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Core Java APIs on Mac OS X

In general, the Core Java APIs behave as you would expect them to on other platforms so most of them are not discussed in this chapter. There are a couple of details concerning Preferences that you should be aware of, as discussed in “Other Tools.” Basic information on using JNI in Mac OS X is provided in “Input Methods.”

Contents:

Networking
Preferences
JNI

Networking

Mac OS X v.10.3 and beyond supports IPv6 (Internet Protocol version 6). Because J2SE 5.0 and Java 1.4.2 use IPv6 on platforms that support it, the default networking stack in Mac OS X is the IPv6 stack. You can make Java use the IPv4 stack by setting the java.net.preferIPv4Stack system property to true.

Preferences

The Preferences API is fully supported in Mac OS X, but there are two details you should be aware of to provide the best experience to users:

• The preferences files generated by the Preferences API are named com.apple.java.util.prefs. The user’s preferences file is stored in /Library/Preferences/ in their home directory (~/Library/Preferences/). The system preferences are stored in /Library/Preferences/.

• To be consistent with the Mac OS X user experience, your preferences should be available from the application menu. The com.apple.eawt.Application class provides a mechanism for doing this. See Java 1.4 API: Apple Extensions and J2SE 5.0 Apple Extensions Reference for more information.

JNI

JNI libraries are named with the library name used in the System.loadLibrary method of your Java code prefixed by lib and suffixed with .jnilib. For example, System.loadLibrary("hello") loads the library named libhello.jnilib.

If you are developing a Cocoa Java application, you need to load your JNI library using a different mechanism. If your library is called hello.jnilib, you should call System.load(NSBundle.mainBundle().pathForResource("hello", "jnilib", "Java")); Note that this assumes that your library is located in Resources/Java/.

In building your JNI libraries, you have two options. You can either build them as bundles or as dynamic shared libraries (sometimes called dylibs). If you are concerned about maintaining backward compatibility with Mac OS X version 10.0, you should build them as a bundle; otherwise you probably want to build them as a dylib. Dylibs have the added value of being able to be prebound, which speeds up the launch time of your application. They are also easier to build if you have multiple libraries to link together.

To build as a dynamic shared library, use the -dynamiclib flag. Since your .h file produced by javah includes jni.h, you need to make sure you include its source directory. Putting all of that together looks something like this:

cc -c -I/System/Library/Frameworks/JavaVM.framework/Headers sourceFile.c

cc -dynamiclib -o libhello.jnilib sourceFile.o -framework JavaVM

To build a JNI library as a bundle use the -bundle flag:

cc -bundle -I/System/Library/Frameworks/JavaVM.framework/Headers -o libName.jnilib -framework JavaVM sourceFiles

For example, if the files hello.c and hola.c contain the implementations of the native methods to be built into a dynamic shared JNI library that will be called with System.loadLibrary("hello"), you would build the resultant library, libhello.jnilib, with this code:

cc -c -I/System/Library/Frameworks/JavaVM.framework/Headers hola.c

cc -c -I/System/Library/Frameworks/JavaVM.framework/Headers hello.c

cc -dynamiclib -o libhello.jnilib hola.o hello.o -framework JavaVM

Often JNI libraries have interdependencies. For example assume the following:

• libA.jnilib contains a function foo().

• libB.jnilib needs to link against libA.jnilib to make use of foo().

Such an interdependency is not a problem if you build your JNI libraries as dynamic shared libraries, but if you build them as bundles it does not work since symbols are private to a bundle. If you need to use bundles for backward compatibility, one solution is to put the common functions into a separate dynamic shared library and link that to the bundle. For example:

1. Compile the JNI library:

   cc -g -I/System/Library/Frameworks/JavaVM.framework/Headers -c -o myJNILib.o myJNILib.c

2. Compile the file with the common functions:

   cc -g -I/System/Library/Frameworks/JavaVM.framework/Headers -c -o CommonFunctions.o CommonFunctions.c

3. Build the object file for your common functions as a dynamic shared library:

   cc -dynamiclib -o libCommonFunctions.dylib CommonFunctions.o

4. Build your JNI library as a bundle and link against the dynamic shared library with your common functions in it:

   cc -bundle -lCommonFunctions -o libMyJNILib.jnilib myJNILib.o

A complete example of calling a dynamic shared library from a bundle, including both a makefile and an Xcode project, can be found in the MyFirstJNIProject sample code. More details on JNI can be found in Tech Note TN2147: JNI Development on Mac OS X.

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