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 http://www.davidflanagan.com/javaexamples2.
*/
import java.awt.event.*;
import java.beans.*;
import java.lang.reflect.*;
import java.io.*;
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) {
this.target = 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 != ';')
t.pushBack();
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
args.add(Boolean.TRUE);
types.add(boolean.class);
} else if (t.sval.equals("false")) { // Boolean literal
args.add(Boolean.FALSE);
types.add(boolean.class);
} else { // Assume its a string
args.add(t.sval);
types.add(String.class);
}
} else if (c == '"') { // If the token is a quoted string
args.add(t.sval);
types.add(String.class);
} 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));
types.add(int.class);
} else { // Otherwise, its a double
args.add(new Double(t.nval));
types.add(double.class);
}
} 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 != ',')
t.pushBack();
}
} 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\");"));
f.pack();
f.show();
}
}
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