kit-sdq / programming-lecture-logging-proxy

An annotation processor that generates proxy classes which log method calls and then delegate to the original implementation

Logging Proxy

What is this?

A small annotation processor (and example usage) that generates proxy classes at compile-time using an annotation processor.

Why create this?

This is useful if you load the actual implementation from somewhere else (e.g. a student submission), but want a simple interface to program tests against. If you just use your own implementation, the students will only receive the assertion failures — but no history up to them. This project automates generating a proxy at compile-time that traces all calls made by the testing infrastructure and allows you to present the full call history to students in any way you like.

Use in your project

1. Add the jitpack repository

   <repositories>
     <repository>
       <id>jitpack.io</id>
       <url>https://jitpack.io</url>
     </repository>
   </repositories>

2. Add it as a depdendency

    <dependency>
      <groupId>com.github.I-Al-Istannen</groupId>
      <artifactId>logging-proxy</artifactId>
      <version>6880313241395d6350518d0b2b15d5ee068b20e9</version>
    </dependency>

3. Configure your maven compiler plugin to use the processor

   <plugin>
     <groupId>org.apache.maven.plugins</groupId>
     <artifactId>maven-compiler-plugin</artifactId>
     <configuration>
       <annotationProcessorPaths>
         <dependency>
           <groupId>com.github.I-Al-Istannen</groupId>
           <artifactId>logging-proxy</artifactId>
           <version>6880313241395d6350518d0b2b15d5ee068b20e9</version>
         </dependency>
       </annotationProcessorPaths>
     </configuration>
   </plugin>

Example

Consider the following simple shim (or real implementation) for a student task.

@GenerateTestProxy
public class Calculator {
  @ProxyMethod
  public int add(int a, int b) {
    return a + b;
  }

  public void notLogged() {} // Calls are not logged. Maybe useful for setup methods.

  @ProxyMethod
  public static int addTwo(int a, int b) {
    return a + b;
  }
}

These annotations are enough to trigger generation of the following proxy class.

package de.ialistannen.loggingproxy.example;
import de.ialistannen.loggingproxy.TestLogger;
class CalculatorProxy {
    public int add(int a, int b) {
        $testLogger$.onMethodCallStart("add", a, b);
        int result = $inner$.add(a, b);
        $testLogger$.onMethodReturn("add", result);
        return result;
    }

    public int addStatic(int a, int b) {
        $testLogger$.onMethodCallStart("addTwo", a, b);
        int result = Calculator.addTwo(a, b);
        $testLogger$.onMethodReturn("addTwo", result);
        return result;
    }

    TestLogger $testLogger$;

    public CalculatorProxy(TestLogger logger, Calculator inner) {
        $testLogger$ = logger;
        $inner$ = inner;
    }

    Calculator $inner$;
}

This can then easily be used to test the calculator and provide feedback, for example using property based testing:

  @Property(tries = 5)
  void test(@ForAll int a, @ForAll int b) {
    CalculatorProxy proxy = new CalculatorProxy(new MyTestLogger(), new Calculator());
    assertThat(proxy.add(a, b)).isEqualTo(a + b);
  }

  @Property(tries = 5)
  void testStatic(@ForAll int a, @ForAll int b) {
    CalculatorProxy proxy = new CalculatorProxy(new MyTestLogger(), new Calculator());
    assertThat(proxy.addStatic(a, b)).isEqualTo(a + b);
  }

  private static class MyTestLogger implements TestLogger {
    @Override
    public void onMethodCallStart(String method, Object... arguments) {
      String args = Arrays.stream(arguments)
        .map(Objects::toString)
        .collect(Collectors.joining(", ", "(", ")"));
      System.out.println("---> " + method + args);
    }

    @Override
    public void onMethodReturn(String method, Object returnValue) {
      System.out.println("<--- " + returnValue);
    }
  }