我们知道Java是静态语言，而python、ruby是动态语言，Java程序一旦写好很难在运行时更改类的行为，而python、ruby可以。不过基于bytecode层面上我们可以做一些手脚，来使Java程序多一些灵活性和Magic，ASM就是这样一个应用广泛的开源库。
可能我们在开发中几乎对ASM没有印象，我记得当时学习cglib反向代理的时候，就需要映入asm.jar
ASM概述
- ASM是一个Java字节码操纵框架，它可以动态生成类或者增强既有类的功能
- ASM可以直接产生二进制class文件，也可以在类被加载入虚拟机之前动态改变类行为，ASM从类文件中读入信息后，能够改变类行为，分析类信息，甚至能根据要求生成新类
- 目前许多框架如cglib、Hibernate、Spring都直接或间接地使用ASM操作字节码
ASM编程模型
ASM提供了两种编程模型：基于事件形式的编程模型(Core API)和基于树形的编程模型(Tree API)。可以用XML解析做形象的比喻，Core API类似于XML解析的SAX解析，而Tree API类似于XML解析的Document解析。
ASM Core API
该模型不需要一次性将整个类的结构读取到内存中，因此这种方式更快，需要更少的内存，但这种编程方式难度较大(类似于XML解析的SAX解析)。
ASM Core API中操纵字节码的功能基于ClassVisitor接口。这个接口中的每个方法对应了class文件中的每一项。ASM提供了三个基于ClassVisitor接口的类来实现class文件的生成和转换。
- ClassReader: ClassReader解析一个类的class字节码
- ClassAdapter: ClassAdapt是ClassVisitor的实现类，实现要变化的功能
- ClassWriter: ClassWriter也是ClassVisitor的实现类，可以用来输出变化后的字节码
ASM Tree API
该模型需要一次性将一个类的完整结构全部读取到内存当中，所以这种方法需要更多的内存，这种编程方式较简单(类似于XML解析的document解析)。因为Java ASM基本上都是Core API，所以这里不对ASM Tree API进行介绍。
ASMifier工具
前面我们已经知道ASM是字节码操纵框架，那我们要怎样操纵字节码？怎样通过编码的方式改变字节码？
ASM给我们提供了ASMifier工具来帮助开发，可使用ASMifier工具生成ASM结构来对比字节码的变化。
IDEA安装ASM插件
asm插件中集成了ASMifier工具，我们在idea中安装相关的插件，方便开发。在idea插件安装界面搜索asm，出现如下两个插件，任选一个进行安装(或者两个都装上)
在对应的类上右键
通过ASMifier查看ASM结构
注意：asm是字节码操纵框架，前提是有字节码的存在。所有如果是没有看到ASMifier，请查看java类是否已完成编译。
ASM开发
下面我们来使用asm来改变.class文件。创建一个简单的案例
public class AsmTest {
public void m1(){
System.out.println("now in method m1");
/**
* InterruptedException相当于一个局部变量，会影响结果(需将异常注释)
*/
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}ASMifier生成对应的代码如下：
package asm.cn.crazy.dreamer.asm;
import java.util.*;
import org.objectweb.asm.*;
public class AsmTestDump implements Opcodes {
public static byte[] dump() throws Exception {
ClassWriter cw = new ClassWriter(0);
FieldVisitor fv;
MethodVisitor mv;
AnnotationVisitor av0;
cw.visit(52, ACC_PUBLIC + ACC_SUPER, "cn/crazy/dreamer/asm/AsmTest", null, "java/lang/Object", null);
cw.visitSource("AsmTest.java", null);
{
mv = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
mv.visitCode();
Label l0 = new Label();
mv.visitLabel(l0);
mv.visitLineNumber(9, l0);
mv.visitVarInsn(ALOAD, 0);
mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);
mv.visitInsn(RETURN);
Label l1 = new Label();
mv.visitLabel(l1);
mv.visitLocalVariable("this", "Lcn/crazy/dreamer/asm/AsmTest;", null, l0, l1, 0);
mv.visitMaxs(1, 1);
mv.visitEnd();
}
{
mv = cw.visitMethod(ACC_PUBLIC, "m1", "()V", null, new String[]{"java/lang/Exception"});
mv.visitCode();
Label l0 = new Label();
Label l1 = new Label();
Label l2 = new Label();
mv.visitTryCatchBlock(l0, l1, l2, "java/lang/InterruptedException");
Label l3 = new Label();
mv.visitLabel(l3);
mv.visitLineNumber(13, l3);
mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv.visitLdcInsn("now in method m1");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
mv.visitLabel(l0);
mv.visitLineNumber(18, l0);
mv.visitLdcInsn(new Long(100L));
mv.visitMethodInsn(INVOKESTATIC, "java/lang/Thread", "sleep", "(J)V", false);
mv.visitLabel(l1);
mv.visitLineNumber(21, l1);
Label l4 = new Label();
mv.visitJumpInsn(GOTO, l4);
mv.visitLabel(l2);
mv.visitLineNumber(19, l2);
mv.visitFrame(Opcodes.F_SAME1, 0, null, 1, new Object[]{"java/lang/InterruptedException"});
mv.visitVarInsn(ASTORE, 1);
Label l5 = new Label();
mv.visitLabel(l5);
mv.visitLineNumber(20, l5);
