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Having fun with Javassist JEEConf 2017 @antonarhipov

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whoami Anton Arhipov @antonarhipov Javassist inside!

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@Entity
 @Table(name = "owners")
 public class Owner extends Person {
 @Column(name = "address")
 @NotEmpty
 private String address;
 
 @Column(name = "city")
 @NotEmpty
 private String city;
 
 @Column(name = "telephone")
 @NotEmpty
 @Digits(fraction = 0, integer = 10)
 private String telephone;
 
 @OneToMany(cascade = CascadeType.ALL, mappedBy = "owner")
 private Set pets;

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public class JavassistLazyInitializer extends BasicLazyInitializer implements MethodHandler { final JavassistLazyInitializer instance = new JavassistLazyInitializer(…); 
 ProxyFactory factory = new ProxyFactory();
 factory.setSuperclass(interfaces.length == 1?persistentClass:null);
 factory.setInterfaces(interfaces);
 factory.setFilter(FINALIZE_FILTER); 
 Class cl = factory.createClass();
 final HibernateProxy proxy = (HibernateProxy) cl.newInstance();
 ((ProxyObject)proxy).setHandler(instance);
 instance.constructed = true;
 return proxy;

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public class JavassistLazyInitializer extends BasicLazyInitializer implements MethodHandler { final JavassistLazyInitializer instance = new JavassistLazyInitializer(…); 
 ProxyFactory factory = new ProxyFactory();
 factory.setSuperclass(interfaces.length == 1?persistentClass:null);
 factory.setInterfaces(interfaces);
 factory.setFilter(FINALIZE_FILTER); 
 Class cl = factory.createClass();
 final HibernateProxy proxy = (HibernateProxy) cl.newInstance();
 ((ProxyObject)proxy).setHandler(instance);
 instance.constructed = true;
 return proxy;

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The main use case for bytecode generation in Java frameworks is to generate proxies

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Why? Programming model (AOP, ORM, итд) Monitoring agent (NewRelic, XRebel) Specialized debuggers Developer tools (JRebel) Obfuscation (Proguard)

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Javassist 101 www.javassist.org

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API ClassPool CtClass CtClass CtClass CtClass CtField CtMethod CtConst CtMethod insertBefore insertAfter instrument

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https://speakerdeck.com/antonarhipov

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ct = cp.makeClass("com.zt.A", cp.get("com.zt.Clazz")); CtMethod[] methods = ct.getMethods();
 for (CtMethod method : methods) {
 //… } ct.writeFile("/output"); }

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ct = cp.makeClass("com.zt.A", cp.get("com.zt.Clazz")); CtMethod[] methods = ct.getMethods();
 for (CtMethod method : methods) {
 //… } ct.writeFile("/output"); } ClassPool cp = new ClassPool(null); cp.appendSystemPath();

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ct = cp.makeClass("com.zt.A", cp.get("com.zt.Clazz")); CtMethod[] methods = ct.getMethods();
 for (CtMethod method : methods) {
 //… } ct.writeFile("/output"); } public class A extends Clazz { 
 public A() {
 }
 }

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ct = cp.makeClass("com.zt.A", cp.get("com.zt.Clazz")); CtMethod[] methods = ct.getMethods();
 for (CtMethod method : methods) {
 //… } ct.writeFile("/output"); }

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ct = cp.makeClass("com.zt.A", cp.get("com.zt.Clazz")); CtMethod[] methods = ct.getMethods();
 for (CtMethod method : methods) {
 //… } ct.writeFile("/output"); } mars:output anton$ javap -c com/zt/A.class public class com.zt.A extends com.zt.Clazz { public com.zt.A(); Code: 0: aload_0 1: invokespecial #10 4: return

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ct = cp.makeClass("com.zt.A", cp.get("com.zt.Clazz")); CtMethod[] methods = ct.getMethods();
 for (CtMethod method : methods) {
 //… } ct.writeFile("/output"); } Can generate classes from metadata at build time

