Chapter 1. About this manual

This manual is mostly examples-based. It starts with two simple step-by-step examples (showing how to deal with static and dynamic libraries), which should give enough information for basic JEL usage (but don't forget to read the rest of this manual to learn how to get the top performance from JEL). The additional information can be found in API documentation.

Chapter 2. JEL design goals

The main design goal was to create light weight expression compiler generating extremely fast code. The main emphasis is the code execution time and not the compilation time (it is nevertheless small). The other goal was to make JEL language to be very close to Java language with direct access to all built-in Java data types and functions.

Chapter 3. Features

• Support for all Java data types (boolean, byte, char, short, long, int, float, double, arrays, references)

• Octal (0456) and hexadecimal (0x1FFF) literals.

• Support for all Java arithmetic operators: + (add),- (subtract), * (multiply), / (divide), % (remainder), & (bitwise and),| (bitwise or), ^ (bitwise xor),~ (bitwise complement), << (left shift), >> (right signed shift), >>> (right unsigned shift); on most of supported data types according to Java Language Specification (JLS)

• Comparison operators (==,!=,<,>=,>,<=) as defined by JLS.

• dot (".") operator on objects ("abc".length()==3).

• dot (".") operator on objects ("abc".length()==3).

• Boolean logical operators (&&,||,!) with lazy evaluation (i.e. in the expression false&&complexBooleanFunction() the function is never called).

• Conditionals (true?2:3 = 2)

• Direct access to methods and fields of Java objects.

• Method overloading according to JLS.

• Dynamic variables interface allowing to add variables to JEL namespace without supplying the class file defining them.

• Automatic unwrapping of designated objects to Java primitive types.

• Support for strings. Objects of class java.lang.String can be directly entered into expressions using double quotes, also the standard Java escape codes are parsed. Example : "a string\n\015".

• String concatenation ("a"+2+(2>3)+object = "a2false"+object.toString()).

• User definable string comparison using usual relational operators "<", "<=", ">", ">=", "==", "!=", which employs locale by default.

• User-controllable object down-casting using "(class.name)var" syntax. It is possible to assign names to classes in JEL expressions to be different from their real Java class names.

• Constants folding, extended (by default, but can be controlled) to static methods (which are automatically called at compile time) and static fields (which are considered constants).

• High performance of generated code.

Chapter 4. How to use JEL.

In this section a simple example of a program using JEL is given, and explained with references to more detailed sections of this manual. The example program evaluates the expression given on its command line (similar program exists in the distribution under the name ./samples/Calculator.java), let's follow it step by step.

    public static void main(String[] args) {
    
    // Assemble the expression
    StringBuffer expr_sb=new StringBuffer();
    for(int i=0;i<args.length;i++) {
      expr_sb.append(args[i]);
      expr_sb.append(' ');
      };
    String expr=expr_sb.toString();
  

This first part of the program is not related to JEL. It's purpose is to assemble the expression, possibly, containing spaces into the single line. This has to be done, because shells tend to tokenize parameters but we don't need it here.

    // Set up the library
    Class[] staticLib=new Class[1];
    try {
      staticLib[0]=Class.forName("java.lang.Math");
    } catch(ClassNotFoundException e) {
      // Can't be ;)) ...... in java ... ;)
    };
    Library lib=new Library(staticLib,null,null,null,null);
    try {
    lib.markStateDependent("random",null);
    } catch (NoSuchMethodException e) {
      // Can't be also
    };
  

There can be two types of names in the Library : static and virtual (dynamic).

Methods and variables of the first class are assumed (by default) to be dependent only on their arguments i.e. not to save any information from call to call (they are "stateless")... Examples are mathematical functions like sin, cos, log, constants E, PI in java.lang.Math. For such methods (fields) it does not matter how many times (when) they will be called (their value will be taken) the result will always be the same provided arguments (if they are present) are the same. Stateless methods will be evaluated by JEL at compile time if their arguments are constants (known at compile time). To define set of static functions(fields) it is needed to pass the array of Class objects, defining those functions, as the first parameter of the library constructor (see example above). Note ONLY STATIC functions of the Classes, passed in the first argument of the gnu.jel.Library constructor will be defined in the namespace. By default all static functions are considered "stateless" by JEL.

However, some static functions still save their state (in static variables) in between calls. Thus they return different results, depending on when (how many times) they are is called even if their arguments are the same. If such function is evaluated at compile time, we have troubles, because it will be evaluated only once during expression lifetime and it's state dependence will be lost. Typical example of the static function, having a state is java.lang.Math.random. JEL has special mechanism, provided by gnu.jel.Library class to mark static functions as state dependent. (see the above example to find out how it was done for the java.lang.Math.random)

The virtual functions, which are explicitly state dependent, will be discussed later in this document. The example we currently consider does not use them. However, virtual functions are, actually, most important to JEL because expression, containing all stateless functions, is a constant, it will be completely evaluated at compile time, there is absolutely no sense to evaluate such expression repeatedly (this is what JEL was designed for). Still we shall continue with this simple example as the following code is mostly independent of whether we use virtual functions or not...

