Dynamic proxy is used in Java to create proxy objects that implement a specific interface at runtime. Its core is implemented through the java.lang.reflect.Proxy class and the InvocationHandler interface. The specific steps are: 1. Define the interface; 2. Create a real object to implement the interface; 3. Write an InvocationHandler to handle method calls; 4. JVM automatically generates proxy classes and intercepts method calls. Common application scenarios include logging, security checking, performance monitoring, and testing simulation. Dynamic proxy has problems such as only supporting interfaces (default), slight performance overhead caused by reflection, and increased debugging complexity. The example shows how to implement the logging function of the proxy through LoggingHandler.
Dynamic proxies in Java are a way to create a proxy object at runtime that implements one or more interfaces. This proxy can intercept method calls before delegating them to the actual object, allowing you to add behavior like logging, security checks, or performance monitoring without modifying the original class.
They're especially useful when you want to apply cross-cutting concerns consistently across multiple classes that share an interface.
How Dynamic Proxies Work
At the core of dynamic proxies is the java.lang.reflect.Proxy class and the InvocationHandler interface.
Here's how it works:
- You define an interface (or multiple interfaces).
- You have a real object that implements that interface.
- You create an
InvocationHandlerthat defines what should happen when methods are called on the proxy. - The JVM generates a new class at runtime that implements your interface and routes all method calls through your handler.
The key point: you don't write the proxy class yourself — it's generated dynamically.
When Should You Use Them?
Dynamic proxies come in handy in several practical scenarios:
- Logging method calls : Automatically log entry/exit of methods without adding print statements everywhere.
- Security checks : Intercept method calls and verify permissions before execution.
- Performance monitoring : Time how long method calls take and collect metrics.
- Mocking/stubbing in tests : Frameworks like Mockito use proxies to simulate behavior.
If you find yourself repeating logic around method invocations across many classes, dynamic proxies might be a good fit.
Basic Example
Let's say you have a simple interface:
public interface Service {
void doSomething();
}And a class implementing it:
public class RealService implements Service {
public void doSomething() {
System.out.println("Doing something...");
}
}Now, here's how you'd create a proxy that logs each method call:
public class LoggingHandler implements InvocationHandler {
private Object target;
public LoggingHandler(Object target) {
this.target = target;
}
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("Before method: " method.getName());
Object result = method.invoke(target, args);
System.out.println("After method: " method.getName());
return result;
}
}Finally, create and use the proxy:
Service realService = new RealService();
Service proxy = (Service) Proxy.newProxyInstance(
realService.getClass().getClassLoader(),
new Class[]{Service.class},
new LoggingHandler(realService)
);
proxy.doSomething();You'll see both the log messages and the actual behavior executed.
Limitations and Considerations
There are a few things to keep in mind:
- They only work with interfaces by default (though libraries like CGLIB can handle classes too).
- Performance overhead is minimal but measurable — not ideal for ultra-low-lateency code.
- Debugging can get trickier since the proxy class is generated at runtime.
- Stack traces may show synthetic classes, which can confuse developers unfamiliar with proxies.
Also, because everything goes through reflection, some IDEs and static analysis tools might miss what's really going on unless properly configured.
So while dynamic proxies aren't needed every day, they're a powerful tool when you need to wrap behavior consistently around interface methods. Once you understand how Proxy and InvocationHandler fit together, they're not hard to use — just easy to overlook until you really need them.
Basically that's it.
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