JavaAgent的那点秘密
前言大家都知道Java文件编译成Class文件之后才能在Jvm中运行的，而Class文件自有一套规范:
- 不必一定是Java文件经由javac编译产生；
- 其它编程语言也可以直接生成Class文件交由Jvm运行；
既然其它语言都可以生成Class文件，理论上Java语言自己是不是也可以修改/篡改Class文件呢？答案是肯定的。那么是否有这方面的需求呢（用Java语法去直接修改Class文件） 我举个例子：
- 需要去增强别人的二方包，需要针对其中大量的方法进行增强
- 程序运行期间，动态去修改某个类（有点类似于ide中的热编译热部署）从而达到线上快速调试的目的公司级别基础框架，应用层毫不感知地进行慢sql监控，traceid链路跟踪
目前市面上还有很多字节码技术基本上都是这么做的，而他们都是利用的jdk5官方提供JavaAgent技术 字节码技术
直接操纵字节码技术有目前已经有很多工具了，比如asm，javaassist，arthas，这里的示例选用的是asm来进行举例讲解
asm maven坐标：
基于asm生成一个class文件org.ow2.asm asm-all6.0_BETA
这个示例中，可以学到：
- 演示了生成类变量的生成
- for循环的编写方式
- 组装集合对象ArrayList，并作为出参返回
- 集合对象ArrayList作为入参进行遍历
package com.example.lzcjavaagent;
import org.objectweb.asm.*;
import java.io.File;
import java.io.FileOutputStream;
import java.lang.reflect.Method;
import java.util.List;
public class HelloGeneratorClass implements Opcodes {
public static byte[] generatorHelloClass() throws Exception {
ClassWriter cw = new ClassWriter(0);
FieldVisitor fv;
MethodVisitor mv;
cw.visit(V1_7, ACC_PUBLIC + ACC_SUPER, "com/lzc/Hello", null, "java/lang/Object", null);
cw.visitSource("Hello.java", null);
{
fv = cw.visitField(ACC_PUBLIC + ACC_FINAL + ACC_STATIC, "FLAG", "Ljava/lang/String;", null, "u6211u662fu5e38u91cf");
fv.visitEnd();
}
{
mv = cw.visitMethod(ACC_PUBLIC, "", "()V", null, null);
mv.visitCode();
Label l0 = new Label();
mv.visitLabel(l0);
mv.visitVarInsn(ALOAD, 0);
mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "", "()V", false);
mv.visitInsn(RETURN);
Label l1 = new Label();
mv.visitLabel(l1);
mv.visitMaxs(1, 1);
mv.visitEnd();
}
{
mv = cw.visitMethod(ACC_PUBLIC, "display", "()V", null, null);
mv.visitCode();
Label l0 = new Label();
mv.visitLabel(l0);
mv.visitLineNumber(16, l0);
mv.visitInsn(ICONST_0);
mv.visitVarInsn(ISTORE, 1);
Label l1 = new Label();
mv.visitLabel(l1);
mv.visitframe(Opcodes.F_APPEND, 1, new Object[]{Opcodes.INTEGER}, 0, null);
mv.visitVarInsn(ILOAD, 1);
mv.visitIntInsn(BIPUSH, 6);
Label l2 = new Label();
mv.visitJumpInsn(IF_ICMPGE, l2);
Label l3 = new Label();
mv.visitLabel(l3);
mv.visitLineNumber(17, l3);
mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv.visitLdcInsn(">>>>>>>>>>u6211u662fu5e38u91cf");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
Label l4 = new Label();
mv.visitLabel(l4);
mv.visitLineNumber(16, l4);
mv.visitIincInsn(1, 1);
mv.visitJumpInsn(GOTO, l1);
mv.visitLabel(l2);
mv.visitLineNumber(20, l2);
mv.visitframe(Opcodes.F_CHOP, 1, null, 0, null);
mv.visitInsn(RETURN);
Label l5 = new Label();
mv.visitLabel(l5);
mv.visitLocalVariable("var1", "I", null, l1, l2, 1);
mv.visitLocalVariable("this", "Ldemo/Hello;", null, l0, l5, 0);
mv.visitMaxs(2, 2);
mv.visitEnd();
}
{
mv = cw.visitMethod(ACC_PUBLIC, "testList", "()Ljava/util/List;", "()Ljava/util/List;", null);
mv.visitCode();
Label l0 = new Label();
mv.visitLabel(l0);
mv.visitTypeInsn(NEW, "java/util/ArrayList");
mv.visitInsn(DUP);
mv.visitMethodInsn(INVOKESPECIAL, "java/util/ArrayList", "", "()V", false);
mv.visitVarInsn(ASTORE, 1);
Label l1 = new Label();
mv.visitLabel(l1);
mv.visitVarInsn(ALOAD, 1);
mv.visitLdcInsn("Tome");
mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "add", "(Ljava/lang/Object;)Z", true);
mv.visitInsn(POP);
Label l2 = new Label();
mv.visitLabel(l2);
mv.visitVarInsn(ALOAD, 1);
mv.visitLdcInsn("Jack");
mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "add", "(Ljava/lang/Object;)Z", true);
