Spring AOP mechanisms
in a word, Spring AOP is proxy-based. That’s, AOP by proxying. and Spring use one of the following two ways to create the proxy for a given target bean.
-
JDK dynamic proxies, preferred wheneven the proxied target implements at least one interface.
-
CGLIB, if the target object does not implement any interfaces, can be force to use by:
-
set the value of the proxy-target-class attribute of the element to true:
-
set the ‘proxy-target-class’ attribute of the element to true when using the @AspectJ autoproxy support:
-
NOTES
- Spring AOP其实只是兼容了AspectJ的注解，但是底层其实跟AspjectJ一点关系都没有。
- 因为Spring AOP是proxy-based和method-based proxy，所以他有如下的局限性：
- 不能增强final或者静态方法。
- 内部方法调用(selfs-call)不会被AOP。因为target-object没有被增强，this引用的是target-object。Spring AOP top problem #1 - aspects are not applied
使用AspectJ可以无限制的使用AOP，但是使用起来相对复杂很多，需要仔细权衡:
NOTES Spring AOP VS AspectJ AOP
Spring-AOP : Runtime weaving through proxy using concept of dynamic proxy if interface exists or cglib library if direct implementation provided.
AspectJ: Compile time weaving through AspectJ Java Tools(ajc compiler) if source available or post compilation weaving (using compiled files).Also, load time weaving with Spring can be enabled – it needs the aspectj definition file and offers flexibility. Compile time weaving can offer benefits of performance (in some cases) and also the joinpoint definition in Spring -aop is restricted to method definition only which is not the case for AspectJ.
TIPS warving process time
The AspectJ weaver takes class files as input and produces class files as output. The weaving process itself can take place at one of three different times: compile-time, post-compile time, and load-time. The class files produced by the weaving process (and hence the run-time behaviour of an application) are the same regardless of the approach chosen.
Compile-time weaving is the simplest approach. When you have the source code for an application, ajc will compile from source and produce woven class files as output. The invocation of the weaver is integral to the ajc compilation process. The aspects themselves may be in source or binary form. If the aspects are required for the affected classes to compile, then you must weave at compile-time. Aspects are required, e.g., when they add members to a class and other classes being compiled reference the added members.
Post-compile weaving (also sometimes called binary weaving) is used to weave existing class files and JAR files. As with compile-time weaving, the aspects used for weaving may be in source or binary form, and may themselves be woven by aspects.
Load-time weaving (LTW) is simply binary weaving defered until the point that a class loader loads a class file and defines the class to the JVM. To support this, one or more “weaving class loaders”, either provided explicitly by the run-time environment or enabled through a “weaving agent” are required.
You may also hear the term “run-time weaving”. We define this as the weaving of classes that have already been defined to the JVM (without reloading those classes). AspectJ 5 does not provide explicit support for run-time weaving although simple coding patterns can support dynamically enabling and disabling advice in aspects.
Spring AOP Implement
difine a AopProxy Interface, see AopProxy
package org.springframework.aop.framework;
/**
* Delegate interface for a configured AOP proxy, allowing for the creation
* of actual proxy objects.
*
* <p>Out-of-the-box implementations are available for JDK dynamic proxies
* and for CGLIB proxies, as applied by {@link DefaultAopProxyFactory}.
*
* @author Rod Johnson
* @author Juergen Hoeller
* @see DefaultAopProxyFactory
*/
public interface AopProxy {
Object getProxy();
Object getProxy(ClassLoader classLoader);
}
There are two implements for the AopProxy:
The JdkDynamicAopProxy is implemented based on JDK java.lang.reflect.Proxy. It’s quite easy to create a proxy base on the target object’s interface:
final class JdkDynamicAopProxy implements AopProxy, InvocationHandler, Serializable {
public Object getProxy(ClassLoader classLoader) {
if (logger.isDebugEnabled()) {
logger.debug("Creating JDK dynamic proxy: target source is " + this.advised.getTargetSource());
}
// Determine the complete set of interfaces to proxy for the given AOP configuration.
Class[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised);
findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
// Returns an instance of a proxy class for the specified interfaces that dispatches method invocations to the specified invocation handler.
return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
}
}
with Proxy class’s newProxyInstance method:
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
note that the InvocationHandler argument above is the JdkDynamicAopProxy instance itselft. This is ture cause the JdkDynamicAopProxy implements the InvocationHandler interface too.
public interface InvocationHandler {
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable;
}
The implement code for invoke method is too long, and I am not going to paste the code here. See the source code directly if you are interested. Just remember that it’s nothing but the proxy pattern. The proxy implements the interfaces of the target object, and has a reference to the target.
