SWT/Developer Guide

Introduction

SWT use native OSes toolkits so expect some OSes specific stuffs. The most common native toolkits for common platforms are:

• Win32 on Windows
• GTK on Linux
• Cocoa on OS X

SWT is a Java framework. In order to be able to use the native toolkits, SWT has to use a feature of Java to call native code. This feature is called Java Native Interface (JNI). JNI lets you define methods in Java with the keyword native and the behavior of this method will be delegated to some native code (C/C++ or Objective C most of the time).

Usually with JNI, you would have to write the native code manually, and run the javah command on the Java files with native method to generate header that would bind your Java code and the native code (before compiling it). You would end up with something like that:

• HelloWorld.java with a method declared as "native". This class is compiled with javac to a .class file.
• HelloWorld.h is a C header file with the signature of the native method. It can be generated by the program javah (available with the Java SDK).
• libHelloWorld.c, the native C code using system calls and JNI data structures to manipulate Java objects. This file requires to #include HelloWorld.h.

This method has several drawbacks. First, writing native code is very error prone. It is especially the case with SWT because the team chose to have a very thin layer between Java and the native toolkit. Basically they chose to create a native Java method for one native system call. Lets take the gdk_window_set_cursor GTK system call. In the file OS.java, SWT developers has to write

public static final native void gdk_window_set_cursor(long window, long cursor);

and in the file os.c

JNIEXPORT void JNICALL gdk_window_set_cursor
	(JNIEnv *env, jclass that, jintLong arg0, jintLong arg1)
{
	OS_NATIVE_ENTER(env, that, _1gdk_1window_1set_1cursor_FUNC);
	gdk_window_set_cursor((GdkWindow *)arg0, (GdkCursor *)arg1);
	OS_NATIVE_EXIT(env, that, _1gdk_1window_1set_1cursor_FUNC);
}

This is pretty straightforward and cumbersome to write manually.

A second drawback about using javah is that it does not handle 32/64 bits automatically. You have to duplicate your code if you want to handle both of them. The main difference when interacting with a 64 bits OS instead of a 32 bits one, is that pointer are 64 bits long instead of 32 bits. From a programmer point of view, Java is agnostic to the CPU's word size... except when doing native calls. So every integers representing a pointer in a system call has to mapped to Java long integer when using a 64 bits OS and mapped to a simple int when runnning on a 32 bits OS. In the previous example gdk_window_set_cursor, window and cursor are both pointers. This native method will work well with the 64 bits version of GTK, but not with the 32 bits as windows and cursor are declared as long integers. The overhead of writing both versions is heavyweight so SWT's team tackles the two drawbacks at once. Read more about it later.

JNI Generator

SWT uses a home made tool call JNI Generator to generate both .c and .h files. The source of this tool are located in the project org.eclipse.swt.tools (JNI Generation subfolder). A introductory documentation of this tool is available online. Basically, it parses the file OS.java in the current classpath and generates os.c and os.h accordingly. Whenever you change something in OS.java in your IDE, a dedicated builder (org.eclipse.swt.tools.jnibuilder) is called along with the Java compilation to update os.c and os.h.