lisp: (jlinker-init :lisp-advertises [:file path] [:timeout n] ...)

     Java: LispCall.connect(path, -1, -1);


     Advertise at a pre-determined port (and host):

     Lisp: (jlinker-init :lisp-advertises :port port ... )

     Java: LispCall.connect(host, port, -1, -1);

   Lisp must advertise before Java issues the connect() call;
   otherwise the connect() call will fail. 

   To advertise for a limited time, call jlinker-init
   with :timeout n where n is
   the number of seconds to advertise.

Java keeps looking for Lisp to advertise

     Java: LispCall.connect(path, pollInterval, pollCount);

     Lisp: (jlinker-init :lisp-advertises [:lisp-file path] [:timeout n] ...)

   If Java makes the call first, then the Java program will keep
   checking every pollInterval (milliseconds) until Lisp starts or the
   count runs out.

   If Lisp has made the call first, Java will connect immediately.

Java advertises, THEN, Lisp connects

     Java: LispCall.advertise(path, host, port, -1)
        or LispCall.advertise(path, host, port, timeout)

     Lisp: (jlinker-init :java-advertises [:file path] ...)

   Java must make the call first, otherwise, the call to jlinker-init
   will fail. Java advertises a port number in the
   given file that defaults to "LispToJava.trp".

   Java may simply advertise at a pre-determined port known to the
   Lisp application.

     Java: LispCall.advertise(port, -1)
        or LispCall.advertise(port, timeout)

     Lisp: (jlinker-init :java-advertises :port port ... )

4.4 One Lisp and Several Java Client/Server connections

The function jlinker-listen sets up a process that creates a new listener every time Java makes a new connection, so that it is always possible for Java to connect to Lisp, except for a narrow time slot when Lisp is processing a new connection. In this case, the style is always for Lisp to advertise and Java to connect.

When multiple connections are active, the code for each must run in a separate Lisp process, and in the scope of a separate binding of *jlinker-connection*.


4.5 Native Java Issues

This section covers some issues that apply only to the native jlinker implementation.

On some UNIX or Linux versions, it may be necessary to modify the environment variable LD_LIBRARY_PATH to include the directory where the Java VM shared library (libjvm.so or libjvm.dylib) is located. This setting is required if the Lisp/Java application exits with the message

Error occurred during initialization of VM
Unable to load native library: libjvm.so: cannot open shared object file:

4.5.1 LD_LIBRARY_PATH

If the environment variable LD_LIBRARY_PATH is needed by the Java VM, the value must be set before Allegro CL is started. Calling (setf sys:getenv) (after Lisp has started) is not sufficient in this case. If the Lisp variable net.jlinker:*jni-ld-path-p* is set to a non-nil value we search the locations specified in LD_LIBRARY_PATH for a file named libjni.so, libjni.dll, libjvm.dylib, or if the value of *jni-ld-path-p* is a string, then a file with that name. If the file is not found, we signal a continuable error.


4.5.2 Dumplisp

When a Lisp image is saved with dumplisp and a native jlinker connection was running at the time of the dumplisp, the jlinker interface must be re-initialized again when the saved image is restarted. To re-initialize the jlinker interface, use the following sequence:

(jlookup :connection-instance :reset)
(jlinker-init :native :load t 
              :library "correct path to Java VM shared library")

4.5.3 Event Polling

In the Unix versions of Allegro CL we do not use native OS thread implementations. Consequently, Java methods can only call Lisp when called from Lisp initially. To allow GUI event callbacks to function, the listener and adapter classes described above can be used to queue AWT events in Java. The queue is polled periodically from Lisp and the events transferred to a Lisp scheduler. The following calls may be used to manage the poll behavior:

(jlinker-slot :max-interval [new-value])
   The longest interval (in seconds) between polls.
   The initial value is 0.5.
(jlinker-slot :min-interval [new-value])
   The shortest interval (in seconds) between polls.
   The initial value 0.075.
(jlinker-slot :event-group [new-value])
   The maximum number of events to dequeue at each poll.
   The initial value is 5.

