ASM之FieldVisitor创建变量

FieldVisitor使用abstract 修饰，用于创建变量，在使用时调用 ClassWriter.visitField即可创建FieldVisitor

方法介绍

visitField(Opcodes.ACC_PUBLIC, “str”, “Ljava/lang/String;”, null, “Hello World”)

第一个参数是修饰类型，第二个参数是变量名，第三个是变量类型，第四个签名，第五个是变量的值（设置值好像没什么用，所以我在下面代码的初始化中重新初始化了str的值）

示例代码如下

package com.example.asmapplicationimport org.junit.Test
import org.objectweb.asm.ClassWriter
import org.objectweb.asm.Opcodes
import java.io.File
import java.io.FileOutputStream
import java.net.URLClassLoaderclass DemoASMGenerateField {@Testfun generate() {val filePath ="E:\\Develop\\ASMApplication2\\app\\src\\test\\java\\com\\example\\asmapplication\\generate\\GenerateField.class"val file = File(filePath)if (!file.parentFile.exists()) {file.parentFile.mkdir()}//创建ClassWriterval cw = ClassWriter(ClassWriter.COMPUTE_FRAMES)//设定包名和类名cw.visit(Opcodes.V1_8,Opcodes.ACC_PUBLIC,"com/example/asmapplication/generate/GenerateField",null,"java/lang/Object",null)//创建全局变量val vfStr =cw.visitField(Opcodes.ACC_PUBLIC, "str", "Ljava/lang/String;", null, "Hello World")vfStr.visitEnd()//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.visitVarInsn(Opcodes.ALOAD, 0)//初始化定义的变量的值mvInit.visitLdcInsn("Hello World")mvInit.visitFieldInsn(Opcodes.PUTFIELD, "com/example/asmapplication/generate/GenerateField", "str", "Ljava/lang/String;")mvInit.visitInsn(Opcodes.RETURN)mvInit.visitMaxs(0, 0)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 WordmvTest.visitVarInsn(Opcodes.ALOAD, 0);mvTest.visitFieldInsn(Opcodes.GETFIELD,"com/example/asmapplication/generate/GenerateField","str","Ljava/lang/String;");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(0, 0)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()val classLoader = URLClassLoader(arrayOf(File("E:\\Develop\\ASMApplication2\\app\\src\\test\\java").toURI().toURL()))val clazz = classLoader.loadClass("com.example.asmapplication.generate.GenerateField")val obj = clazz.newInstance()clazz.getMethod("test").invoke(obj)}
}

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 GenerateField {public String str = "Hello World";public GenerateField() {}public void test() {System.out.println(this.str);}
}

技巧分享

假如我们想在MainActivity中动态生成两个变量

public int age = 100;
public String str = "Hello World";

可以这样做：

1、先在一个Test类中创建两个变量，代码如下：

public class Test {public int age = 100;public String str = "Hello World";private int sum(int i, int j) {return i+j;}
}

编译后，通过ASM Bytecode Outline Rebooted等工具查看Test.class的字节码信息：

public class com/baidu/main/test/Test {// access flags 0x1public I age// access flags 0x1public Ljava/lang/String; str// access flags 0x1public <init>()VALOAD 0INVOKESPECIAL java/lang/Object.<init> ()VALOAD 0BIPUSH 100PUTFIELD com/baidu/main/test/Test.age : IALOAD 0LDC "Hello World"PUTFIELD com/baidu/main/test/Test.str : Ljava/lang/String;RETURNMAXSTACK = 2MAXLOCALS = 1// access flags 0x2private sum(II)IILOAD 1ILOAD 2IADDIRETURNMAXSTACK = 2MAXLOCALS = 3
}

对应的ASM信息如下：

public class TestDump implements Opcodes {public static byte[] dump() throws Exception {ClassWriter classWriter = new ClassWriter(0);FieldVisitor fieldVisitor;MethodVisitor methodVisitor;AnnotationVisitor annotationVisitor0;classWriter.visit(V1_8, ACC_PUBLIC | ACC_SUPER, "com/baidu/main/test/Test", null, "java/lang/Object", null);{fieldVisitor = classWriter.visitField(ACC_PUBLIC, "age", "I", null, null);fieldVisitor.visitEnd();}{fieldVisitor = classWriter.visitField(ACC_PUBLIC, "str", "Ljava/lang/String;", null, null);fieldVisitor.visitEnd();}{methodVisitor = classWriter.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);methodVisitor.visitCode();methodVisitor.visitVarInsn(ALOAD, 0);methodVisitor.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);methodVisitor.visitVarInsn(ALOAD, 0);methodVisitor.visitIntInsn(BIPUSH, 100);methodVisitor.visitFieldInsn(PUTFIELD, "com/baidu/main/test/Test", "age", "I");methodVisitor.visitVarInsn(ALOAD, 0);methodVisitor.visitLdcInsn("Hello World");methodVisitor.visitFieldInsn(PUTFIELD, "com/baidu/main/test/Test", "str", "Ljava/lang/String;");methodVisitor.visitInsn(RETURN);methodVisitor.visitMaxs(2, 1);methodVisitor.visitEnd();}{methodVisitor = classWriter.visitMethod(ACC_PRIVATE, "sum", "(II)I", null, null);methodVisitor.visitCode();methodVisitor.visitVarInsn(ILOAD, 1);methodVisitor.visitVarInsn(ILOAD, 2);methodVisitor.visitInsn(IADD);methodVisitor.visitInsn(IRETURN);methodVisitor.visitMaxs(2, 3);methodVisitor.visitEnd();}classWriter.visitEnd();return classWriter.toByteArray();}
}

