is there any way of replace this code of this way?

Question

I' am trying to replace this code:

public void doThing(){
    if(User.getInstance().isLoggin()){
    .....
    }
}

by this

@withUser
public void doThing(){
    ....
}

I saw about the interceptors and annotation, but I cannot do it.

Is this possible?

Answer 1

It is absolutely possible. Project Lombok provides the exact feature what you are looking for. You just need to create your own annotation by extending EclipseAnnotationHandler or JavacAnnotationHandler depending upon your requirement.

A sample annotation handler (WithUser) is given below for reference. If used on method, the entire method will become enclosed by if(false){...} block. Replace false with your own expression on Handler (HandleWithUser.java).

@WithUser
public void doThing(){
    System.out.println("Hello World");
}

After annotation processing

public void doThing(){
    if (false){
        System.out.println("Hello World");
    }
}

Setup Project Lombok

git clone https://github.com/rzwitserloot/lombok.git
cd lombok
ant eclipse

Open the Lombok project on Eclipse and create the following files.

WithUser.java

package lombok;

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

@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
@Retention(RetentionPolicy.SOURCE)
public @interface WithUser {    
}

HandleWithUser.java

package lombok.javac.handlers;

import static lombok.javac.handlers.JavacHandlerUtil.setGeneratedBy;
import org.eclipse.jdt.internal.compiler.ast.ASTNode;
import org.eclipse.jdt.internal.compiler.ast.AbstractMethodDeclaration;
import org.eclipse.jdt.internal.compiler.ast.Annotation;
import org.eclipse.jdt.internal.compiler.ast.IfStatement;
import org.eclipse.jdt.internal.compiler.ast.Statement;
import org.mangosdk.spi.ProviderFor;
import com.sun.tools.javac.tree.JCTree;
import com.sun.tools.javac.tree.JCTree.JCAnnotation;
import com.sun.tools.javac.tree.JCTree.JCIf;
import com.sun.tools.javac.tree.JCTree.JCLiteral;
import com.sun.tools.javac.tree.JCTree.JCMethodDecl;
import com.sun.tools.javac.tree.JCTree.JCStatement;
import com.sun.tools.javac.util.Context;
import com.sun.tools.javac.util.List;
import org.eclipse.jdt.internal.compiler.ast.FalseLiteral;
import lombok.WithUser;
import lombok.core.AnnotationValues;
import lombok.javac.JavacAnnotationHandler;
import lombok.javac.JavacNode;
import lombok.javac.JavacTreeMaker;

@ProviderFor(JavacAnnotationHandler.class)
public class HandleWithUser extends JavacAnnotationHandler<WithUser>{

    @Override
    public void handle(AnnotationValues<WithUser> annotationValues, JCAnnotation ast, JavacNode annotationNode) {
        JavacNode annotation = annotationNode.up();
        JCMethodDecl method = (JCMethodDecl)annotation.get();
        List<JCStatement> contents = (List<JCStatement>) method.body.stats;
        JCTree source = annotation.get();
        JavacTreeMaker maker = annotationNode.getTreeMaker();
        JCLiteral falseLiteral = maker.Literal(false);
        JCIf ifStatement = maker.If(falseLiteral, maker.Block(0, contents), null);
        Context context = annotationNode.getContext();
        JCStatement ifBlock = setGeneratedBy(ifStatement, source, context);
        method.body.stats = List.of(ifBlock);
        annotation.rebuild();
    }

}

Generate lombok.jar

ant dist

Test

Make sure you are including lombok.jar in classpath

javac -cp lombok.jar Test.java

For more info, visit https://projectlombok.org/contributing/contributing