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POjavassist not injecting annotation at existing field
 

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    <p>I'm trying to inject JAXB annotation at runtime using Javassist. I have written following code:</p> <pre><code>public class AssistAnnotationInjector { public static void addAnnotationRunTime(String className, String fieldName) throws NotFoundException, CannotCompileException, IOException, ClassNotFoundException{ CtClass ctClass = ClassPool.getDefault().get(className); ClassFile ccFile = ctClass.getClassFile(); ConstPool constPool = ccFile.getConstPool(); AnnotationsAttribute attr = new AnnotationsAttribute(constPool, AnnotationsAttribute.visibleTag); Annotation annot = new Annotation("javax.xml.bind.annotation.XmlTransient",constPool); attr.addAnnotation(annot); CtField field = ctClass.getDeclaredField(fieldName); field.getFieldInfo().addAttribute(attr); System.out.println(field.getAnnotation(XmlTransient.class)); ccFile.setVersionToJava5(); ctClass.writeFile(); } public static void main (String args[]) throws CannotCompileException, NotFoundException, IOException, SecurityException, NoSuchMethodException, ClassNotFoundException, JAXBException, NoSuchFieldException{ Person&lt;Student&gt; p = new Person&lt;Student&gt;(); p.setName("XYZ"); Student s = new Student(); s.setName("ABC"); s.setId("239423"); p.setPayload(s); addAnnotationRunTime("RuntimeAnnotation.Person", "name"); Field f = p.getClass().getDeclaredField("name"); System.out.println(f.getAnnotation(XmlTransient.class)); JAXBContext context = JAXBContext.newInstance(p.getClass()); Marshaller mr = context.createMarshaller(); mr.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE); mr.marshal(p, System.out); } } </code></pre> <p>And Person.java class is:</p> <pre><code>@XmlRootElement(name="Person") @XmlAccessorType(XmlAccessType.FIELD) @XmlSeeAlso({Student.class}) public class Person &lt;T&gt;{ private T payload; private String name; public void setPayload(T payload){ this.payload = payload; } public T getPayload(){ return payload; } public void setName(String name){ this.name = name; } public String getName(){ return name; } } </code></pre> <p>In AssistAnnotationInjector.java, I am trying to add XmlTransient annotation to 'name' field. But the name field is still coming in marshalling output. Why is it so?</p> <p>PS: marshal output is :</p> <pre><code>@javax.xml.bind.annotation.XmlTransient null &lt;?xml version="1.0" encoding="UTF-8" standalone="yes"?&gt; &lt;Person&gt; &lt;payload xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="student"&gt; &lt;name&gt;ABC&lt;/name&gt; &lt;id&gt;239423&lt;/id&gt; &lt;/payload&gt; **&lt;name&gt;XYZ&lt;/name&gt;** &lt;/Person&gt; </code></pre> <p>name tag was not expected to present in output..</p>
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    1. COKeep in mind that the way your code currently is, Person class is loaded by the classloader as soon as you start your AssistAnnotationInjector, that means even if you injection is successful you already have the non injected class loaded into memory and that will be the one used. To check if your injection was successful use `java -c -l -v -p` in the compiled class and check if the field is annotated. Also you can run another `TestClass` that just does the marshalling.
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    2. COso isn't there any way to change the byte code of loaded class? in such case do I have to implement new class loader?
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    3. COAs far as I know, once the default class loaders (by default read the standard ones in the JVM) loads the bytecode for a class you cannot change it. The only way to reload a changed class that is loaded is by loading it through another class loader instance. Alternatively you can also have your own class loader implementation that tracks your beans and hot swaps them when needed. Also, keep in mind that once you added the annotation via bytecode modification it sticks in the bytecode. So won't serialize name ever again (until you manage to remove the annotation again via instrumentation).
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