CODEWITHSUNDEEP
aopaspectj
aepurniet
aepurniet

Reputation: 1727

declare ITD static method on multiple types

I would like to declare a static method (like void main(String..args)) on every subtype of my App class.

public aspect Aspects pertypewithin(App+) {

    protected Class appClass;

    after() : staticinitialization(App+) && !staticinitialization(App) {
        StaticPart point = thisJoinPointStaticPart;
        Signature signature = point.getSignature();
        Class declaringType = signature.getDeclaringType();
        this.appClass = declaringType;
    }

    public static void App.main(String...args) {
        // how do i make this appear on every subtype of App, not just App
    }

}

Is this possible with AspectJ?

Upvotes: 0

Views: 348

Answers (1)

kriegaex
kriegaex

Reputation: 67297

The usual pattern for adding a set of non-static methods to multiple classes is to define an interface + implementing methods within an aspect and use declare parents in order to make the target classes implement the interface.

Unfortunately this does not work for static methods like main because static methods cannot be defined via an interface. On the other hand, if you have a class MyApp extends App you can call java -cp ... MyApp, i.e. its parent main method will automatically be used.

Now let us assume that for some reason you want the contents of the generated main methods to be somehow different. In this case you really need to generate one method per class. For that purpose you can use a new feature introduced in AspectJ 1.8.2 and described in the release notes: annotation processing support. Here is some self-consistent sample code. I compiled it from the command line because from Eclipse I failed to get it running according to AspectJ maintainer Andy Clement's description.

Okay, let's say we have two project directories somewhere in a base directory called java-src. The directory layout be like this:

java-src
    SO_AJ_ITD_AddMainMethodToAllSubclasses_AJ
        src\de\scrum_master\app\App.java
        src\de\scrum_master\app\BarApp.java
        src\de\scrum_master\app\FooApp.java
        compile_run.bat
    SO_AJ_ITD_AddMainMethodToAllSubclasses_APT
        src\de\scrum_master\app\EntryPoint.java
        src\de\scrum_master\aspect\EntryPointProcessor.java
        src\META-INF\services\javax.annotation.processing.Processor

In project SO_AJ_ITD_AddMainMethodToAllSubclasses_AJ we have our Java classes:

package de.scrum_master.app;

@EntryPoint
public class App {
    public void doSomething() {
        System.out.println("Doing something");
    }
}

package de.scrum_master.app;

public class FooApp extends App {
    public void doFoo() {
        System.out.println("Doing foo");
    }

    public int add(int a, int b) {
        return a + b;
    }
}

package de.scrum_master.app;

public class BarApp extends App {
    public void doBar() {
        System.out.println("Doing bar");
    }

    public int multiply(int a, int b) {
        return a * b;
    }
}

In project SO_AJ_ITD_AddMainMethodToAllSubclasses_APT we have our marker annotation, the annotation processor and processor description file for the META-INF directory of our processor.jar:

Please note: The annotation is @Inherited when applied to classes. This is important in order to make the annotation processor find it.

package de.scrum_master.app;

import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Inherited
public @interface EntryPoint {}

The annotation processor creates one aspect per annotated class. Each aspect does the following:

  • Add a main method which
  • instantiates an object of the target class,
  • always calls the base method doSomething() upon the target instance,
  • searches the target class for other simple non-static methods without parameters and calls these methods, just so as to show some fancy and dynamic stuff and make the resulting aspects look a bit different.
package de.scrum_master.aspect;

import java.io.*;
import javax.tools.*;
import java.util.*;
import javax.annotation.processing.*;
import javax.lang.model.*;
import javax.lang.model.element.*;

import de.scrum_master.app.EntryPoint;

@SupportedAnnotationTypes(value = { "*" })
@SupportedSourceVersion(SourceVersion.RELEASE_7)
public class EntryPointProcessor extends AbstractProcessor {
    private Filer filer;

    @Override
    public void init(ProcessingEnvironment env) {
        filer = env.getFiler();
    }

    @Override
    public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {
        // Discover elements marked with @EntryPoint
        for (Element element : env.getElementsAnnotatedWith(EntryPoint.class)) {
            // Skip non-class elements
            if (element.getKind() != ElementKind.CLASS)
                continue;

            String packageName = element.getEnclosingElement().toString().substring(8);
            String className = element.getSimpleName().toString();
            String aspectName = "MainMethodAspect_" + className;

            // For each marked class, create an aspect adding a 'main' method
            String aspectSource = createAspectSource(element, packageName, className, aspectName);
            writeAspectSourceToDisk(element, packageName, aspectName, aspectSource);
        }
        return true;
    }

    private String createAspectSource(Element element, String packageName, String className, String aspectName) {
        String variableName = className.substring(0, 1).toLowerCase() + className.substring(1);

