DZone
Thanks for visiting DZone today,
Edit Profile
  • Manage Email Subscriptions
  • How to Post to DZone
  • Article Submission Guidelines
Sign Out View Profile
  • Post an Article
  • Manage My Drafts
Over 2 million developers have joined DZone.
Log In / Join
Refcards Trend Reports
Events Video Library
Over 2 million developers have joined DZone. Join Today! Thanks for visiting DZone today,
Edit Profile Manage Email Subscriptions Moderation Admin Console How to Post to DZone Article Submission Guidelines
View Profile
Sign Out
Refcards
Trend Reports
Events
View Events Video Library
Zones
Culture and Methodologies Agile Career Development Methodologies Team Management
Data Engineering AI/ML Big Data Data Databases IoT
Software Design and Architecture Cloud Architecture Containers Integration Microservices Performance Security
Coding Frameworks Java JavaScript Languages Tools
Testing, Deployment, and Maintenance Deployment DevOps and CI/CD Maintenance Monitoring and Observability Testing, Tools, and Frameworks
Culture and Methodologies
Agile Career Development Methodologies Team Management
Data Engineering
AI/ML Big Data Data Databases IoT
Software Design and Architecture
Cloud Architecture Containers Integration Microservices Performance Security
Coding
Frameworks Java JavaScript Languages Tools
Testing, Deployment, and Maintenance
Deployment DevOps and CI/CD Maintenance Monitoring and Observability Testing, Tools, and Frameworks

Integrating PostgreSQL Databases with ANF: Join this workshop to learn how to create a PostgreSQL server using Instaclustr’s managed service

Mobile Database Essentials: Assess data needs, storage requirements, and more when leveraging databases for cloud and edge applications.

Monitoring and Observability for LLMs: Datadog and Google Cloud discuss how to achieve optimal AI model performance.

Automated Testing: The latest on architecture, TDD, and the benefits of AI and low-code tools.

Related

  • User-Friendly API Publishing and Testing With Retrofit
  • Top Java Collection Interview Questions for 2021
  • Composite Design Pattern in Java
  • Iterator Design Pattern In Java

Trending

  • Getting Started With Prometheus Workshop: Instrumenting Applications
  • Java Parallel GC Tuning
  • Microservices With Apache Camel and Quarkus (Part 5)
  • Agile Metrics and KPIs in Action
  1. DZone
  2. Coding
  3. Languages
  4. The Power of Proxies in Java

The Power of Proxies in Java

Nicolas Fränkel user avatar by
Nicolas Fränkel
CORE ·
May. 11, 10 · Analysis
Like (1)
Save
Tweet
Share
150.38K Views

Join the DZone community and get the full member experience.

Join For Free

In this article, I’ll show you the path that leads to true Java power, the use of proxies.

They are everywhere but only a handful of people know about them. Hibernate for lazy loading entities, Spring for AOP, LambdaJ for DSL, only to name a few: they all use their hidden magic. What are they? They are… Java’s dynamic proxies.

Everyone knows about the GOFProxy design pattern:

Allows for object level access control by acting as a pass through entity or a placeholder object.

Likewise, in Java, a dynamic proxy is an instance that acts as a pass through to the real object. This powerful pattern let you change the real behaviour from a caller point of view since method calls can be intercepted by the proxy.

Pure Java proxies

Pure Java proxies have some interesting properties:

  • They are based on runtime implementations of interfaces
  • They are public, final and not abstract
  • They extend java.lang.reflect.Proxy

In Java, the proxy itself is not as important as the proxy’s behaviour. The latter is done in an implementation of java.lang.reflect.InvocationHandler. It has only a single method to implement:

public Object invoke(Object proxy, Method method, Object[] args)
  • proxy: the proxy instance that the method was invoked on
  • method: the Method instance corresponding to the interface method invoked on the proxy instance. The declaring class of the Method object will be the interface that the method was declared in, which may be a superinterface of the proxy interface that the proxy class inherits the method through
  • args: an array of objects containing the values of the arguments passed in the method invocation on the proxy instance, or null if interface method takes no arguments. Arguments of primitive types are wrapped in instances of the appropriate primitive wrapper class, such as java.lang.Integer or java.lang.Boolean

Let’s take a simple example: suppose we want a List that can’t be added elements to it. The first step is to create the invocation handler:

public class NoOpAddInvocationHandler implements InvocationHandler {

  private final List proxied;

  public NoOpAddInvocationHandler(List proxied) {

    this.proxied = proxied;
  }

  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

    if (method.getName().startsWith("add")) {

      return false;
    }

    return method.invoke(proxied, args);
  }
}

The invoke method will intercept method calls and do nothing if the method starts with “add”. Otherwise, it will the call pass to the real proxied object. This is a very crude example but is enough to let us understand the magic behind.

