GitHub - jboss-javassist/javassist: Java bytecode engineering toolkit
示例：Point.java
package com.ks.test.app;
public class Point {
private int x;
private int y;
public String name;
public Point(int x, int y) {
this.x = x;
this.y = y;
}
public int getX() {
return x;
}
public void setX(int x) {
this.x = x;
}
public int getY() {
return y;
}
public void setY(int y) {
this.y = y;
}
public void move(int x, int y){
this.x = x;
this.y = y;
}
}
编译java文件并查看字节码
mvn clean compile
javap -c target.classes.com.ks.test.app.Point
Java 字节码
public class com.ks.test.app.Point {
public java.lang.String name;
public com.ks.test.app.Point(int, int);
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: iload_1
6: putfield #2 // Field x:I
9: aload_0
10: iload_2
11: putfield #3 // Field y:I
14: return
public int getX();
Code:
0: aload_0
1: getfield #2 // Field x:I
4: ireturn
public void setX(int);
Code:
0: aload_0
1: iload_1
2: putfield #2 // Field x:I
5: return
public int getY();
Code:
0: aload_0
1: getfield #3 // Field y:I
4: ireturn
public void setY(int);
Code:
0: aload_0
1: iload_1
2: putfield #3 // Field y:I
5: return
public void move(int, int);
Code:
0: aload_0
1: iload_1
2: putfield #2 // Field x:I
5: aload_0
6: iload_2
7: putfield #3 // Field y:I
10: return
}
分析一下move()方法的字节码
aload_0: 从局部变量0加载一个引用到堆栈上
iload_1: 从局部变量1加载 int 值
putfield：给对象赋值
return：方法返回
Java代码都会编译成字节码，使用 Javasisst 可以非常容易的修改字节码；
生产一个Java类
import java.lang.reflect.Field;
import javassist.ClassPool;
import javassist.bytecode.AccessFlag;
import javassist.bytecode.ClassFile;
import javassist.bytecode.FieldInfo;
public class Demo1 {
public static void main(String[] args) throws Exception {
// 生产一个Demo类，并添加一个id字段
ClassFile cf = new ClassFile(false, "com.ks.test.app.Demo", null);
cf.setInterfaces(new String[]{"java.io.Serializable"});
FieldInfo fieldInfo = new FieldInfo(cf.getConstPool(), "id", "I");
fieldInfo.setAccessFlags(AccessFlag.PUBLIC);
cf.addField(fieldInfo);
ClassPool classPool = ClassPool.getDefault();
Field[] fields = classPool.makeClass(cf).toClass().getFields();
//
System.out.println(fields[0].getName());
}
}
获取Java Class 的字节码
import javassist.ClassPool;
import javassist.bytecode.ClassFile;
import javassist.bytecode.CodeAttribute;
import javassist.bytecode.CodeIterator;
import javassist.bytecode.MethodInfo;
import javassist.bytecode.Mnemonic;
public class Demo2 {
public static void main(String[] args) throws Exception {
ClassPool classPool = ClassPool.getDefault();
ClassFile classFile = classPool.get("com.ks.test.app.Point").getClassFile();
MethodInfo methodInfo = classFile.getMethod("move");
CodeAttribute codeAttribute = methodInfo.getCodeAttribute();
CodeIterator codeIterator = codeAttribute.iterator();
// 打印 move 方法的字节码
System.out.println("start print move byte code....");
System.out.println();
while (codeIterator.hasNext()) {
int index = codeIterator.next();
int op = codeIterator.byteAt(index);
System.out.println(Mnemonic.OPCODE[op]);
}
System.out.println();
System.out.println("end print move byte code....");
}
}
给已有Java Class 添加字段
import java.lang.reflect.Field;
import javassist.ClassPool;
import javassist.bytecode.AccessFlag;
import javassist.bytecode.ClassFile;
import javassist.bytecode.FieldInfo;
public class Demo3 {
public static void main(String[] args) throws Exception {
ClassPool classPool = ClassPool.getDefault();
ClassFile classFile = classPool.get("com.ks.test.app.Point").getClassFile();
// 给 Point 类添加一个id字段
FieldInfo fieldInfo = new FieldInfo(classFile.getConstPool(), "id", "I");
fieldInfo.setAccessFlags(AccessFlag.PUBLIC);
classFile.addField(fieldInfo);
// only get public fields
Field[] fields = classPool.makeClass(classFile).toClass().getFields();
for (Field field : fields) {
System.out.println(field.getName());
}
}
}
添加构造方法
import javassist.ClassPool;
import javassist.bytecode.Bytecode;
import javassist.bytecode.ClassFile;
import javassist.bytecode.CodeIterator;
import javassist.bytecode.MethodInfo;
import javassist.bytecode.Mnemonic;
