Java Interface Library/en

Documentation for expecco 2.1

Introduction

The Java Interface library ("Java Bridge") is used to interact with Java objects inside an external Java Virtual Machine (JVM).

The access to Java objects, classes and programs is done via a framework called "Java-Bridge", which is similar in operation to the dotNET bridge. This framework implements transparent forwarding of method (virtual function) calls to Java objects, which exist in a local or remote Java Virtual Machine (JVM). Also, return values, callBacks and exception information are passed back from the Java program to expecco. This is done by a proxy-object mechanism, which catches all function calls, wraps the arguments, sends a datagram to the other bridge side, awaits the reply and returns the result to the original caller. Thus, remote procedure calls are almost completely transparent to the Smalltalk/JavaScript code inside expecco. In your elementary code, you can write remote function calls as if they were to local objects.

Architecture

On the expecco side, the bridge consists of a number of Proxy objects, which behave like regular Smalltalk objects as seen from elementary expecco code. However, instead of performing an action when one of their methods (virtual functions) is called, they send a message over a socket connection to the Java VM (actually: to a bridge software inside the Java application) which decodes the message and sends it to the destination Java object. The same is done in reverse direction with the return value.

+---------------------+     +----------+     +----------------+      +----------------+     +---------+ 
|                     |     |          |     |                |      |                |     |   Java  |
|       expecco       |---->|  Proxy   |---->|      Bridge    |==>>==|      Bridge    |---->|  Object |
|  (elementary code)  |<----|          |<----| (expecco side) |==<<==|   (Java side)  |<----|         |
+---------------------+     +----------+     +----------------+      +----------------+     +---------+

This setup allows for transparent communication with objects inside a local or remote Java VM. This may be either utility functions which happen to be convenient for the test application (for example: file parsing, syntax analysis, protocol implementaitons or access to specific hardware via driver libraries), or the tested application in the system under test itself.

Especially, it allows for the objects of the tested application to be looked into, manipulated and for functions of it or underlying frameworks to be called.

For expecco code, we tried to make this as transparent as possible, however there are a few exceptions, when function names need to be translated (for example, because the function naming syntax is different) or due to the fact that for some operations no corresponding name exists in Smalltalk (array access, for example).

Of course, multiple such connections can coexist and be served/handled in parallel. This enables an expecco test suite to communicate with both sides of a tested client-server Java application, for example.

Notice that a similar mechanism is used in expecco to communicate with DOTNET, Qt (C++) and C applications.

 

Initializing / Releasing the Bridge

Before any communication can take place between expecco and any Java object, the Java side of the bridge has to be started, and a communication has to be established. All of the bridges classes are in the JAVA namespace. The main interface class is "Java", in the "JAVA" namespace:

    java := JAVA::Java newWithServer.

or (in JavaScript):

    java = JAVA::Java.newWithServer();

This starts the Java-side of the bridge (actually executes java as a background command), and establishes a connection to that JVM.

The bridge connection should be closed, if the bridge is no longer needed. Release the bridge with:

    java.closeBridge();

which terminates the connection.

Start and Connect Remote

The above calls automatically startup a java VM and connect to it. Alternatively, you may want to connect to an already running java program (typically, your system under test). In order for your program to be reachable, it execute the server code cound in the "JavaBridge.jar" file. To pass parameters you will typically start it from a command line or a little shell/batch script. Call:

   java -jar <PATH_TO_JAVABRIDGE.JAR> <PARAMETERS>

The "JavaBridge.jar" file is found in the installation directory of expecco at:

   \exept\expecco\packages\exept\technologyBridge\javaBridge\javaBridge_Server_Client\JavaBridge.jar

When the bridge is running on the remote machine as server (default) use newConnectedTo:port:withTimeout: to connect to it.

   JAVA::Java newConnectedTo:'myHost' port:4567 withTimeout:30.

Or if started in client mode use newWaitingForConnectOnHost:port:withTimeout:.

