Java tutorial
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We also recommend that a * file or class name and description of purpose be included on the * same "printed page" as the copyright notice for easier * identification within third-party archives. * * Copyright 2016 Alibaba Group * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. * * */ package com.taobao.asm; import org.objectweb.asm.ClassReader; import org.objectweb.asm.ClassVisitor; import org.objectweb.asm.ClassWriter; import org.objectweb.asm.MethodVisitor; import org.objectweb.asm.Opcodes; import java.io.FileOutputStream; import java.io.IOException; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; public class AsmExample extends ClassLoader implements Opcodes { public static class Foo { public static void execute() { System.out.println("test changed method name"); } public static void changeMethodContent() { System.out.println("test change method"); } } public static void main(String[] args) throws IOException, IllegalArgumentException, SecurityException, IllegalAccessException, InvocationTargetException { ClassReader cr = new ClassReader(Foo.class.getName()); ClassWriter cw = new ClassWriter(cr, ClassWriter.COMPUTE_MAXS); ClassVisitor cv = new MethodChangeClassAdapter(cw); cr.accept(cv, Opcodes.ASM4); // MethodVisitor mw = cw.visitMethod(ACC_PUBLIC + ACC_STATIC, "add", "([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("this is add method print!"); // 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(0, 0); mw.visitEnd(); // gets the bytecode of the Example class, and loads it dynamically byte[] code = cw.toByteArray(); AsmExample loader = new AsmExample(); Class<?> exampleClass = loader.defineClass(Foo.class.getName(), code, 0, code.length); for (Method method : exampleClass.getMethods()) { System.out.println(method); } System.out.println("*************"); // uses the dynamically generated class to print 'Helloworld' exampleClass.getMethods()[0].invoke(null); //changeMethodContent System.out.println("*************"); exampleClass.getMethods()[1].invoke(null); //execute,?? // gets the bytecode of the Example class, and loads it dynamically FileOutputStream fos = new FileOutputStream("e:\\logs\\Example.class"); fos.write(code); fos.close(); } static class MethodChangeClassAdapter extends ClassVisitor implements Opcodes { public MethodChangeClassAdapter(final ClassVisitor cv) { super(Opcodes.ASM4, cv); } @Override public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) { if (cv != null) { cv.visit(version, access, name, signature, superName, interfaces); } } @Override public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) { if (cv != null && "execute".equals(name)) { //??execute??execute1 return cv.visitMethod(access, name + "1", desc, signature, exceptions); } if ("changeMethodContent".equals(name)) //changeMethodContent?? { MethodVisitor mv = cv.visitMethod(access, name, desc, signature, exceptions);// MethodVisitor newMethod = null; newMethod = new AsmMethodVisit(mv); //? return newMethod; } if (cv != null) { return cv.visitMethod(access, name, desc, signature, exceptions); } return null; } } static class AsmMethodVisit extends MethodVisitor { public AsmMethodVisit(MethodVisitor mv) { super(Opcodes.ASM4, mv); } @Override public void visitMethodInsn(int opcode, String owner, String name, String desc) { super.visitMethodInsn(opcode, owner, name, desc); } @Override public void visitCode() { // //?? super.visitCode(); } @Override public void visitInsn(int opcode) { //????? // if (opcode == Opcodes.RETURN) { // pushes the 'out' field (of type PrintStream) of the System class mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;"); // pushes the "Hello World!" String constant mv.visitLdcInsn("this is a modify method!"); // invokes the 'println' method (defined in the PrintStream class) mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V"); // mv.visitInsn(RETURN); } super.visitInsn(opcode); } } }