mv.visitInsn(POP);
Label l3 = new Label();
mv.visitLabel(l3);
mv.visitVarInsn(ALOAD, 1);
mv.visitLdcInsn("Lily");
mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "add", "(Ljava/lang/Object;)Z", true);
mv.visitInsn(POP);
Label l4 = new Label();
mv.visitLabel(l4);
mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv.visitTypeInsn(NEW, "java/lang/StringBuilder");
mv.visitInsn(DUP);
mv.visitLdcInsn(">>>>>>>>>>testList > list.size = ");
mv.visitMethodInsn(INVOKESPECIAL, "java/lang/StringBuilder", "", "(Ljava/lang/String;)V", false);
mv.visitVarInsn(ALOAD, 1);
mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "size", "()I", true);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(I)Ljava/lang/StringBuilder;",
false);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;", false);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
Label l5 = new Label();
mv.visitLabel(l5);
mv.visitVarInsn(ALOAD, 1);
mv.visitInsn(ARETURN);
Label l6 = new Label();
mv.visitLabel(l6);
mv.visitMaxs(4, 2);
mv.visitEnd();
}
{
mv = cw.visitMethod(ACC_PUBLIC, "testMap", "(Ljava/util/List;)Ljava/util/Map;", "(Ljava/util/List;)Ljava/util/Map;", null);
mv.visitCode();
Label l0 = new Label();
mv.visitLabel(l0);
mv.visitLineNumber(32, l0);
mv.visitTypeInsn(NEW, "java/util/HashMap");
mv.visitInsn(DUP);
mv.visitMethodInsn(INVOKESPECIAL, "java/util/HashMap", "", "()V", false);
mv.visitVarInsn(ASTORE, 2);
Label l1 = new Label();
mv.visitLabel(l1);
mv.visitLineNumber(34, l1);
mv.visitInsn(ICONST_0);
mv.visitVarInsn(ISTORE, 3);
Label l2 = new Label();
mv.visitLabel(l2);
mv.visitframe(Opcodes.F_APPEND, 2, new Object[]{"java/util/Map", Opcodes.INTEGER}, 0, null);
mv.visitVarInsn(ILOAD, 3);
mv.visitVarInsn(ALOAD, 1);
mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "size", "()I", true);
Label l3 = new Label();
mv.visitJumpInsn(IF_ICMPGE, l3);
Label l4 = new Label();
mv.visitLabel(l4);
mv.visitLineNumber(35, l4);
mv.visitVarInsn(ALOAD, 1);
mv.visitVarInsn(ILOAD, 3);
mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List", "get", "(I)Ljava/lang/Object;", true);
mv.visitTypeInsn(CHECKCAST, "java/lang/String");
mv.visitVarInsn(ASTORE, 4);
Label l5 = new Label();
mv.visitLabel(l5);
mv.visitLineNumber(36, l5);
mv.visitVarInsn(ALOAD, 2);
mv.visitVarInsn(ALOAD, 4);
mv.visitTypeInsn(NEW, "java/lang/StringBuilder");
mv.visitInsn(DUP);
mv.visitMethodInsn(INVOKESPECIAL, "java/lang/StringBuilder", "", "()V", false);
mv.visitVarInsn(ALOAD, 4);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
mv.visitVarInsn(ALOAD, 4);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;", false);
mv.visitMethodInsn(INVOKEINTERFACE, "java/util/Map", "put", "(Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;", true);
mv.visitInsn(POP);
Label l6 = new Label();
mv.visitLabel(l6);
mv.visitLineNumber(34, l6);
mv.visitIincInsn(3, 1);
mv.visitJumpInsn(GOTO, l2);
mv.visitLabel(l3);
mv.visitLineNumber(39, l3);
mv.visitframe(Opcodes.F_CHOP, 1, null, 0, null);
mv.visitVarInsn(ALOAD, 2);
mv.visitInsn(ARETURN);
Label l7 = new Label();
mv.visitLabel(l7);
mv.visitLocalVariable("str", "Ljava/lang/String;", null, l5, l6, 4);
mv.visitLocalVariable("i", "I", null, l2, l3, 3);
mv.visitLocalVariable("this", "Ldemo/Hello;", null, l0, l7, 0);
mv.visitLocalVariable("list", "Ljava/util/List;", "Ljava/util/List;", l0, l7, 1);
mv.visitLocalVariable("map", "Ljava/util/Map;", "Ljava/util/Map;", l1, l7, 2);