从ASM生成的代码中可以看到，age和str两个变量后并没有立刻赋值（赋值了不一定生效），而是在init方法里面进行了赋值。

2.对MainActivity进行插装

package com.baidu.plugin.printimport org.objectweb.asm.AnnotationVisitor
import org.objectweb.asm.Attribute
import org.objectweb.asm.ClassVisitor
import org.objectweb.asm.FieldVisitor
import org.objectweb.asm.Label
import org.objectweb.asm.MethodVisitor
import org.objectweb.asm.Opcodes
import org.objectweb.asm.commons.AdviceAdapterclass PrintClassVisitor(nextVisitor: ClassVisitor, private val className: String): ClassVisitor(Opcodes.ASM5, nextVisitor) {override fun visit(version: Int,access: Int,name: String?,signature: String?,superName: String?,interfaces: Array<out String>?) {println("visit access $access className $className name $name  signature $signature")val numFiledVisitor = cv.visitField(Opcodes.ACC_PRIVATE, "age", "I", null, 25)numFiledVisitor.visitEnd()val strFiledVisitor = cv.visitField(Opcodes.ACC_PUBLIC, "str", "Ljava/lang/String;", null, "Hello World")strFiledVisitor.visitEnd()super.visit(version, access, name, signature, superName, interfaces)}override fun visitEnd() {super.visitEnd()}override fun visitField(access: Int,name: String?,descriptor: String?,signature: String?,value: Any?): FieldVisitor {println("visitField access $access className $className name $name  descriptor $descriptor")val filedVisitor = super.visitField(access, name, descriptor, signature, value)return MyPrintFieldVisitor(Opcodes.ASM5,filedVisitor)}override fun visitMethod(access: Int,name: String?,descriptor: String?,signature: String?,exceptions: Array<out String>?): MethodVisitor {val methodVisitor = super.visitMethod(access, name, descriptor, signature, exceptions)return MyPrintMethodVisitor(Opcodes.ASM5,methodVisitor,access,className,name,descriptor)}}class MyPrintFieldVisitor(api: Int, filedVisitor: FieldVisitor) : FieldVisitor(api, filedVisitor) {override fun visitAnnotation(descriptor: String?, visible: Boolean): AnnotationVisitor {println("visitAnnotation descriptor $descriptor visible $visible")return super.visitAnnotation(descriptor, visible)}}class MyPrintMethodVisitor(api: Int,methodVisitor: MethodVisitor?,private val access: Int,private val className: String,private val name: String?,private val descriptor: String?
) : AdviceAdapter(api, methodVisitor, access, name, descriptor) {override fun onMethodEnter() {println("onMethodEnter access $access className $className name $name  descriptor $descriptor")if (name.equals("<init>") && descriptor.equals("()V")) {//每个classFile都有一个<init>的初始化方法(固定写法)mv.visitVarInsn(Opcodes.ALOAD, 0)mv.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false)mv.visitVarInsn(Opcodes.ALOAD, 0)//初始化定义的变量的值mv.visitIntInsn(BIPUSH, 100)mv.visitFieldInsn(PUTFIELD, "com/baidu/main/MainActivity", "age", "I" )//String的初始化会在构造方法中mv.visitVarInsn(Opcodes.ALOAD, 0)mv.visitLdcInsn("Hello World")mv.visitFieldInsn(PUTFIELD, "com/baidu/main/MainActivity", "str", "Ljava/lang/String" )mv.visitInsn(RETURN)}if (name.equals("sum999")) {//System.out.println("999999");mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");mv.visitLdcInsn("999999-begin")mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);}super.onMethodEnter()}override fun onMethodExit(opcode: Int) {println("onMethodExit access $access className $className name $name  descriptor $descriptor")if (name.equals("sum999")) {//System.out.println("999999");mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");mv.visitLdcInsn("999999-end")mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);}super.onMethodExit(opcode)}override fun visitFieldInsn(opcode: Int, owner: String?, name: String?, descriptor: String?) {println("visitFieldInsn opcode $opcode owner $owner name $name  descriptor $descriptor")super.visitFieldInsn(opcode, owner, name, descriptor)}override fun visitLocalVariable(name: String?,descriptor: String?,signature: String?,start: Label?,end: Label?,index: Int) {println("visitLocalVariable name $name  descriptor $descriptor start $start end $end index $index")super.visitLocalVariable(name, descriptor, signature, start, end, index)}override fun visitAttribute(attribute: Attribute?) {println("visitAttribute attribute $attribute")super.visitAttribute(attribute)}override fun visitLineNumber(line: Int, start: Label?) {println("visitLineNumber line $line start $start")super.visitLineNumber(line, start)}override fun visitFrame(type: Int,numLocal: Int,local: Array<out Any>?,numStack: Int,stack: Array<out Any>?) {println("visitFrame type $type  numLocal $numLocal local $local numStack $numStack stack $stack")super.visitFrame(type, numLocal, local, numStack, stack)}override fun visitLabel(label: Label?) {println("visitLabel label $label")super.visitLabel(label)}override fun visitCode() {println("visitCode")super.visitCode()}override fun visitMaxs(maxStack: Int, maxLocals: Int) {println("visitMaxs maxStack $maxStack maxLocals $maxLocals")super.visitMaxs(maxStack, maxLocals)}override fun visitEnd() {println("visitEnd")super.visitEnd()}
}

