PROPOSAL: Compiletime information access

[Bruce Chapman]

I think the implementation of this could be far cleaner if the values 
you want were represented as static final fields not methods, which 
would make them compile time constants. This wouldn't work for exec() 
and getBindings() but those are probably better done using annotation 
processing.

The strategy would then be that the compiler would contain a custom 
implementation of javax.lang.model.TypeElement representing only this 
class. When the compile time constants were being resolved during 
compilation, this customer TypeElement with Custom VariableElements 
would be called by the compiler to get the constant value, the 
VariableElement could (with only a little magic) return the appropriate 
value for the site where the compile time constant was being copied to.

The only concern would be that the compile time constants would have non 
constant values which varied depending on where they were referenced 
from. I suspect this might be a small compiler change, the question 
would then be whether the utility of such a mechanism carried enough 
benefit to counteract the rather strange behaviour of this particular class.

It will be interesting to see what other think.

Bruce

rssh at gradsoft.com.ua wrote:
> AUTHOR:  Ruslan Shevchenko
>
> OVERVIEW:
>
>  FEATURE SUMMARY:
>
>    Add API to access compile-time context of current compiled element from
>  programs and annotation processors.
>
>  MAJOR ADVANTAGE:
>
>    Now compile time properties are unaccessible for programmer. This meaning
>  that some functionality (such as access to location of file and line) is
>  accessible only from debug version of program with technique, which is not
>  officially supported, some (such as access to time of compilation or
>   to binding context of current program location) is not avaible at all.
>
>  MAJOR DISADVANTAGE
>
>    Inaccurate usage of this feature can cause producing of unmaintable code.
>
>  ALTERNATIVES:
>
>    External preprocessing of java sources.
>
> EXAMPLES
>
> SIMPLE EXAMPLE:
>
> Example 1
>
>   if (traceCondigion) {
>     log.trace(Compiletime.getFileName()+":"+
>               Compiletime.getLineNumber());
>   }
>
> Example 2
>
>   if (system.currentTimeMillis() - Compiletime.getTimeMillis() > 86400) {
>      System.out.println("This file was compiled more than day ago");
>   }
>
> Example 3
>
>   System.out.println("Superprogram by AAA AC, version:"+
>                                       Compiletime.exec("svnversion"));
>
> ADVANCED EXAMPLE:
>
> Example 1
>   // assuming that multiline strings are implemented:
>
>   int a=0;
>   int b=1;
>   Binding bindings = Compiletime.getInstance().getCurrentBindings();
>   ScriptEngine velocity = ScriptEngineManager.get("velocity");
>   try {
>     String fname = Compiletime.getFileName();
>     int    line  = Compiletime.getLineNumber();
>     String s = velocity.eval("""
>     #if ($a==$b)
>         Something interesting here may be,
>        Are you know that value of b is $b ?
>     #else
>        no mistics here.
>     #end
>     """,
>     bindings);
>   } catch (ScriptException ex){
>      Sytem.err.println("error in inline velocity in "+fname+", "
>                        "line "+line+1+ex.getLineNumber());
>   }
>
>
> Example 2
>
>   boolean isDemo = (Compiletime.eval(Properties.class,
>                                    "getProperty","is.demo")!=null);
>
>   if (!isDemo) {
>     String key = (String)Compiletime.eval(GenerateUniqueKey.class,
>                                         "generate");
>     LoadNotDemoClass();
>   }
>
>
> DETAILS:
>
>
>
>  Add to Java Library pseudo-objects java.lang.Compiletime with access to
> compile-time properties and next signature:
>
> public class Compiletime
> {
>    /**
>     * get filename of compiled call.
>     * in case of generated source and avaible JSR-45 SMAP file
>     * return file name of translated-source.
>     **/
>    public static String getFileName();
>
>    /**
>     * get line number of compiled call.
>     * in case of generated source and avaible JSR-45 SMAP file
>     * return line number in translated-source.
>     **/
>    public static int getLineNumber();
>
>    /**
>     * get class name where this call is placed.
>     * in case of generated source and avaible JSR-45 SMAP file
>     * return class name in translated-source.
>     **/
>    public static int getClassName();
>
>    /**
>     * get method name where this call is placed.
>     * in case of generated source and avaible JSR-45 SMAP file
>     * return method name in translated-source.
>     **/
>    public static int getMethodName();
>
>
>    /**
>     * generate JSR223 bindings for given names in current compilation
>     *context.
>     *Names array must be empty or consists from string literals.
>     **/
>    public static Bindings getBindings(String ... names)
>
>
>    /**
>     * get time of compilation in miliseconds.
>     **/
>    public static long getTimeMillis();
>
>    /**
>     * Execute os command in compile-time.
>     *@command - must be a string literal or result of call of Compiletime
>     * method, otherwise compile-time error is thrown
>     **/
>    public static String exec(String command)
>    /**
>     *  call java class at compile-time.
>     *During processing this directive, compiler will
>     *1. determinate, if class available in user path.
>     *2. determinate, if exists method with appropriative number and types
>     *  of parameters.
>     *3. If such method is static - call one, otherwise
>     *  3.1 if class have default constructor - create instance of object
>     *      (otherwise throw compile-time error)
>     *  3.2. Call methid with new-created instance.
>     *  3.3. If method return some result - substitute output to result,
>     *       on exception throw compile-time error.
>     *@param classLiteral - must be a class literal for object to call.
>     *@param methodName - must be a string literal with name of method to
> call.
>     *@param parameters - parameters of method to call. Must be a literals, or
>     *    calls of Compiletime methods.
>     **/
>    public static Object eval(Class<?> classLiteral, String methodName,
> Object .. params);
>
> }
>
>
>
> COMPILATION:
>
>  During compilation calls to compile time are changed to generation of
> appriative constant expressions.
>
> String x = Compiletime.getFileName();
> changed to
> String x = "com/mycompany/package/MyClass.java";
>
> int x = Compiletime.getLinNumber();
>  changed to
> int x = 33;
>
> String x = getClassName()
>  changed to
> String x = "com.mycompany.package.MyClass";
>
>
> class X
> {
>   int p1;
>   String p2;
>
>   ...
>   public void doSomething(int x, int y)
>   {
>     int local = x+y;
>     Bindings bindings = Compiletime.getBindings();
>     evalSomeScript(bindings);
>   }
>
> }
>
>
>  will translated to
>
> class X
> {
>   int p1;
>   String p2;
>   ...
>
>   public void doSomething(int x, int y)
>   {
>     int local = x+y;
>     SimpleBinding binding=(SimpleBinging uniqueName= new SimpleBinding();
>                            uniqueName.put("this",this);
>                            uniqueName.put("p1",p1);
>                            uniqueName.put("p2",p2);
>                            uniqueName.put("x",x);
>                            uniqueName.put("y",y);
>                            uniqueName.put("local",local);
>                            uniqueName );
>     evalSomeScript(bindings);
>   }
>
>
> }
>
>  (assuming that Block Expressions for Java will be avaible. If not - it
> will be necessory create own implementation of Bindings as part of
> library).
>
> exec will be changed to Sting literal, with result of execution, i. e.
> on Unix
> String compiledBy = Compiletime.eval("whoami");
> whill be translated to 'rssh'
>
> At last eval call is translated to
>  - appropriative literal, if result of eval is primitive type or boxed
>    primitivew type.
>  - Call of code to unserialize constant byte array, which as serialized
>    during compilation
>  or throw compile-time error is object is not primitive type and not
> implements Serializable.
>
> JLS changes: current scope of JLS is not touched.
>
> Btw, may be add to JLS chapter with name 'Compilation process' where
> describe:
>  - high level description of transforming source code to JVM classes.
>  - process of automatic loading of annotation processors and when ones are
> used.
>  - this API.
>
>
> TESTING:
>
>  Special cases are:
>   *  compability with JSR45
>   *  testing of exec function is
>
> LIBRARY SUPPORT:
>   None, or adding two classes (for Bindings implementation with filling in
> constructor and Unserializing Object utility) in depends of avaibility
> of block expressions for Java
>
>  (May be exists sence add methods for retrieveing source location (i.e. file
>  name and line number retrieving) to javax.lang.model.element.Element to
>  support better diagnostics.
>
> REFLECTIVE APIS: None
>
> OTHER CHANGES: None
>
> MIGRATION: None
>
> COMPABILITY
>  None
>
> REFERENCES
>
>  http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4411102
>  http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6439965
>  http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4649007
>
> IMPLEMENTATION URL (optional)
>
>  None yet.
>
>
>
>
>
>
>