Invoke method through Java Reflection API : Method « Reflection « Java

Invoke method through Java Reflection API

Invoke method through Java Reflection API
 * Copyright (c) 2000 David Flanagan.  All rights reserved.
 * This code is from the book Java Examples in a Nutshell, 2nd Edition.
 * It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied.
 * You may study, use, and modify it for any non-commercial purpose.
 * You may distribute it non-commercially as long as you retain this notice.
 * For a commercial use license, or to purchase the book (recommended),
 * visit

import java.awt.event.*;
import java.beans.*;
import java.lang.reflect.*;
import java.util.*;

 * This class represents a Method, the list of arguments to be passed to that
 * method, and the object on which the method is to be invoked. The invoke()
 * method invokes the method. The actionPerformed() method does the same thing,
 * allowing this class to implement ActionListener and be used to respond to
 * ActionEvents generated in a GUI or elsewhere. The static parse() method
 * parses a string representation of a method and its arguments.
public class Command implements ActionListener {
  Method m; // The method to be invoked

  Object target; // The object to invoke it on

  Object[] args; // The arguments to pass to the method

  // An empty array; used for methods with no arguments at all.
  static final Object[] nullargs = new Object[] {};

  /** This constructor creates a Command object for a no-arg method */
  public Command(Object target, Method m) {
    this(target, m, nullargs);

   * This constructor creates a Command object for a method that takes the
   * specified array of arguments. Note that the parse() method provides
   * another way to create a Command object
  public Command(Object target, Method m, Object[] args) { = target;
    this.m = m;
    this.args = args;

   * Invoke the Command by calling the method on its target, and passing the
   * arguments. See also actionPerformed() which does not throw the checked
   * exceptions that this method does.
  public void invoke() throws IllegalAccessException,
      InvocationTargetException {
    m.invoke(target, args); // Use reflection to invoke the method

   * This method implements the ActionListener interface. It is like invoke()
   * except that it catches the exceptions thrown by that method and rethrows
   * them as an unchecked RuntimeException
  public void actionPerformed(ActionEvent e) {
    try {
      invoke(); // Call the invoke method
    } catch (InvocationTargetException ex) { // but handle the exceptions
      throw new RuntimeException("Command: "
          + ex.getTargetException().toString());
    } catch (IllegalAccessException ex) {
      throw new RuntimeException("Command: " + ex.toString());

   * This static method creates a Command using the specified target object,
   * and the specified string. The string should contain method name followed
   * by an optional parenthesized comma-separated argument list and a
   * semicolon. The arguments may be boolean, integer or double literals, or
   * double-quoted strings. The parser is lenient about missing commas,
   * semicolons and quotes, but throws an IOException if it cannot parse the
   * string.
  public static Command parse(Object target, String text) throws IOException {
    String methodname; // The name of the method
    ArrayList args = new ArrayList(); // Hold arguments as we parse them.
    ArrayList types = new ArrayList(); // Hold argument types.

    // Convert the string into a character stream, and use the
    // StreamTokenizer class to convert it into a stream of tokens
    StreamTokenizer t = new StreamTokenizer(new StringReader(text));

    // The first token must be the method name
    int c = t.nextToken(); // read a token
    if (c != t.TT_WORD) // check the token type
      throw new IOException("Missing method name for command");
    methodname = t.sval; // Remember the method name

    // Now we either need a semicolon or a open paren
    c = t.nextToken();
    if (c == '(') { // If we see an open paren, then parse an arg list
      for (;;) { // Loop 'till end of arglist
        c = t.nextToken(); // Read next token

        if (c == ')') { // See if we're done parsing arguments.
          c = t.nextToken(); // If so, parse an optional semicolon
          if (c != ';')
          break; // Now stop the loop.

        // Otherwise, the token is an argument; figure out its type
        if (c == t.TT_WORD) {
          // If the token is an identifier, parse boolean literals,
          // and treat any other tokens as unquoted string literals.
          if (t.sval.equals("true")) { // Boolean literal
          } else if (t.sval.equals("false")) { // Boolean literal
          } else { // Assume its a string
        } else if (c == '"') { // If the token is a quoted string
        } else if (c == t.TT_NUMBER) { // If the token is a number
          int i = (int) t.nval;
          if (i == t.nval) { // Check if its an integer
            // Note: this code treats a token like "2.0" as an int!
            args.add(new Integer(i));
          } else { // Otherwise, its a double
            args.add(new Double(t.nval));
        } else { // Any other token is an error
          throw new IOException("Unexpected token " + t.sval
              + " in argument list of " + methodname + "().");

        // Next should be a comma, but we don't complain if its not
        c = t.nextToken();
        if (c != ',')
    } else if (c != ';') { // if a method name is not followed by a paren
      t.pushBack(); // then allow a semi-colon but don't require it.

    // We've parsed the argument list.
    // Next, convert the lists of argument values and types to arrays
    Object[] argValues = args.toArray();
    Class[] argtypes = (Class[]) types.toArray(new Class[argValues.length]);

    // At this point, we've got a method name, and arrays of argument
    // values and types. Use reflection on the class of the target object
    // to find a method with the given name and argument types. Throw
    // an exception if we can't find the named method.
    Method method;
    try {
      method = target.getClass().getMethod(methodname, argtypes);
    } catch (Exception e) {
      throw new IOException("No such method found, or wrong argument "
          + "types: " + methodname);

    // Finally, create and return a Command object, using the target object
    // passed to this method, the Method object we obtained above, and
    // the array of argument values we parsed from the string.
    return new Command(target, method, argValues);
  public static void main(String[] args) throws IOException {
    javax.swing.JFrame f = new javax.swing.JFrame("Command Test");
    javax.swing.JButton b1 = new javax.swing.JButton("Tick");
    javax.swing.JButton b2 = new javax.swing.JButton("Tock");
    javax.swing.JLabel label = new javax.swing.JLabel("Hello world");
    java.awt.Container pane = f.getContentPane();

    pane.add(b1, java.awt.BorderLayout.WEST);
    pane.add(b2, java.awt.BorderLayout.EAST);
    pane.add(label, java.awt.BorderLayout.NORTH);

    b1.addActionListener(Command.parse(label, "setText(\"tick\");"));
    b2.addActionListener(Command.parse(label, "setText(\"tock\");"));



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