   JAVA::Java newWaitingForConnectOnHost:'localhost' port:4567 withTimeout:30.

For more details see API

Startup Parameters

You can use any parameter in any order. If no parameter is used then the bridge listenes on port 14014 in server mode without keepAlive.

-help

• Shows the help

-ip <aHostnameOrIP>

• In client mode this specifies the ip or hostname to connect to.

-port <aPortNumber>

• In client mode this specifies the port to connect to. In server mode this specifies the listening port.

-asClient

• The bridge will start in client mode. If not set the bridge is started in server mode.

-keepAlive

• By setting this flag the bridge will not exit after the connection has closed and starts to connect or listen again.

Loading Applications

By accessing a classloader, additional classes, or applications as contained in JAR files can be loaded.

This can be used to add a JAR file to the class path:

    java addJarByPath:'pathToJarFile'.

Java Package Import and Class Access

The import of required packages is done indirectly, when one of the package's classes is accessed for the very first time. Or in other words, if you access a class and the package of the class is not imported, then it is for the next class access in the same package and you don't need to specify the package again for the next class you use from the same package.

This code access the class JFrame from the "javax.swing" package for the first time. The whole class name in Java would be "javax.swing.JFrame". Note that the dots are replaced by underscores and the full class name is a message send to the Java handle.

    java javax_swing_JFrame. 

After that call the JavaVM now knows the package "javax.swing" and will lookup short form class names in the set of known packages. Now we can access all classes of that package without specifying the package before the class name. For example we now want to access the class "javax.swing.JButton". The following code shows how to do this now in short form.

    java JButton. 

ATTENTION: If a classes' short form exists more than once on the Java side, then you have to specify the full name of the class. Otherwise, the first matching class will be taken and that's probably not the class you were looking for.

Instantiating a Class

Object instances are created via the "new"-message, sent to a proxy of a Java class. You can get this proxy as described in the previous section. For example we want to create a new instance of a JFrame and a JButton.

    frame := java javax_swing_JFrame new.
    button := java JButton new. 

The first statement results in a proxy object for the JFrame instance, this also does the import of the "javax.swing" package indirectly. The second line results in a proxy for a new JButton object.

Calling Methods

A method call is done by a message send to a proxy object where the message is the method name to call. For example to call the "setVisible(boolean isVisible)" method on a JFrame object. First instantiate such an object and send the message to that proxy.

    frame setVisible:true.

To call a method with more than one parameter like "setSize(int width,int height)" on a JFrame we can write this.

    frame setSize:300 _:200.

but just as well:

    frame setSize:300 anyWord:200.

or any other selector with "setSize:" as its first component. The selector translation mechanism simply takes the first part as the java selector. In practice, you would take a reasonably descriptive name, such as: "frame setSize:300 height:200".

Or for a method with 4 parameters it could look like this.

    frame setBounds:100 y:50 width:300 height:200.

Also here the "y: width: height:" can be named as you want, but the first part must be "setBounds:".

Calling Static Methods

Calling a static method on an object is not different from calling non static methods. In most cases you will call a static method not on an object but directly on the class. This is done by just calling the method on the class object. For example we want to call the static method "isDefaultLookAndFeelDecorated()" of the JFrame class.

    java javax_swing_JFrame isDefaultLookAndFeelDecorated.

or if the package is already known:

    java JFrame isDefaultLookAndFeelDecorated.

Accessing Fields

Accessing a field of a Java object or class is not different from calling methods. To access a field, send a message where the message is the field name. To access the field of a class the field must be static. For example we want to get the value of the static field "EXIT_ON_CLOSE" of the JFrame class.

    value := java javax_swing_JFrame EXIT_ON_CLOSE.

or if the package is already known:

    value := java JFrame EXIT_ON_CLOSE.

The variable "value" now holds the integer value of the "EXIT_ON_CLOSE" constant.

