org.springframework.aop.TargetSource
TargetSource的使用
在 Spring AOP 中，通常被代理对象就是 Bean 对象，由 BeanFactory 创建。
同时 Spring AOP 中提供了 TargetSource 接口，可以自定义逻辑创建被代理对象。例如 @Lazy 注解，当加在属性上时，会产生一个代理对象赋值给这个属性，产生代理对象的代码为：
org.springframework.context.annotation.ContextAnnotationAutowireCandidateResolver#buildLazyResolutionProxy
public class ContextAnnotationAutowireCandidateResolver {
protected Object buildLazyResolutionProxy(final DependencyDescriptor descriptor, final @Nullable String beanName) {
BeanFactory beanFactory = getBeanFactory();
Assert.state(beanFactory instanceof DefaultListableBeanFactory,
"BeanFactory needs to be a DefaultListableBeanFactory");
final DefaultListableBeanFactory dlbf = (DefaultListableBeanFactory) beanFactory;
TargetSource ts = new TargetSource() {
@Override
public Class<?> getTargetClass() {
return descriptor.getDependencyType();
}
@Override
public boolean isStatic() {
return false;
}
@Override
public Object getTarget() {
Set<String> autowiredBeanNames = (beanName != null ? new LinkedHashSet<>
(1) : null);
Object target = dlbf.doResolveDependency(descriptor, beanName,
autowiredBeanNames, null);
if (target == null) {
Class<?> type = getTargetClass();
if (Map.class == type) {
return Collections.emptyMap();
} else if (List.class == type) {
return Collections.emptyList();
} else if (Set.class == type || Collection.class == type) {
return Collections.emptySet();
}
throw new
NoSuchBeanDefinitionException(descriptor.getResolvableType(),
"Optional dependency not present for lazy injection point");
}
if (autowiredBeanNames != null) {
for (String autowiredBeanName : autowiredBeanNames) {
if (dlbf.containsBean(autowiredBeanName)) {
dlbf.registerDependentBean(autowiredBeanName, beanName);
}
}
}
return target;
}
@Override
public void releaseTarget(Object target) {
}
};
ProxyFactory pf = new ProxyFactory();
pf.setTargetSource(ts);
Class<?> dependencyType = descriptor.getDependencyType();
if (dependencyType.isInterface()) {
pf.addInterface(dependencyType);
}
return pf.getProxy(dlbf.getBeanClassLoader());
}
}
上述代码利用 ProxyFactory 生成代理对象，使用了 TargetSource，以达到代理对象在执行某个方法时，调用 TargetSource 的 getTarget() 方法实时得到一个被代理对象。