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    <p>With AspectJ you can modify methods and fields with advises.</p> <p>My example is written with <code>@AspectJ</code> syntax which modifies the code at compile-time or load-time. If you want the modification at runtime, you can use Spring AOP which also supports this <code>@AspectJ</code> syntax.</p> <p>An example with a simple Person class and a stub repository. All information about which fields are updated is handled by an aspect called SetterAspect. It monitors which fields that are updated when the fields is written to. </p> <p>The other advice in this example is around the update method in the repository. This is to retrieve the data collected from the first aspect.</p> <p>The Person class:</p> <pre><code>public class Person { private String firstName; private String lastName; public String getFirstName() { return firstName; } public void setFirstName(String firstName) { this.firstName = firstName; } public String getLastName() { return lastName; } public void setLastName(String lastName) { this.lastName = lastName; } public static void main(String[] args) { Person person = new Person(); person.setFirstName("James"); person.lastName = "Jameson"; DtoRepository&lt;Person&gt; personRepository = new DtoRepository&lt;Person&gt;(); personRepository.update(person); } } </code></pre> <p>The stub repository:</p> <pre><code>public class DtoRepository&lt;T&gt; { public void update(T t) { System.out.println(t.getClass().getSimpleName() + " updated.."); } public void updatePerson(T t, Set&lt;String&gt; updatedFields) { System.out.print("Updated the following fields on " + t.getClass().getSimpleName() + " in the repository: " + updatedFields); } } </code></pre> <p>The output for executing the <code>main()</code> method in the Person class with AspectJ:</p> <blockquote> <p>Updated the following fields on Person in the repository: [lastName, firstName]</p> </blockquote> <p>Important to note here is that the main() method calls on the <code>DtoRepository.update(T t)</code> but the <code>DtoRepository.update(T t, Set&lt;String&gt; updatedFields)</code> gets executed because of the around advice in the repository aspect.</p> <p>The aspect that monitors all writing to private fields in the demo package:</p> <pre><code>@Aspect public class SetterAspect { private UpdatableDtoManager updatableDtoManager = UpdatableDtoManager.INSTANCE; @Pointcut("set(private * demo.*.*)") public void setterMethod() {} @AfterReturning("setterMethod()") public void afterSetMethod(JoinPoint joinPoint) { String fieldName = joinPoint.getSignature().getName(); updatableDtoManager.updateObjectWithUpdatedField( fieldName, joinPoint.getTarget()); } } </code></pre> <p>The repository aspect:</p> <pre><code>@Aspect public class UpdatableDtoRepositoryAspect { private UpdatableDtoManager updatableDtoManager = UpdatableDtoManager.INSTANCE; @Pointcut("execution(void demo.DtoRepository.update(*)) " + "&amp;&amp; args(object)") public void updateMethodInRepository(Object object) {} @Around("updateMethodInRepository(object)") public void aroundUpdateMethodInRepository( ProceedingJoinPoint joinPoint, Object object) { Set&lt;String&gt; updatedFields = updatableDtoManager.getUpdatedFieldsForObject(object); if (updatedFields.size() &gt; 0) { ((DtoRepository&lt;Object&gt;)joinPoint.getTarget()). updatePerson(object, updatedFields); } else { // Returns without calling the repository. System.out.println("Nothing to update"); } } } </code></pre> <p>Finally, the two helper classes used by the aspects:</p> <pre><code>public enum UpdatableDtoManager { INSTANCE; private Map&lt;Object, UpdatedObject&gt; updatedObjects = new HashMap&lt;Object, UpdatedObject&gt;(); public void updateObjectWithUpdatedField( String fieldName, Object object) { if (!updatedObjects.containsKey(object)) { updatedObjects.put(object, new UpdatedObject()); } UpdatedObject updatedObject = updatedObjects.get(object); if (!updatedObject.containsField(fieldName)) { updatedObject.add(fieldName); } } public Set&lt;String&gt; getUpdatedFieldsForObject(Object object) { UpdatedObject updatedObject = updatedObjects.get(object); final Set&lt;String&gt; updatedFields; if (updatedObject != null) { updatedFields = updatedObject.getUpdatedFields(); } else { updatedFields = Collections.emptySet(); } return updatedFields; } } </code></pre> <p>and</p> <pre><code>public class UpdatedObject { private Map&lt;String, Object&gt; updatedFields = new HashMap&lt;String, Object&gt;(); public boolean containsField(String fieldName) { return updatedFields.containsKey(fieldName); } public void add(String fieldName) { updatedFields.put(fieldName, fieldName); } public Set&lt;String&gt; getUpdatedFields() { return Collections.unmodifiableSet( updatedFields.keySet()); } } </code></pre> <p>My example does all the update logic with aspects. If all the DTOs implemented an interface that returns a <code>Set&lt;String&gt;</code>, you could have avoided the last aspect.</p> <p>I hope this answered your question!</p>
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