4.5.4 Miscellaneous Caveats

If the Lisp application call the AWT "dispose" method for an object where "isDisplayable" is "false", the Java VM will not return to Lisp and the entire application will hang in a non-interruptible state. We recommend a form such as

(when (jcall "isDisplayable" x) (jcall "dispose" x))


5.0 A Complete Code Example

All the following classes and methods are defined in Java package com.franz.jlinker and supplied in the file jlinker.jar.

We include here a complete example of a simple program.

(in-package :user)

;;(set-case-mode :case-sensitive-lower)

(require :jlinker)

(use-package :net.jlinker)
(defpackage :net.jlinker (:nicknames :jl))

;; Make sure the required files are locally visible
;; customized copy of [Allegro directory]/jlinker/jl-config.cl
;;                    [Allegro directory]/jlinker/jlinker.jar

(load "jl-config")



(defun new-tokenizer (&optional (string "A B C D ")
				(delimiters " "))
  (jnew (jconstructor "java.util.StringTokenizer" 
	       "java.lang.String" "java.lang.String") 
	string delimiters))

(defun next-token (inst)
  (jcall (jmethod "java.util.StringTokenizer" "nextToken")
		   inst))

(defun run-tokenizer (&optional (string "A B C D ")
				(delimiters " "))

  (or (jlinker-query) (jlinker-init))
  
  (let ((inst (new-tokenizer string delimiters))
	res)
    
    (dotimes (i (jcall (jmethod "java.util.StringTokenizer" "countTokens") 
		       inst))
      (push (next-token inst)
	    res))
    
    (values inst (reverse res))))

------------------- console log: ---------------------
cl-user(4): :ld example
; Loading C:\mmWork\java\fi\java-cur\example.cl
;   Loading C:\mmWork\java\fi\java-cur\jl-config.cl
cl-user(5): (run-tokenizer)
; Fast loading from bundle code\acldns.fasl.
#<tran-struct Java IP 1004,118185548 java.util.StringTokenizer>
("A" "B" "C" "D")

	;; the following example shows how a Java error
	;; is mapped to a Lisp error

cl-user(6): (next-token *)
Error: Java error: java.util.NoSuchElementException
result= "java.util.NoSuchElementException"

Restart actions (select using :continue):
 0: Supply another value.
 1: Return to Top Level (an "abort" restart)
 2: Abort #<process Initial Lisp Listener(6d8)>
[1c] cl-user(7): :pop
cl-user(8): 

There are additional code examples in <Allegro directory>/examples/jlinker/*, including:

applet/      examples of Java applets connected to Lisp.
javabean/    examples of Java Beans connected to Lisp.
servlet/     examples of Java servlets connected to Lisp.
timecard/    a complete Lisp application using the Java
		AWT classes for the user interface.


6.0 Packaging Lisp applications as Java beans and servlets

The jLinker Java Bean API facilitates the creation of Java Bean classes that call Allegro CL functions to do the work of the Java Bean. The jLinker Servlet API facilitates the creation of Java Servlets that call Allegro CL functions to do the work of the Servlet.

The extensions are loaded with the forms

(require :jlinker)    ;; Available to all customers.
                      ;; returns NIL if jlinker is already 
                      ;; loaded. jlinker module must be
                      ;; loaded before jlinkent module.
(require :jlinkent)   

jLinker includes support for Java Servlets and Java Beans. The Java support consists of Java classes that implement communication between a Java HttpServlet and a Lisp image. The Lisp support consists of classes and functions that implement the Lisp side of the interface. We also include examples of simple servlets where the work of the servlet is performed in Lisp.


6.1 The jLinker Java Bean API

The jLinker Java Bean API facilitates the creation of Java Bean classes that call Allegro CL functions to do the work of the Java Bean.

All Lisp symbols are in package javatools.jlinker.

