| Calling Procedural COBOL from Java | Calling Object COBOL from Java | |
This chapter describes how you can access Java objects from COBOL programs.
Micro Focus Java support enables you to send messages to Java objects from Object COBOL programs and classes. Java is supported through a Java domain in Object COBOL. The Java domain enables you to declare Java classes inside a COBOL program. You can then send messages to the Java classes. You can also send messages from Java classes to COBOL - this is covered in the chapter Calling Object COBOL from Java.
The Java domain support works by creating a COBOL proxy object for each Java object, as shown in Figure 3-1. The class itself that you declare is a proxy for the static methods of the Java class.
Figure 3-1: The COBOL Proxy
Object
The COBOL run-time system converts parameters sent with messages from COBOL data types to Java data types. If a method returns a parameter, it is converted from a Java data type to a COBOL data type.
You must have set up your COBOL and Java environments before you can use this technology; see the chapter Before You Start.
This section shows you how to code a COBOL program to call Java methods using the Object COBOL Java domain. Your program needs to have a Class-Control Section, and use the INVOKE verb each time you want to call a Java method, although it does not have to be written as an Object COBOL class.
Each Java class you want to use in a COBOL program must be declared in the Class-Control Section. You need to provide the full name of the package the class is in, prefixed by $java$. The prefix tells the COBOL run-time system to load the class from the Java domain.
For example:
class-control.
jRectangle is class "$java$java.awt.Rectangle"
.
This declares jRectangle as a COBOL proxy object for the Rectangle class in the java.awt package. The package must be available on your Java classpath or your program will fail at run time.
Each Java class has one or more constructor methods to instantiate objects. In Java, constructor methods have the same name as the class. To enable you to invoke these methods from COBOL, they are mapped to the "new" method name on the COBOL proxy object.
The different constructors on a Java class take different numbers and combinations of parameters to initialize the instance you are creating. For example, the Java Rectangle class can be instantiated in several different ways, including the two shown below in Java code:
Rectangle r1 = new Rectangle ()
// rectangle (x,y) = 0,0, width=0, height=0
Rectangle r2 = new Rectangle(4, 5, 10, 20)
// rectangle (x,y) = (4,5), width=10, height=20
The equivalent COBOL code is shown below:
working-storage section.
01 r1 object reference.
01 r2 object reference.
...
procedure division.
...
invoke jRectangle "new" returning r1
*> rectangle (x,y) = 0,0, width=0, height=0
invoke jRectangle "new" using 4, 5, 10, 20
returning r2
*> rectangle (x,y) = (4,5), width=10, height=20
The COBOL run-time system uses the number and type of parameters to call the appropriate constructor on the Java class. You must be careful to provide parameters of the types expected by the Java class, as Java is a strongly typed language. The chapter Java Data Types explains how COBOL data types are mapped on to Java data types. The copyfile javatypes.cpy also defines a set of data types for use in COBOL which correspond directly to Java data types; you are recommended to use these for transferring data between Java and COBOL.
You can call any of the methods on a Java object by sending it a message with the same name as the method. You can also call the static methods of a Java class - send the message to the classname declared in the Class-Control paragraph of your program. Method names in Java are case sensitive, so the case of the message name in COBOL must match the case of the method in Java.
Java allows method overloading - where one method name has different implementations according to the number and type of parameters passed. COBOL handles this transparently for you, so that the correct Java method is always called.
For example, the Rectangle class has three different add() methods, which take different parameters. The Java code below shows three different ways you can call the add() method on a rectangle.
Rectangle r1 = new Rectangle(0,0,0,0) ;
Point pt = new Point(6,6) ;
Rectangle r2 = new Rectangle(3,4,9,9) ;
r1.add(4,5) ; // changes r1 to smallest rectangle
// containing r1 and point 4,5
r1.add(pt) ; // changes r1 to smallest rectangle
// containing r1 and Point pt.
r1.add(r2) ; // changes r1 to union of r1 & r2
The equivalent code in COBOL looks like this:
class-control.
jRectangle is class "$java$java.awt.Rectangle"
jPoint is class "$java$java.awt.Point"
.
working-storage section.
01 r1 object reference.
01 r2 object reference.
01 pt object reference.
procedure division.
invoke jRectangle "new" returning r1
invoke jPoint "new" using 4 5 returning pt
invoke jRectangle "new" using 3 4 9 9 returning r2
invoke r1 "add" using 4 5
invoke r1 "add" using pt
invoke r1 "add" using r2
Although r2 and pt are both data items of type object reference, the COBOL run-time system determines the type of Java object represented and calls the correct Java method.
You can access the public members and static variables of a Java class by invoking "setname" and "getname" methods on the Object COBOL proxy. Variable names are case-sensitive in Java, so the case of name in your COBOL code must match the case declared in the Java code.
For example, the Java class below has public variables classVal and instVal:
public class x {
static int classVal;
int instVal;
};
This sample of COBOL code sets the static variable classVal, and then retrieves the member instVal.