mv.visitVarInsn(ALOAD, 1);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/InterruptedException", "printStackTrace", "()V", false);
mv.visitLabel(l4);
mv.visitLineNumber(22, l4);
mv.visitFrame(Opcodes.F_SAME, 0, null, 0, null);
mv.visitInsn(RETURN);
Label l6 = new Label();
mv.visitLabel(l6);
mv.visitLocalVariable("e", "Ljava/lang/InterruptedException;", null, l5, l4, 1);
mv.visitLocalVariable("this", "Lcn/crazy/dreamer/asm/AsmTest;", null, l3, l6, 0);
mv.visitMaxs(2, 2);
mv.visitEnd();
}
cw.visitEnd();
return cw.toByteArray();
}
}修改java代码，我们添加一个计时器，计算方法执行的时间
public class AsmTest {
public void m1(){
long start = System.currentTimeMillis();
System.out.println("now in method m1!!!");
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
long end = System.currentTimeMillis();
System.out.println("total time = " + (end - start));
}
}ASMifier生成对应的代码如下：
{
methodVisitor = classWriter.visitMethod(ACC_PUBLIC, "m1", "()V", null, null);
methodVisitor.visitCode();
Label label0 = new Label();
Label label1 = new Label();
Label label2 = new Label();
methodVisitor.visitTryCatchBlock(label0, label1, label2, "java/lang/InterruptedException");
Label label3 = new Label();
methodVisitor.visitLabel(label3);
methodVisitor.visitLineNumber(12, label3);
methodVisitor.visitMethodInsn(INVOKESTATIC, "java/lang/System", "currentTimeMillis", "()J", false);
methodVisitor.visitVarInsn(LSTORE, 1);
Label label4 = new Label();
methodVisitor.visitLabel(label4);
methodVisitor.visitLineNumber(13, label4);
methodVisitor.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
methodVisitor.visitLdcInsn("now in method m1!!!");
methodVisitor.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
methodVisitor.visitLabel(label0);
methodVisitor.visitLineNumber(15, label0);
methodVisitor.visitLdcInsn(new Long(100L));
methodVisitor.visitMethodInsn(INVOKESTATIC, "java/lang/Thread", "sleep", "(J)V", false);
methodVisitor.visitLabel(label1);
methodVisitor.visitLineNumber(18, label1);
Label label5 = new Label();
methodVisitor.visitJumpInsn(GOTO, label5);
methodVisitor.visitLabel(label2);
methodVisitor.visitLineNumber(16, label2);
methodVisitor.visitFrame(Opcodes.F_FULL, 2, new Object[]{"cn/crazy/dreamer/asm/AsmTest", Opcodes.LONG}, 1, new Object[]{"java/lang/InterruptedException"});
methodVisitor.visitVarInsn(ASTORE, 3);
Label label6 = new Label();
methodVisitor.visitLabel(label6);
methodVisitor.visitLineNumber(17, label6);
methodVisitor.visitVarInsn(ALOAD, 3);
methodVisitor.visitMethodInsn(INVOKEVIRTUAL, "java/lang/InterruptedException", "printStackTrace", "()V", false);
methodVisitor.visitLabel(label5);
methodVisitor.visitLineNumber(19, label5);
methodVisitor.visitFrame(Opcodes.F_SAME, 0, null, 0, null);
methodVisitor.visitMethodInsn(INVOKESTATIC, "java/lang/System", "currentTimeMillis", "()J", false);
methodVisitor.visitVarInsn(LSTORE, 3);
Label label7 = new Label();
methodVisitor.visitLabel(label7);
methodVisitor.visitLineNumber(20, label7);
methodVisitor.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
methodVisitor.visitTypeInsn(NEW, "java/lang/StringBuilder");
methodVisitor.visitInsn(DUP);
methodVisitor.visitMethodInsn(INVOKESPECIAL, "java/lang/StringBuilder", "<init>", "()V", false);
methodVisitor.visitLdcInsn("total time = ");
methodVisitor.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
