This article discusses the differences between the Core Java APIs on Mac OS X and other platforms. 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 article. There are a couple of details concerning Preferences that you should be aware of, as discussed in “Other Tools.”
Mac OS X v10.3 and later supports IPv6 (Internet Protocol version 6). Because Java SE 6 uses 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
.
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 their home directory (~/Library/Preferences/
). The system preferences are stored in /Library/Preferences/
and are only persisted to disk if the user is an administrator.
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 J2SE 5.0 Apple Extensions Reference for more information.
It is recommended that you use the Java JNI template in Xcode as a starting point for your JNI development. This template creates a bundled Mac OS X application with a JNI library inside its Resources folder.
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
. Java HotSpot also recognizes .dylib
as a valid JNI library format as of Mac OS X v10.5.
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
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:
Compile the JNI library.
cc -g -I/System/Library/Frameworks/JavaVM.framework/Headers -c -o myJNILib.o myJNILib.c
Compile the file with the common functions.
cc -g -I/System/Library/Frameworks/JavaVM.framework/Headers -c -o CommonFunctions.o CommonFunctions.c
Build the object file for your common functions as a dynamic shared library.
cc -dynamiclib -o libCommonFunctions.dylib CommonFunctions.o
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
Note: When building JNI libraries, you need to explicitly designate the path to jni.h
. This is in /System/Library/Frameworks/JavaVM.framework/Headers/
, not /usr/include/
as on some other platforms.
Note: After you have built your JNI libraries, make sure to let Java know where they are. It is recommended that you do this by putting your libraries into your application bundle and passing in the path with the -Djava.library.path
option. It is also possible to do this by putting your libraries in /Library/Java/Extensions/
, but this is discouraged, as it breaks the encapsulation of your bundle.
A complete example of JNI development 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.
To interoperate with the Objective-C runtime from JNI, link against JavaNativeFoundation.framework, which is a sub-framework of JavaVM.framework. It contains Objective-C classes and macros to automatically set up and tear down autorelease pools, catch and re-throw Java and Cocoa exceptions, hold JNI global references in Foundation container classes, and convert object graphs of strings, numbers, lists, maps, and sets.
The Java implementation for Mac OS X includes the Java HotSpot VM runtime and the Java HotSpot client and server VMs, all from Sun. The VM options available with the Java VM in Mac OS X vary slightly from those available on other platforms. The available options are presented in Java Virtual Machine Option Reference for Mac OS X.
When calling the Runtime.exec()
method, be sure to call Runtime.exec(String[] cmdarray)
or one of its counterparts, instead of the single-string version (Runtime.exec(String command)
). Passing a string outside of an array can have unpredictable results. Runtime.exec(String[] cmdarray)
behaves as it does in a standard Unix environment.
“ JVM properties ” lists the basic properties of the Java VM in Mac OS X. You can use System.getProperties().list(System.out)
to obtain a complete list of system properties.
Note : The mrj.version
system property is still exposed by the VM in Java SE 6. Although you may still use this to determine if you are running in the Mac OS, for forward compatibility consider using the os.name
property to determine if you are running in Mac OS X. The reason is that this property may go away in future attempts to further synchronize the Apple source with the source from Sun.
Last updated: 2009-10-09