Anatomy of a Hibernate Proxy

2023 December 2

Hibernate ORM is well known for its use of proxy classes to enable lazy loading.

A Hibernate proxy class

extends the entity class
implements org.hibernate.proxy.HibernateProxy
implements org.hibernate.proxy.ProxyConfiguration
has a field of type org.hibernate.proxy.ProxyConfiguration.Interceptor
has a static final field of type java.lang.reflect.Method for every superclass method
routes all methods to org.hibernate.proxy.ProxyConfiguration.InterceptorDispatcher#intercept which then dispatches to org.hibernate.proxy.ProxyConfiguration.Interceptor#intercept

the generated proxy class looks like this

public class EntityClass$HibernateProxy$iPrgCr9u extends EntityClass implements HibernateProxy, ProxyConfiguration {

  private ProxyConfiguration.Interceptor $$_hibernate_interceptor;
   
  private static final Method cachedValue$xVIlXpzP$4cscpe1 = Object.class.getMethod("toString");
   
  private static final Method cachedValue$xVIlXpzP$uoilpf3 = ChildEntity.class.getMethod("setId", Long.class);

  private static final Method cachedValue$xVIlXpzP$o23rrk2 = HibernateProxy.class.getMethod("getHibernateLazyInitializer");
   
  private static final Method cachedValue$xVIlXpzP$5j4bem0 = Object.class.getMethod("equals", Object.class);
   
  private static final Method cachedValue$xVIlXpzP$gpia792 = HibernateProxy.class.getMethod("writeReplace");
   
  private static final Method cachedValue$xVIlXpzP$7m9oaq0 = Object.class.getDeclaredMethod("clone");
   
  private static final Method cachedValue$xVIlXpzP$9pqdof1 = Object.class.getMethod("hashCode");
   
  private static final Method cachedValue$xVIlXpzP$8j4rtp1 = ChildEntity.class.getMethod("getId");

  @Override
  public boolean equals(Object obj) {
    return (Boolean)InterceptorDispatcher.intercept(this, cachedValue$xVIlXpzP$5j4bem0, new Object[]{obj}, false, this.$$_hibernate_interceptor);
  }

  @Override
  public String toString() {
    return (String)InterceptorDispatcher.intercept(this, cachedValue$xVIlXpzP$4cscpe1, new Object[0], null, this.$$_hibernate_interceptor);
  }

  @Override
  public int hashCode() {
    return (Integer)InterceptorDispatcher.intercept(this, cachedValue$xVIlXpzP$9pqdof1, new Object[0], 0, this.$$_hibernate_interceptor);
  }

  @Override
  protected Object clone() throws CloneNotSupportedException {
    return InterceptorDispatcher.intercept(this, cachedValue$xVIlXpzP$7m9oaq0, new Object[0], null, this.$$_hibernate_interceptor);
  }

  @Override
  public void setId(Long id) {
    InterceptorDispatcher.intercept(this, cachedValue$xVIlXpzP$uoilpf3, new Object[]{id}, null, this.$$_hibernate_interceptor);
  }

  @Override
  public Long getId() {
    return (Long) InterceptorDispatcher.intercept(this, cachedValue$xVIlXpzP$8j4rtp1, new Object[0], null, this.$$_hibernate_interceptor);
  }

  @Override
  public Object writeReplace() {
    return InterceptorDispatcher.intercept(this, cachedValue$xVIlXpzP$gpia792, new Object[0], null, this.$$_hibernate_interceptor);
  }

  @Override
  public LazyInitializer getHibernateLazyInitializer() {
    return (LazyInitializer) InterceptorDispatcher.intercept(this, cachedValue$xVIlXpzP$o23rrk2, new Object[0], null, this.$$_hibernate_interceptor);
  }

  @Override
  public void $$_hibernate_set_interceptor(ProxyConfiguration.Interceptor interceptor) {
    this.$$_hibernate_interceptor = interceptor;
  }
}

A few things are important to note here:

All methods that can be intercepted are intercepted
Proxies can be cast to HibernateProxy or ProxyConfiguration and their methods can be invoked by anybody
Every object can have its own Interceptor

If you want to have a look at Hibernate proxy classes for yourself give Hibernate Proxy Dumper a try.