methodVisitor.visitVarInsn(LLOAD, 3);
methodVisitor.visitVarInsn(LLOAD, 1);
methodVisitor.visitInsn(LSUB);
methodVisitor.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(J)Ljava/lang/StringBuilder;", false);
methodVisitor.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;", false);
methodVisitor.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
Label label8 = new Label();
methodVisitor.visitLabel(label8);
methodVisitor.visitLineNumber(21, label8);
methodVisitor.visitInsn(RETURN);
Label label9 = new Label();
methodVisitor.visitLabel(label9);
methodVisitor.visitLocalVariable("e", "Ljava/lang/InterruptedException;", null, label6, label5, 3);
methodVisitor.visitLocalVariable("this", "Lcn/crazy/dreamer/asm/AsmTest;", null, label3, label9, 0);
methodVisitor.visitLocalVariable("start", "J", null, label4, label9, 1);
methodVisitor.visitLocalVariable("end", "J", null, label7, label9, 3);
methodVisitor.visitMaxs(6, 5);
methodVisitor.visitEnd();
}我们用文本比较工具Beyond Compare比较上面两段代码中m1方法部分的代码(红色的为不同部分)：
注：如下代码意义不大，可忽略
Label label7 = new Label();
methodVisitor.visitLabel(label7);
methodVisitor.visitLineNumber(20, label7);现在我们用ASM来实现统计程序用时。
编写ClassVisitor类：
/**
* @Description 自定义ClassVisitor
* @Author zhangli
* @Date 2020/10/13 21:18
* @Version 1.0
**/
public class MyClassVisitor extends ClassVisitor {
// vision与jdk版本有关
public MyClassVisitor(int vision, ClassVisitor classVisitor) {
super(vision, classVisitor);
}
/* public MyClassVisitor(ClassVisitor classVisitor) {
super(Opcodes.ASM7, classVisitor);
}*/
@Override
public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {
cv.visit(version, access, name, signature, superName, interfaces);
}
@Override
public MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {
MethodVisitor mv = cv.visitMethod(access, name, descriptor, signature, exceptions);
// 对非init方法增加记录方法执行时间的功能
if(!"<init>".equals(name) && mv != null){
// 增加功能
mv = new MyMethodVisitor(mv);
}
return mv;
}
class MyMethodVisitor extends MethodVisitor{
public MyMethodVisitor(MethodVisitor methodVisitor) {
super(Opcodes.ASM5, methodVisitor);
}
// 相当于开始
@Override
public void visitCode() {
mv.visitCode();
// 将上面asm代码前一部分新增的部分添加在visitCode()中
mv.visitMethodInsn(Opcodes.INVOKESTATIC, "java/lang/System", "currentTimeMillis", "()J", false);
mv.visitVarInsn(Opcodes.LSTORE, 1);
}
// 相当于结束
@Override
public void visitInsn(int opcode) {
// 如果是return或异常，说明程序结束
if(opcode >= Opcodes.IRETURN && opcode <= Opcodes.RETURN
|| opcode == Opcodes.ATHROW){
//将上面asm代码后一部分新增的添加到visitInsn()中
mv.visitMethodInsn(Opcodes.INVOKESTATIC, "java/lang/System", "currentTimeMillis", "()J", false);
mv.visitVarInsn(Opcodes.LSTORE, 3);
Label label7 = new Label();
mv.visitLabel(label7);
mv.visitLineNumber(20, label7);
mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv.visitTypeInsn(Opcodes.NEW, "java/lang/StringBuilder");
mv.visitInsn(Opcodes.DUP);
mv.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/StringBuilder", "<init>", "()V", false);
mv.visitLdcInsn("total time = ");
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
mv.visitVarInsn(Opcodes.LLOAD, 3);
mv.visitVarInsn(Opcodes.LLOAD, 1);
mv.visitInsn(Opcodes.LSUB);
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(J)Ljava/lang/StringBuilder;", false);
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;", false);
