A. Sundararajan's Weblog

Script Beans - part 2

This is continuation of my earlier post on script beans. On rethinking, I've made few (minor) changes to ScriptObject and Callable. The changes are:

• Using varargs instead of Object[] arguments in few places.
• Added invoke method in ScriptObject -- this is used to directly invoke a method on a script object instead of getting a method/function valued property using get and then invoking Callable.call method. Why? Many dynamically typed languages have Smalltalk's doesNotUnderstand equivalent. i.e., if a method is not found, then a method missing handler is invoked. - like JRuby's method_missing and Groovy's invokeMethod. ScriptObject.invoke may be implemented by calling ScriptObject.get and Callable.call on result and possibly incorporating language specific method missing method as well.
• ScriptObject.put returns boolean to tell whether the put was successful or not (may be read-only property was assigned or property addition is not possible and so on).

import javax.script.ScriptException;

/**
 * Any Java object supporting this interface can be
 * accessed from scripts with "simpler" access pattern.
 * For example, a script engine may support natural
 * property/field access syntax for the properties exposed
 * via this interface. We use this interface so that we
 * can dynamically add/delete/modify fields exposed to
 * scripts. Also, script engines may expose this interface for
 * it's own objects.
 */
public interface ScriptObject {
  // special value to denote no-result -- so that
  // null could be used as proper result value
  public static final Object UNDEFINED = new Object();
  // empty object array
  public static final Object[] EMPTY_ARRAY = new Object[0];

  /*
   * Returns all property names supported by this object.
   * Property "name" is either a String or an Integer".
   */
  public Object[] getIds();

  /**
   * Get the value of the named property.
   */
  public Object get(String name);

  /**
   * Get the value of the "indexed" property. 
   * Returns UNDEFINED if the property does not exist.
   */
  public Object get(int index);

  /**
   * Set the value of the named property. Returns
   * whether the put was successful or not.
   */
  public boolean put(String name, Object value);

  /**
   * Set the value of the indexed property. Returns
   * whether the put was successful or not.
   */
  public boolean put(int index, Object value);

  /**
   * Returns whether the named property exists or not.
   */
  public boolean has(String name);

  /**
   * Returns whether the indexed property exists or not.
   */
  public boolean has(int index);

  /**
   * Deletes the named property. Returns true on success.
   */
  public boolean delete(String name);

  /**
   * Deletes the indexed property. Returns true on success.
   */
  public boolean delete(int index);

  /**
   * Call the named method on this script object.
   */
  public Object invoke(String name, Object... arguments)
      throws NoSuchMethodException, ScriptException;
}

import javax.script.ScriptException;

/**
 * This interface is used to represent "function/method" valued
 * properties in ScriptObjects.
 */
public interface Callable {
  /**
   * Call the underlying function passing the given
   * arguments and return the result.
   */
  public Object call(Object... args) throws ScriptException;
}

import javax.script.ScriptException;

/**
 * Simple dummy implementation of ScriptObject.
 */
public abstract class AbstractScriptObject 
  implements ScriptObject {

  public Object[] getIds() {
    return EMPTY_ARRAY;
  }    

  public Object get(String name) {
    return UNDEFINED;
  }

  public Object get(int index) {
    return UNDEFINED;
  }

  public boolean put(String name, Object value) {
    return false;
  }

  public boolean put(int index, Object value) {
    return false;
  }

  public boolean has(String name) {
    return false;
  }

  public boolean has(int index) {
    return false;
  }

  public boolean delete(String name) {
    return false;
  }

  public boolean delete(int index) {
    return false;
  }

  public Object invoke(String name, Object... arguments) 
    throws NoSuchMethodException, ScriptException {
    Object value =  get(name);
    if (value instanceof Callable) {
      return ((Callable)value).call(arguments);
    } else {
      throw new NoSuchMethodException(name);
    }
  }
}

( Mar 24 2007, 02:12:41 PM IST ) Permalink Comments [3] | | | | |