How the Bridge Works

The term bridge refers to the fact that there is a connection between two different worlds: the Objective-C world and the Java world. To use the bridge effectively, it's helpful to understand what's happening on each side of the bridge.

The Java bridge allows you to write code in one language that references an object from the other language. For example, you can send a message to a Java object from Objective-C:

myInt = [myJavaObject numberOfItems];

And you can send a message to an Objective-C object from Java:

myInt = myObjCObject.numberOfItems();

The bridge uses two basic approaches to allow this type of communication:

• Objects of certain commonly used classes are transparently converted, or morphed , into a corresponding object on the other side of the bridge. An example would be using a Java String object in a method call that expects an Objective-C NSString. See Morphing for more information.
• For all other objects, the bridge builds a proxy object that references the actual object on the other side of the bridge that is being messaged.

Exposing Objective-C classes to Java code requires a bit more work than the converse. Because Java is a strongly typed language, it requires more information about the classes and interfaces it manipulates at compile time. Therefore, before using Objective-C classes as Java ones, a description of them has to be written and compiled. This is called wrapping the Objective-C classes. The wrapping process involves creating a Java to Objective-C Bridging Specification ( .jobs ) file--either with the WrapIt application or by hand--and processing it with a tool called bridget . See Wrapping Objective-C Frameworks for more information.

Java has a facility called the Java Native Interface, which allows a Java programmer to write methods that are implemented in another language (such as C or Objective-C). The Java bridge makes use of this capability to allow you to send messages from Java to Objective-C:

• It provides a native method for each Objective-C method that is being wrapped.
• It generates stub code that dispatches to the Objective-C method.

Suppose you have an Objective-C framework containing a class called Foo with a method called ping , as follows:

@interface Foo:NSObject
     - (void) ping;  //the actual Objective-C method
@end

To use this method from Java, you specify in your .jobs file that you want to expose the Foo class, and that its corresponding Java name should be Foo (you must also specify the package name, for example, com.yourFirm.whatever.Foo ). You also specify that you want to expose the ping method (and its Java name, if different).

The bridget tool generates a native method declaration (in Foo.java ) that looks like this:

The native keyword indicates that the code for the method is in a language other than Java.

Bridget also generates a file containing stub code, which is the actual code that gets called on the Objective-C side. This stub code lives in a file called packageName_className_ stubs.m . By convention, the stub routine for the native method has the name Java_className_methodName . The following code illustrates what such a stub routine might look like:

void Java_Foo_ping(JAVAHandle object) {
    [BRIDGEJavaHandleToId(object) ping]; //the Objective-C method call
}

Note: JAVAHandle is the way the bridge points to a Java object. BRIDGEJavaHandleToId is a function that converts the Java handle to an Objective-C id. These examples are intended to give a sense of how the bridge works. Normally, you don't need to understand these internal bridge data types in detail, unless you are writing morphing conversion routines, for example. In that case, you can refer to the bridge header files (located in /System/Developer/Java/Headers/ ) for more information.

To call this method from Java, you'd write code such as the following:

myFooObject = new Foo();
myFooObject.ping();

When you instantiate a Foo object, the Java bridge creates a proxy object on the Java side and a "real" object on the Objective-C side, which is the receiver of any messages that you send to it.

@interface Foo:NSObject
     - (void) ping:(NSString *) theString); 
@end

myFooObject.ping(myString);

Java_Foo_ping(JAVAHandle object, JAVAHandle str) {
    [BRIDGEJavaHandleToId(object) ping:JavaStringToNSString(str)];
}

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Using the Java Bridge
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