上文探讨了应用JDK动态代理实现Spring AOP功能的方式，下面将继续探讨Spring AOP功能的另外一种实现方式 -- CGLIB。
首先，来看看类名CglibAopProxy，该类实现了两个接口：一个是AopProxy接口，一个是Serializable接口。直接忽略Serializable接口，AopProxy接口中只定义了两个同名方法getProxy。如下所示：
class CglibAopProxy implements AopProxy, Serializable
类中有一个非常重要的属性advised，该属性在JdkDynamicAopProxy中也存在，主要用于配置代理信息，是一个非常重要的属性，如下
protected final AdvisedSupport advised;
属性为final类型，可以被子类继承，一旦赋值后便不能改变。在CglibAopProxy中，advised在构造器中被赋值，后续不可更改：
public CglibAopProxy(AdvisedSupport config) throws AopConfigException {
Assert.notNull(config, "AdvisedSupport must not be null");
if (config.getAdvisors().length == 0 && config.getTargetSource() == AdvisedSupport.EMPTY_TARGET_SOURCE) {
throw new AopConfigException("No advisors and no TargetSource specified");
}
this.advised = config;
this.advisedDispatcher = new AdvisedDispatcher(this.advised);
}
跟JdkDynamicAopProxy一样，实现了AopProxy接口，实现了getProxy()方法，如下：
@Override
public Object getProxy(ClassLoader classLoader) {
if (logger.isDebugEnabled()) {
logger.debug("Creating CGLIB proxy: target source is " + this.advised.getTargetSource());
}
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, classLoader);
// 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.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
enhancer.setStrategy(new ClassLoaderAwareUndeclaredThrowableStrategy(classLoader));
Callback[] callbacks = getCallbacks(rootClass);
Class<?>[] types = new Class<?>[callbacks.length];
for (int x = 0; x < types.length; x++) {
types[x] = callbacks[x].getClass();
}
// fixedInterceptorMap only populated at this point, after getCallbacks call above
enhancer.setCallbackFilter(new ProxyCallbackFilter(
this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
enhancer.setCallbackTypes(types);
// Generate the proxy class and create a proxy instance.
return createProxyClassAndInstance(enhancer, callbacks);
}
catch (CodeGenerationException ex) {
throw new AopConfigException("Could not generate CGLIB subclass of class [" +
this.advised.getTargetClass() + "]: " +
"Common causes of this problem include using a final class or a non-visible class",
ex);
}
catch (IllegalArgumentException ex) {
throw new AopConfigException("Could not generate CGLIB subclass of class [" +
this.advised.getTargetClass() + "]: " +
"Common causes of this problem include using a final class or a non-visible class",
ex);
}
catch (Throwable ex) {
// TargetSource.getTarget() failed
throw new AopConfigException("Unexpected AOP exception", ex);
}
}
在创建代理过程中需要判断被代理类是否已经进行过验证，若没有验证过则执行验证。验证方法如下：
private void doValidateClass(Class<?> proxySuperClass, ClassLoader proxyClassLoader, Set<Class<?>> ifcs) {
if (proxySuperClass != Object.class) {
Method[] methods = proxySuperClass.getDeclaredMethods();
for (Method method : methods) {
int mod = method.getModifiers();
if (!Modifier.isStatic(mod)) {
if (Modifier.isFinal(mod)) {
if (implementsInterface(method, ifcs)) {
logger.warn("Unable to proxy interface-implementing method [" + method + "] because " +
"it is marked as final: Consider using interface-based JDK proxies instead!");
}
logger.info("Final method [" + method + "] cannot get proxied via CGLIB: " +
"Calls to this method will NOT be routed to the target instance and " +
"might lead to NPEs against uninitialized fields in the proxy instance.");
}
else if (!Modifier.isPublic(mod) && !Modifier.isProtected(mod) && !Modifier.isPrivate(mod) &&
proxyClassLoader != null && proxySuperClass.getClassLoader() != proxyClassLoader) {
logger.info("Method [" + method + "] is package-visible across different ClassLoaders " +
"and cannot get proxied via CGLIB: Declare this method as public or protected " +
"if you need to support invocations through the proxy.");
}
}
}
doValidateClass(proxySuperClass.getSuperclass(), proxyClassLoader, ifcs);
}
}
接着配置CGLIB加强createEnhancer()，最后创建代理类生成代理实例createProxyClassAndInstance(enhancer, callbacks)