字节码ASM使用

BuildClass.java

package com.lzy.research;

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;

/**
 * Description:
 *
 * @author liuzhengyang
 * @version 1.0
 * @since 2016-04-18
 */
public class BuildClass extends ClassLoader implements Opcodes {
	public static void main(String[] args) throws IOException {
		// 定义一个类
		ClassWriter classWriter = new ClassWriter(0);
		classWriter.visit(V1_8, ACC_PUBLIC, "Example", null, "java/lang/Object", null);

		// 生成默认的构造方法
		MethodVisitor methodVisitor = classWriter.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);

		// 生成构造方法的字节码指令
		methodVisitor.visitVarInsn(ALOAD, 0);
		methodVisitor.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);
		methodVisitor.visitInsn(RETURN);
		methodVisitor.visitMaxs(1, 1);
		methodVisitor.visitEnd();

		// 生成main方法
		methodVisitor = classWriter.visitMethod(ACC_PUBLIC + ACC_STATIC, "main", "([Ljava/lang/String;)V", null, null);
		// 生成main方法的字节码指令
		methodVisitor.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");

		methodVisitor.visitLdcInsn("Hello world!");
		methodVisitor.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
		methodVisitor.visitInsn(RETURN);
		methodVisitor.visitMaxs(2, 2);

		// 字节码生成 完成
		methodVisitor.visitEnd();

		// 获取生成的的class文件对应的二进制流
		byte[] code = classWriter.toByteArray();

		// 将二进制流写到本地磁盘上
		FileOutputStream fos = new FileOutputStream("Example.class");
		fos.write(code);
		fos.flush();
		fos.close();


		// 直接将二进制流加载到内存中
		BuildClass buildClass = new BuildClass();
		Class<?> example = buildClass.defineClass("Example", code, 0, code.length);
		try {
			example.getMethods()[0].invoke(null, new Object[] {null});
		} catch (IllegalAccessException e) {
			e.printStackTrace();
		} catch (InvocationTargetException e) {
			e.printStackTrace();
		}

	}
}