Evaluating Expression with Java 

These are some notes about evaluating expressions in Java. This is not a lesson explaining every detail from the beginning.

Evaluating Expression by Compiling and Running them

The following program was tested using the Java SE 6 JDK.

Main.java

public class Main
{
  public static void main
  ( final java.lang.String[] args ) 
  throws java.lang.Throwable
  { java.lang.System.out.printf( "%.02f%n", System.nanoTime() / 1E9 );
    java.lang.System.out.println( eval( "new java.lang.Boolean( true )" )); 
    java.lang.System.out.printf( "%.02f%n", System.nanoTime() / 1E9 );
    java.lang.System.out.println( eval( "2 + 3" )); 
    java.lang.System.out.printf( "%.02f%n", System.nanoTime() / 1E9 ); }
  
  public static boolean writeSource
  ( final java.lang.String sourcePath,
    final java.lang.String sourceCode )
  throws java.io.FileNotFoundException
  { java.io.PrintWriter writer = 
    new java.io.PrintWriter( sourcePath );
    writer.println( sourceCode );
    writer.close(); return true; }
  public static boolean compile
  ( final java.lang.String sourcePath )
  throws java.io.IOException
  { final javax.tools.JavaCompilerTool compiler = 
    javax.tools.ToolProvider.defaultJavaCompiler();
    final javax.tools.JavaFileManager manager = 
    compiler.getStandardFileManager();
    final javax.tools.JavaFileObject source = 
    manager.getFileForInput( sourcePath ); /* java.io.IOException */
    final javax.tools.JavaCompilerTool.CompilationTask task = 
    compiler.run( null, source );
    return task.getResult(); }
  public static final java.lang.Class loadExpression
  ( final java.lang.String path, final java.lang.String className )
  throws 
  java.net.MalformedURLException, 
  java.lang.ClassNotFoundException
  { /* java.net.MalformedURLException */
    final java.net.URLClassLoader myLoader = 
    new java.net.URLClassLoader
    ( new java.net.URL[]{ new java.io.File( path ).toURI().toURL() });
    /* java.lang.ClassNotFoundException */
    return java.lang.Class.forName( className, true, myLoader ); }
  public static java.lang.Object evalExpression
  ( final java.lang.Class test )
  throws 
  java.lang.NoSuchMethodException, 
  java.lang.IllegalAccessException,
  java.lang.reflect.InvocationTargetException
  { final java.lang.Class[] parameterType = null;
    /* java.lang.NoSuchMethodException */
    final java.lang.reflect.Method method = 
    test.getMethod( "expression", parameterType );
    final java.lang.Object[] argument = null; Object instance = null; 
    /* java.lang.IllegalAccessException, 
    java.lang.reflect.InvocationTargetException */
    return method.invoke( instance, argument ); }
  public static java.lang.Object eval
  ( final java.lang.String expression ) 
  throws
  java.io.FileNotFoundException, 
  java.io.IOException,
  java.net.MalformedURLException, 
  java.lang.ClassNotFoundException,
  java.lang.NoSuchMethodException, 
  java.lang.IllegalAccessException,
  java.lang.reflect.InvocationTargetException
  { final java.lang.Object result;
    final java.lang.String path = "c:/";
    final java.lang.String className = "ExpressionWrapper";
    final java.lang.String sourcePath = path + className + ".java";
    writeSource( /* to */ sourcePath, 
      "public final class " + className + "\n" +
      "{ public static java.lang.Object expression()\n" +
      "  { return " + expression + "; }}\n" );
    if( compile( sourcePath ))
    { final java.lang.Class class_ = 
      loadExpression( /* from */ path, className );
      result = evalExpression( class_ ); }
    else { result = null; }
    return result; }}

System.out

33432,94
true
33441,99
5
33443,41

Aussprachehinweis

close cloz