/***
* ASM examples: examples showing how ASM can be used
* Copyright (c) 2000-2007 INRIA, France Telecom
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. Neither the name of the copyright holders nor the names of its
* contributors may be used to endorse or promote products derived from
* this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
* THE POSSIBILITY OF SUCH DAMAGE.
*/
import org.ow2.asm.ClassWriter;
import org.ow2.asm.MethodVisitor;
import org.ow2.asm.Opcodes;
import org.ow2.asm.Type;
import org.ow2.asm.commons.GeneratorAdapter;
import org.ow2.asm.commons.Method;
import java.io.FileOutputStream;
import java.io.PrintStream;
/**
* @author Eric Bruneton
*/
public class Helloworld extends ClassLoader implements Opcodes {
public static void main(final String args[]) throws Exception {
// Generates the bytecode corresponding to the following Java class:
//
// public class Example {
// public static void main (String[] args) {
// System.out.println("Hello world!");
// }
// }
// creates a ClassWriter for the Example public class,
// which inherits from Object
ClassWriter cw = new ClassWriter(0);
cw.visit(V1_1, ACC_PUBLIC, "Example", null, "java/lang/Object", null);
// creates a MethodWriter for the (implicit) constructor
MethodVisitor mw = cw.visitMethod(ACC_PUBLIC,
"<init>",
"()V",
null,
null);
// pushes the 'this' variable
mw.visitVarInsn(ALOAD, 0);
// invokes the super class constructor
mw.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V");
mw.visitInsn(RETURN);
// this code uses a maximum of one stack element and one local variable
mw.visitMaxs(1, 1);
mw.visitEnd();
// creates a MethodWriter for the 'main' method
mw = cw.visitMethod(ACC_PUBLIC + ACC_STATIC,
"main",
"([Ljava/lang/String;)V",
null,
null);
// pushes the 'out' field (of type PrintStream) of the System class
mw.visitFieldInsn(GETSTATIC,
"java/lang/System",
"out",
"Ljava/io/PrintStream;");
// pushes the "Hello World!" String constant
mw.visitLdcInsn("Hello world!");
// invokes the 'println' method (defined in the PrintStream class)
mw.visitMethodInsn(INVOKEVIRTUAL,
"java/io/PrintStream",
"println",
"(Ljava/lang/String;)V");
mw.visitInsn(RETURN);
// this code uses a maximum of two stack elements and two local
// variables
mw.visitMaxs(2, 2);
mw.visitEnd();
// gets the bytecode of the Example class, and loads it dynamically
byte[] code = cw.toByteArray();
FileOutputStream fos = new FileOutputStream("Example.class");
fos.write(code);
fos.close();
Helloworld loader = new Helloworld();
Class<?> exampleClass = loader.defineClass("Example", code, 0, code.length);
// uses the dynamically generated class to print 'Helloworld'
exampleClass.getMethods()[0].invoke(null, new Object[] { null });
// ------------------------------------------------------------------------
// Same example with a GeneratorAdapter (more convenient but slower)
// ------------------------------------------------------------------------
cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
cw.visit(V1_1, ACC_PUBLIC, "Example", null, "java/lang/Object", null);
// creates a GeneratorAdapter for the (implicit) constructor
Method m = Method.getMethod("void <init> ()");
GeneratorAdapter mg = new GeneratorAdapter(ACC_PUBLIC,
m,
null,
null,
cw);
mg.loadThis();
mg.invokeConstructor(Type.getType(Object.class), m);
mg.returnValue();
mg.endMethod();
// creates a GeneratorAdapter for the 'main' method
m = Method.getMethod("void main (String[])");
mg = new GeneratorAdapter(ACC_PUBLIC + ACC_STATIC, m, null, null, cw);
mg.getStatic(Type.getType(System.class),
"out",
Type.getType(PrintStream.class));
mg.push("Hello world!");
mg.invokeVirtual(Type.getType(PrintStream.class),
Method.getMethod("void println (String)"));
mg.returnValue();
mg.endMethod();
cw.visitEnd();
code = cw.toByteArray();
loader = new Helloworld();
exampleClass = loader.defineClass("Example", code, 0, code.length);
// uses the dynamically generated class to print 'Helloworld'
exampleClass.getMethods()[0].invoke(null, new Object[] { null });
}
}