Let us imagine that a JFrame object would have a field named "myField". To access this field, had to write something like:

    value := frame myField.

and "value" now holding whatever "myField" returns. This could be another object reference or any primitive value.

Callbacks from Java

Sometimes we want to execute a small piece of smalltalk code during the execution of the Java code. For example, we may want to install a Smalltalk observer to be notified when a button on a Java GUI was pressed. The following code registers a Smalltalk block as a callback for a mouse press event of a button:

    listener := MouseListener new.
    listener mousePressed:[ Transcript showCR:'Hello from Java' ].

This above creates the callback and the listener, which can now be registered on a button:

    button addMouseListener:listener.

Now, with the press of the button in the Java GUI, the Smalltalk block will be executed and writes "Hello from Java" on the Smalltalk console.

Another example, using an observer would look like:

    observer := java Observer new.
    observer update:[:sourceObservable :argument| 
        argument notNil ifTrue:[ Transcript showCR:('I was notifyed with: ',argument toString) ]
        ifFalse:[ Transcript showCR:'I was notifyed and the argument was nil' ].
    ].
    myObservable addObserver:observer.

When the observable calls "notifyObservers" the Smalltalk block will be executed.

Exceptions in Java

Exceptions on the Java side are signalled back to the Smalltalk side and raise a corresponding exception there. However, on the Java side, the call stack which lead to the exception has already been unwound at that time (Java exceptions are not proceedable). So a proceed in the Smalltalk-side exception handler only affects the Smalltalk caller, but may leave the Java side in an undefined state.

Examples

This example creates a JFrame with one button inside. Then a Mouselistener is created and registered on the button. When the button is pressed, the title of the window changes to "onPressed" and the Transcript shows the message "onPressed". When the button is released, the title of the window is changed to "onReleased" and the Transcript shows "onReleased". If the button lost the mouse focus, the bridge will exit (disconnect), and the Java side is terminated.

    |java frame button listener|
 
    "/getting a bridge
    java := JAVA::JavaBridge newWithServer.

    "/creating a new frame and button object
    frame := java javax_swing_JFrame new.
    button := java JButton new:'Click'.

    "/creating a new mouse listener and adding callback blocks
    listener := java MouseListener new.
    listener mousePressed:[ Transcript showCR:'onPress'. frame setTitle:'onPressed' ].
    listener mouseReleased:[ Transcript showCR:'onReleased'. frame setTitle:'onReleased' ].
    listener mouseExited:[ Transcript showCR:'onExited'. java closeBridge. ].

    "/register the mouse listener on the button and make the frame with button visible
    button addMouseListener:listener.
    frame add:button.
    frame setSize:300 y:100.
    frame setVisible: true.

Simple RMI Call

The next example is written in JavaScript syntax. It shows how an RMI host is queried for an RMI object and then the "getHello" method is called on it. The following code is not directly executable: you must replace the path to the compiled MyRemoteObject class by the directory containing the class file, and to replace the RMI host to a real one.

    var java, reg, myRemoteObject, callResult;

    // Start a local JavaVM and connect the Communication-Bridge to it
    java = JAVA::Java.singletonInstance;  

    // Add the class path where the description class of the shared RMI object can be found. RMI could also load the class from the server, but then we would need a RMI SecurityManager with download rights
    java.bridgeSide.addClassSearchPath("my\added\class\path"); 

    // java.rmi.registry.LocateRegistry gets us the RMI Registry on the host
    reg = java.java_rmi_registry_LocateRegistry.getRegistry("rmiHost",4567);  

    // get the rmi object
    myRemoteObject = reg.lookup("myObject");   

    // get the hello string that was written by the RMI server into the rmi object
    callResult = myRemoteObject.getHello();

The RMI server written in Java:

