Proxetta advices are unique: they are written in the same way as one would write overridden method in a subclass. Accessing target method or info is done with special macro markers.
Advices implement the ProxyAdvice interface that has just one method: execute(). Now, the question is how Proxetta references the proxy target, i.e. the pointcut method that is proxified. Instead of having some custom (handler) object filled with data about the target method (pointcut), Proxetta introduces special 'macro' class ProxyTarget for this purpose. ProxyTarget is just a dummy class and all its methods are empty! ProxyTarget methods serves just as macros that will be replaced by appropriate bytecode that does what macro method is specified for! After replacing ProxyTarget macro methods, all dependencies on ProxyTarget are gone. Macro methods are replaced with the bytecode that mimic the code that developer would write by himself, if he would like to subclass target class and override target method (pointcut).
Here is the definition of the advice that should log some data (on all log pointcuts):
public class LogProxyAdvice implements ProxyAdvice {
public Object execute() {
int totalArgs = ProxyTarget.argumentsCount();
Class target = ProxyTarget.targetClass();
String methodName = ProxyTarget.targetMethodName();
System.out.println(">>>" + target.getSimpleName()
+ '#' + methodName + ':' + totalArgs);
Object result = ProxyTarget.invoke();
System.out.println("<<<" + result);
return result;
When Proxetta finds some pointcut, i.e. proxy target method on which to apply this advice, it will replace all ProxyTarget method invocations (i.e. macros) with appropriate bytecode. For example, ProxyTarget.argumentsCount() will be replaced by appropriate number of target method arguments. The macro method call is replaced by a number.
To continue the example, if the pointcut is the method in the following class:
public class Foo {
public String someMethod(Integer first, double second) {...}
then the proxy bytecode generated using above advice will look like:
public class Foo$Proxy extends Foo {
public String someMethod(Integer first, double second) {
int totalArgs = 2; // arguments count
Class target = Foo.class; // target class
String methodName = "someMethod" // target method name
System.out.println(">>>" + target.getSimpleName()
+ '#' + methodName + ':' + totalArgs);
Object result = super.someMethod(first, second);
System.out.println("<<<" + result);
return result;
Replacements occurs in the run-time, during proxy creation, using bytecode manipulation. But don't forget that changes are done in the class, meaning, all the replaced info is on class-level scope, as you would write them before compilation!
All macro methods replacement is done on class-level. Think of it as constant data you put there before compilation! {: .attn}
And one more thing, for the sake of good health:
It is advisable to use ProxyTarget macro-methods just in simple assignment expressions. {: .attn}
During creation of proxy class and methods, Proxetta also copies advice's constructors, static initialization blocks, fields, etc. Advices should be written very carefully, always having in mind that advice's code will be added to the target proxy class. Common mistake is accessing package scoped class from advice: while it is valid for advice, it will be not valid for target class, since it is in different package. Another mistake is usage of static attributes declared in advice's class - since they are copied to every class, each one will have its own static attribute, instead of having one field for all classes. This can be solved by using some external class that will hold this static attribute.
Using inner classes in advices is not supported. {: .attn}
We just didn't want to complicate your world :)
Last modified 1yr ago