The CglibAopProxy use the CGLIB library to generate the proxy object.
final class CglibAopProxy implements AopProxy, Serializable {
public Object getProxy() {
return getProxy(null);
}
public Object getProxy(ClassLoader classLoader) {
try {
Class<?> rootClass = this.advised.getTargetClass();
Assert.state(rootClass != null, "Target class must be available for creating a CGLIB proxy");
Class<?> proxySuperClass = rootClass;
if (ClassUtils.isCglibProxyClass(rootClass)) {
proxySuperClass = rootClass.getSuperclass();
Class<?>[] additionalInterfaces = rootClass.getInterfaces();
for (Class<?> additionalInterface : additionalInterfaces) {
this.advised.addInterface(additionalInterface);
}
}
// Validate the class, writing log messages as necessary.
validateClassIfNecessary(proxySuperClass);
// Configure CGLIB Enhancer...
Enhancer enhancer = createEnhancer();
if (classLoader != null) {
enhancer.setClassLoader(classLoader);
if (classLoader instanceof SmartClassLoader &&
((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {
enhancer.setUseCache(false);
}
}
enhancer.setSuperclass(proxySuperClass);
enhancer.setStrategy(new UndeclaredThrowableStrategy(UndeclaredThrowableException.class));
enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
enhancer.setInterceptDuringConstruction(false);
Callback[] callbacks = getCallbacks(rootClass);
enhancer.setCallbacks(callbacks);
enhancer.setCallbackFilter(new ProxyCallbackFilter(
this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
Class<?>[] types = new Class[callbacks.length];
for (int x = 0; x < types.length; x++) {
types[x] = callbacks[x].getClass();
}
enhancer.setCallbackTypes(types);
// Generate the proxy class and create a proxy instance.
Object proxy;
if (this.constructorArgs != null) {
proxy = enhancer.create(this.constructorArgTypes, this.constructorArgs);
}
else {
proxy = enhancer.create();
}
return proxy;
}
catch () {
// ..Exception handle..
}
}
}
Note that how CglibAopProxy different from the JdkDynamicAopProxy. The CglibAopProxy will construct a class(Enhancer) that extends the target class, instead of implements the target class’s interfaces.
enhancer.setSuperclass(proxySuperClass);
That’s why it can proxy target object that has no interface.
Also note that both the JdkDynamicAopProxy and CglibAopProxy construct the proxy object on the fly, that is on runtime, not compile time. And because it has to construct the proxy class first before it can instance the proxy object, this may be a performance bottleneck. But in spring, this offen happen once when startup spring application context.
Spring use the DefaultAopProxyFactory to create the AOP proxy object. DefaultAopProxyFactory will automatically create CGLIB-based proxies if one the following is true for a given AdvisedSupport instance:
- the “optimize” flag is set
- the “proxyTargetClass” flag is set
- no proxy interfaces have been specified
DefaultAopProxyFactory
public class DefaultAopProxyFactory implements AopProxyFactory, Serializable {
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {
Class targetClass = config.getTargetClass();
if (targetClass == null) {
throw new AopConfigException("TargetSource cannot determine target class: " +
"Either an interface or a target is required for proxy creation.");
}
if (targetClass.isInterface()) {
return new JdkDynamicAopProxy(config);
}
return CglibProxyFactory.createCglibProxy(config);
}
else {
return new JdkDynamicAopProxy(config);
}
}
/**
* Inner factory class used to just introduce a CGLIB dependency
* when actually creating a CGLIB proxy.
*/
private static class CglibProxyFactory {
public static AopProxy createCglibProxy(AdvisedSupport advisedSupport) {
return new CglibAopProxy(advisedSupport);
}
}
}
Resources
- 9. Aspect Oriented Programming with Spring
- 8.6 Proxying mechanisms
- Java Reflection: Dynamic Proxies
- Weaving with AspectJ AspectJ非常好的介绍文章
- Spring AOP APIs Spring AOP API介绍。介绍了Spring AOP底层使用到的核心类和接口。比如ProxyFactoryBean。强烈推荐阅读。
- Spring Auto proxy creator example 介绍怎样使用ProxyFactoryBean创建AOP代理。