    // Compile
    CompiledExpression expr_c=null;
    try {
      expr_c=Evaluator.compile(expr,lib);
    } catch (CompilationException ce) {
      System.err.print("–––COMPILATION ERROR :");
      System.err.println(ce.getMessage());
      System.err.print("                       ");
      System.err.println(expr);
      int column=ce.getColumn(); // Column, where error was found
      for(int i=0;i<column+23-1;i++) System.err.print(' ');
      System.err.println('^');
    };

      if (expr_c !=null) {
      
      // Evaluate (Can do it now any number of times FAST !!!)
      Number result=null;
      try {
	result=(Number)expr_c.evaluate(null);
      } catch (Throwable e) {
	System.err.println("Exception emerged from JEL compiled"+
			   " code (IT'S OK) :");
	System.err.print(e);
      };
  

Result of the evaluate method is always an object. JEL converts primitive numeric types into instances of corresponding Java reflection classes (read the section Making things faster to find out how to avoid this conversion). For example, a value of primitive type long will be returned as an instance of java.lang.Long class (int maps to java.lang.Integer, float to java.lang.Float, etc.). If result is an arbitrary Java object it is returned as the reference to that object.

The

try ... catch

      // Print result
      if (result==null) 
	System.out.println("void");
      else
	System.out.println(result.toString());
   };
};

Chapter 5. Using libraries

The namespace of JEL expressions is represented by gnu.jel.Library class. Its constructor:

Library(Class[]�staticLib, Class[]�dynamicLib,
Class[]�dotClasses, DVMap� resolver,
Hashtable�cnmap)

The details on usage of each of these arguments are given in a separate sections below.

The working example using all current functionality of JEL namespace is given in the examples/YourTestBed directory in the distribution. You'll want to check it after reading this section.

dynamicLib[0]=com.mycompany.MyClass.class)

public class VariableProvider {
  public double xvar;
  
  public double x() {return xvar;};
};

    // Set up library
    Class[] staticLib=new Class[1];
    try {
      staticLib[0]=Class.forName("java.lang.Math");
    } catch(ClassNotFoundException e) {
      // Can't be ;)) ...... in java ... ;)
    };

    Class[] dynamicLib=new Class[1];
    VariableProvider variables=new VariableProvider();
    Object[] context=new Object[1];
    context[0]=variables;
    dynamicLib[0]=variables.getClass();
    
    Library lib=new Library(staticLib,dynamicLib,null,null,null);
    try {
    lib.markStateDependent("random",null);
    } catch (NoSuchMethodException e) {
      // Can't be also
    };

      if (expr_c !=null) {
    
        try {
           for(int i=0;i<100;i++) {
              variables.xvar=i;      // <- Value of the variable
              System.out.println(expr_c.evaluate(context));
                               //^^^^^^^^^^^^^^^ evaluating 100 times
           };
        } catch (Throwable e) {
	   System.err.println("Exception emerged from JEL compiled"+
		              " code (IT'S OK) :");
           System.err.print(e);
        };
    };

YYY getXXXProperty(String name)

  double getDoubleProperty(String name); // YYY=double XXX=Double
  String getStringProperty(String name); // YYY=java.lang.String XXX=String

getStringProperty("x") +
  (getDoubleProperty("y")+1.0)

public String getTypeName(String  name)

YYY getXXXProperty(String name)

resolver.getTypeName("x")=="String" &&
resolver.getTypeName("y")=="Double"

getStringProperty("x")+(getDoubleProperty("y")+1.0)

resolver.getTypeName("x")!=null && 
resolver.getTypeName("x.f1")=="String" && 
resolver.getTypeName("x.f2")=="Double"

getStringProperty("x.f1")+(getDoubleProperty("x.f2")+1.0)

public Object translate(String name) {
   if (name.equals("x")) return new Integer(1);
   return name;
   };

Chapter 6. Automatic unwrapping of objects to primitive types.

This problem appears mostly when one uses dynamic variables, but may also arise in other cases. Suppose a reference to the object of the class Weight (representing a weight of a certain item) appeared in the expression. It is clear that Weight is always represented by a floating point number (although it may have other properties, like units). If the class Weight has the method

public double getValue()

To save typing (since version 0.9.3 of JEL) one may have the class Weight implement gnu.jel.reflect.Double interface. Then, the aforementioned getValue method will be called automatically by JEL (or object w will be "unwrapped" to primitive type). This unwrapping will be performed automatically when needed: one can have expressions "w+1.0" meaning "w.getValue()+1" and "w.getUnits()" both valid (in the second case w is not "unwrapped").

