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J2ME Development using Eclipse
"From zero to mobile in minutes" |
Preprocessing
Device fragmentation is a difficult issue in the Micro Edition world. Devices are constrained in a variety of different and inconsistent ways. While there are many techniques that can be used to deal with these inconsistencies, the limited memory and processing power of many mobile devices points to a solution that occurs off-device rather than something that happens at runtime.
EclipseME provides some help in the area of device fragmentation through the inclusion of a source code "preprocessor". Using the limited processing power of the preprocessor, it is possible to do some build time source code transformations.
Enabling and Disabling Preprocessing
Use of the preprocessing functionality is an optional part of the EclipseME functionality. Preprocessing is enabled or disabled on a project-level basis, allowing significant flexibility in the structure of workspace projects.
Enabling Preprocessing
Enabling preprocessing functionality can be accomplished in two different ways. During the creation of a new Midlet Suite, preprocessing may be enabled on the first page of the wizard.
After a project has been created preprocessing may be enabled via the J2ME project context menu option.
Disabling Preprocessing
Projects with preprocessing functionality enabled, may also disable the functionality if it is no longer necessary or useful. Disabling the preprocessor is done via the project context menu, just as enablement is done. For projects with processing enabled, there will be a "Disable Preprocessing" menu item rather than an item to enable processing.
Controlling Compilation
Preprocessing controls the final compiled code using a set of preprocessor directives. The preprocessor supports the following directives inside a Java source file. All directives must follow a "//" comment that starts at the beginning of a line (whitespace is allowed left of them, but no Java code). That way, they don't interfere with normal Java compilation. Directives must not span multiple lines of code.
| Directive | Decription |
|---|---|
| #define <identifier> | Defines an identifier, thus making its value "true" when it is referenced in further expressions. |
| #undefine <identifier> | Undefines an identifier, thus making its value "false" when it is referenced in further expressions. |
| #ifdef <identifier> | The following lines are compiled only if the given identifier is defined (or undefined, in the case of an "#ifndef" directive). "#else" does exactly what your think it does. Each directive must be ultimately closed by an "#endif" directive. The "#elifdef" and "#elifndef" directives help to specify longer conditional cascades without having to nest each level. The "#if" and "#elif" directives even allow to use complex expressions. These expressions are very much like Java boolean expressions. They may consist of identifiers, parentheses and the usual "&&", "||", "^", and "!" operators. Please note that "#ifdef" and "#ifndef" don't support complex expressions. They only expect a single argument - the symbol to be checked. |
| #ifndef <identifier> | |
| #else | |
| #endif | |
| #elifdef <identifier> | |
| #elifndef <identifier> | |
| #if <expression> | |
| #elif <expression> | |
| #include <filename> | Includes the given source file at the current position. Must be terminated by "#endinclude" for technical reasons. Note that relative file names are interpreted as relative to the project root directory. |
| #endinclude |
Defining Symbols
The compilation directive expressions are dependent on the definition of a set of "symbols". These symbols are either defined/true or undefined/false. It is possible to control whether a particular symbol is defined or undefined directly via preprocessing directives or indirectly through the definition of symbol definition sets.
Direct Definitions
The #define and #undefine directives directly control whether
a particular symbol is currently defined or undefined. Using these directives, it is possible
to override the current state of a symbol from a previous setting.
Symbol Definition Sets
Symbol definition sets provide a means to define a group of related symbols with a particular name to reference that group. A definition set may then be specified for a project via the user interface to control the compilation.
Symbol definition sets are most useful for controlling definitions without the need to alter source code. For example, symbol definition sets can be used to remove debugging information to create a production build. Start by defining a "Debug" symbol definition set in which the symbol "DEBUG" has been defined. Wrap the debugging code in your project with "#ifdef DEBUG" directives. When building a production version of the code, use a different symbol definition set that does not have DEBUG defined and the debugging code will be removed.
...
// #ifdef DEBUG
System.out.println("Some debug output");
// #endif
...
Defining Symbol Definition Sets
Symbol set definitions are accessed via the EclipseME preferences. From the Window menu, choose the Preferences... menu item. Expand the J2ME category and select the Symbol Set Definitions category.
Creating a New Set
Creating a new symbol definition set is accomplished by specifying the name of the definition set and selecting the Add button. An empty set will be created.
Defining Symbols
To define a new symbol:
- Select the set via the definition set drop-down.
- Press the Add button to the right of the symbols list.
- Select the newly created default symbol name and edit the name within the cell.
Removing Symbols
To remove a symbol from the set:
- Select the set via the definition set drop-down.
- Select symbol definition to be removed.
- Press the Remove button to the right of the symbols list.
Removing a Set
If a symbol definition set is no longer necessary, it can be removed:
- Select the set via the definition set drop-down.
- Press the Remove button to the right of the definition set drop-down.
Referencing Symbol Definition Sets
Once a Symbol Definition Set has been defined, it may be referenced by a project. This is accomplished from the J2ME category of the project properties dialog.
The symbols that are defined in the set will then be used by the preprocessor directives in processing the source code.