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
}
mv.visitInsn(opcode);
}
}
}生成ASM代码：
/**
* @Description asm生成对应的ClassVisitor
* @Author zhangli
* @Date 2020/10/13 21:46
* @Version 1.0
**/
public class Generator {
public static void main(String[] args) throws Exception {
ClassReader reader = new ClassReader("cn/crazy/dreamer/asm/AsmTest");
ClassWriter writer = new ClassWriter(ClassWriter.COMPUTE_MAXS);
ClassVisitor cv = new MyClassVisitor(Opcodes.ASM5,writer);
reader.accept(cv, ClassReader.SKIP_DEBUG);
byte[] data = writer.toByteArray();
// 输出
File file = new File("F:\\crazy\\freedom\\ifd-jvm\\target\\classes\\cn\\crazy\\dreamer\\asm\\AsmTest.class");
FileOutputStream fos = new FileOutputStream(file);
fos.write(data);
fos.close();
System.out.println("Generator AsmTest class success");
}
}编写测试类：
/**
* @Description TODO
* @Author zhangli
* @Date 2020/10/13 23:02
* @Version 1.0
**/
public class Test {
public static void main(String[] args) throws Exception {
AsmTest test = new AsmTest();
test.m1();
}
}现在我们将java代码还原为：
public class AsmTest {
public void m1(){
System.out.println("now in method m1");
/**
* InterruptedException相当于一个局部变量，会影响结果(需将异常注释)
*/
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}执运行程序,出现如下错误：
Exception in thread "main" java.lang.VerifyError: Bad local variable type
Exception Details:
Location:
cn/crazy/dreamer/asm/AsmTest.m1()V @46: lload_1
Reason:
Type top (current frame, locals[1]) is not assignable to long
Current Frame:
bci: @46
flags: { }
locals: { 'cn/crazy/dreamer/asm/AsmTest', top, top, long, long_2nd }
stack: { 'java/io/PrintStream', 'java/lang/StringBuilder', long, long_2nd }
Bytecode:
0x0000000: b800 1140 b200 1712 19b6 001f 1400 20b8
0x0000010: 0027 a700 084c 2bb6 002a b800 1142 b200
0x0000020: 17bb 002c 59b7 002d 122f b600 3321 1f65
0x0000030: b600 36b6 003a b600 1fb1
Exception Handler Table:
bci [12, 18] => handler: 21
Stackmap Table:
same_locals_1_stack_item_frame(@21,Object[#19])
same_frame(@26)
at cn.crazy.dreamer.asm.Test.main(Test.java:12)上面的错误提示错误的变量类型，原因：is not assignable to long,即不支持long类型，是因为我们使用ASM添加的是统计耗时的代码，是long类型的，但是实际上是将InterruptedException异常类型赋值给long类型了，所以我们要将异常抛出，而不是try(try的话会将异常也出现在m1方法中)
将代码改成：
public class AsmTest {
public void m1() throws Exception{
System.out.println("now in method m1");
/**
* InterruptedException相当于一个局部变量，会影响结果(需将异常注释)
*/
/*try {*/
Thread.sleep(100);
/*} catch (InterruptedException e) {
e.printStackTrace();
}*/
}
}再次执行Test.java，运行结果如下
now in method m1
total time = 101虽然我们实现了计时功能，但是效果并不理想，代码侵入严重。要怎么完善呢？下面我们用ASM实现类型于AOP的功能
ASM模拟AOP功能
java代码如下
public class JvmTest {
public void test(){
System.out.println("init test() method!!!");
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}我们简单定义一个logger类，用于计时
public class MyTimeLogger {
private static long start = 0;
public static void start(){
start = System.currentTimeMillis();
}
public static void end(){
long end = System.currentTimeMillis();
System.out.println("total = " + (end - start));
}
}JvmTest.java中test()方法对应的ASM代码如下：
{
methodVisitor = classWriter.visitMethod(ACC_PUBLIC, "test", "()V", null, null);
methodVisitor.visitCode();
Label label0 = new Label();
Label label1 = new Label();
Label label2 = new Label();
methodVisitor.visitTryCatchBlock(label0, label1, label2, "java/lang/InterruptedException");
Label label3 = new Label();