$set ooctrl(-f+p)
class-control.
x is class "$Java$x"
.
working-storage section.
copy "javatypes.cpy".
01 anX object reference.
01 anInt jint.
procedure division.
invoke x "setclassVal" using by value 4
invoke x "new" returning anX
invoke anX "getinstVal" returning anInt
An exception thrown by Java is passed back to COBOL as an Object COBOL exception raised against the javaexpt class. The default exception behavior is for the COBOL run-time system to display a message warning of the exception, and then terminate. Alternatively, you can trap the exception by adding an exception handler to your COBOL program.
The instructions below assume you have first read the chapter Exception Handling Frameworks in your OO Programming with Object COBOL.
To trap Java exceptions:
class-control.
...
JavaExceptionManager is class "javaexpt"
ExceptionManager is class "exptnmgr"
Callback is class "callback"
EntryCallback is class "entrycll"
...invoke Callback "new" using anObject z"methodName"
returning aHandler
An EntryCallback looks like this:
invoke EntryCallback "new" using z"entryPointname"
returning aHandlerinvoke ExceptionManager "register"
using JavaExceptionManager aHandlerNow all Java exceptions thrown by classes you are calling through the Object COBOL Java domain get sent to your exception handler.
This is an example of a COBOL program which catches an exception thrown by a Java program:
$set ooctrl (+p-f)
program-id. ExceptionCatcher.
class-control.
SimpleClass is class "$JAVA$SimpleClass"
EntryCallback is class "entrycll"
JavaExceptionManager is class "javaexpt"
ExceptionManager is class "exptnmgr"
.
working-storage section.
01 theInstance object reference.
01 wsCallback object reference.
local-storage section.
01 filler pic x. *> dummy storage to allow the local entry
*> point to be used for the callback
linkage section.
01 lnkException object reference.
procedure division.
*>---Set up Exception handler
invoke EntryCallback "new" using z"JException"
returning wsCallback
invoke ExceptionManager "register"
using javaexceptionmanager
wsCallback
*>---Instantiate the class
invoke SimpleClass "new" returning theInstance
display "instantiated"
invoke theInstance "TestException"
display "excepted"
stop run.
entry "Jexception" using lnkException.
invoke lnkException "display"
.
The Local-Storage Section in the above program is simply to allow recursion - the COBOL run-time system treats invoking the EntryCallback as a recursive call. Without a Local-Storage Section, this will result in a run-time system error.
This is the Java code for SimpleClass:
import java.lang.* ;
public class SimpleClass {
public SimpleClass() {
}
public void TestException() throws Exception
{
Exception e = new Exception ("Test error" );
throw e;
}
}
Using the Object COBOL Java domain means that you never access Java objects directly - you are always going through a proxy as explained in the section Overview. You can obtain a pointer to the actual Java object using the "getJavaObject" method of the javasup class. You can use this in conjunction with the Java Native Interface (JNI) if you want access to Java functionality not provided by the Object COBOL Java domain.
To get a JNI pointer, invoke "getEnv" on the javasup class. The JNI pointer is a pointer to a table of functions; data type JNINativeInterface in copyfile javatypes.cpy provides a structure that makes the JNI function table easier to use, as shown in the example below:
working-storage section.
01 JEnv pointer.
01 jobject pointer.
linkage section.
01 lnk-JNINativeInterface JNINativeInterface.
*>
procedure division.
invoke javasup "getEnv" returning jEnv
*> Map the pointer passed in JEnv to the
*> JNINativeInterface structure so that we
*> can call JNI functions.
set address of lnk-JNINativeInterface to JEnv
*>
You can now call JNI functions using the names provided by the JNINativeInterface type definition. You can see an example of this in the section Example of Throwing an Exception in the chapter Calling Procedural COBOL from Java. For more information on JNI, see Sun's Java site.
For more information on the javasup class, see the section Java Domain Class Library in your Class Library Reference.
The Java run-time environment includes a garbage collector which automatically destroys unwanted objects. The garbage collector deletes any object which is not referenced by any other object. Once you have a proxy to a Java object in a COBOL program, the COBOL run-time system has a reference to the Java object which prevents the Java garbage collector from removing it.
When you have finished with a Java object, you must release the reference to it so that the garbage collector can delete the object - otherwise your application has a memory leak. To finalize the object:
invoke javaobject "finalize" returning javaobject
The returning parameter is important, as otherwise the COBOL run-time system passes the message on to the actual Java object instead of destroying the COBOL proxy. If the message is passed to the Java object, it invokes this method:
public void finalize()
This method is either inherited or implemented by all Java classes, and is called by the Java garbage collector before it destroys an object.
In the unlikely event that your Java class implements a finalize() method which returns a value, use the "delete" method to destroy the Object COBOL proxy instead:
invoke javaobject "delete" returning javaobject
Copyright © 2002 Micro Focus International Limited. All rights reserved.
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| Calling Procedural COBOL from Java | Calling Object COBOL from Java | |