mv.visitMaxs(4, 5);
mv.visitEnd();
}
cw.visitEnd();
return cw.toByteArray();
}
public static byte[] createClass() {
try {
byte[] data = generatorHelloClass();
// File file = new File("/Users/mac/Download/classes/com/lzc/Hello.class");
File file = new File("/Users/mac/work/gitstudy/lzcjavaagent/target/classes/com/lzc/Hello.class");
File parent1 = new File(file.getParent());
boolean mkdirs = parent1.mkdirs();
System.out.println("mkdirs=" + mkdirs + ",exist=" + file.exists());
boolean createNewFile = file.createNewFile();
System.out.println("createNewFile=" + createNewFile + ",exist=" + file.exists());
FileOutputStream fos = new FileOutputStream(file);
fos.write(data);
fos.close();
return data;
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
public static void useClass(byte[] data) {
try {
MyClassLoader myClassLoader = new MyClassLoader();
Class helloClass = myClassLoader.defineClass("com.lzc.Hello", data);
Object obj = helloClass.newInstance();
Method method = helloClass.getMethod("display", null);
method.invoke(obj, null);
method = helloClass.getMethod("testList", null);
Object result = method.invoke(obj, null);
System.out.println(result);
method = helloClass.getMethod("testMap", List.class);
Object result2 = method.invoke(obj, result);
System.out.println(result2);
} catch (Exception e) {
e.printStackTrace();
}
}
static class MyClassLoader extends ClassLoader {
public MyClassLoader() {
super();
}
@Override
public Class loadClass(String name) throws ClassNotFoundException {
return super.loadClass(name);
}
public Class defineClass(String name, byte[] b) {
return super.defineClass(name, b, 0, b.length);
}
}
public static void main(String[] args) {
try {
byte[] data = createClass();
useClass(data);
} catch (Exception e) {
e.printStackTrace();
}
}
}
生成的class文件，反编译之后的java文件如下：
package com.lzc;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class Hello {
public static final String FLAG = "我是常量";
public Hello() {
}
public void display() {
for(int var1 = 0; var1 < 6; ++var1) {
System.out.println(">>>>>>>>>>我是常量");
}
}
public List testList() {
ArrayList var1 = new ArrayList();
var1.add("Tome");
var1.add("Jack");
var1.add("Lily");
System.out.println(">>>>>>>>>>testList > list.size = " + var1.size());
return var1;
}
public Map testMap(List list) {
Map map = new HashMap();
for(int i = 0; i < list.size(); ++i) {
String str = (String)list.get(i);
map.put(str, str + str);
}
return map;
}
}
基于asm篡改一个class文件
本文将以一个实际案例进行讲解，在这里我们假设无法直接去修改MathGame.java，但是我们有MathGame.class文件，计划将MathGame.class进行增强。
package com.example.lzcjavaagent;
import org.objectweb.asm.*;
import org.objectweb.asm.commons.AnalyzerAdapter;
import org.objectweb.asm.commons.LocalVariablesSorter;
import org.objectweb.asm.util.TraceClassVisitor;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.PrintWriter;
public class TimeCountAdpter extends ClassVisitor implements Opcodes {
private String owner;
private boolean isInterface;
private String methodName;
private String filedName = "UDASMCN";
private int acc = Opcodes.ACC_PUBLIC + Opcodes.ACC_STATIC + Opcodes.ACC_FINAL;
private boolean isPresent = false;
public TimeCountAdpter(ClassVisitor classVisitor) {
super(ASM6, classVisitor);
}
@Override
public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {
super.visit(version, access, name, signature, superName, interfaces);
