Sometimes, I want to log (through slf4j and log4j) every execution of a method, seeing what arguments it receives, what it returns and how much time every execution takes. This is how I'm doing it, with help of AspectJ, jcabi-aspects and Java 6 annotations:

public class Foo {
  @Loggable
  public int power(int x, int p) {
    return Math.pow(x, p);
  }
}

This is what I see in log4j output:

[INFO] com.example.Foo #power(2, 10): 1024 in 12μs
[INFO] com.example.Foo #power(3, 3): 27 in 4μs

Nice, isn't it? Now, let's see how it works.

Annotation with Runtime Retention

Annotations is a technique introduced in Java 6. It is a meta-programming instrument that doesn't change the way code works, but gives marks to certain elements (methods, classes or variables). In other words, annotations are just markers attached to the code that can be seen and read. Some annotations are designed to be seen at compile time only — they don't exist in .class files after compilation. Others remain visible after compilation and can be accessed in runtime.

For example, @Override is of the first type (its retention type is SOURCE), while @Test from JUnit is of the second type (retention type is RUNTIME). @Loggable — the one I'm using in the script above — is an annotation of the second type, from jcabi-aspects. It stays with the bytecode in the .class file after compilation.

Again, it is important to understand that even though method power() is annotated and compiled, it doesn't send anything to slf4j so far. It just contains a marker saying "please, log my execution".

Aspect Oriented Programming (AOP)

AOP is a useful technique that enables adding executable blocks to the source code without explicitly changing it. In our example, we don't want to log method execution inside the class. Instead, we want some other class to intercept every call to method power(), measure its execution time and send this information to slf4j.

We want that interceptor to understand our @Loggable annotation and log every call to that specific method power(). And, of course, the same interceptor should be used for other methods where we'll place the same annotation in the future.

This case perfectly fits the original intent of AOP — to avoid re-implementation of some common behavior in multiple classes.

Logging is a supplementary feature to our main functionality, and we don't want to pollute our code with multiple logging instructions. Instead, we want logging to happen behind the scenes.

In terms of AOP, our solution can be explained as creating an aspect that cross-cuts the code at certain join points and applies an around advice that implements the desired functionality.

AspectJ

Let's see what these magic words mean. But, first, let's see how jcabi-aspects implements them using AspectJ (it's a simplified example, full code you can find in MethodLogger.java):

@Aspect
public class MethodLogger {
  @Around("execution(* *(..)) && @annotation(Loggable)")
  public Object around(ProceedingJoinPoint point) {
    long start = System.currentTimeMillis();
    Object result = point.proceed();
    Logger.info(
      "#%s(%s): %s in %[msec]s",
      MethodSignature.class.cast(point.getSignature()).getMethod().getName(),
      point.getArgs(),
      result,
      System.currentTimeMillis() - start
    );
    return result;
  }
}

This is an aspect with a single around advice around() inside. The aspect is annotated with @Aspect and advice is annotated with @Around. As discussed above, these annotations are just markers in .class files. They don't do anything except provide some meta-information to those w ho are interested in runtime.

Annotation @Around has one parameter, which — in this case — says that the advice should be applied to a method if:

  1. its visibility modifier is * (public, protected or private);

  2. its name is name * (any name);

  3. its arguments are .. (any arguments); and

  4. it is annotated with @Loggable

When a call to an annotated method is to be intercepted, method around() executes before executing the actual method. When a call to method power() is to be intercepted, method around() receives an instance of class ProceedingJoinPoint and must return an object, which will be used as a result of method power().

In order to call the original method, power(), the advice has to call proceed() of the join point object.

We compile this aspect and make it available in classpath together with our main file Foo.class. So far so good, but we need to take one last step in order to put our aspect into action — we should apply our advice.

Binary Aspect Weaving

Aspect weaving is the name of the advice applying process. Aspect weaver modifies original code by injecting calls to aspects. AspectJ does exactly that. We give it two binary Java classes Foo.class and MethodLogger.class; it gives back three — modified Foo.class, Foo$AjcClosure1.class and unmodified MethodLogger.class.

In order to understand which advices should be applied to which methods, AspectJ weaver is using annotations from .class files. Also, it uses reflection to browse all classes on classpath. It analyzes which methods satisfy the conditions from the @Around annotation. Of course, it finds our method power().

So, there are two steps. First, we compile our .java files using javac and get two files. Then, AspectJ weaves/modifies them and creates its own extra class. Our Foo class looks something like this after weaving:

public class Foo {
  private final MethodLogger logger;
  @Loggable
  public int power(int x, int p) {
    return this.logger.around(point);
  }
  private int power_aroundBody(int x, int p) {
    return Math.pow(x, p);
  }
}

AspectJ weaver moves our original functionality to a new method, power_aroundBody(), and redirects all power() calls to the aspect class MethodLogger.

Instead of one method power() in class Foo now we have four classes working together. From now on, this is what happens behind the scenes on every call to power():

UML

Original functionality of method power() is indicated by the small green lifeline on the diagram.

As you see, the aspect weaving process connects together classes and aspects, transferring calls between them through join points. Without weaving, both classes and aspects are just compiled Java binaries with attached annotations.

jcabi-aspects

jcabi-aspects is a JAR library that contains Loggable annotation and MethodLogger aspect (btw, there are many more aspects and annotations). You don't need to write your own aspect for method logging. Just add a few dependencies to your classpath and configure jcabi-maven-plugin for aspect weaving (get their latest versions in Maven Central):

<project>
  <dependencies>
    <dependency>
      <groupId>com.jcabi</groupId>
      <artifactId>jcabi-aspects</artifactId>
    </dependency>
    <dependency>
      <groupId>org.aspectj</groupId>
      <artifactId>aspectjrt</artifactId>
    </dependency>
  </dependencies>
  <build>
    <plugins>
      <plugin>
        <groupId>com.jcabi</groupId>
        <artifactId>jcabi-maven-plugin</artifactId>
        <executions>
          <execution>
            <goals>
              <goal>ajc</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
</project>

Since this weaving procedure takes a lot of configuration effort, I created a convenient Maven plugin with an ajc goal, which does the entire aspect weaving job. You can use AspectJ directly, but I recommend that you use jcabi-maven-plugin.

That's it. Now you can use @com.jcabi.aspects.Loggable annotation and your methods will be logged through slf4j.

If something doesn't work as explained, don't hesitate to submit a Github issue.