Notice that in case you want your method call to pass through, you need to call the method on the real object. For this, you’ll need a reference to the latter, something the invoke method does not provide. That’s why in most cases, it’s a good idea to pass it to the constructor and store it as an attribute.

Note: under no circumstances should you call the method on the proxy itself since it will be intercepted again by the invocation handler and you will be faced with a StackOverflowError.

To create the proxy itself:

 List proxy = (List) Proxy.newProxyInstance(
  NoOpAddInvocationHandlerTest.class.getClassLoader(),
  new Class[] { List.class },
  new NoOpAddInvocationHandler(list));

The newProxyInstance method takes 3 arguments:

  • the class loader
  • an array of interfaces that will be implemented by the proxy
  • the power behind the throne in the form of the invocation handler

Now, if you try to add elements to the proxy by calling any add methods, it won’t have any effect.

CGLib proxies

Java proxies are runtime implementations of interfaces. Objects do not necessarily implement interfaces, and collections of objects do not necessarily share the same interfaces. Confronted with such needs, Java proxies fail to provide an answser.

Here begins the realm of CGLib. CGlib is a third-party framework, based on bytecode manipulation provided by ASM that can help with the previous limitations. A word of advice first, CGLib’s documentation is not on par with its features: there’s no tutorial nor documentation. A handful of JavaDocs is all you can count on. This said CGLib waives many limitations enforced by pure Java proxies:

  • you are not required to implement interfaces
  • you can extend a class

For example, since Hibernate entities are POJO, Java proxies cannot be used in lazy-loading; CGLib proxies can.

There are matches between pure Java proxies and CGLib proxies: where you use Proxy, you use net.sf.cglib.proxy.Enhancer class, where you use InvocationHandler, you use net.sf.cglib.proxy.Callback. The two main differences is that Enhancer has a public constructor and Callback cannot be used as such but only through one of its subinterfaces:

  • Dispatcher: Dispatching Enhancer callback
  • FixedValue: Enhancer callback that simply returns the value to return from the proxied method
  • LazyLoader: Lazy-loading Enhancer callback
  • MethodInterceptor: General-purpose Enhancer callback which provides for “around advice”
  • NoOp: Methods using this Enhancer callback will delegate directly to the default (super) implementation in the base class

As an introductory example, let’s create a proxy that returns the same value for hash code whatever the real object behind. The feature looks like a MethodInterceptor, so let’s implement it as such:

<public class HashCodeAlwaysZeroMethodInterceptor implements MethodInterceptor {

  public Object intercept(Object object, Method method, Object[] args,
    MethodProxy methodProxy) throws Throwable {

    if ("hashCode".equals(method.getName())) {

      return 0;
    }

    return methodProxy.invokeSuper(object, args);
  }
}

Looks awfully similar to a Java invocation handler, doesn’t it? Now, in order to create the proxy itself:

Object proxy = Enhancer.create(
  Object.class,
  new HashCodeAlwaysZeroMethodInterceptor());

Likewise, the proxy creation isn’t suprising. The real differences are:

  • there’s no interface involved in the process
  • the proxy creation process also creates the proxied object. There’s no clear cut between proxy and proxied from the caller point of view
  • thus, the callback method can provide the proxied object and there’s no need to create and store it in your own code

Conclusion

This article only brushed the surface of what can be done with proxies. Anyway, I hope it let you see that Java has some interesting features and points of extension, whether out-of-the-box or coming from some third-party framework

You can find the sources for this article in Eclipse/Maven format here.

 

From http://blog.frankel.ch/the-power-of-proxies-in-java

Java (programming language) Object (computer science) Interface (computing) Lazy loading

Opinions expressed by DZone contributors are their own.

Related

  • User-Friendly API Publishing and Testing With Retrofit
  • Top Java Collection Interview Questions for 2021
  • Composite Design Pattern in Java
  • Iterator Design Pattern In Java

Comments

Partner Resources

X

ABOUT US

  • About DZone
  • Send feedback
  • Careers
  • Sitemap

ADVERTISE

  • Advertise with DZone

CONTRIBUTE ON DZONE

  • Article Submission Guidelines
  • Become a Contributor
  • Visit the Writers' Zone

LEGAL

  • Terms of Service
  • Privacy Policy

CONTACT US

  • 3343 Perimeter Hill Drive
  • Suite 100
  • Nashville, TN 37211
  • support@dzone.com

Let's be friends: