PROPOSAL: Compiletime information access (original) (raw)

rssh at gradsoft.com.ua rssh at gradsoft.com.ua
Tue Mar 24 08:02:07 PDT 2009

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

I think any compiler can load or spawn external JVM, if needed.

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.

Yes, it was Compile-Time error.

- "myMethod" can't be called because we haven't compiled it's class yet.

Or, thenks, I forgott sentence: 'Class, avaible in user classpath durong compilation time'.

(I. e. I mean call classes from classpath, not sourcepath)

- MessageFormat.format relies on the current locale, which may be different at compile time to run time.

Yes. And the same with AnnotationProcessing (which can be called in compile-time now).

- 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.

No. Compile-time error on supplying 'i' will be thrown, because parameters of Compiletime.eval() must be literal or result of compile-time call.

I.e. Compiletime.eval() is also constant expression.

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

Yes, I forgott to write sentence, that classes must be avaible at user classpath before start of compilation.

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

They will be disappeared at all.

evaluating during compilation?) and security (what security model does it operate under?).

Security model of java compiler. We already have al this potential problems with annotations processing, where any java code can be auto-discovered and run. And practice shows that this is not the biggest problem of language.

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

Are you know, that you can execute arbitrary command during compilation in current javac compiler by call System.exec from auto discovered annotation processor ?

So, nothing new here.

code without having to worry about those.

Possibility to write unmaintainable code exists.

But if think in such way, than creation of turing-complete programming language was fatal security error ;)

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/viewbug.do?bugid=4411102 http://bugs.sun.com/bugdatabase/viewbug.do?bugid=6439965 http://bugs.sun.com/bugdatabase/viewbug.do?bugid=4649007 IMPLEMENTATION URL (optional) None yet.

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