One of the goals of the application framework is to make it easier to allow applications to run as stand-alone Java command line applications, as well as run within the Eclipse IDE. The main problem faced when supporting general applications to run within Eclipse, is that such applications all run within the same instance of the Java Virtual Machine (JVM). In fact, a single application may have multiple instances running at the same time, within a single instance of the IDE. The following sections address the issues that arise when running within the IDE, and how the application framework handles them.
Within Java programs, members can be defined with the
static modifier. Since multiple instances of an application may be running simultaneously, within a single instance of the IDE, one should avoid using static variables that contain information that is specific to a single instance of the application. For instance, assume an application that maintains an integer counter, used to generate unique identifiers. If defined in a class as follows:
public static int count = 0;
and incremented when needed, the first instance of the application will run just fine. Variable
count starts at zero, and is incremented over and over again. When a second instance of the application starts however, the static variable keeps its value, as the new application is started within the same Eclipse instance, and thus within the same JVM. The count won’t start from zero, thus leading to different results for the application.
The conclusion is that one should be careful to avoid static variables that hold information specific to an application instance.
Applications often have settings, and they are generally passed as command line arguments. GUI applications however, often use a dialog to configure the options instead. To allow applications within the application framework to work in both scenarios, all applications should use the option framework.
See also the option framework section.
Command line applications generally obtain input from stdin, and write output to stdout and/or stderr streams. For applications running within the IDE, those streams are connected to the Eclipse application (IDE) as a whole, and not to the applications running within the IDE. To provide a uniform I/O interface, the application framework includes an I/O framework.
See also the I/O framework section.
The Eclipse IDE uses the Standard Widget Toolkit (SWT) for its graphical user interface (GUI). To be compatible with the Eclipse IDE, all GUI applications should use SWT as well. In order for GUI applications to work seamlessly within the Eclipse IDE as well as stand-alone, the application framework automatically registers the main SWT display thread for stand-alone applications, and uses the Eclipse SWT display thread when running within the Eclipse IDE. This reduces the burden of having to register the main SWT display threads, but also avoids blocking and other thread related issues.
Using the GUI option, the GUI can be enabled or disabled. If disabled, headless execution mode is used, which disables creation of a SWT display thread, and thus disables all GUI functionality, including the option dialog.
Within Java, the
System.exit method can be used to immediately terminate an application, by terminating the JVM. For applications running within the Eclipse IDE, this not only terminates the application, but the IDE as well. As such, the
System.exit method should never be used in applications that are intended to be executed within the IDE.
Stand-alone applications can typically be started from a the command line terminal window. Pressing Ctrl+C at such a command line terminal window terminates the currently running application (on Unix-based systems, this generates a SIGINT). Applications running within the Eclipse IDE however, don’t run in an actual command line terminal. Instead, they run within the IDE, and the stdin, stdout, and stderr streams are coupled to the Eclipse console view. The Eclipse console view does not support termination using the Ctrl+C key combination. Instead, Ctrl+C is used to copy console output to the clipboard. To remedy this situation, application framework applications running within Eclipse get a Terminate button with their console within the IDE, to allow for easy termination.
Furthermore, the application framework allows termination requests via the
AppEnv.terminate method. Application framework applications and threads should regularly call the
AppEnv.isTerminationRequested method to see whether they should terminate.
See also the termination features of the Applications view.
Exceptions are Java feature that allows applications to report error conditions. Exceptions can generally be divided into two categories: internal errors, and end-user errors. Internal errors should generally not happen, and make the application crash. The application framework provides crash reports for end users to report crashes due to internal errors. The application framework also provides exception classes for end-user errors, to provide nice error messages, instead of stack traces.
See also the exception framework section.
Java uses system properties (
System.getProperty method etc). Those properties are global to the entire JVM, meaning they are shared between applications running within the Eclipse IDE. The application framework provides functionality to maintain system properties on a per application basis, turning them into application properties. All application framework applications should use the
setProp* methods in the
AppEnv class instead of the property related methods in the
System class. This ensures that the application properties are used instead of the global system properties.
One of the standard system properties in Java is the
user.dir property, which refers to the current working directory, or more precisely, the directory from which the JVM was started. Java doesn’t allow changing the current working directory. The application framework however, maintains the current working directory on a per application basis. Changing the current working directory is also supported. Application framework applications should use the methods in the
org.eclipse.escet.common.app.framework.Paths class to get and set the current working directory, to resolve relative paths, etc. These methods also allow both Windows (
\) and Unix (
/) separators to be used in paths, on all platforms, transparently.
Furthermore, within the Eclipse IDE all projects are visible in the Project Explorer and Package Explorer views. In order to allow importing of resources from other projects etc, it may be nice to allow end users to specify platform paths (plug-in paths or workspace paths). Eclipse Modeling Framework (EMF) URIs, besides local file system paths, provide functionality for platform URIs as well. EMF URIs can for instance be used to load models that are instances of an Ecore. The
Paths class mentioned above features methods to create such EMF URIs, from various sources. Those methods also feature smart handling of
platform:/auto/... paths, an addition to platform URIs, added by the application framework. Such URIs are first resolved in the workspace, and if they can’t be found there, they are resolved in the plug-ins. This allows for easier debugging, as the workspace always overrides the plug-ins.