The example code in examples/jlinker/javabean is described in the file readme.txt.

Lisp API

See def-java-to-lisp-wrapper, gen-output-lang, and gen-java-stream.

6.2 The jLinker Servlet API

The jLinker Servlet API facilitates the creation of Java Servlets that call Allegro CL functions to do the work of the Servlet.

All Lisp symbols are in package javatools.jlinker.

Java signatures are taken from "Java Servlet API Specification - Version 2.1a - November 1998" from Sun Microsystems at http://java.sun.com/products/servlet/.

The example code in examples/jlinker/servlet is described in the file readme.txt.

Lisp API

The javatools.jlinker::servlet class is the superclass of all the Lisp implementation classes that support the servlet interface. It has slots:

The following functions and methods are defined:

The http-servlet class is a subclass of javatools.jlinker::servlet. This is the Lisp counterpart to the Java class LispHttpServlet.

This class implements dummy methods for all the Java methods in the Java class HttpServlet. User code should subclass this class and override any method definitions that are actually used by the application. The subclass must also define a value for java-classes slot.

The predefined dummy methods are:

These are classes that should be subclassed by the application. The subclass defines working methods for the above generic functions. The subclass also defines a value for the java-classes slot:

Two start-work methods are defined on instances of those classes. The argument list is (self  async-http-servlet) work request response gate and (self  multi-async-http-servlet) work request response gate.

Java API

Methods implemented in Java class LispHttpServlet.

public void init(ServletConfig config)			Java Method

The Java method invokes the Lisp function new-servlet to propagate this method call.

public void service(...)				Java Method

Handled by the Java super-class implementation.

public void destroy()					Java Method

The Java method calls the Lisp destroy method.

protected void doDelete(HttpServletRequest request,	Java Method
                        HttpServletResponse response) 
          throws ServletException;

The Java method calls the Lisp do-delete method.

protected void doGet(HttpServletRequest request,	Java Method
                     HttpServletResponse response) 
          throws ServletException;

The Java method calls the Lisp do-get method.

protected void doHead(HttpServletRequest request,	Java Method
                      HttpServletResponse response)
          throws ServletException;

The Java method calls the Lisp do-head method.

protected void doOptions(HttpServletRequest request,	Java Method
                         HttpServletResponse response)
          throws ServletException;

The Java method calls the Lisp do-options method.

protected void doPost(HttpServletRequest request,	Java Method
                      HttpServletResponse response)
          throws ServletException;

The Java method calls the Lisp do-post method.

protected void doPut(HttpServletRequest request,	Java Method
                     HttpServletResponse response)
          throws ServletException;

The Java method calls the Lisp do-put method.

protected void doTrace(HttpServletRequest request,	Java Method
                       HttpServletResponse response)
          throws ServletException;

The Java method calls the Lisp do-trace method.

Methods implemented in Java class com.franz.jlinker.JavaLinkCommon.

public static Object[] newGate()			Java Method

Return a new closed gate.

public static void testGate(Object[] gate)		Java Method

Wait for gate to open and return a String x.

	x.length()=0 if operation completed 
	x.length()>0 if error or failure, string contains message
public static Object[] lispValues			Java Method
              (res, called, min, max, firstRefP)

Utility function called by the sample implementations of LispHttpServlet and LispAsyncHttpServlet to decode the result array returned from a call to Lisp.

	res    - result array returned from Lisp
	called - the name of the Lisp function called
	min    - the minimum number of expected values
	max    - the maximum number of expected values
	firstRefP - first returned value should be a remote reference
			to a Lisp object

returned value is an array Object[2] where the first element is an Integer return code and the second element a String error message.



7.0 Index of classes, operators, and variables


Copyright (c) 1998-2012, Franz Inc. Oakland, CA., USA. All rights reserved.
Documentation for Allegro CL version 8.2. This page was not revised from the 8.1 page.
Created 2010.1.21.

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Allegro CL version 8.2
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