Generating the SWT JNI Code

All of the C code used by SWT is generated by the JNIGeneratorApp application included in the SWT Tools bundle and available on the SWT Tools Update Sites. This page describes how to use this tool when fixing bugs or adding features to SWT.

Note that this process has changed for SWT 3.5. For older versions see Generating the SWT JNI Code for SWT 3.4.

Adding native methods

Add the function prototype in OS.java, copying the style and structure of the other functions in that file. Note that for Mac/Cocoa, portions of OS.java are also generated.
Install the latest SWT Tools from the SWT Tools Update Site.
Restart Eclipse.
Use code assist templates like jni* (i.e. jnifield and jnimethod) to add javadoc tags that describe flags and casts for any primitive types. See metadata for more documentation.
Save the file. This should build the appropriate C files.
Compile the new C code and copy the new libraries to the appropriate fragment. To compile the code, right-click on the build.xml file in the Eclipse SWT PI/ws/library directory for your window system, and choose "Run As -> Ant Build...". Select the JRE tab, and check "Run in the same JRE as the workspace". Select the Refresh tab, and check "Refresh resources upon completion" to refresh your workspace after running the build to ensure Eclipse picks up the fresh binaries. Now select the "Run" button.

Adding structs

Often, methods return values or require parameters as a C struct. To add a new structure, simply add a new class in the same package as the OS.java file which matches the definition of the C structure. This will be picked up and correctly handled by JNIGeneratorApp.

If a struct is input-only or output-only, this information can be used by JNIGeneratorApp to generate more efficient C code. Use the flags attribute of the native function to indicate this in the metadata.

More Hints

For GTK+ and Motif, all native functions must be wrapped in locking code. When adding functions, simply copy the style and structure of the other native functions.
Many functions on Mac OS X return structs which use unions. For an example of how to add the markup for a union, see the ControlFontStyleRec structure and its metadata.

And there you go! If you're having trouble, please post to the SWT mailing list or file a new bug in bugzilla.