There are gnu.jel.reflect.* interfaces for all Java primitive types. To use the automatic unwrapping one just needs to make his classes to implement one of these interfaces.

There is a similar mechanism for strings (since version 0.9.6) and a corresponding empty interface gnu.jel.reflect.String to denote objects automatically convertible to java.lang.String by means of their .toString() method. For example, if x is of a class implementing gnu.jel.reflect.String interface the expression x+"a" will be compiled into x.toString()+"a" (otherwise this expression produces a error message). The objects automatically convertible to strings can also be supplied as arguments of methods requiring java.lang.String (usual method overloading rules apply). Still, in the current version of JEL it is impossible to cast methods of java.lang.String on such objects. That is x.substring(1) is a syntax error (unless x> itself has the .substring(int) method). This deficiency can be addressed in future.

Chapter 7. Error detection and reporting

Expressions are made by human, and making errors is the natural property of humans, consequently, JEL has to be aware of that.

There are two places, where errors can appear. First are the compilation errors, which are thrown in the form of gnu.jel.CompilationException by the gnu.jel.Evaluator.compile. These errors signal about syntax problems in the entered expressions, wrong function names, illegal types combinations, but NOT about illegal values of arguments of functions. The second source of errors is the compiled code itself, Throwables, thrown out of gnu.jel.CompiledExpression.evaluate are primarily due to the invalid values of function arguments.

Compilation errors are easy to process. Normally, you should surround compilation by the

   try {
      // ... compilation
   catch (CompilationException e) {
      // ... process and report the error
   }

Errors of the second type are appearing during the function evaluation and can not be so nicely dealt with by JEL. They depend on the actual library, supplied to the compiler. For example methods of java.lang.Math do not generate any checked exceptions at all (still, Errors are possible), but you may connect library, of functions throwing exceptions. As a general rule : exceptions thrown by functions from the library are thrown from evaluate method

Chapter 8. Making things faster

In the above text the result of the computation, returned by evaluate was always an object. While this is very flexible it is not very fast. Objects have to be allocated on heap and garbage collected. When the result of computation is the Java primitive type it can be desirable to retrieve it without creation of the object. This can be done (since the version 0.2 of JEL) with evaluateXX() family of calls (see gnu.jel.CompiledExpression. There is an evaluateXX() method for each Java primitive type, if you know what type expression has you can just call the corresponding method.

If you do not know the type of the compiled expression you can query it using getType. Be warned, that the call to wrong evaluateXX method will result in exception. Another tricky point is that JEL always selects smallest data type for constant representation. Namely, expression "1" has type byte and not int, thus in most cases you will have to query the type, and only then, call the proper evaluateXX method.

It is anyway possible to eliminate type checks at evaluation time completely. There is a version of compile method in gnu.jel.Evaluator, which allows to fix the type of the result. It directs the compiler to perform the widening conversion to the given type, before returning the result. For example: if you fix the type to be int (passing java.lang.Integer.TYPE as an argument to compile) all expressions (such as "1", "2+5", "2*2") will be evaluated by evaluate_int method of the compiled expression. Also, the attempt to evaluate "1+2L" will be rejected by compiler, asking to insert the explicit narrowing conversion (such as "(int)(1+2L)").

Chapter 9. Serialization of compiled expressions

There used to be a specialized serialization interface in JEL up to version 0.8.3. The need for such interface was dictated by the fact that JEL allowed to use constants of arbitrary reference types in expressions, which is not supported directly by the Java class file format. Starting with version 0.9 this feature was removed and now JEL generates ordinary Java class files.

To store compiled expressions into a file just grab their code with gnu.jel.Evaluator.compileBits. The code is returned as a byte array which is easy to save/restore. Then, the expression can be instantiated using gnu.jel.ImageLoader with the code

byte[] image; 
// ... code to read the JEL-generated class file into the "image" ...
CompiledExpression expression=(CompiledExpression)(ImageLoader.load(image)).newInstance();

Chapter 10. Limitations of JEL

There is one serious limitation, which should be mentioned. Actually it is not a JEL limitation but rather a limitation of the typical Java run-time

To load compiled expressions into the Java virtual machine memory JEL uses a custom java.lang.ClassLoader. While there is nothing wrong with that, setting up a classLoader is a privileged operation in Java. This means either JEL should run in a Java application (there are no security restrictions on Java applications), or , if JEL is distributed in some custom applet the applet should be signed.

Chapter 11. Summarizing remarks

I hope you found JEL useful. Don't hesitate to contact me if there are any problems with JEL, please, report BUGS, suggest tests, send me your patches,... There are still many improvements to be done.

Most current information about JEL should be available at http://galaxy.fzu.cz/JEL/.

JEL is the "free software" and is distributed to you under terms of GNU General Public License. Find the precise terms of the license in the file ./COPYING in the root of this distribution.

Please, contact the author directly if you'd like JEL to be commercially licensed to you on a different terms.