RMIServer.java:

    import java.rmi.AlreadyBoundException;
    import java.rmi.RemoteException;
    import java.rmi.registry.LocateRegistry;
    import java.rmi.registry.Registry;

    public class RMIServer {

        public static void 
        main( String[] args ) throws RemoteException, AlreadyBoundException, InterruptedException {
            int port = 4567;

            if(args.length > 0)
                port = Integer.parseInt(args[0]);
                    
            Registry reg = LocateRegistry.createRegistry(port);
            reg.bind("myObject", new MyRemoteObject("Hello from RMIServer: "+port));
            while(true) {
                Thread.sleep(100);
            }
        }
    }

The implementation of the shared RMI object written in Java:

MyRemoteObject.java
    import java.io.Serializable;
    import java.rmi.Remote;

    public class MyRemoteObject implements Remote,Serializable {
            
        private static final long serialVersionUID = 1L;
            
        private String hello;
            
        public MyRemoteObject(String hello) {
            this.hello = hello;
        }

        public String getHello(){
            return hello;
        }
    }

API

JAVA::Java

class protocol

newWithServer

• This will start a local JavaVM, running the JavaBridge, and establishes a connection on a random free port.

A new connected instance of JAVA::Java class is returned.

newWithServerForJavaPath:aJavaPathFilename

• This is the same as newWithServer but the path to the java executable can be specified. The JavaVM version must be 1.6 or higher to run the JavaBridge.

aJavaPathFilename - must be a instance of Filename

   Smalltalk: JAVA::Java newWithServerForJavaPath:('my\path\to\java.executable'asFilename).
   JavaScript: JAVA::Java.newWithServerForJavaPath("my\path\to\java.executable".asFilename());

newConnectedTo:aHhostString port:aPortNumber withTimeout:aTimeoutSeconds

• This is used to connect to a running JavaBridge on the network. The JavaBridge must be started in server mode on the remote host. After connecting a new connected instance of JAVA::Java is returned.

aHostString - the hostname or IP of the remote machine as String

aPortNumber - the port on which the remote machine is listening as Number

aTimeoutSeconds - a connect timeout in seconds as Number

   Smalltalk: JAVA::Java newConnectedTo:'myHostName' port:4567 withTimeout:30.
   JavaScript: JAVA::Java.newConnectedTo_port_withTimeout("myHostName",4567,30);

newWaitingForConnectOnHost:aListeningAddressString port:aListeningPortNumber withTimeout:aTimeoutSeconds

• This is used to wait for incomming connecions of a running JavaBridge on the network. The JavaBridge must be started in client mode on the remote host. After connecting a new connected instance of JAVA::Java is returned.

deprecated: aListeningAddressString - no longer used. Can be nil.

aListeningPortNumber - the port on which to listen for incomming connections as Number

aTimeoutSeconds - a connect timeout in seconds as Number, If nil it will wait endless.

singletonInstance

• This will call newWithServer for the first time and then always returns the same connected JAVA::Java instance until the connection was closed, then a new connected instance is returned.

exitAllInstances

• This will close all instances of JAVA::Java.

instance protocol

addJarByPath:aPath

• Adds a jar to the class path of the running JavaVM. If the jar depends on other jars or librarys you have to add them too.

aPath - the path to the jar file as String.

addLibraryByPath:aPath

• Adds a library to the library path of the running JavaVM. If the library depends on other librarys you have to add them too.

aPath - the path to the library file as String.

isConnected

• Tests if a connection is still established.

return: - true if the connection is still established, else false.

isAlive

• Tests if a connection is still responding.

return: - true if the connection is still alive, else false. Does a message round trip.

closeBridge

• This will close the bridge connection. Depending on in which mode the JavaBridge was started, the JavaVM is also closed (-keepAlive startparameter not set).

exit

• Same as closeBridge.

See Also

The DOTNET Interface Plugin & Library, which implements a likewise interface for .NET applications/libraries.

---

Back to Plugins
Back to Online Documentation.

---

Copyright © 2014-2024 eXept Software AG