methodVisitor.visitLabel(label3);
methodVisitor.visitLineNumber(12, label3);
methodVisitor.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
methodVisitor.visitLdcInsn("init test() method!!!");
methodVisitor.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
methodVisitor.visitLabel(label0);
methodVisitor.visitLineNumber(14, label0);
methodVisitor.visitLdcInsn(new Long(100L));
methodVisitor.visitMethodInsn(INVOKESTATIC, "java/lang/Thread", "sleep", "(J)V", false);
methodVisitor.visitLabel(label1);
methodVisitor.visitLineNumber(17, label1);
Label label4 = new Label();
methodVisitor.visitJumpInsn(GOTO, label4);
methodVisitor.visitLabel(label2);
methodVisitor.visitLineNumber(15, label2);
methodVisitor.visitFrame(Opcodes.F_SAME1, 0, null, 1, new Object[]{"java/lang/InterruptedException"});
methodVisitor.visitVarInsn(ASTORE, 1);
Label label5 = new Label();
methodVisitor.visitLabel(label5);
methodVisitor.visitLineNumber(16, label5);
methodVisitor.visitVarInsn(ALOAD, 1);
methodVisitor.visitMethodInsn(INVOKEVIRTUAL, "java/lang/InterruptedException", "printStackTrace", "()V", false);
methodVisitor.visitLabel(label4);
methodVisitor.visitLineNumber(18, label4);
methodVisitor.visitFrame(Opcodes.F_SAME, 0, null, 0, null);
methodVisitor.visitInsn(RETURN);
Label label6 = new Label();
methodVisitor.visitLabel(label6);
methodVisitor.visitLocalVariable("e", "Ljava/lang/InterruptedException;", null, label5, label4, 1);
methodVisitor.visitLocalVariable("this", "Lcn/crazy/dreamer/asm/logger/JvmTest;", null, label3, label6, 0);
methodVisitor.visitMaxs(2, 2);
methodVisitor.visitEnd();
}修改JvmTest.java,加入程序计时功能
public class JvmTest {
public void test(){
MyTimeLogger.start();
System.out.println("init test() method!!!");
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
MyTimeLogger.end();
}
}查看此时test()对应ASM代码：
{
methodVisitor = classWriter.visitMethod(ACC_PUBLIC, "test", "()V", null, null);
methodVisitor.visitCode();
Label label0 = new Label();
Label label1 = new Label();
Label label2 = new Label();
methodVisitor.visitTryCatchBlock(label0, label1, label2, "java/lang/InterruptedException");
Label label3 = new Label();
methodVisitor.visitLabel(label3);
methodVisitor.visitLineNumber(12, label3);
methodVisitor.visitMethodInsn(INVOKESTATIC, "cn/crazy/dreamer/asm/logger/MyTimeLogger", "start", "()V", false);
Label label4 = new Label();
methodVisitor.visitLabel(label4);
methodVisitor.visitLineNumber(13, label4);
methodVisitor.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
methodVisitor.visitLdcInsn("init test() method!!!");
methodVisitor.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
methodVisitor.visitLabel(label0);
methodVisitor.visitLineNumber(15, label0);
methodVisitor.visitLdcInsn(new Long(100L));
methodVisitor.visitMethodInsn(INVOKESTATIC, "java/lang/Thread", "sleep", "(J)V", false);
methodVisitor.visitLabel(label1);
methodVisitor.visitLineNumber(18, label1);
Label label5 = new Label();
methodVisitor.visitJumpInsn(GOTO, label5);
methodVisitor.visitLabel(label2);
methodVisitor.visitLineNumber(16, label2);
methodVisitor.visitFrame(Opcodes.F_SAME1, 0, null, 1, new Object[]{"java/lang/InterruptedException"});
methodVisitor.visitVarInsn(ASTORE, 1);
Label label6 = new Label();
methodVisitor.visitLabel(label6);
methodVisitor.visitLineNumber(17, label6);
methodVisitor.visitVarInsn(ALOAD, 1);