public class Demo4 {
public static void main(String[] args) throws Exception {
ClassPool classPool = ClassPool.getDefault();
ClassFile classFile = classPool.get("com.ks.test.app.Point").getClassFile();
Bytecode bytecode = new Bytecode(classFile.getConstPool());
bytecode.addLload(0);
bytecode.addInvokespecial("java/lang/Object", MethodInfo.nameInit, "()V");
bytecode.addReturn(null);
// addInvokespecial
// 调用 java.lang.Object <init> 方法
MethodInfo methodInfo = new MethodInfo(classFile.getConstPool(), MethodInfo.nameInit, "()V");
methodInfo.setCodeAttribute(bytecode.toCodeAttribute());
classFile.addMethod(methodInfo);
CodeIterator codeIterator = bytecode.toCodeAttribute().iterator();
// 打印字节码
while (codeIterator.hasNext()) {
int index = codeIterator.next();
int op = codeIterator.byteAt(index);
System.out.println(Mnemonic.OPCODE[op]);
}
}
}
运行时替换 Java Class，可以通过以下2种方式：
RedefineClassAgent 工具类，生成 jvm agent，并load agent，利用 Instrumentation redefineClasses 更新运行时类字节码信息；
以下为关键代码：
创建和加载Agent
/**
* Lazy loads the agent that populates {@link #instrumentation}. OK to call multiple times.
*
* @throws FailedToLoadAgentException if agent either failed to load or if the agent wasn't able to get an
* instance of {@link Instrumentation} that allows class redefinitions.
*/
private static void ensureAgentLoaded() throws FailedToLoadAgentException {
if (instrumentation != null) {
// already loaded
return;
}
// load the agent
try {
File agentJar = createAgentJarFile();
// Loading an agent requires the PID of the JVM to load the agent to. Find out our PID.
String nameOfRunningVM = ManagementFactory.getRuntimeMXBean().getName();
String pid = nameOfRunningVM.substring(0, nameOfRunningVM.indexOf('@'));
// load the agent
VirtualMachine vm = VirtualMachine.attach(pid);
vm.loadAgent(agentJar.getAbsolutePath(), "");
vm.detach();
} catch (Exception e) {
throw new FailedToLoadAgentException(e);
}
// wait for the agent to load
for (int sec = 0; sec < AGENT_LOAD_WAIT_TIME_SEC; sec++) {
if (instrumentation != null) {
// success!
return;
}
try {
LOGGER.info("Sleeping for 1 second while waiting for agent to load.");
Thread.sleep(1000);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
throw new FailedToLoadAgentException();
}
}
// agent didn't load
throw new FailedToLoadAgentException();
}
/**
* An agent must be specified as a .jar where the manifest has an Agent-Class attribute. Additionally, in order
* to be able to redefine classes, the Can-Redefine-Classes attribute must be true.
*
* This method creates such an agent Jar as a temporary file. The Agent-Class is this class. If the returned Jar
* is loaded as an agent then {@link #agentmain(String, Instrumentation)} will be called by the JVM.
*
* @return a temporary {@link File} that points at Jar that packages this class.
* @throws IOException if agent Jar creation failed.
*/
private static File createAgentJarFile() throws IOException {
File jarFile = File.createTempFile("agent", ".jar");
jarFile.deleteOnExit();
// construct a manifest that allows class redefinition
Manifest manifest = new Manifest();
Attributes mainAttributes = manifest.getMainAttributes();
mainAttributes.put(Attributes.Name.MANIFEST_VERSION, "1.0");
mainAttributes.put(new Attributes.Name("Agent-Class"), RedefineClassAgent.class.getName());
mainAttributes.put(new Attributes.Name("Can-Retransform-Classes"), "true");
mainAttributes.put(new Attributes.Name("Can-Redefine-Classes"), "true");
try (JarOutputStream jos = new JarOutputStream(new FileOutputStream(jarFile), manifest)) {
// add the agent .class into the .jar
JarEntry agent = new JarEntry(RedefineClassAgent.class.getName().replace('.', '/') + ".class");
jos.putNextEntry(agent);
// dump the class bytecode into the entry
ClassPool pool = ClassPool.getDefault();
CtClass ctClass = pool.get(RedefineClassAgent.class.getName());
jos.write(ctClass.toBytecode());
jos.closeEntry();
} catch (CannotCompileException | NotFoundException e) {
// Realistically this should never happen.
