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HelloWorld.java
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executable file
·108 lines (91 loc) · 4.23 KB
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package playground;
import act.asm.ClassWriter;
import act.asm.MethodVisitor;
import act.asm.Opcodes;
import act.asm.Type;
import act.asm.commons.GeneratorAdapter;
import act.asm.commons.Method;
import java.io.FileOutputStream;
import java.io.PrintStream;
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",
false);
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", false);
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 });
}
}