After.java

public class Test{
  @interface var{}
  public Object a = new function().get();
  
  class function implements java.util.function.Supplier<Integer>{
    @Override
    public Integer get(){
      return 2;
    }
  }
  public static void main(String[] args){
    System.out.println(new Test().a);
  }
}

Test.java

public class Test{
  @interface var{}
  public var a = function();
  class function implements java.util.function.Supplier<Integer>{
    @Override
    public Integer get(){
      return 2;
    }
  }
  public static void main(String[] args){
    System.out.println(new Test().a);
  }
}

TestProcessor.java

import java.util.Set;

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.Processor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.annotation.processing.SupportedSourceVersion;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.TypeElement;

import com.google.auto.service.AutoService;
import com.sun.source.tree.MethodInvocationTree;
import com.sun.source.tree.VariableTree;
import com.sun.source.util.TreePath;
import com.sun.source.util.TreeScanner;
import com.sun.source.util.Trees;
import com.sun.tools.javac.model.JavacElements;
import com.sun.tools.javac.processing.JavacProcessingEnvironment;
import com.sun.tools.javac.tree.JCTree.JCExpression;
import com.sun.tools.javac.tree.JCTree.JCMethodInvocation;
import com.sun.tools.javac.tree.JCTree.JCNewClass;
import com.sun.tools.javac.tree.JCTree.JCVariableDecl;
import com.sun.tools.javac.tree.TreeMaker;
import com.sun.tools.javac.util.List;

@AutoService(Processor.class)
@SupportedSourceVersion(SourceVersion.RELEASE_8)
@SupportedAnnotationTypes("*")
public class TestProcessor extends AbstractProcessor {

	@Override
	public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
		JavacProcessingEnvironment environment = (JavacProcessingEnvironment) processingEnv;
		Trees trees = Trees.instance(environment);
		roundEnv.getRootElements().stream().map(trees::getPath).map(TreePath::getCompilationUnit).forEach((t) -> {
			t.accept(new Visitor(environment), null);
		});
		return false;
	}

	private class Visitor extends TreeScanner<Void, Void> {
		public Visitor(JavacProcessingEnvironment environment) {
			super();
			maker = TreeMaker.instance(environment.getContext());
			elements = JavacElements.instance(environment.getContext());
		}

		TreeMaker maker;
		JavacElements elements;

		@Override
		public Void visitVariable(VariableTree node, Void p) {
			System.out.println(node);
			if (node instanceof JCVariableDecl) {
				JCVariableDecl declaration = (JCVariableDecl) node;
				System.out.println(declaration);
				System.out.println(declaration.vartype);
				System.out.println(declaration.getClass());
				JCExpression expression = declaration.getInitializer();
				if (expression != null) {
					expression.getTree().accept(new TreeScanner<Void, Void>() {

						public Void visitMethodInvocation(MethodInvocationTree methodInvocationTree, Void p2) {
							JCMethodInvocation methodInvocation = (JCMethodInvocation) methodInvocationTree;
							System.out.println(methodInvocation.getMethodSelect().toString());
							JCNewClass class1 = maker.NewClass(null, (List.<JCExpression> nil()),
									maker.Ident(elements.getName(methodInvocation.getMethodSelect().toString())),
									List.<JCExpression> nil(), null);
							JCMethodInvocation invocation = maker.Apply(List.nil(),
									maker.Select(class1, elements.getName("get")), List.nil());
							System.out.println(invocation);
							declaration.init = invocation;
							System.out.println("methodInvocationTree2");
							return null;
						};
					}, null);
				}
				if (declaration.vartype.toString().equals("var")) {
					JCExpression id = maker.Ident(elements.getName("java"));
					id = maker.Select(id, elements.getName("lang"));
					id = maker.Select(id, elements.getName("Object"));
					System.out.println(id);
					declaration.vartype = maker.Ident(elements.getTypeElement("java.lang.Object"));
				}
			}
			return null;
		}
	}

}