Java Gurus,
I am pretty new for annotations and haven't searched for this a lot, so please bear with me...
I would like to implement a Custom Annotation which will intercept a method call; to start with something very basic it can just print the methods name and parameters so that I could avoid the logger statement.
A sample call like this:
public MyAppObject findMyAppObjectById(Long id) throws MyCustomException {
log.debug("in findMyAppObjectById(" + id + ")");
//....
}
can be converted into:
@LogMethodCall(Logger.DEBUG)
public MyAppObject findMyAppObjectById(Long id) throws MyCustomException {
//....
}
Could I get some hints about this?
Based in your answers of my comments, you will not be able to do this with just annotations. You can, of course, create your annotations and create some reflective code that will detected then and execute some code, but this will not change your code too much, because you will need to call the parser method before you call your methods and I think that will not help you too much, since you will need to call the parser method before each call.
If you need the behavior that you mentioned (automatic call), you will need to combine your annotations with some AOP framework like Spring (plain Java) or AspectJ (AspectJ code). With then, you can set pointcuts and everytime this point is reached, some code may be executed. You can configure then to execute some code before and/or after method execution.
If the first scenario is sufficient, you can do something like:
Logger: enum
public enum Logger {
INFO,
DEBUG;
}
LogMethodCall: annotation
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Retention( RetentionPolicy.RUNTIME ) // the annotation will be available during runtime
@Target( ElementType.METHOD ) // this can just be used in methods
public @interface LogMethodCall {
Logger logLevel() default Logger.INFO;
}
Person: annotated class
public class Person {
// will use the default log level (INFO)
@LogMethodCall
public void foo( int a ) {
System.out.println( "foo! " + a );
}
@LogMethodCall( logLevel = Logger.DEBUG )
public void bar( int b ) {
System.out.println( "bar! " + b );
}
}
Utils: class with the log static method (this will perform the "parsing")
public class Utils {
public static void log( Object o, String methodName ) {
// gets the object class
Class klass = o.getClass();
// iterate over its methods
for ( Method m : klass.getMethods() ) {
// verify if the method is the wanted one
if ( m.getName().equals( methodName ) ) {
// yes, it is
// so, iterate over its annotations
for ( Annotation a : m.getAnnotations() ) {
// verify if it is a LogMethodCall annotation
if ( a instanceof LogMethodCall ) {
// yes, it is
// so, cast it
LogMethodCall lmc = ( LogMethodCall ) a;
// verify the log level
switch ( lmc.logLevel() ) {
case INFO:
System.out.println( "performing info log for \"" + m.getName() + "\" method" );
break;
case DEBUG:
System.out.println( "performing debug log for \"" + m.getName() + "\" method" );
break;
}
}
}
// method encountered, so the loop can be break
break;
}
}
}
}
AnnotationProcessing: class with code to test the annotation processing
public class AnnotationProcessing {
public static void main(String[] args) {
Person p = new Person();
Utils.log( p, "foo" );
p.foo( 2 );
Utils.log( p, "bar" );
p.bar( 3 );
}
}
Of course, you will need to improve my code to fit your needs. It is just a start point.
More about annotations:
More about AOP: