PROPOSAL: Compiletime information access

[Vilya Harvey]

Wouldn't this effectively kill off jikes, or any other compiler that wasn't
itself implemented in Java?

Regardless of that, there's some potentially big problems with this that I
don't see addressed in the proposal. Consider the code snippet below:

import java.text.MessageFormat;
import java.util.Date;

class Bad {
    private static final String HELLO = "Hello #{0} at {1,time} on
{1,date}!";

    public void myMethod(int i) {
        System.out.println(MessageFormat.format(HELLO, i, new Date()));
    }

    public static void main(String[] args) {
        for (int i = 0; i < 10; ++i)
            Compiletime.eval(getClass(), "myMethod", i);
    }
}

Although it looks like valid Java, it contains a lot of bugs:

   - The arguments to Compiletime.eval (getClass() and i) aren't available
   until runtime.
   - "myMethod" can't be called because we haven't compiled it's class yet.
   - MessageFormat.format relies on the current locale, which may be
   different at compile time to run time.
   - Compiletime.eval will only be called once, although it looks like it
   should be called 10 times; then at runtime the program will iterate 10 times
   over an empty loop and exit.


The main problem is the execution of arbitrary Java code at compile time.
The code can contain dependencies on things which haven't been compiled yet,
or libraries which are dynamically loaded at runtime; worse, it may depend
on an environment which may be different between compile time and runtime
(quite likely when developing J2EE apps, for example). It also raises the
questions around verification (how do you verify classes that you are
evaluating during compilation?) and security (what security model does it
operate under?).

The other problem with the proposal is the Compiletime.exec() method.
Allowing arbitrary commands to be executed introduces serious security
implications to the compilation step. Currently, at least, you can compile
code without having to worry about those.

Finally, because it looks like ordinary Java code it's easy to confuse the
two, leading to mistakes like the loop body in the example.

In my opinion, this is more suited to a separate preprocessor. A quick
google found a few of those already available.

Vil.


2009/3/24 <rssh at gradsoft.com.ua>

>
> 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.
>
>
>
>
>
>
>