Menu
Consider this code:
public void example(String s, int i, @Foo Bar bar) {
/* ... */
}
I'm interested in the value of the argument annotated with @Foo. Assume that I have already figured out via reflection (with Method#getParameterAnnotations()) which method parameter has the @Foo annotation. (I know it is the third parameter of the parameter list.)
How can I now retrieve the value of bar for further usage?
525
You can obtain the names of the formal parameters of any method or constructor with the method java. lang. reflect.
Overview. Method Parameter Reflection support was added in Java 8. Simply put, it provides support for getting the names of parameters at runtime. In this quick tutorial, we'll take a look at how to access parameter names for constructors and methods at runtime – using reflection.
We can use newInstance() method on the constructor object to instantiate a new instance of the class. Since we use reflection when we don't have the classes information at compile time, we can assign it to Object and then further use reflection to access it's fields and invoke it's methods.
reflect package is used to fetch the parameter types using method parameter reflection. Reflection is a process of analyzing and modifying all capabilities of class at runtime. It is also used to manipulate private members of the class which includes fields, methods and constructors, etc..
You can't. Reflection does not have access to local variables, including method parameters.
If you want that functionality, you need to intercept the method call, which you can do in one of several ways:
In all of these, you would gather the parameters from the method call and then tell the method call to execute. But there's no way to get at the method parameters through reflection.
Update: here's a sample aspect to get you started using annotation-based validation with AspectJ
public aspect ValidationAspect {
pointcut serviceMethodCall() : execution(public * com.yourcompany.**.*(..));
Object around(final Object[] args) : serviceMethodCall() && args(args){
Signature signature = thisJoinPointStaticPart.getSignature();
if(signature instanceof MethodSignature){
MethodSignature ms = (MethodSignature) signature;
Method method = ms.getMethod();
Annotation[][] parameterAnnotations =
method.getParameterAnnotations();
String[] parameterNames = ms.getParameterNames();
for(int i = 0; i < parameterAnnotations.length; i++){
Annotation[] annotations = parameterAnnotations[i];
validateParameter(parameterNames[i], args[i],annotations);
}
}
return proceed(args);
}
private void validateParameter(String paramName, Object object,
Annotation[] annotations){
// validate object against the annotations
// throw a RuntimeException if validation fails
}
}
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