        StringBuilder aspectSource = new StringBuilder()
            .append("package " + packageName + ";\n\n")
            .append("public aspect " + aspectName + " {\n")
            .append("    public static void " + className + ".main(String[] args) {\n")
            .append("        " + className + " " + variableName + " = new " + className + "();\n")
            .append("        " + variableName + ".doSomething();\n");

        for (Element childElement : element.getEnclosedElements()) {
            // Skip everything which is not a non-static method
            if (childElement.getKind() != ElementKind.METHOD || childElement.getModifiers().contains(Modifier.STATIC))
                continue;
            ExecutableElement method = (ExecutableElement) childElement;
            // Skip methods with parameters or named 'doSomething'
            if (!method.getParameters().isEmpty() || method.getSimpleName().toString().equals("doSomething"))
                continue;
            // Add call to found method
            aspectSource.append("        " + variableName + "." + method.getSimpleName() + "();\n");
        }

        aspectSource
            .append("    }\n")
            .append("}\n");

        return aspectSource.toString();
    }

    private void writeAspectSourceToDisk(Element element, String packageName, String aspectName, String aspectSource) {
        try {
            JavaFileObject file = filer.createSourceFile(packageName + "." + aspectName, element);
            file.openWriter().append(aspectSource).close();
            System.out.println("Generated aspect " + packageName + "." + aspectName + " to advise " + element);
        } catch (IOException ioe) {
            // Message "already created" can appear if processor runs more than once
            if (!ioe.getMessage().contains("already created"))
                ioe.printStackTrace();
        }
    }
}

The processor description file src\META-INF\services\javax.annotation.processing.Processor for APT looks like this:

de.scrum_master.aspect.EntryPointProcessor

How to compile and run: Last, but not least here is a (Windows) batch file SO_AJ_ITD_AddMainMethodToAllSubclasses_AJ\compile_run.bat which

  • compiles the APT project and packages it into a JAR,
  • compiles the other project including APM aspect code generation and finally
  • runs each of the three Java classes, testing if their main methods actually work as expected:
@echo off

set SRC_PATH=C:\Users\Alexander\Documents\java-src
set ASPECTJ_HOME=C:\Program Files\Java\AspectJ

echo Building annotation processor
cd "%SRC_PATH%\SO_AJ_ITD_AddMainMethodToAllSubclasses_APT"
rmdir /s /q bin
del /q processor.jar
call "%ASPECTJ_HOME%\bin\ajc.bat" -8 -sourceroots src -d bin -cp "c:\Program Files\Java\AspectJ\lib\aspectjrt.jar"
jar -cvf processor.jar -C src META-INF -C bin .

echo.
echo Generating aspects and building project
cd "%SRC_PATH%\SO_AJ_ITD_AddMainMethodToAllSubclasses_AJ"
rmdir /s /q bin .apt_generated
call "%ASPECTJ_HOME%\bin\ajc.bat" -8 -sourceroots src -d bin -s .apt_generated -cp "c:\Program Files\Java\AspectJ\lib\aspectjrt.jar";..\SO_AJ_ITD_AddMainMethodToAllSubclasses_APT\processor.jar

echo.
echo Running de.scrum_master.app.App
java -cp bin;"c:\Program Files\Java\AspectJ\lib\aspectjrt.jar" de.scrum_master.app.App

echo.
echo Running de.scrum_master.app.FooApp
java -cp bin;"c:\Program Files\Java\AspectJ\lib\aspectjrt.jar" de.scrum_master.app.FooApp

echo.
echo Running de.scrum_master.app.BarApp
java -cp bin;"c:\Program Files\Java\AspectJ\lib\aspectjrt.jar" de.scrum_master.app.BarApp

Console output: If you run the batch file, the output should look as follows:

Building annotation processor
Manifest wurde hinzugefügt
Eintrag META-INF/ wird ignoriert
META-INF/services/ wird hinzugefügt(ein = 0) (aus = 0)(0 % gespeichert)
META-INF/services/javax.annotation.processing.Processor wird hinzugefügt(ein = 43) (aus = 45)(-4 % verkleinert)
de/ wird hinzugefügt(ein = 0) (aus = 0)(0 % gespeichert)
de/scrum_master/ wird hinzugefügt(ein = 0) (aus = 0)(0 % gespeichert)
de/scrum_master/app/ wird hinzugefügt(ein = 0) (aus = 0)(0 % gespeichert)
de/scrum_master/app/EntryPoint.class wird hinzugefügt(ein = 430) (aus = 253)(41 % verkleinert)
de/scrum_master/aspect/ wird hinzugefügt(ein = 0) (aus = 0)(0 % gespeichert)
de/scrum_master/aspect/EntryPointProcessor.class wird hinzugefügt(ein = 5782) (aus = 2617)(54 % verkleinert)

Generating aspects and building project
Generated aspect de.scrum_master.app.MainMethodAspect_App to advise de.scrum_master.app.App
Generated aspect de.scrum_master.app.MainMethodAspect_BarApp to advise de.scrum_master.app.BarApp
Generated aspect de.scrum_master.app.MainMethodAspect_FooApp to advise de.scrum_master.app.FooApp

Running de.scrum_master.app.App
Doing something

Running de.scrum_master.app.FooApp
Doing something
Doing foo

Running de.scrum_master.app.BarApp
Doing something
Doing bar

If you look at the files generated by the annotation processor under SO_AJ_ITD_AddMainMethodToAllSubclasses_AJ\.apt_generated, you will find three classes looking like this (I am showing just one of them as a sample):

package de.scrum_master.app;

public aspect MainMethodAspect_FooApp {
    public static void FooApp.main(String[] args) {
        FooApp fooApp = new FooApp();
        fooApp.doSomething();
        fooApp.doFoo();
    }
}

Sorry for this lenghty answer, but other than creating a GitHub repo and just pointing there, I had to mention it all in order to make it reproducible.

Enjoy!

Upvotes: 1

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