For almost every function I wrote in Java, I add null checks to the method header as follows:

import javax.annotation.Nonnull;
import static com.google.common.base.Preconditions.checkNotNull;

public void foo(@Nonnull String bar, @Nonnull String baz) {
    checkNotNull(bar, "bar == NULL");
    checkNotNull(baz, "baz == NULL");
    // ...
}

When the code is examined, checkNotNull calls look totally redundant given that the parameters are already annotated with @Nonnull. However, @Nonnull just provides an insight to the static code analysis tools and does not have an effect at runtime, which makes checkNotNull calls necessary. I thought as programmers we should be able to do better than this and found myself working on a way to intercept methods with one or more @Nonnull annotated parameters and check if they are null at runtime. Finally, I managed to find a solution using Spring AOP. All you have to do is it include spring-boot-starter-aop artifact in your dependencies, add the following Spring aspect to your project, and you are done.

import org.aspectj.lang.JoinPoint;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;
import org.aspectj.lang.reflect.CodeSignature;
import org.aspectj.lang.reflect.MethodSignature;
import org.springframework.stereotype.Component;

import javax.annotation.Nonnull;
import java.lang.annotation.Annotation;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

@Aspect
@Component
public class NullMonitor {

    private static class MethodArgument {

        private final int index;
        private final String name;
        private final List<Annotation> annotations;
        private final Object value;

        private MethodArgument(
                int index,
                String name,
                List<Annotation> annotations,
                Object value) {
            this.index = index;
            this.name = name;
            this.annotations = Collections.unmodifiableList(annotations);
            this.value = value;
        }

        public int getIndex() { return index; }

        public String getName() { return name; }

        public List<Annotation> getAnnotations() { return annotations; }

        public boolean hasAnnotation(Class<? extends Annotation> type) {
            for (Annotation annotation : annotations)
                if (annotation.annotationType().equals(type))
                    return true;
            return false;
        }

        public Object getValue() { return value; }

        public static List<MethodArgument> of(JoinPoint joinPoint) {
            List<MethodArgument> arguments = new ArrayList<>();
            CodeSignature codeSignature = (CodeSignature) joinPoint.getSignature(); 
            String[] names = codeSignature.getParameterNames();
            MethodSignature methodSignature =
                    (MethodSignature) joinPoint.getStaticPart().getSignature();
            Annotation[][] annotations = methodSignature.getMethod().getParameterAnnotations();
            Object[] values = joinPoint.getArgs();
            for (int i = 0; i < values.length; i++)
                arguments.add(new MethodArgument(
                        i, names[i], Arrays.asList(annotations[i]), values[i]));
            return Collections.unmodifiableList(arguments);
        }

    }

    @Before("execution(* *(.., @javax.annotation.Nonnull (*), ..))")
    public void nullCheck(JoinPoint joinPoint) {
        MethodSignature methodSignature = (MethodSignature) joinPoint.getSignature();
        for (MethodArgument argument : MethodArgument.of(joinPoint))
            if (argument.hasAnnotation(Nonnull.class) && argument.getValue() == null)
                throw new NullPointerException(String.format(
                        "%s: argument \"%s\" (at position %d) cannot be null",
                        methodSignature.getMethod(), argument.getName(), argument.getIndex()));
    }

}

The source code is available in null-check project at GitHub with some unit tests and a little bit more detailed installation instructions.