owner = name;
isInterface = (access & ACC_INTERFACE) != 0;
}
@Override
public MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {
MethodVisitor mv = cv.visitMethod(access, name, descriptor, signature, exceptions);
if (!isInterface && mv != null && !name.equals("") && !name.equals("") && !name.startsWith("java")) {
methodName = name;
AddTimerMethodAdapter ca = new AddTimerMethodAdapter(mv);
AnalyzerAdapter aa = new AnalyzerAdapter(owner, access, name, descriptor, ca);
LocalVariablesSorter lvs = new LocalVariablesSorter(access, descriptor, aa);
ca.aa = aa;
ca.lvs = lvs;
return ca.lvs;
}
return mv;
}
@Override
public FieldVisitor visitField(int access, String name, String descriptor, String signature, Object value) {
if (name.equals(filedName)) {
isPresent = true;
}
return super.visitField(access, name, descriptor, signature, value);
}
@Override
public void visitEnd() {
if (!isInterface) {
FieldVisitor fv = cv.visitField(acc, filedName, "Ljava/lang/String;", null, owner);
if (fv != null) {
fv.visitEnd();
}
}
cv.visitEnd();
}
class AddTimerMethodAdapter extends MethodVisitor {
private int time;
private int maxStack;
public LocalVariablesSorter lvs;
public AnalyzerAdapter aa;
public AddTimerMethodAdapter(MethodVisitor methodVisitor) {
super(ASM6, methodVisitor);
}
@Override
public void visitCode() {
mv.visitCode();
mv.visitMethodInsn(INVOKESTATIC, "java/lang/System", "nanoTime", "()J", false);
time = lvs.newLocal(Type.LONG_TYPE);
mv.visitVarInsn(LSTORE, time);
maxStack = 4;
}
@Override
public void visitInsn(int opcode) {
if ((opcode >= IRETURN && opcode <= RETURN) || opcode == ATHROW) {
mv.visitMethodInsn(INVOKESTATIC, "java/lang/System", "nanoTime", "()J", false);
mv.visitVarInsn(LLOAD, time);
mv.visitInsn(LSUB);
mv.visitVarInsn(LSTORE, time);
mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
mv.visitTypeInsn(NEW, "java/lang/StringBuilder");
mv.visitInsn(DUP);
mv.visitMethodInsn(INVOKESPECIAL, "java/lang/StringBuilder", "", "()V", false);
mv.visitLdcInsn(" " + owner.replaceAll("/", "\."));
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
mv.visitLdcInsn("#" + methodName + ":");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
mv.visitVarInsn(LLOAD, time);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(J)Ljava/lang/StringBuilder;", false);
mv.visitLdcInsn("(ns)");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;", false);
mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
maxStack = Math.max(aa.stack.size() + 4, maxStack);
}
mv.visitInsn(opcode);
}
@Override
public void visitMaxs(int maxStack, int maxLocals) {
super.visitMaxs(Math.max(maxStack, this.maxStack), maxLocals);
}
}
public static void main(String[] args) {
try {
ClassWriter classWriter = new ClassWriter(ClassWriter.COMPUTE_frameS);
TraceClassVisitor traceClassVisitor = new TraceClassVisitor(classWriter, new PrintWriter(System.out));
TimeCountAdpter timeCountAdpter = new TimeCountAdpter(traceClassVisitor);
ClassReader classReader = new ClassReader("demo.MathGame");
classReader.accept(timeCountAdpter, ClassReader.EXPAND_frameS);
File file = new File("/Users/mac/Download/classes/demo/MathGame.class");
String parent = file.getParent();
File parent1 = new File(parent);
boolean mkdirs = parent1.mkdirs();
System.out.println("mkdirs=" + mkdirs + ",exist=" + file.exists());
boolean createNewFile = file.createNewFile();
