Calling the Closure Compiler from Java

Someone asked how to deal with Compiler input and output as strings on the closure-compiler-discuss Google group, so here is a code snippet that should help out:

  /**
   * @param code JavaScript source code to compile.
   * @return The compiled version of the code.
   */
  public static String compile(String code) {
    Compiler compiler = new Compiler();

    CompilerOptions options = new CompilerOptions();
    // Advanced mode is used here, but additional options could be set, too.
    CompilationLevel.ADVANCED_OPTIMIZATIONS.setOptionsForCompilationLevel(
        options);

    // To get the complete set of externs, the logic in
    // CompilerRunner.getDefaultExterns() should be used here.
    JSSourceFile extern = JSSourceFile.fromCode("externs.js",
        "function alert(x) {}");

    // The dummy input name "input.js" is used here so that any warnings or
    // errors will cite line numbers in terms of input.js.
    JSSourceFile input = JSSourceFile.fromCode("input.js", code);

    // compile() returns a Result, but it is not needed here.
    compiler.compile(extern, input, options);

    // The compiler is responsible for generating the compiled code; it is not
    // accessible via the Result.
    return compiler.toSource();
  }

This is packaged in a runnable example in CallCompilerProgrammaticallyExample.java.

Note that the recommended way of extending compiler behavior is to subclass CompilerRunner (most likely createOptions() will be the method you are interested in overriding). Your subclass should have a main() method and be set as the Main-Class in a jar. Such a jar can then be used with the -c option in the calcdeps.py utility.