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Spring normally chooses a JDK dynamic proxy when a bean implements at least one interface, and a CGLIB subclass proxy when it implements none. You can force class-based CGLIB proxying with proxy-target-class=true. Choose JDK proxying when an interface is the complete contract your callers need; choose CGLIB when advice must reach concrete-class methods or callers must inject the implementation type. The choice is rarely a performance decision. More important are API visibility, subclassing constraints, configuration, and whether calls actually pass through the proxy.
What are Spring proxies, and why do they matter?
Spring AOP is proxy-based. Instead of rewriting the target method, Spring places a proxy in front of the bean. Calls that enter through that proxy can trigger advice such as transactions, caching, security checks, logging, or asynchronous execution before and after the target method runs.
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The object you receive from the application context may therefore not be the concrete object you wrote. It may be an interface proxy that delegates to the target, or a runtime-generated subclass of the target. That difference affects which methods callers can see, what type can be injected, which methods can be advised, and whether a call reaches the advice chain at all.
The practical rule is simple: design around the API your callers need, then verify that the selected proxy can expose and override every method that must be advised. Do not choose a proxy style merely because its generated class name looks familiar.
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How does Spring decide between a JDK proxy and CGLIB?
With Spring’s normal automatic choice, an interface leads to a JDK dynamic proxy and the absence of an interface leads to a CGLIB proxy.
When the target implements an interface
Spring uses a JDK dynamic proxy when the target implements at least one interface. The proxy presents the interface contract to callers and delegates those operations to the target object. Methods that exist only on the concrete implementation are not part of that proxy’s interface surface.
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When the target implements no interface
Without a target interface, Spring creates a CGLIB proxy. CGLIB generates a subclass at runtime, allowing the proxy to expose concrete methods that can legally be overridden.
This is useful for class-based services, but it does not remove Java’s inheritance rules. The target class must be subclassable, and each method that needs advice must be visible and overridable.
When configuration forces class-based proxying
Setting proxy-target-class=true tells Spring to use class-based CGLIB proxying even when the target also implements interfaces. In Java configuration, the equivalent is:
@EnableAspectJAutoProxy(proxyTargetClass = true)
@Configuration
class AopConfig {
}XML configuration can express the same intent with the proxy-target-class="true" attribute on the relevant auto-proxy element. Transaction auto-proxy configuration can also participate in this choice.
Spring combines several auto-proxy facilities through a unified auto-proxy creator. If multiple configuration elements contribute settings, the strongest setting applies. A single component enabling class-based proxying can therefore affect other advice facilities that use that creator, not just the aspect you were editing.
What is the difference between JDK dynamic proxies and CGLIB?
Both proxy styles can run the same Spring advice. Their important difference is how they represent the target and what Java members they can expose or override.
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| Axis | JDK dynamic proxy | CGLIB proxy |
|---|---|---|
| Generation model | Runtime-generated object implementing interfaces | Runtime-generated subclass of the target class |
| Spring’s automatic trigger | Target implements at least one interface | Target implements no interface |
| Best fit | The interface fully describes the operations callers need | Advice must cover concrete methods, or callers require a concrete type |
| Visible API | Interface methods exposed by the proxy | Overridable methods inherited from the target class; interfaces may also be implemented |
| Type requirement | Callers should depend on an exposed interface | Target must be subclassable and the injection type must be compatible with the generated subclass |
| Methods that cannot be advised | Methods not on an exposed interface | Final, private, or otherwise non-visible methods, plus methods that cannot be overridden |
| How to select explicitly | Default interface-oriented behavior | proxy-target-class=true, or automatic selection when no interface exists |
| Performance | No documented decisive advantage | No documented decisive advantage |
The table describes proxy capability, not a guarantee that every eligible method is advised. The call still has to enter through the proxy, and the method must match the pointcut and other advice conditions in your application.
When should you choose a JDK dynamic proxy?
Choose the JDK style when the interface is a complete and intentional boundary for the service.
The interface is the contract
Interface-oriented proxying keeps callers coupled to behavior rather than implementation details. A controller, scheduled job, or other service can depend on PaymentService without knowing which class implements it. The proxy naturally preserves that boundary.
You want implementation-only methods hidden
If a concrete class contains helper or maintenance operations that should not be part of the service API, a JDK proxy prevents callers holding the proxied reference from treating those methods as public service operations.
You want fewer inheritance constraints
JDK proxies do not need to subclass the target. A class that cannot be subclassed is not automatically disqualified from interface-based proxying, provided the required operations are on an interface and the proxy is used through that interface.