LOGGER.log(Level.SEVERE, "Exception while creating RedefineClassAgent jar.", e);
throw new IOException(e);
}
return jarFile;
}
运行时更新类字节码
/**
* Attempts to redefine class bytecode.
* <p>
* On first call this method will attempt to load an agent into the JVM to obtain an instance of
* {@link Instrumentation}. This agent load can introduce a pause (in practice 1 to 2 seconds).
*
* @see Instrumentation#redefineClasses(ClassDefinition...)
*
* @param definitions classes to redefine.
* @throws UnmodifiableClassException as thrown by {@link Instrumentation#redefineClasses(ClassDefinition...)}
* @throws ClassNotFoundException as thrown by {@link Instrumentation#redefineClasses(ClassDefinition...)}
* @throws FailedToLoadAgentException if agent either failed to load or if the agent wasn't able to get an
* instance of {@link Instrumentation} that allows class redefinitions.
*/
public static void redefineClasses(ClassDefinition... definitions) throws UnmodifiableClassException, ClassNotFoundException, FailedToLoadAgentException {
ensureAgentLoaded();
instrumentation.redefineClasses(definitions);
}
测试代码
import java.lang.instrument.ClassDefinition;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
public class Demo5 {
public static void main(String[] args) throws Exception {
ClassPool classPool = ClassPool.getDefault();
CtClass ctClass = classPool.get("com.ks.test.app.Point");
ctClass.stopPruning(true);
// javaassist freezes methods if their bytecode is saved
// defrost so we can still make changes.
if (ctClass.isFrozen()) {
ctClass.defrost();
}
CtMethod ctMethod = ctClass.getDeclaredMethod("move");
ctMethod.insertBefore("{ System.out.println(\"Wheeeeee!\"); }");
byte[] bytecode = ctClass.toBytecode();
ClassDefinition definition = new ClassDefinition(Class.forName("com.ks.test.app.Point"), bytecode);
RedefineClassAgent.redefineClasses(definition);
Point point = new Point(1, 1);
point.move(1, 2);
}
}
使用 javassist HotSwapper 工具，该工具依赖 JDWP；
JDWP: Provides the implementation of the Java Debug Wire Protocol (JDWP) agent.
使用方法：
java -Xdebug -Xrunjdwp:transport=dt_socket,address=8888,server=y,suspend=y Test参考文档：
测试代码：
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
import javassist.util.HotSwapper;
public class Demo6 {
//执行命令
//java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=10240 Demo6
public static void main(String[] args) throws Exception {
HotSwapper hs = new HotSwapper(10240);
new Hello().print();
ClassPool classPool = ClassPool.getDefault();
byte[] originBytes = classPool.get("com.ks.test.app.Hello").toBytecode();
System.out.println("** reload a v2 version");
CtClass ctClass = classPool.get("com.ks.test.app.Hello");
ctClass.stopPruning(true);
if(ctClass.isFrozen()){
ctClass.defrost();
}
CtMethod ctMethod = ctClass.getDeclaredMethod("print");
ctMethod.insertBefore("{System.out.println(\"** HelloWorld.print()\");}");
hs.reload("com.ks.test.app.Hello", ctClass.toBytecode());
new Hello().print();
System.out.println("** reload the original version");
hs.reload("com.ks.test.app.Hello", originBytes);
new Hello().print();
}
}
测试命令：
mvn clean package
java -cp "$JAVA_HOME/lib/*:target/javassit-tour-jar-with-dependencies.jar" -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=10240 com.ks.test.app.Demo6
注意：需要把 jdk 目录里 lib 下 jar 加载到 classpath
运行结果：
Listening for transport dt_socket at address: 10240
hello world
** reload a v2 version
** HelloWorld.print()
hello world
** reload the original version
hello world