System.out.println("mkdirs=" + mkdirs + ",createNewFile=" + createNewFile + ",exist=" + file.exists());
FileOutputStream fileOutputStream = new FileOutputStream(file);
fileOutputStream.write(classWriter.toByteArray());
fileOutputStream.flush();
fileOutputStream.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
增强前的MathGame完整代码
package demo;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.TimeUnit;
//-javaagent:/Users/mac/work/gitstudy/lzcjavaagent/target/lzcjavaagent-0.0.1-SNAPSHOT.jar
public class MathGame {
private static Random random = new Random();
private int illegalArgumentCount = 0;
public static void main(String[] args) throws InterruptedException {
MathGame game = new MathGame();
while (true) {
game.run();
TimeUnit.SECONDS.sleep(2);
}
}
public void run() throws InterruptedException {
try {
int x=5;
int y=2;
int z=x-y;
int number = random.nextInt()/1000;
List primeFactors = primeFactors(number);
print(number, primeFactors);
} catch (Exception e) {
System.out.println(String.format("debug illegalArgumentCount:%3d, ", illegalArgumentCount) + e.getMessage());
}
}
public static void print(int number, List primeFactors) {
StringBuffer sb = new StringBuffer(number + "=");
for (int factor : primeFactors) {
sb.append(factor).append('*');
}
if (sb.charAt(sb.length() - 1) == '*') {
sb.deleteCharAt(sb.length() - 1);
}
System.out.println("debug=="+sb);
}
public List primeFactors(int number) {
if (number < 2) {
illegalArgumentCount++;
throw new IllegalArgumentException("debug number is: " + number + ", need >= 2");
}
List result = new ArrayList();
int i = 2;
while (i <= number) {
if (number % i == 0) {
result.add(i);
number = number / i;
i = 2;
} else {
i++;
}
}
return result;
}
}
增强后的MathGame
package demo;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Random;
import java.util.concurrent.TimeUnit;
public class MathGame {
private static Random random;
private int illegalArgumentCount = 0;
public static final String UDASMCN = "demo/MathGame";
public MathGame() {
}
public static void main(String[] var0) throws InterruptedException {
long var1 = System.nanoTime();
MathGame game = new MathGame();
while(true) {
game.run();
TimeUnit.SECONDS.sleep(2L);
}
}
public void run() throws InterruptedException {
long var1 = System.nanoTime();
try {
int x = 5;
int y = 2;
int var10000 = x - y;
int number = random.nextInt() / 1000;
List primeFactors = this.primeFactors(number);
print(number, primeFactors);
} catch (Exception var8) {
System.out.println(String.format("debug illegalArgumentCount:%3d, ", this.illegalArgumentCount) + var8.getMessage());
}
var1 = System.nanoTime() - var1;
System.out.println(" demo.MathGame" + "#run:" + var1 + "(ns)");
}
public static void print(int number, List primeFactors) {
long var2 = System.nanoTime();
StringBuffer sb = new StringBuffer(number + "=");
Iterator var5 = primeFactors.iterator();
while(var5.hasNext()) {
int factor = (Integer)var5.next();
sb.append(factor).append('*');
}
if (sb.charAt(sb.length() - 1) == '*') {
sb.deleteCharAt(sb.length() - 1);
}
System.out.println("debug==" + sb);
var2 = System.nanoTime() - var2;
System.out.println(" demo.MathGame" + "#print:" + var2 + "(ns)");
}
public List primeFactors(int number) {
long var2 = System.nanoTime();
if (number < 2) {
++this.illegalArgumentCount;
IllegalArgumentException var10000 = new IllegalArgumentException("debug number is: " + number + ", need >= 2");