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); cp.appendClassPath(new ClassPath(){ … }); CtClass ct = cp.get("com.zt.A"); CtMethod[] methods = ct.getMethods();
 for (CtMethod method : methods) {
 //… } ct.writeFile("/output"); } … or you can post process the compiled classes

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ctClass = cp.get("com.zt.A"); CtMethod foo = ctClass.getMethod("foo", "()V"); foo.insertBefore("System.out.println();"); Class c = ctClass.toClass();
 A a = (A) c.newInstance();
 a.foo("Hello");
 }

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ctClass = cp.get("com.zt.A"); CtMethod foo = ctClass.getMethod("foo", "()V"); foo.insertBefore("System.out.println();"); Class c = ctClass.toClass();
 A a = (A) c.newInstance();
 a.foo("Hello");
 }

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ctClass = cp.get("com.zt.A"); CtMethod foo = ctClass.getMethod("foo", "()V"); foo.insertBefore("System.out.println();"); Class c = ctClass.toClass();
 A a = (A) c.newInstance();
 a.foo("Hello");
 } public void foo() { 
 }

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ctClass = cp.get("com.zt.A"); CtMethod foo = ctClass.getMethod("foo", "(Ljava/lang/String;)V"); foo.insertBefore("System.out.println();"); Class c = ctClass.toClass();
 A a = (A) c.newInstance();
 a.foo("Hello");
 } public void foo(String s) { 
 }

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ctClass = cp.get("com.zt.A"); CtMethod foo = ctClass.getMethod("foo", "(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;Lja foo.insertBefore("System.out.println();"); Class c = ctClass.toClass();
 A a = (A) c.newInstance();
 a.foo("Hello");
 } Descriptors might get quite long ;)

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ctClass = cp.get("com.zt.A"); CtMethod foo = ctClass.getMethod("foo", "(Ljava/lang/String;)V"); foo.insertBefore("System.out.println($1)"); Class c = ctClass.toClass();
 A a = (A) c.newInstance();
 a.foo("Hello");
 } $1, $2, $3 — local variables $0 — this for non-static methods

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ctClass = cp.get("com.zt.A"); CtMethod foo = ctClass.getMethod("foo", "(Ljava/lang/String;)V"); foo.insertBefore("System.out.println($1)"); Class c = ctClass.toClass();
 A a = (A) c.newInstance();
 a.foo("Hello");
 } Exception in thread "main" javassist.CannotCompileException: [source error] ; is missing at javassist.CtBehavior.insertBefore(CtBehavior.java:774) at javassist.CtBehavior.insertBefore(CtBehavior.java:734) at com.zt.basics.Ex.main(Ex.java:35)

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public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass ctClass = cp.get("com.zt.A"); CtMethod foo = ctClass.getMethod("foo", "(Ljava/lang/String;)V"); foo.insertBefore("System.out.println($1);"); Class c = ctClass.toClass();
 A a = (A) c.newInstance();
 a.foo("Hello");
 }

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CtMethod foo = … foo.insertBefore(…); foo.insertAfter(…); Can implement tracing

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… or add logging CtMethod foo = … foo.insertBefore(…); foo.insertAfter(…);

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… or implement AOP CtMethod foo = … foo.insertBefore(…); foo.insertAfter(…);

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CtMethod foo = … foo.instrument(new ExprEditor() { @Override
 public void edit(NewExpr e) throws CannotCompileException {
 e.replace("{" +
 "$_ = $proceed($$);" +
 "System.out.println($_);" +
 "}");
 } });

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CtMethod foo = … foo.instrument(new ExprEditor() { @Override
 public void edit(NewExpr e) throws CannotCompileException {
 e.replace("{" +
 "$_ = $proceed($$);" +
 "System.out.println($_);" +
 "}");
 } });

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CtMethod foo = … foo.instrument(new ExprEditor() { @Override
 public void edit(NewExpr e) throws CannotCompileException {
 e.replace("{" +
 "$_ = $proceed($$);" +
 "System.out.println($_);" +
 "}");
 } }); Intercept new instances