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When should you choose CGLIB?
Use CGLIB when the concrete class is the required boundary and its methods are suitable for subclass interception.
The target has no interface
For a class with no interface, CGLIB is Spring’s normal automatic option. This lets Spring apply method advice without inventing a separate interface solely for proxying.
An advised method exists only on the class
If callers legitimately need a concrete-class method and that method must be advised, a JDK proxy cannot expose it. A CGLIB subclass can proxy it when the method is visible and overridable.
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A concrete injection type is required
Some injection boundaries are written against the implementation type rather than an interface. A JDK proxy is not a substitute for that concrete type, while a compatible CGLIB subclass can satisfy the class-based boundary when the target is subclassable.
That convenience comes with stronger constraints. Before forcing CGLIB, check the class declaration and every method that must receive advice. A final class cannot be subclassed, and final or private methods cannot be overridden for interception. Non-visible methods are excluded as well.
What does proxy-target-class change?
proxy-target-class changes the proxying model from interface-oriented selection to class-based CGLIB proxying. It is not a switch that makes self-invocation work, nor does it make final methods interceptable.
Java configuration
Set proxyTargetClass = true on @EnableAspectJAutoProxy when your configuration owns the aspect auto-proxy creator:
@EnableAspectJAutoProxy(proxyTargetClass = true)
@Configuration
class AopConfig {
}The spelling is different in Java and XML: Java uses proxyTargetClass, while XML uses the hyphenated proxy-target-class.
XML configuration
An XML auto-proxy element can opt into class-based proxies with:
<aop:aspectj-autoproxy proxy-target-class="true" />Use the equivalent attribute on the transaction auto-proxy configuration when that facility is the one creating the proxy. Because Spring may consolidate these facilities into one auto-proxy creator, inspect all relevant configuration rather than changing only the annotation or XML element nearest the failing method.
Why a global-looking change can have broad effects
The unified creator means the strongest proxying setting can apply to several kinds of advice. Enabling class-based proxying for one concern may change the proxy type observed by beans receiving transactions, caching, security, or other proxy-based advice. Treat the setting as an application configuration decision, not merely a local annotation tweak.
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Which methods can CGLIB actually advise?
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- Final class: cannot be subclassed, so a CGLIB proxy cannot be created for it.
- Final method: cannot be overridden, so subclass-based advice cannot intercept it.
- Private method: is not inherited as an overridable method and cannot be intercepted through the subclass.
- Non-visible method: methods that are not visible to the generated subclass are excluded.
- Eligible method with no matching advice: may be proxyable but still run without advice if the pointcut does not match.
Changing a method from private to protected or public can make it structurally eligible, but that is an API and encapsulation decision. Do not widen visibility solely to hide an architectural problem; consider moving the operation behind a separate service boundary when it represents a distinct advised action.
Why does self-invocation skip @Transactional, @Async, caching, or security advice?
A call made through the proxy can run advice. A call from one method to another through this stays inside the target object and bypasses the proxy, so the advice chain is not entered.
The failing call path
Suppose processOrder() is advised and calls sendReceipt() as this.sendReceipt(). The external caller enters the proxy for processOrder(), but the internal call goes directly from the target object to its own method. Spring never gets a second proxy interception point for sendReceipt().
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Move the operation that needs independent advice into another Spring bean and inject that bean. The call then crosses a bean boundary and enters the second bean through its proxy. This design also makes the transaction, asynchronous task, cache operation, or security check explicit in the service structure.
Alternative: self-injection
Inject the proxied bean reference and call the advised method through that reference instead of this. This can work, but it introduces a self-reference and should be documented so future refactoring does not replace the proxy call with a direct one.
Last resort: AopContext.currentProxy()
AopContext.currentProxy() requires the proxy to be exposed. It couples the class directly to Spring AOP and is discouraged by Spring’s guidance. Use it only when refactoring or self-injection is impractical and the coupling is an accepted trade-off.
Switching from JDK to CGLIB does not remove the self-invocation rule. Both are proxies, and neither can intercept a call that never passes through the proxy object.
How do proxy types affect injection and casting?
JDK proxies should be consumed through one of their exposed interfaces. If a dependency is declared as the concrete implementation, the interface proxy cannot satisfy that class-based expectation.
CGLIB proxies are subclasses, so a compatible concrete-class injection point can work, but only when the target class can be subclassed. Forcing CGLIB solely to satisfy a concrete injection point may solve the immediate type mismatch while increasing coupling to implementation details.