关键代码如下：

创建变量：

override fun visit(version: Int,access: Int,name: String?,signature: String?,superName: String?,interfaces: Array<out String>?) {println("visit access $access className $className name $name  signature $signature")val numFiledVisitor = cv.visitField(Opcodes.ACC_PRIVATE, "age", "I", null, 25)numFiledVisitor.visitEnd()val strFiledVisitor = cv.visitField(Opcodes.ACC_PUBLIC, "str", "Ljava/lang/String;", null, "Hello World")strFiledVisitor.visitEnd()super.visit(version, access, name, signature, superName, interfaces)}

注意：

cv.visitField(Opcodes.ACC_PUBLIC, "str", "Ljava/lang/String;", null, "Hello World")也给变量赋值了，但是没有生效。

在init方法中给变量赋值：

 override fun onMethodEnter() {println("onMethodEnter access $access className $className name $name  descriptor $descriptor")if (name.equals("<init>") && descriptor.equals("()V")) {//每个classFile都有一个<init>的初始化方法(固定写法)mv.visitVarInsn(Opcodes.ALOAD, 0)mv.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false)mv.visitVarInsn(Opcodes.ALOAD, 0)//初始化定义的变量的值mv.visitIntInsn(BIPUSH, 100)mv.visitFieldInsn(PUTFIELD, "com/baidu/main/MainActivity", "age", "I" )//String的初始化会在构造方法中mv.visitVarInsn(Opcodes.ALOAD, 0)mv.visitLdcInsn("Hello World")mv.visitFieldInsn(PUTFIELD, "com/baidu/main/MainActivity", "str", "Ljava/lang/String" )mv.visitInsn(RETURN)}super.onMethodEnter()}

如果不会写，可以参考Test.class的ASM生成代码，直接照抄就行。

插桩后的代码如下：

package com.baidu.main;import android.os.Bundle;
import androidx.activity.ComponentActivity;
import kotlin.Metadata;/* compiled from: MainActivity.kt */
@Metadata(d1 = {"\u0000&\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0002\b\u0002\n\u0002\u0010\u0002\n\u0000\n\u0002\u0018\u0002\n\u0000\n\u0002\u0010\b\n\u0002\b\u0005\n\u0002\u0010\t\n\u0000\u0018\u00002\u00020\u0001B\u0005¢\u0006\u0002\u0010\u0002J\u0012\u0010\u0003\u001a\u00020\u00042\b\u0010\u0005\u001a\u0004\u0018\u00010\u0006H\u0014J\u0018\u0010\u0007\u001a\u00020\b2\u0006\u0010\t\u001a\u00020\b2\u0006\u0010\n\u001a\u00020\bH\u0002J\u0018\u0010\u000b\u001a\u00020\b2\u0006\u0010\t\u001a\u00020\b2\u0006\u0010\n\u001a\u00020\bH\u0002J\u0010\u0010\f\u001a\u00020\u00042\u0006\u0010\r\u001a\u00020\u000eH\u0002¨\u0006\u000f"}, d2 = {"Lcom/baidu/main/MainActivity;", "Landroidx/activity/ComponentActivity;", "()V", "onCreate", "", "savedInstanceState", "Landroid/os/Bundle;", "sum", "", "i", "j", "sum999", "test", "time", "", "app_debug"}, k = 1, mv = {1, 8, 0}, xi = 48)
/* loaded from: classes4.dex */
public final class MainActivity extends ComponentActivity {private int age = 100;public String str;public MainActivity() {this.str = "Hello World";}/* access modifiers changed from: protected */@Override // androidx.activity.ComponentActivity, androidx.core.app.ComponentActivity, android.app.Activitypublic void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);test(100);sum(1, 2);}private final void test(long time) {Thread.sleep(time);}private final int sum(int i, int j) {return i + j;}private final int sum999(int i, int j) {System.out.println("999999-begin");int i2 = i + j;System.out.println("999999-end");return i2;}
}