Xcode Build Rules

Most Xcode project types support build rules, which allow developers to modify how existing file types are built, and to define how new file types are built. The J2ObjC scripts are intentionally designed to plug into build systems like Xcode.

The j2objc-sample-reversi project is an example of how to add Java sources to a simple iOS game.

A minimal build requires updating build settings and then adding an J2ObjC build rule.

Update the Build Settings

  1. Click the project in the Project Navigator to open the Project Editor and ensure the application target is selected.
  2. Click the Build Settings tab.
  3. Link the JRE emulation library (jre_emul) by adding -ljre_emul to Other Linker Flags. It should look like this:

Linking JRE Emulation Library in Xcode

  1. Click the + to and select Add User-Defined Setting.
  2. Name the setting J2OBJC_HOME and set its value to the location of J2ObjC. This should either be the folder resulting from unzipping the release zip or the j2objc/dist folder if you compiled from source and your root is j2objc.
  3. Under Search Paths modify the following:
  • Framework Search Paths add ${J2OBJC_HOME}/frameworks
  • Library Search Paths add ${J2OBJC_HOME}/lib (for each build configuration).
  • User Header Search Paths add ${J2OBJC_HOME}/include.
  1. Confirm your settings by searching for J2OBJC_HOME. You should see something similar to this:

Xcode Build Settings

Adding a J2ObjC Build Rule

  1. Determine the root directory of your Java source files, which we'll call $source-root. The root directory is the directory that contains the top package of your source files.
  • If you using git and would like to pull in your Java files from another git project, you can add a submodule tracking the project with your Java sources. For example, suppose your Xcode project (xcodeproj) is in ~/dev/MyProject then you likely have another MyProject directory with your Objective-C source. In the ~/dev/MyProject/MyProject directory run git submodule add git@github.com:user/javaproject to create a ~/dev/MyProject/MyProject/javaproject directory with source for your Java project right along side your Objective-C sources. You can then drag that folder into your Xcode project under the MyProject group, which has Xcode mirroring your filesystem. ${PROJECT_DIR}/MyProject/javaproject/src is the $source-root.

  • If your Java sources are in a group or directory in your Xcode project, the $source-root is ${PROJECT_DIR}/__group_or_directory_name__.

  • If in doubt, right click on that group or directory and select Show in Finder to see the directory and use the absolute path.

  • For example, if you have a Java package foo.bar in a directory called ~/myproject/src, that package's Java files should be in ~/myproject/src/foo/bar/**.java – that means ~/myproject/src is the root directory for your project.

  • If the Java source files are external to the Xcode project, enter the full path used when listing them in a Terminal window.

  1. Click the project in the Project Navigator to open the Project Editor and ensure the application target is selected.
  2. Click the Build Rules tab.
  3. Click the + to add a build rule.
  4. For the new rule's Process option, select "Java source files". The Using option should be "Custom script:".
  5. In the custom script text box, add the following (remember to substitute $source-root):
if [ ! -f "${J2OBJC_HOME}/j2objc" ]; then echo "J2OBJC_HOME is not correctly defined, currently set to '${J2OBJC_HOME}'"; exit 1; fi;
"${J2OBJC_HOME}/j2objc" -d ${DERIVED_FILES_DIR} -sourcepath "$source-root" --no-package-directories -g ${INPUT_FILE_PATH};
  1. In the Output Files panel, click the + button and add: ${DERIVED_FILES_DIR}/${INPUT_FILE_BASE}.h.
  2. Click the + button again, and add ${DERIVED_FILES_DIR}/${INPUT_FILE_BASE}.m.

When you are finished, the settings panel should look something like this:

Xcode Build Rules

Linking Additional Libraries

The link build step (Xcode's "Link Binary With Libraries" build phase) needs J2ObjC-specific flags, which vary depending on how your application uses translated Java classes. For a complete list see Required Link Settings. Here are a couple examples what additional libraries you may need to link:

  • To use the java.util.zip package you must link the libz.dylib library by adding -ljre_zip to your Other Linker Flags.
  • To do secure hash generation, you must add the Security Framework to your project.

Debugging Build Problems

If Xcode reports a build failure with these steps, open the Issue Navigator and click on an error to inspect the details. This will show the details of what command-line statement was executed. Here are a few common errors:

  • _Class_ cannot be resolved - Most likely, the $source-path you used was incorrect. Look for the -sourcepath option in the executed command and ensure it points to the root directory of your Java source files.

  • "JreEmulation.h" file not found - Most likely, the problem is a bad User Header Search Paths value. Search the build command for the -I option with the path; if it looks okay, copy the path (not the -I) and in a terminal window run ls with that path to verify there isn't a typo.

  • "_IOSClass_FromClass", referenced from: or "_OBJC_CLASS_$_Java" - Either the Library Search Paths is incorrect or you forgot to link the JRE emulation library by setting Other Linker Flags to -ljre_emul. You may also need to link additional libraries.

If you still have problems, ask the j2objc-discuss group.