methodVisitor.visitMethodInsn(INVOKEVIRTUAL, "java/lang/InterruptedException", "printStackTrace", "()V", false);
methodVisitor.visitLabel(label5);
methodVisitor.visitLineNumber(19, label5);
methodVisitor.visitFrame(Opcodes.F_SAME, 0, null, 0, null);
methodVisitor.visitMethodInsn(INVOKESTATIC, "cn/crazy/dreamer/asm/logger/MyTimeLogger", "end", "()V", false);
Label label7 = new Label();
methodVisitor.visitLabel(label7);
methodVisitor.visitLineNumber(20, label7);
methodVisitor.visitInsn(RETURN);
Label label8 = new Label();
methodVisitor.visitLabel(label8);
methodVisitor.visitLocalVariable("e", "Ljava/lang/InterruptedException;", null, label6, label5, 1);
methodVisitor.visitLocalVariable("this", "Lcn/crazy/dreamer/asm/logger/JvmTest;", null, label3, label8, 0);
methodVisitor.visitMaxs(2, 2);
methodVisitor.visitEnd();
}通过对比两段代码，我们发现基本上只是增加了MyTimeLogger类的start()和end()：
methodVisitor.visitMethodInsn(INVOKESTATIC, "cn/crazy/dreamer/asm/logger/MyTimeLogger", "start", "()V", false);
methodVisitor.visitMethodInsn(INVOKESTATIC, "java/lang/Thread", "sleep", "(J)V", false);编写ClassVisitor类
public class MyLoggerClassVisitor extends ClassVisitor {
public MyLoggerClassVisitor(int vision, ClassVisitor classVisitor) {
super(vision, classVisitor);
}
@Override
public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {
cv.visit(version, access, name, signature, superName, interfaces);
}
@Override
public MethodVisitor visitMethod(int access, String name, String description, String signature, String[] exceptions) {
MethodVisitor mv = cv.visitMethod(access, name, description, signature, exceptions);
// 对非init方法增加功能
if(!"<init>".equals(name) && mv != null){
// 增加功能
mv = new MyLoggerMethodVisitor(Opcodes.ASM5, mv);
}
return mv;
}
class MyLoggerMethodVisitor extends MethodVisitor{
public MyLoggerMethodVisitor(int vision, MethodVisitor methodVisitor) {
super(vision, methodVisitor);
}
// 相当于开始
@Override
public void visitCode() {
mv.visitCode();
mv.visitMethodInsn(Opcodes.INVOKESTATIC, "cn/crazy/dreamer/asm/logger/MyTimeLogger", "start", "()V", false);
}
// 相当于结束
@Override
public void visitInsn(int opcode) {
// 结束标识
if(opcode >= Opcodes.IRETURN && opcode <= Opcodes.RETURN
|| opcode == Opcodes.ATHROW){
mv.visitMethodInsn(Opcodes.INVOKESTATIC, "cn/crazy/dreamer/asm/logger/MyTimeLogger", "end", "()V", false);
}
mv.visitInsn(opcode);
}
}
}通过ASM生成class文件，我们将测试方法一起实现：
public class MyLoggerGenerator {
public static void main(String[] args) throws Exception {
ClassReader reader = new ClassReader("cn/crazy/dreamer/asm/logger/JvmTest");
ClassWriter writer = new ClassWriter(ClassWriter.COMPUTE_MAXS);
ClassVisitor cv = new MyLoggerClassVisitor(Opcodes.ASM5,writer);
reader.accept(cv, ClassReader.SKIP_DEBUG);
byte[] data = writer.toByteArray();
// 输出（先要编译产生class文件）
File file = new File("F:\\crazy\\freedom\\ifd-jvm\\target\\classes\\cn\\crazy\\dreamer\\asm\\logger\\JvmTest.class");
FileOutputStream fos = new FileOutputStream(file);
fos.write(data);
fos.close();
System.out.println("Generator JvmTest class success");
test();
}
public static void test(){
JvmTest jvmTest = new JvmTest();
jvmTest.test();
}
}运行结果如下：
Generator JvmTest class success
init test() method!!!
total = 100我们通过ASM实现了类似AOP的功能，其他AOP框架的实现都是类似的，当然要复杂的多，我们这里只演示基本原理。
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