var2 = System.nanoTime() - var2;
System.out.println(" demo.MathGame" + "#primeFactors:" + var2 + "(ns)");
throw var10000;
} else {
List result = new ArrayList();
int i = 2;
while(i <= number) {
if (number % i == 0) {
result.add(i);
number /= i;
i = 2;
} else {
++i;
}
}
var2 = System.nanoTime() - var2;
System.out.println(" demo.MathGame" + "#primeFactors:" + var2 + "(ns)");
return result;
}
}
static {
long var0 = System.nanoTime();
random = new Random();
var0 = System.nanoTime() - var0;
System.out.println(" demo.MathGame" + "#:" + var0 + "(ns)");
}
}
JavaAgent
前面已经演示了基于asm增强class文件，接下来进一步演示运行期增强class文件。
Java Agent定义从本质上讲，Java Agent 是一个遵循一组严格约定的常规 Java 类。 上面说到 javaagent命令要求指定的类中必须要有premain()方法，并且对premain方法的签名也有要求，签名必须满足以下两种格式：
public static void premain(String agentArgs, Instrumentation inst) public static void premain(String agentArgs)
JVM 会优先加载 带 Instrumentation 签名的方法，加载成功忽略第二种，如果第一种没有，则加载第二种方法。简单来讲，就是 premain 方法，在 main 方法之前执行。
如何使用javaagent？使用 javaagent 需要几个步骤：
- 定义一个 MANIFEST.MF 文件，必须包含 Premain-Class 选项，通常也会加入Can-Redefine-Classes 和 Can-Retransform-Classes 选项。
- 创建一个Premain-Class 指定的类，类中包含 premain 方法，方法逻辑由用户自己确定（就是下面示例中的类TraceAgent）。
- 将 premain 的类和 MANIFEST.MF 文件打成 jar 包。
- 使用参数 -javaagent: jar包路径 启动要代理的方法。（示例： -javaagent:
/Users/mac/work/gitstudy/lzcjavaagent/target/lzcjavaagent-0.0.1-SNAPSHOT.jar）
package com.example.lzcjavaagent;
import java.lang.instrument.Instrumentation;
public class TraceAgent {
public static void premain(String agentArgs, Instrumentation instrumentation){
instrumentation.addTransformer(new LogTransformer());
}
}
package com.example.lzcjavaagent;
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassWriter;
import java.io.IOException;
import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;
public class LogTransformer implements ClassFileTransformer {
@Override
public byte[] transform(ClassLoader loader, String className, Class classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
try {
if(className.startsWith("java")||className.startsWith("sun")||className.startsWith("com/intellij/rt/debugger")){
return classfileBuffer;
}
ClassReader cr = new ClassReader(className);
ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
TimeCountAdpter timeCountAdpter = new TimeCountAdpter(cw);
cr.accept(timeCountAdpter, ClassReader.EXPAND_frameS);
return cw.toByteArray();
} catch (IOException e) {
System.out.println("alioo IOException:"+className+":"+e.getMessage());
e.printStackTrace();
} catch (Error e) {
System.out.println("alioo Error:"+className+":"+e.getMessage());
e.printStackTrace();
}
return classfileBuffer;
}
}
除了上述类之外，生成的jar文件中的meta-INF/MANIFEST.MF中还需要增加如下内容：
Premain-Class: com.example.lzcjavaagent.TraceAgent
示例：
~/work/gitstudy/lzcjavaagent/target>more lzcjavaagent-0.0.1-SNAPSHOT/meta-INF/MANIFEST.MF Manifest-Version: 1.0 Premain-Class: com.example.lzcjavaagent.TraceAgent Archiver-Version: Plexus Archiver Built-By: mac Created-By: Apache Maven 3.6.3 Build-Jdk: 1.8.0_192
如果嫌手写比较麻烦的话可以使用maven插件maven-shade-plugin
org.apache.maven.plugins maven-shade-pluginpackage shade com.example.lzcjavaagent.TraceAgent
本文完整的代码git地址：https://github.com/lzc-alioo/lzcjavaagent，欢迎下载运行。
参考文章：
https://www.cnblogs.com/rickiyang/p/11368932.html
https://blog.csdn.net/catoop/article/details/50629921
https://blog.csdn.net/wenwen513/article/details/86498687
