Package org.apache.bsf.engines.java

# Class JavaEngine

java.lang.Object
org.apache.bsf.util.BSFEngineImpl
org.apache.bsf.engines.java.JavaEngine

All Implemented Interfaces:
  `PropertyChangeListener`, `EventListener`, `BSFEngine`

public class JavaEngine extends BSFEngineImpl
This is the interface to Java from the Bean Scripting Framework.

The Java code must be written script-style -- that is, just the body of the function, without class or method headers or footers. The JavaEngine will generate those via a "boilerplate" wrapper:

     import java.lang.*;
     import java.util.*;
     public class $$CLASSNAME$$ {
       static public Object BSFJavaEngineEntry(org.apache.bsf.BSFManager bsf) {
         // Your code will be placed here
       }
     }

$$CLASSNAME$$ will be replaced by a generated classname of the form BSFJava*, and the bsf parameter can be used to retrieve application objects registered with the Bean Scripting Framework.

If you use the placeholder string $$CLASSNAME$$ elsewhere in your script -- including within text strings -- BSFJavaEngine will replace it with the generated name of the class before the Java code is compiled.

## Hazards:

NOTE that it is your responsibility to convert the code into an acceptable Java string. If you're invoking the JavaEngine directly (as in the JSPLikeInJava example) that means \"quoting\" characters that would otherwise cause trouble.

ALSO NOTE that it is your responsibility to return an object, or null in lieu thereof!

Since the code has to be compiled to a Java classfile, invoking it involves a fair amount of computation to load and execute the compiler. We are currently making an attempt to manage that by caching the class after it has been loaded, but the indexing is fairly primitive. It has been suggested that the Bean Scripting Framework may want to support preload-and-name-script and execute-preloaded-script-by-name options to provide better control over when and how much overhead occurs.

Author:
  Joe Kesselman, Rony G. Flatscher (added BSF_Log[Factory] to allow BSF to run without org.apache.commons.logging present)

- ## Field Summary

  ### Fields inherited from class org.apache.bsf.util.BSFEngineImpl

  `classLoader, classPath, declaredBeans, lang, mgr, tempDir`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `JavaEngine()`
  Constructor.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `Object`
  `call(Object object, String method, Object[] args)`
  This is used by an application to call into the scripting engine to make a function/method call.
  `void`
  `compileScript(String source, int lineNo, int columnNo, Object script, CodeBuffer cb)`
  Default impl of compileScript - generates code that'll create a new manager, and execute the script.
  `Object`
  `eval(String source, int lineNo, int columnNo, Object oscript)`
  This is used by an application to evaluate a string containing some expression.
  `void`
  `initialize(BSFManager mgr, String lang, Vector declaredBeans)`
  initialize the engine; called right after construction by the manager.

  ### Methods inherited from class org.apache.bsf.util.BSFEngineImpl

  `apply, compileApply, compileExpr, declareBean, exec, iexec, propertyChange, terminate, undeclareBean`

  ### Methods inherited from class java.lang.Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Constructor Details

  - ### JavaEngine

    public JavaEngine()
    Constructor.

- ## Method Details

  - ### call

    public Object call(Object object, String method, Object[] args) throws BSFException
    Description copied from interface: `BSFEngine`
    This is used by an application to call into the scripting engine to make a function/method call. The "object" argument is the object whose method is to be called, if that applies. For non-OO languages, this is typically ignored and should be given as null. For pretend-OO languages such as VB, this would be the (String) name of the object. The arguments are given in the args array.
    Parameters:
      `object` - object on which to make the call
      `method` - name of the method / procedure to call
      `args` - the arguments to be given to the procedure
    Throws:
      `BSFException` - if anything goes wrong while eval'ing a BSFException is thrown. The reason indicates the problem.

  - ### compileScript

    public void compileScript(String source, int lineNo, int columnNo, Object script, CodeBuffer cb) throws BSFException
    Description copied from class: `BSFEngineImpl`
    Default impl of compileScript - generates code that'll create a new manager, and execute the script.
    Specified by:
      `compileScript` in interface `BSFEngine`
    Overrides:
      `compileScript` in class `BSFEngineImpl`
    Parameters:
      `source` - (context info) the source of this script (e.g., filename)
      `lineNo` - (context info) the line number in source for script
      `columnNo` - (context info) the column number in source for script
      `script` - the script to compile
      `cb` - the CodeBuffer to compile into
    Throws:
      `BSFException` - if anything goes wrong while compiling a BSFException is thrown. The reason indicates the problem.

  - ### eval

    public Object eval(String source, int lineNo, int columnNo, Object oscript) throws BSFException
    This is used by an application to evaluate a string containing some expression. It should store the "bsf" handle where the script can get to it, for callback purposes.

    Note that Java compilation imposes serious overhead, but in exchange you get full Java performance once the classes have been created (minus the cache lookup cost).

    Nobody knows whether javac is threadsafe. I'm going to serialize access to protect it.

    There is no published API for invoking javac as a class. There's a trick that seems to work for Java 1.1.x, but it stopped working in Java 1.2. We will attempt to use it, then if necessary fall back on invoking javac via the command line.

    Parameters:
      `source` - (context info) the source of this expression (e.g., filename)
      `lineNo` - (context info) the line number in source for expr
      `columnNo` - (context info) the column number in source for expr
      `oscript` - the expression to evaluate
    Throws:
      `BSFException` - if anything goes wrong while eval'ing a BSFException is thrown. The reason indicates the problem.

  - ### initialize

    public void initialize(BSFManager mgr, String lang, Vector declaredBeans) throws BSFException
    Description copied from class: `BSFEngineImpl`
    initialize the engine; called right after construction by the manager. Declared beans are simply kept in a vector and that's it. Subclasses must do whatever they want with it.
    Specified by:
      `initialize` in interface `BSFEngine`
    Overrides:
      `initialize` in class `BSFEngineImpl`
    Parameters:
      `mgr` - The BSFManager that's hosting this engine.
      `lang` - Language string which this engine is handling.
      `declaredBeans` - Vector of BSFDeclaredObject containing beans that should be declared into the language runtime at init time as best as possible.
    Throws:
      `BSFException` - if anything goes wrong while init'ing a BSFException is thrown. The reason indicates the problem.