CSC/ECE 517 Summer 2008/wiki1 7 n1

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Ruby's eval can parse and execute an arbitrary string of Ruby code. Does Java have a similar facility? Provide an illustration of what this facility is good for. Compare the ease of writing such code in Ruby vs. writing equivalent code in Java.

Flavors of eval

The eval method is one of the most powerful features of Ruby. The Kernel.eval will parse and execute an arbitrary string of legal Ruby source code. To put it plainly, if your Ruby program can generate a string of valid Ruby code, the Kernel.eval method can evaluate that code. The eval method gives developers the ability to modify the runtime behavior of application.

Using eval method is straightforward in Ruby. For example: 

eval("puts \"Hello World\"") #--> Hello World

This is overly simplistic example, but illustrate the concept well. You can optionally specify a binding with the eval method. If a binding is given, the evaluation will be performed in the context of the binding. For example: 

def get_binding
  a = 1
  binding
end
a = 2
the_binding = get_binding
eval("puts a", the_binding)   #--> 1
eval("puts a")                #--> 2

Object.instance_eval

In addition to switching context with binding, the instance_eval method of Object allows you to evaluate a string or block in the context of an instance of a class. It allows you to create a block of code in any context and evaulate it later in the context of an individual instance. 

class Paginator
  def initialize
    @page = 1
  end
  def next_page
    @page += 1
  end
end
paginator = Paginator.new
paginator.next_page
paginator.instance_eval("puts @page") #=> 2

Module.class_eval

Similarly, the class_eval or module_eval method of Module allows you to evaluate a string or block in the context of a class or module.

You can use class_eval to add methods to a class as well as include other modules in a class. 

klass = Class.new
klass.class_eval("def hello() puts \"Hello World\" end")
klass.new.hello                                             #==> Hello World

Benefits of Runtime Evaluation

The evaluation facility is good for Metaprogramming. It allows developers to write programs that are modifiable at run time, and have program write programs. The simple examples shown in the previous section already demonstrate several powerful concepts, such as calling methods dynamically, and add method to a class at run time. Here are couple of real-world applications of this facility:

  • The ability for program to write programs combine with generative programming concept could save developers tons of time.
  • Development of Shell Command application similar to irb (Interactive Ruby).
  • Mathematical Expression Evaluator
  • Program that gives users the ability to write scripts that enhance its functions or dictates certain behaviors at runtime. For example, scripting character behavior in game.

Evaluation Options in Java

Unlike Ruby, Java has static typing and is a compiled language, not an interpreted language. Java does not have eval method in its standard library. However it is entirely possible to write an interpreter in Java that would provide similar function. Below are a few approaches.

You can use Hashtable with String lookup to dynamically load delegate objects. However delegate objects would still contain prewritten pieces of code that perform predetermined functions. It is nothing close to the capability of Ruby's eval method, but it works for simple application.

Java Reflection API gives you the ability to examine or modify the runtime behavior of applications. Developers can dynamically load classes, examine private members reflectively, and invoke methods from a string. However it cannot be used to parse and evaluate just any arbitrary string of Java code. One workaround is to use generative programming. Your program generates code on the fly, compile the generated source code into Java bytecodes at runtime, load the compiled class and invoke its methods dynamically. For instance, in Java 5 you can generate and write code to a file on hard disk, use sun.tools.javac.Main to invoke javac and compile the file into class file, finally using Class.forName of Reflection API to instantiate a new object. Java 6 Compiler API makes it even easier by providing JavaCompiler interface.

Another approach is to write a parser using JavaCC. JavaCC is a parser generator. As a developer, you writes a grammar specification, and JavaCC converts it to a Java program that can recognize matches to the grammar. Advanced developer can implement a programming language and translate it to work with Java. When combined with the concept of compilation at runtime and reflection, this is a powerful solution that can evaluate not only a string of Java code.

But not every developer has the knowledge, time, and resource to implement a parser using JavaCC. Fortunately there are third party script engines that run in JVM, such as BeanShell, Jpython, etc. Some of the engines utilize the approach from above, generally translating well-known scripting language into Java bytecodes at runtime. These engines often provide eval function. For example in BeanShell: 

Interpreter i = new Interpreter();
Object result = i.eval("long time = 1212807831703l; new Date(time)");

returns a Date object.

Java 6 introduces Scripting framework. It allows Java applications to host script engines. This approach is similar to the previous approach, except the framework provides a standardized method for integrating script engines. Ideally third party script engine is supported by dropping any JSR-223 compliant script engine jar in the CLASSPATH and uniformly access it from your Java applications. This mechanism is similar to the way JDBC drivers, JNDI implementations are accessed. Below is an example from Sun, loading the script engine (based on Rhino: JavaScript for Java version 1.6R2) included in Java SE 6: 

import javax.script.*;
public class EvalScript {
   public static void main(String[] args) throws Exception {
       // create a script engine manager
       ScriptEngineManager factory = new ScriptEngineManager();
       // create a JavaScript engine
       ScriptEngine engine = factory.getEngineByName("JavaScript");
       // evaluate JavaScript code from String
       engine.eval("print('Hello, World')");
   }
}

Conclusion

Java does not have function like the eval method in its standard library, but it is possible to develop a similar facility. That said, it is rather difficult. If using third party script engine, Java developers need to determine if third-party libraries are compatible with the Java version the project is currently using or will be using. And there is the risk of library dependency conflict. Once a script engine is chosen, Java developers also need to learn the API and likely new syntax used by the script engine. The learning curve is much more steeper for those choose to write a parser using JavaCC. In addition to learning JavaCC syntax, developers must have knowledge in compiler design concepts, such as top-down, parse-tree, etc. Developers must also have thorough understanding of Java language and grammar. Java 1.6 introduces Scripting for Java Platform, a powerful scripting framework, but it does not eliminate the fact developers must learn another scripting language. Consider Java 1.6 scripting framework the simplest solution, a simple program printing "Hello World" requires about 8 lines of code (see example from the previous section) as opposite to just 1 line of code in Ruby. Fortunately having frameworks like Apache Bean Framework and JRuby to bridge Java and Ruby, developers won't have to choose one or another.

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