ASM之ClassWriter生成.class

ASM之ClassWriter生成.class

我们可以使用ClassWriter来生成一个类

如果不知道如何编写ASMified代码，可以直接使用插件ASMPlugin，将你需要的功能编写成正常的java代码，然后使用ASM Bytecode Viewer将编写的类转换成ASMified代码后作为参考

代码如下

package com.example.asmapplication

import org.junit.Test
import org.objectweb.asm.ClassWriter
import org.objectweb.asm.Opcodes
import java.io.File
import java.io.FileOutputStream

class DemoASMGenerateClass {
    @Test
    fun generate(){
        val filePath = "E:\\Develop\\ASMApplication2\\app\\src\\test\\java\\com\\example\\asmapplication\\generate\\GenerateTest.class"
        val file = File(filePath)
        if (!file.parentFile.exists()){
            file.parentFile.mkdir()
        }
        //创建ClassWriter
        val cw = ClassWriter(ClassWriter.COMPUTE_FRAMES)
        //设定包名和类名
        cw.visit(Opcodes.V1_8,Opcodes.ACC_PUBLIC,"com/example/asmapplication/generate/GenerateTest",null,"java/lang/Object",null)
        //每个classFile都有一个<init>的初始化方法
        val mvInit = cw.visitMethod(Opcodes.ACC_PUBLIC,"<init>","()V",null,null)
        mvInit.visitCode()
        mvInit.visitVarInsn(Opcodes.ALOAD, 0)
        mvInit.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false)
        mvInit.visitInsn(Opcodes.RETURN)
        mvInit.visitMaxs(1, 1);
        mvInit.visitEnd()
        //创建一个test()方法
        val mvTest = cw.visitMethod(Opcodes.ACC_PUBLIC,"test","()V",null,null)
        mvTest.visitCode()
        //打印Hello Word
        mvTest.visitLdcInsn("Hello World!!!")
        mvTest.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;")
        mvTest.visitInsn(Opcodes.SWAP)
        mvTest.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/Object;)V", false)
        mvTest.visitInsn(Opcodes.RETURN)
        mvTest.visitMaxs(2, 1)
        mvTest.visitEnd()
        //类的访问结束
        cw.visitEnd()
        //输出为class文件
        val outputStream = FileOutputStream(file)
        outputStream.write(cw.toByteArray())
        outputStream.flush()
        outputStream.close()
    }
}

最终生成代码：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package com.example.asmapplication.generate;

public class GenerateTest {
    public GenerateTest() {
    }

    public void test() {
        System.out.println("Hello Word!!!");
    }
}