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When an implementation-only operation is genuinely part of the service contract, expose it through a deliberate interface or use class-based proxying with a subclassable class. Avoid casting a proxied reference opportunistically; a cast that succeeds with one configuration can fail when another auto-proxy setting changes the proxy style.
Is JDK or CGLIB faster?
Spring’s reference guidance does not identify a decisive performance advantage for either style. The documented difference is described qualitatively as small, with no universal benchmark number that applies across JDK versions, JVMs, Spring releases, and workloads.
Choose based on API shape, method eligibility, and type requirements. If proxy overhead is material in a particular application, benchmark the actual JDK, JVM, Spring version, advice chain, call frequency, and target workload. A synthetic result from a different environment cannot establish a general winner.
How can you diagnose the wrong proxy or missing advice?
Follow the call path and configuration in a fixed order instead of changing proxy settings at random.
- Inspect the target API. Record every interface the bean implements and identify whether the failing method is declared on one of them.
- Check the effective proxy type. Determine whether the injected object is interface-based or class-based using your debugger or runtime type inspection. A JDK proxy indicates interface-oriented selection; a generated subclass indicates CGLIB.
- Search configuration. Look for
proxyTargetClass = true,proxy-target-class="true", and transaction auto-proxy settings. Remember that the unified auto-proxy creator applies the strongest setting. - Verify method eligibility. For CGLIB, remove final or private declarations only when that matches the intended design, and ensure the method is visible to the subclass. For JDK proxying, confirm the operation is on an exposed interface.
- Trace the caller. Confirm that the call comes through the Spring-managed proxy rather than a direct reference to the target or a
thiscall from another method in the same class. - Check the advice match. A proxy can be present while a pointcut, annotation condition, or method signature prevents the specific advice from matching.
- Retest the boundary. Call the method from an external bean through the injected reference. If advice appears externally but not internally, the remaining issue is self-invocation rather than proxy selection.
What is the safest design rule for Spring proxying?
Start with the caller-facing contract, not the proxy technology. If an interface completely describes the operations that need advice, let Spring use a JDK proxy and keep dependencies interface-based. If the application intentionally relies on concrete methods or class-based injection, use CGLIB only after confirming that the class and advised methods are subclassable.
Keep advised operations at bean boundaries so calls enter through a proxy. Treat proxy-target-class as a coordinated configuration choice because it can affect multiple auto-proxy facilities. Finally, test behavior through the same external call paths production code uses; inspecting only the target class can conceal proxy and self-invocation failures.
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| Situation | Recommended approach | Reason |
|---|---|---|
| Service has a complete interface and callers use it | JDK dynamic proxy | Preserves the explicit interface contract and avoids implementation coupling |
| Service has no interface | Default CGLIB proxy | Spring needs a class-based proxy to apply advice |
| Advice must cover a concrete method absent from interfaces | CGLIB, if the method is overridable | JDK proxies cannot expose implementation-only methods |
| Concrete-class injection is intentional | CGLIB, if the class is subclassable | The generated subclass can satisfy the class-based boundary |
| Target class or required method is final/private/non-visible | Refactor the boundary or method; do not expect CGLIB to advise it | Subclass interception cannot override those members |
| Only concern is presumed speed | Keep the API-driven default and benchmark if necessary | Spring documents no decisive general performance winner |
Frequently Asked Questions
Can one application use JDK proxies for some beans and CGLIB for others?
The effective style depends on which auto-proxy creator handles each bean and on the settings contributed to that creator. Because Spring can consolidate several facilities into a unified creator, a class-based setting may have a wider effect than the single bean or advice you intended; verify the resulting proxy types instead of assuming complete isolation.
Will changing a proxy from JDK to CGLIB change transaction propagation rules?
Proxy style does not define propagation semantics. Once a call is intercepted, the transaction advice applies according to its own configuration; changing the proxy type mainly changes type exposure and which methods can be intercepted.
Should tests assert the generated proxy class name?
Usually no. Generated names and implementation details can vary with configuration, while the stable contract is whether an external call receives the expected advice and result. Test through the interface or intended class boundary and include a case that exercises an internal call when self-invocation is a risk.
Is forcing CGLIB a fix for advice that works only from some callers?
Not by itself. If the missing advice is caused by a call through this, both JDK and CGLIB proxies are bypassed. First prove that the invocation enters the proxy; then address the bean boundary or self-reference before changing proxy style.
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