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CtMethod foo = … foo.instrument(new ExprEditor() { @Override
 public void edit(NewExpr e) throws CannotCompileException {
 e.replace("{" +
 "$_ = $proceed($$);" +
 "System.out.println($_);" +
 "}");
 } }); Intercept new instances

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CtMethod foo = … foo.instrument(new ExprEditor() { @Override
 public void edit(MethodCall m) throws CannotCompileException {
 if(m.getMethodName().contains("println")) {
 m.replace("{}");
 } } }); Remove unwanted invocations

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CtMethod foo = … foo.instrument(new ExprEditor() { @Override
 public void edit(FieldAccess f) throws CannotCompileException {
 if (f.isWriter()) {
 CtField field = f.getField();
 String setterName = findSetter(field);
 f.replace("{" + "$0." + setterName + "($$);" + "}");
 } } }); Replace direct field access with setter calls

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java.lang.ClassFormatError: LVTT entry for 'callbackTypes' in class file com/google/inject/internal/ProxyFactory does not match any LVT entry

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Java agents

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Java Agent import java.lang.instrument.ClassFileTransformer; import java.lang.instrument.Instrumentation; public class Agent { public static void premain(String args, Instrumentation inst) throws Exception { inst.addTransformer(new ClassFileTransformer { // here be code }); } } META-INF/MANIFEST.MF Premain-Class: Agent $> java –javaagent:agent.jar application.Main

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ClassFileTransformer new ClassFileTransformer() { public byte[] transform(ClassLoader loader, String className, Class> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer){ ClassPool cp = ClassPool.getDefault(); CtClass ct = cp.makeClass(new ByteArrayInputStream(classfileBuffer)); // here we can do all the things to ‘ct’ return ct.toBytecode(); } }

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new ClassFileTransformer() { public byte[] transform(ClassLoader loader, String className, Class> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer){ ClassPool cp = ClassPool.getDefault(); CtClass ct = cp.makeClass(new ByteArrayInputStream(classfileBuffer)); // here we can do all the things to ‘ct’ return ct.toBytecode(); } } ClassFileTransformer

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new ClassFileTransformer() { public byte[] transform(ClassLoader loader, String className, Class> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer){ ClassPool cp = ClassPool.getDefault(); CtClass ct = cp.makeClass(new ByteArrayInputStream(classfileBuffer)); // here we can do all the things to ‘ct’ return ct.toBytecode(); } } ClassFileTransformer

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new ClassFileTransformer() { public byte[] transform(ClassLoader loader, String className, Class> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer){ ClassPool cp = ClassPool.getDefault(); CtClass ct = cp.makeClass(new ByteArrayInputStream(classfileBuffer)); // here we can do all the things to ‘ct’ return ct.toBytecode(); } } ClassFileTransformer

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https://github.com/zeroturnaround/callspy

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Javassist in

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JRebel core Spring plugin Hibernate plugin EJB plugin

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JRebel core Spring plugin Hibernate plugin EJB plugin jrebel.jar

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Reloads classes JRebel core Spring plugin Hibernate plugin EJB plugin

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Notifies plugins JRebel core Spring plugin Hibernate plugin EJB plugin

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JRebel core Spring plugin Hibernate plugin EJB plugin Refresh configurations

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Javassist lives here Spring plugin Hibernate plugin EJB plugin JRebel core

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Spring Hibernate OpenEJB JRebel core Spring plugin Hibernate plugin EJB plugin

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class Framework { public void configure(){ //…
 } } CtClass framework = cp.get("com.zt.Framework"); framework.addInterface( cp.get("com.zt.jrebel.Listener")); class Framework implements Listener { public void configure(){
 } } framework.addMethod( CtNewMethod.make( "public void onEvent(){" + " configure();" + "}", framework ));

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https://github.com/antonarhipov/jpoint HowItWorks

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Your task Javassist

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https://speakerdeck.com/antonarhipov http://www.slideshare.net/arhan @antonarhipov anton@zeroturnaround.com